PRODUCTS AND RATES OF THE REACTION
OF SELECTED ARYL CHLOROFORMATES
WITH SILVER NITRATE

Thu}: §or HM Degree of DH. D.
MCEEQAN STATE UNEVERSITY
Matthew J. Zabik

1965

masts

 
 
 
 

 
  

LIBRARY

Michigan State
University

This is to certify that the

thesis entitled

PRODUCTS AND RATE OF THE RE‘CTION
OF 5mm ARYL GEOROFORMATES
WITH SILVER NITRATE

presented by

lhtthow J. Zabik

has been accepted towards fulfillment
of the requirements for

_ph.'_p.degree :1an

 

Date April 23, 1965

 

0-169

,,._-

ABSTRACT

PRODUCTS AND RATES OF THE REACTIONS OF SELECTED
ARYL CHLOROFORMATES WITH SILVER NITRATE

by Matthew J. Zabik

The purpose of this investigation was to obtain in-
formation concerning the nature of the reactions of aryl
chloroformates with silver salts including product analyses,
reaction kinetics, and mechanistic studies. Thirteen aryl
chloroformates were prepared by the reaction of phosgene
with the appropriate phenol. The product studies and re-
action kinetics of the reaction of the aryl chloroformates
with silver nitrate were conducted in acetonitrile as a
reaction medium. The reaction kinetics were determined for
the 4-substituted phenyl chloroformates with equal molar
concentrations of reactants and also in the presence of
excess silver nitrate.

The products obtained in high yield from the reaction
of the 4—substituted phenylchloroformates (4—methoxy,
4-methyl, 4-phenyl, 4-bromo, 4-chloro, 4—nitro) with silver
nitrate were the 4-substituted-2—nitro phenols. From the
interaction of 2,6-disubstituted phenylchloroformates (2.6-

dimethyl, 2,6-diisopropyl) with silver nitrate, two products

Matthew J. Zabik

were obtained; the expected 4—nitro-2,6—disubstituted phenols
and biphenyl quinones. The 2,4,6-trisubstituted phenylchloro-
formate (2,4,6-trimethyl) on reaction with silver nitrate
yielded substituted stilbenequinones as the major product.

The d— and 8-naphthylchloroformates on treatment with silver
nitrate yielded 2-nitro-1-naphthol and 1-nitro—2-naphthol,
respectively, as the major products.

At equal molar concentrations the initial rates of
reaction of the 4—substituted phenylchloroformates with silver
nitrate at 10, 21, and 51° were found to be second order.

In the presence of a five—fold molar excess of silver nitrate,
the initial rates at 21° were determined to be pseudo first
order. This was accepted as good evidence that the rate
determining step is the initial reaction of the 4—substituted
phenylchloroformate with silver nitrate to form a nitrato-
carbamate.

The energy of activations determined in the rate con-
trolling step of the reaction of the 4-substituted phenyl
chloroformates with silver nitrate varied from 8.0 kcal/mole
for the 4-nitro to 18.8 kcal/mole for the 4-methoxy substit-
uent. The change in entropy, AS, for the same reactions were
found to be negative and ranged from —40.7 for the 4-nitro
to —5.1 for the 4-methoxy substituent. A plot of the Hammett
equation, log (fi%0 versus 6‘, for the reaction of the 4—sub-

stituted phenylchloroformates gave good straight line

Matthew J. Zabik

plots with positive slopes,v/9, which had values of 1.5017
at 10°, 1.1482 at 21°, and 0.9211 at 51° for the reactions
at equal molar concentrations of reactants and 0.9583 at 210
for the reaction in excess silver nitrate. This indicates
a negative charge was developed in the transistion state.
The isokinetic temperature was determined to be 52° which is
well above the highest reaction temperature used in the
present investigation.

Several cursory investigations involving the re—
actions of 4—substituted phenylchloroformates with silver

acetate and silver trifluoroacetate are also described.

PRODUCTS AND RATES OF THE REACTION OF SELECTED

ARYL CHLOROFORMATES WITH SILVER NITRATE

BY

Matthew J? Zabik

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1965

ACKNOWLEDGMENT S

The author wishes to express his sincere
appreciation to Professor Robert D. Schuetz for his en—
couragement, guidance and friendship throughout the
course of this investigation.

Grateful acknowledgment is also extended to
Professor Morley Russell for his many helpful discus-

sions and suggestions.

***************

ii

TABLE OF CONTENTS

Page
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1
RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . 5
I. Preparation of Aryl Chloroformates. . . . . . . 5
II. Product Analyses. . . . . . . . . . . . . . . . 7

A. Products from the Reaction of Phenylchloro-

B.

C.

D.

formate with Silver Nitrate. . . . . . . . . 7
Products from the Reaction of p—Substituted
Phenylchloroformates with Silver Nitrate . . 25
Products from the Reaction of d- and fi-
Naphthylchloroformates with Silver Nitrate . 26
Products from the Reaction of 2,6-Di-sub-
stituted Phenylchloroformates with Silver
Nitrate. . . . . . . . . . . . . . . . . . . 26
Products from the Reaction of 2,4,6-Tri-
methylphenylchloroformates with Silver

Nitrate. . . . . . . . . . . . . . . . . . . 28

F. Products from the Reactions of p-Substituted
Phenylchloroformates with Silver Acetate or
Silver Trifluoroacetate. . . . . . . . . . . 50
G. Products Obtained from the Reaction of
p-Phenylphenylchloroformate with Silver
Nitrate in the Presence of Various Trapping
Agents . . . . . . . . . . . . . . . . . . . 52
H. Carbon Dioxide Analyses. . . . . . . . . . . 55
I. Silver Chloride Analyses . . . . . . . . . . 56
III. Rates of the Reactions of p-Substituted Phenyl-
chloroformates with Silver Nitrate. . . . . . . 57
EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . . . 59
I. Reagents. . . . . . . . . . . . . . . . . . . . 59
A. Acetonitrile . . . . . . . . . . . . . . . . 59
B. d-Methylstyrene. . . . . . . . . . . . . . . 59
C. Galvinoxyl . . . . . . . . . . . . . . . . . 59
D. Phenylchlorothiolformate . . . . . . . . . . 59

iii

TABLE OF CONTENTS — Continued

II.

III.

E. Standard Silver Nitrate. . . . . . . . . .
F. Standard Tetrabutylammonium Hydroxide. . . .
G. Silver Acetate . . . . . . . . . . .
H. Silver Trifluoroacetate. . . . . . . . . . .
Preparation of Aryl Chloroformates. . . . . . .
A. Typical Preparation. . . . . . . . . .
B. Attempted Preparation of 2, 6—Di- t- -Butyl—
phenylchloroformate. . . . . . . . . . . . .
Product Analyses. . . . . . . . . . . . . . . .
A. Reaction of Naphthyl and p-Substituted
Phenylchloroformates with Silver Nitrate . .
B. Reaction of the Hindered Phenylchloro-
formates with Silver Nitrate . . . . . . .
1. Reaction of 2,6-Dimethylphenylchloro-
formate with Silver Nitrate . . . . . . .
2. Reaction of 2,6-Diisopropylphenylchloro-
formate with Silver Nitrate . . . . . . .
5. Reaction of 2,4,6—Trimethylphenylchloro-
formate with Silver Nitrate . . . . . . .
C. Reaction of Phenylchlorothiolformate . . .
D. Trapping Experiments . . . . . . . . . . . .
1. Reaction of p-Phenylphenylchloroformate
with Silver Nitrate in the Presence of
Sodium p-Methoxyphenoxide . . . . . . . .
2. Reaction of p-Phenylphenylchloroformate
with Silver Nitrate in the Presence of
p-Methoxyphenol . . . . . . . . . . . . .
5. Reaction of p—Phenylphenylchloroformate
with Silver Nitrate in the Presence of
d-Methylstyrene and Galvinoxyl. . . . .
E. Reaction of p-Substituted.Phenylchloro-
formates with Silver Salts Other than Silver
Nitrate. . . . . . . . . . . . . . . . . . .
1. Reaction of p-Substituted Phenylchloro-
formate with Silver Acetate . . . . . . .
2. Reaction of p-Substituted Phenylchloro-
formate with Silver Trifluoroacetate. . .
F. Reactions of p—Methoxyphenylchloroformate

with Silver Nitrate in the Presence of
Pyridine . . . . . . . . . . . . . . . . .

iv

Page
59
60
60
60
60
60
61

65

65
65
65
67
67

68
69

69

7O

71

71
71

71

72

TABLE OF CONTENTS - Continued

Page

IV. Kinetics of the Reactions of p-Substituted
Phenylchloroformates with Silver Nitrate. . . . 75
V. Miscellaneous Preparations. . . . . . . . . . . 78

A. Preparation of Sodium p-Methoxyphenoxide . . 78

B. Preparation of p-Methylphenylcarbamate . . . 78
SUMMARY. . . . . . . . . . . . . . . . . . . . . . . . . 79
LITERATURE CITED . . . . . . . . . . . . . . . . . . . . 81

GRAPH

1.

10.

11.

LIST OF GRAPHS

Page
Titration Curve for a Mixture of o- and p-nitro-
phenol with Tetrabutylammonium Hydroxide (Glass
versus Calomel Electrodes). . . . . . . . . . . . 22

Typical Beers Law Plot (1778 cm'1 p-methoxyphenyl-

chloroformate absorption) . . . . . . . . . . . . 58

Second Order Kinetic Plots for the Reactions of
p—Substituted Phenylchloroformate with Silger
Nitrate (Equal Molar Concentrations) at 10 . . . 42

Second Order Kinetic Plots for the Reactions of
p-Substituted Phenylchloroformate with Silver
Nitrate (Equal Molar Concentrations) at 210 . . . 45

Second Order Kinetic Plots for the Reaction of
p-Substituted Phenylchloroformate with Silver
Nitrate (Equal Molar Concentrations) at 510 . . . 44

Pseudo First Order Kinetic Plot at 210 for the
Reaction of p-Substituted Phenylchloroformates
with a Five-fold Excess of Silver Nitrate . . . . 47

Effect of Water on the Rate of Reaction of
p—Methoxyphenylchloroformate with Silver Nitrate. 48

Effect of Pyridine on the Rate of Reaction of
p-Methoxyphenylchloroformate (0.10 mole) with
Silver Nitrate (0.10 mole). . . . . . . . . . . . 51

Graph of AS* versus AH* for the Determination of

the Isokinetic Temperature for the Reaction of
p-Substituted Phenylchloroformates with Silver
Nitrate (Equal Molar Concentrations). . . . . . . 52

Plot of the.Hammett Equation for the Reaction of
p-Substituted Phenylchloroformates with Silver

. o
N1trate (Equal Molar Concentrations) at 10 . . . 55
Plot of the Hammett Equation for the Reaction of
p-Substituted Phenylchloroformates with Siéver
Nitrate (Equal Molar Concentrations) at 21 . . . 54

vi

LIST OF GRAPHS — Continued

GRAPH

12.

15.

Page
Plot of the Hammett Equation for the Reaction of
p-Substituted Phenylchloroformates with Silver
Nitrate (Equal Molar Concentrations) at 51 . . . 55
Plot of the Hammett Equation for the Reaction of
p-Substituted Phenylchloroformates with a Five-
fold Excess of Silver Nitrate at 21° . . . . . . 56

vii

LIST OF INFRARED SPECTRA

SPECTRA
1. Phenylchloroformate . . . . . . . . . . . . .
2. p—Methoxyphenylchloroformate. . . . . . . . . .
5. p-Methylphenylchloroformate . . . . . . . . .
4. p-Phenylphenylchloroformate . . . . . . . . .
5. p-Bromophenylchloroformate. . . . . . . . . . .
6. p-Chlorophenylchloroformate . . . . . . . . . .
7. p-Nitrophenylchloroformate (Mull) . . . . . . .
8. d-Naphthylchloroformate . . . . . . . . . . . .
9. d-Naphthylchloroformate (Mull). . . . . . . .
10. 2,6-Dimethy1phenylchloroformate . . . . . . . .
11. 2,6-Diisopropylphenylchloroformate. . . . . .
12. 2,4,6—Trimethylphenylchloroformate. . . . . . .
15. Phenylchlorothiolformate. . . . . . . . . . . .
14. 5,5,5',5'-Tetramethylstilbenequinone-(4,4')
(Mull). . . . . . . . . . . . . . . . . . . . .
15. 2-Hydroxy-5—phenylphenyl Trifluoromethyl Ketone
16. Typical Kinetic Run: Reaction of p-Methoxy-

phenylchloroformate with Silver Nitrate (Equal
Molar Concentration) at 210 . . . . . . . . .

viii

Page

10
11
12
15
14
15
16
17
18
19

20

29

55

41

LIST OF TABLES

TABLE Page
I. Physical Data for the Aryl Chloroformates . . . . 6
II. Product Data from the Reaction of p-Substituted
Phenyl and Naphthyl Chloroformates with Silver

Nitrate . . . . . . . . . . . . . . . . . . . . . 24

III. Products Obtained from the Reaction of p-Substi-
tuted Phenylchloroformates with Silver Acetate. . 51

IV. Initial Rate Constants and Activation Parameters
for the Reaction of p-Substituted Phenylchloro-
formates with Silver Nitrate. . . . . . . . . . . 49
V. Initial Pseudo First Order Rate Constants for the
Reaction of p—Substituted Phenylchloroformates
with a Five—fold Excess of Silver Nitrate at 21°. 50

VI. Elemental Analyses of the Aryl Chloroformates . . 62

ix

LIST OF DIAGRAMS

DIAGRAM
1. Carbon Dioxide Absorption Train . .

2. Variable Temperature Infrared Cell.

Page
64

74

INTRODUCT ION

Although considerable research has been reported on
aryl chloroformate, very little investigational work has
been done with reactionsof aryl chloroformates with inor-
ganic salts. Wolfrom and Chaney (1) in 1960, reported on
an attempt to prepare phenyl nitrate by the interaction of
phenylchloroformate with silver nitrate. These investi-
gators obtained o-nitrophenol in a 65% yield as a rearrange-
ment product instead of the expected aryl nitrate. Since
their initial article, no other reports concerned with this
type of aromatic chloroformate rearrangement reaction have
appeared in the literature. The purpose of the present
investigation was to obtain information concerning the
-nature of the reactions of aryl chloroformates with silver
salts including product analysis, reaction kinetics, and
mechanistic studies.

The aryl chloroformates were conveniently prepared
(2,5) by the addition of dimethylaniline to a stirred solu—
tion of phenol and liquid phosgene in benzene as a reaction
media. The“products are obtained pure by distillation or
recrystallization. The tertiary amine is essential for the
reaction to occur at a reasonable rate. Methanistically it
has been postulated (4) that the amine initially forms a
complex with phosgene in addition to binding with the

1

hydrogen chloride
I I g _
-N: + COClg —-——:>- —N: —C1 + Cl ———§- product
I ' .
+ -N:HC1

reaction product. It has also been found that aryl chloro-

formates react with tertiary amines to yield a complex.

9

7‘ _ -O-8-Cl -O-C: N-
l

+ —N: -—-$¥ Cl

 

In contrast to the lack of information on the aryl
chloroformates, considerable research has been reported
on the reactions of aliphatic chloroformates. Some of this
work has a direct bearing on the reactions of aryl chloro—
formates with silver salts. In 1959 Boschan (5) reported
the preparation of aliphatic nitrate esters in high yields
by the reaction of silver nitrate with aliphatic chloro-
formates. He determined that 75% of the R-O bond in the
chloroformate remained intact by 018 labeling experiments
and that the reaction proceeded mainly with retention of

configuration (70% retention, 50% inversion) in the R group.

A

Rog-Cl 'l’ AgN03 J—Afl')‘ R-O > __>‘ RONOg + C02

9%

Hence, it was poStulated that an Sni-type of decomposition
of the intermediate was operative. Some reaction was thbughtto
occur by an ionic mechanism as shown in the following equa-

tion.

o G)
Rod-c1 + AgN03-—+>'AgCl + R0g€>—E93- RONOg

Mortimer (6) found in a study of alkyl nitratocarbonates

that reaction 2 roughly paralleled reaction 1 at moderate

o o
R-o-c-c1 + AgNo3 CH CN R—O-C-ONOE + AgCl (1)
o
R—o—5-0N02 —————->~R0No2 + €02 (2)

temperatures (40°), but was considerably slower than re-
action 1 at temperatures below 0°. However, reaction 1 was
too slow at temperatures below 0° to be of practical value.
He further reported that pyridine was an excellent catalyst
for the metathesis in step 1 of the overall reaction.

In a reaction conducted at -17° and examined by vapor phase
chromatography it was shown that only 40% of the chloro-
formate had reacted after two hours and that no carbon
dioxide evolution had been initiated. This indicated that
reaction 1 was taking place but not reaction 2. When the
reaction temperature was allowed to reach room temperature,
he found only nitrate product (100%) and thus reaction 2
must have occurred at the higher temperature.

Boschan (5) further reported that cholesteryl chloro—
formate reacts with silver trifluoroacetate to yield choles-
teryl trifluoroacetate (77%). This is another example of
a chloroformate reacting with silver salt to yield a re—

arranged product.

 

‘

o
c1-c-o “7* CF3— -0

An additional example of a similar type of a re-
arrangement reaction is that reported by Norris (7) in
which dialkylcarbamyl chlorides react with silver nitrate
to give dialkylnitramines (14w48%) as the main product.

He further determined the rate determining step to be
RgNg-Cl + AgN03 J9 RgNg’ONO‘g + AgCl (5)

R2N8*ON02 ig‘)‘ R2NNOg + C02 (4)

represented by equation (5) with a value of 1.5 x 10-2
B/mole sec at 58.90°, for the rate constant k1.

In all the aliphatic chloroformate rearrangement
reactions reported, it was postulated that the reaction
proceeded in two steps; reaction of the nitrate ion with
the chloroformate to yield a nitratocarbamate and silver
chloride followed by rearrangement of the nitratocarbamate

to yield the nitrate ester and carbon dioxide.

RESULTS AND DISCUSSION

I. Preparation of Aryl Chloroformates

The aryl chloroformates were conveniently prepared,
in half molar quantities, by the addition of N,N-dimethyl-
aniline to a stirred solution of the phenol and phosgene in
benzene as a reaction media. The benzene layer obtained on
addition of cold water to the reaction mixture was washed
with dilute hydrochloric acid, dilute sodium hydroxide,
water, and then dried over anhydrous magnesium sulfate.
Distillation of the aryl chloroformates with the exception of
p—nitrophenylchloroformate gave yields of pure products in
the range 85.4-98.8%. The lower yields were obtained with
the more hindered phenylchloroformates. Attempts to distill
p-nitrophenylchloroformate were unsuccessful even though the
literature (8) reports a boiling point for this material.

On two successive attempts to distill this compound at high
vacuum (0.1 mm) the product decomposed violently. However
the material was readily recrystallized from petroleum ether
(70.2%). The use of the tertiary amine as a catalyst in the
reaction is necessary to have any reasonable reaction rate
since in its absence the starting phenol is quantitatively

recovered. Table (I) summarizes the physical properties

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chloroformates prepared in the course of this investigation.

II. Product Analyses
A. Products from the Reaction of Phenyl-
chloroformates with Silver Nitrate
The reaction of phenylchloroformate with silver
nitrate was reinvestigated to determine whether o-nitrophenol
was the only product obtained as reported by Wolfrom and
Chaney (1). The reaction was conducted on a 0.1 mole scale,
at 20°, using acetonitrile as the reaction solvent. In sub-
sequent reactions, acetonitrile was the choice of reaction
solvents since it dissolves both the chloroformate and the
silver nitrate resulting in a homogeneous reaction solution.
The reaction can be run in a heterogeneous (ether) manner
but the results are not as reproducible. The acetonitrile
solution of the products obtained from the reaction was di-
luted to a specific volume and a 10 ml. aliquot was chromato-
gramed on an alumina column using anhydrous ethyl ether as 3
the elutant. The initial band from the column was light
yellow and was established as o—nitro phenol. On further
elution a second yellow band was obtained which was identified
as p-nitro phenol. The percent of each isomer present varied
from 65-70% and 15-20% ofcr-and p—nitro phenol, respectively.
The yields of carbon dioxide and silver chloride obtained

were 95.2% and 98.6%, respectively. No meta nitrogphenol

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was detected in the reaction products. The total nitro
phenolic content of the reaction mixture was determined by
titration with standard methanolic tetrabutylammonium
hydroxide (29,50). This procedure is incapable of dis-
tinguishing between the o and p-nitro phenols [See Graph (1)].
The total per cent yield (84%) calculated from this titration
corresponds very closely to the total yield obtained from

the column chromatography (84.7%). It was not surprising
that a para nitrated phenol was obtained since numerous

ortho rearrangement reactions reported in the literature

also give the corresponding para products. There are several
possible mechanisms by which this rearrangement reaction

could proceed.

N
/O-g-Cl CO N02 OH
\\k. (%\C=O \\\\EA 2
L. - I
gee
O-% + d})

ZN III8 /©\©
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25

In an effort to determine which mechanism is operative and
the scope of the reaction, a product, kinetic, and mechan-
istic investigation was undertaken of the reaction of

various aryl chloroformates with silver nitrate.

B. Products from the Reaction of p-Substituted
Phenylchloroformates with Silver Nitrate

 

The reaction of p-substituted phenylchloroformates
was carried out at 20° on a 0.05 mole basis using aceto-
nitrile as the reaction media. The p—substituents included
methyl, methoxy, phenyl, bromo, chloro, and nitro. In all
cases the products obtained were 4-substituted-2-nitro
phenols, produced in excellent yields (72.8% to 99.1%).

The products were obtained by the evaporation of the aceto-
nitrile followed by recrystallized of the crude product
from an appropriate solvent. Table (II), summarizes the
pertinent data for these reactions. In all reactions a
small quantity of tarry material was formed from which no
identifiable compounds could be isolated.

The same reactions were rerun using a five—fold ex-
cess of silver nitrate. The reactions proceeded consider-
ably faster than when equal molar-quantities of reactants
were used as evidenced by the visual observation of the rate
of precipitation of silver chloride. This increase in the
concentration of silver nitrate and enhanced rate of re—
action had no major effect on the yields of the 4—subr

stituted—2-nitrophenols as indicated in Table (II).

24

 

 

 

 

 

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25

The p—substituted phenylchloroformate reactions
were also conducted in the presence of pyridine. Here
again the reaction is considerably accelerated. In one run,
started at an initial temperature of 200 and an equal molar
amount of pyridine, the reaction commenced to boil after 30
seconds of reaction and was essentially complete in two
minutes. This reaction time compares to 3 or 4 hours for a
reaction run in the absence of pyridine. It is well—known

(4) that tertiary amines form complexes with aryl

+
'

ArO—fi-Cl + :1:\I- ———>- ArO— :Eq- c1-
chloroformates and this suggests the possibility that the
rate determining step in the rearrangement of aryl chloro—
formates with silver nitrate is the initial attack by the
nitrate ion on the chloroformate. The presence of pyridine
helps break the C-Cl bond and thus enhances the reaction
rate. It was determined that only a trace (0.001 ml) of
pyridine is required for this large rate enhancement, sug—
gesting that the pyridine is regenerated and is reused by
successive chloroformate molecules.

The fact that the yields in these rearrangement
reactions were quite high indicates that the reaction is a

good preparative procedure to obtain o-nitro phenols.

The two step process of preparing the chloroformate and its

26

subsequent rearrangement with silver nitrate gives overall
yields of 70-80% of mononitrated phenol. Besides the mild-
ness of the reaction conditions, this procedure is superior
to the classical nitration procedures employing mixtures

of nitric and sulfuric acids where not only mono nitration

but also large amounts of dinitrated products are obtained.

C. Products from the Reaction of d and 6 Naphthyl-
chloroformates with Silver Nitrate

The reactions of the naphthylchloroformates were
run at ZOOCH1810.05 mole scale in acetonitrile as a
reaction solvent. Following completion of the reaction the
acetonitrile was removed on a rotary evaporator and water
was added to the black oily reaction mixture. The aqueous
suspension on steam distillation yielded 58.6% of 1—nitro—
2—naphthol and 66.6% of 2—nitro—l-naphthol from B and d
naphthylchloroformates, respectively. In each case some
black tarry material remained after steam distillation.
This was chromatogrammed on an alumina column using ether
as the elutent, and in each case yielded numerous bands

which could not be identified.

D. Products from the Reaction of 2,6-Disubstituted
Phenylchloroformates with Silver Nitrate

Reactions of aryl chloroformates with substituents

blocking the ortho positions were conducted to determine if

27

only the para product resulted if a reaction occurred.
Compounds used in this work included the 2,6—dimethyl and
2,6—diisopropyl phenylchloroformates. The highly hindered
and ortho blocked 2,6—di-t-butylphenylchloroformate, an
obvious choice for this type of study, could not be pre-
pared, very probably due to steric hinderence. These re-
actions were run at 200 on a 0.05 mole scale.

On reaction with silver nitrate the 2,6—dimethyl—
phenylchloroformate yielded a redish acetonitrile solution
of the products. This solution on being chromatogramed
yielded 62.6% of 4—nitro—2,6—dimethyl phenol (p-rearrange-
ment, m.p. 172-30) and 0.78 g of a dark red compound which
decomposed at 2240. The latter material was identified as
3,5,3',5'-tetramethyldiphenoquinone-(4,4'), a quinone

dimer (24).

O: z :0

The 2,6-diiSOpropylphenylchloroformate, under the
same reaction conditions and product isolation as described
for the 2,6-dimethyl compound, yielded 4—nitro-2,6-diiso-
propyl phenol (73 %) and again a small amount of a dark
red colored compound identified as 5,5,5',5'-tetrai50pr0pyl

diphenoquinone— (4, 4 ) (24) which melted at Rfll4

i9

28

The fact that the p-nitro compound is the major
product indicates that the p-rearrangement is not hindered
to any great extent by blocking the ortho positions. The
similarity in yields between the diiSOpropyl and dimethyl
compounds further indicates that there is no significant
effect on the p-rearrangement due to the size of groups in

the ortho position.

E. Products from the Reaction of 2,4,6-Trimethyl—
phenylchloroformates with Silver Nitrate

 

The reaction of 2,4,6—trimethylphenylchloroformate
with silver nitrate was run to determine whether the nitro
group could be forced to enter the meta position. The same
experimental procedure was used as already described for
the other hindered phenylchloroformate reactions investi-
gated. As the reaction proceeded in this case, dark red
crystals separated from solution along with the precipi-
tation of the silver chloride. On completion of the re—
action the red crystals and silver chloride were collected
by filtration. Some of the red crystals were picked out of
the product mixture by hand and used in an attempt to de-
termine a solvent from which they could be recrystallized.
These were insoluble in all available solvents and the
silver chloride had to be dissolved away from them with
ammonium hydroxide. The crystals melted (decomposed) at

2450 and had the infrared spectrum shown in spectrum (14).

29

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50

The red crystalline compound was identified as 3,5,3',5'-

tetramethylstilbenequinone (4,4').

F. Products from the Reaction of p-Substituted
Phenylchloroformates with Silver Acetate or
Silver Trifluoroacetate

Reactions of aryl chloroformates with the silver
salts of acetic and trifluoroacetic acid were conducted
in the same experimental manner as with silver nitrate,
to determine whethertfluaacetyl or trifluoroacetyl groups
would enter the aromatic nucleus similarly to the nitro
group.

In the case of silver acetate the only organic
phenolic products obtained are the nonacylated phenols

(90% yields).

O-E-Cl OH

AgOAc \
CH3CN ’

The silver chloride was obtained in practically quanti-

tative yields.

31

Table III. Products Obtained From the Reaction of
p-Substituted Phenylchloroformates with
Silver Acetate

 

 

 

p-Substituted Phenol Percent Percent Yield

Phenylchloroformate Obtained Yield of AgCl
p-Phenyl p-Phenyl 97.5 99.8
p-Chloro p—Chloro 95.2 98.3
p-Methoxy p-Methoxy 96.3 98.5
p-Nitro p—Nitro 94.2 98.9

 

When silver trifluoroacetate was used, substituted
phenyl trifluoromethyl ketones were obtained in greater

than 95% yields. Conceivably this reaction could proceed

0-3-6

OH
3 J63
. + Ago- -CF3 ——>* + co.2 + AgCl
I
Y

Y

Y = phenyl and bromo

through the same type of postulated mechanism for the silver

nitrate.

E6

0-
90
AgN03 01::30 -——%> H'—1>'Product
Y C§ C-

CF3
o
C/F 3 Cy

The carbonyl carbon of the trifluoromethyl acetate is more
positive, due to the electron withdrawing effect (-I) of

the three fluorine atoms, than the acetate carbonyl and

52

therefore would be expected to participate more readily

in the cyclic rearrangement reactions postulated above.
This could account for the observation that silver acetate
failed to yield rearranged products in the reactions
studied. There is similarity between the nitro group and
the trifluoromethyl in that both have positive charges on
the atom involved in the migration (nitrogen atom and
carbonyl carbon atom, respectively). Thus, it appears that
this is a prerequisite for this type of a rearrangement to

occur .

G. Products Obtained from the Reaction of p-Phenyl—
phenylchloroformate with Silver Nitrate in the
Presence of Various Trapping Agents

 

 

 

Experiments were run in which p—methoxyphenol or
p-methoxyphenoxide were added to the reaction medium in an
effort to determine whether a nitronium ion (N029) was
formed in the reaction sequence which in turn reacted with

the aryl ring in an intermolecular process.

-8-C1FON/EUN02
+ A9N03 —> :§82 ——-2-:F>-©//O _'*_'_0_2;. Products

Para methoxy phenol was added to the reaction 10 minutes
after it was initiated, and at the conclusion of the re-
action product isolation was conducted in the usual manner.

The sole product isolated in these experiments was

53

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6:80

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(,2? 1166

 

 

 

54

2-nitro—4-phenylphenol in 95.1% yield. These results
showed that no intermolecular reaction had occurred.
However, this left unanswered the possibility that perhaps
a phenol group could not successfully compete with the inter-
mediate methoxide ion which is normally formed during any
reaction sequence that could produce the N02+ ion.
Therefore, sodium p-methoxyphenoxide was prepared and added
to the reaction medium containing p—phenylphenylchloroform-
ate and silver nitrate at the initiation of the reaction;
then, 10 minutes after the initiation of the reaction; and
finally 60 minutes after initiation of the reaction in an
attempt to trap any N02e ion that might be formed. An im—
mediate problem, encountered with the use of sodium
p-methoxyphenoxide, was that it reacts instantly with the
silver nitrate to yield a black precipitate. The latter
slowly (12 hrs.) changes in color to a light cream—colored
material. Thus, the overall mechanism for the reaction
between phenylphenylchloroformate and silver nitrate may
be changed and the results of this trapping experiment must
be taken with some reservation. In all three cases ex-
amined the sole organic product isolated in high yields
(97-99%) was 4—phenyl—2-nitrophenol, again indicating that
no intermolecular reaction had occurred. These results,
however, do not rule out the porsibility of a closely bound
ion pair where the N02Q if formed was not able to partici-

pate in an intermolecular reaction mechanism.

55

These reactions were also conducted in the presence
of d-methylstyrene and galvinoxyl to determine if the re-
action proceeded by a free radical process. In separate
experiments the galvinoxyl or d—methylstyrene were added at
the initiation of the reaction between p-phenylphenylchloro—
formate and silver nitrate. In both cases, the yield of
4-phenyl-2—nitrophenol ranged between 94-98%. These results
were again very close to the yield obtained without the ad-
dition of either trapping agent. This observation then
rules out the possibility of a free radical mechanism being
Operative in the rearrangement reaction under the experi—

mental conditions studied in the present investigation.

H. Carbon Dioxide Analysis

 

The quantitative determination of the carbon dioxide
liberated during the rearrangement reaction of the various
arylchloroformates studied was carried out on the reactions
conducted at 200 in acetonitrile as a reaction media. The
carbon dioxide evolved was absorbed on Ascarite and weighed
directly. The results are summarized in Table II.

These data indicate that the amount of carbon dioxide
evolved essentially follows the per cent yield of the various
nitrated phenol products formed in the rearrangement. Where
the yields of carbon dioxide are low as in the case of the

hindered phenols, the yield of nitrated phenol was also low.

56

I. Silver Chloride Analysis

 

A semi—quantitative determination of the silver
chloride was made by recovering the silver chloride formed
by filtration, washing it with acetonitrile and anhydrous
ether, drying in an oven at 1050 for 1 1/2 to 2 hours and
then weighing it directly. This procedure was of sufficient
accuracy for the purpose of this investigation. In all cases
the silver chloride was obtained in high yield indicating
that all the chlorine from the chloroformates is lost as
the chloride ion and forms silver chloride. From the me-
chanisms postulated the first step in the rearrangement is
always the loss of the chloride ion followed by either re-

arrangement or decomposition occurring after this step.

O-g-Cl o-E-ONog
+ AgN03 ——>- + AgCl

The above results would indicate that the first step is very
probably quantitative but that subsequent steps (elimination
of carbon dioxide) may or may not be depending on the particu—
lar chloroformate involved in the rearrangement. The yield
of carbon dioxide, lost in a subsequent step of the re—
action, follows the yield of product more closely and there-
fore is probably involved in the final step of the re—

arrangement.

57

III. Rates of the Reactions of the p—Substituted
Phenylchloroformates with Silver Nitrate

 

 

The reaction rates of the various p-substituted
phenylchloroformates were followed by determining the rate

1 chloroformate carbonyl peak

of disappearance of the 1777 cm“
in the infrared spectra. All the p—substituted phenylchloro—
formates examined in this study were checked and found to
follow Beer's Law in the concentrations used in this investi-
gation. Initially, the experimental analyses procedures

were attempted by conductiometric, amperometric, and carbon
dioxide evolution methods. However, these procedures proved
to be unreliable and not reproducible. The conductiometric
and amperometric analyses probably failed due to the use of
acetonitrile as the solvent for the reaction since both of
these procedures are very satisfactory when used in aqueous
solutions.

A Unicam S. P. 200 recording spectrOphotometer was
employed for the infrared measurements. The infrared cell
used was a Research and Industrial Instruments Company
variable temperature cell equipped with silver chloride
windows and modified to permit a sample to be injected or
withdrawn without disassembling the cell. Silver chloride
windows had to be employed since silver nitrate, one of the
reactants, would react with the normal cell windows of
sodium chloride or potassium bromide. Further, the silver

chloride window had an advantage in that the reaction does

58

 

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59

not come in contact with a salt different from that which
the reaction mixture already contained. ThUSq the cell
material could not affect the-rate of the reaction in any
way. A variable temperature cell had to be utilized in this
study since the kinetics were determined at three different
-temperatures, 10, 21, and 51°. The cell temperature-was
controlled to $0.50. Temperature in thewconstant temperature
water bath-was controlled by a Thyratron electronic relay to
i0.01°C and was equipped with a stirrer. The reaction flask
contained a carbon dioxide outlet; a liquid withdrawal tube
(fitted with a fine sintered glass filter on the end immersed
in the reaction solution and a standard taper female syringe
joint at the outlet); a magnetic stirrer bar whose rotational
velocity was controlled by a magnet fitted to an electric
motor (glass enclosed) and immersed in the constant tempera—
ture bath.

The kinetic determinations were conducted by placing
50 ml. of a p-substituted phenylchloroformate solution in a
reaction vessel supported in the constant temperature water
bath. The solution was allowed to reach the bath temperature.
A sample of this solution was injected into the variable
temperature infrared cell to obtain the transmittance of the
initial concentration. The cell was then washed with aceto-
nitrile and air-dried in preparation for the next determin-
ation. To the solution in the bath was injected 4 ml. of

standard silver nitrate solution. Samples were then withdrawn

40

at predetermined times and injected into the cell to determine
the transmittance of the reaction mixture. The time at Which
the minimum transmittance was reached was recorded. Follow-
ing each concentration time determination, the cell was
rinsed with acetonitrile and a acetonitrile versus acetonitrile
(100% line) was determined. This procedure was necessary
since silver chloride precipitates from the-reaction solution
in the cell-and as~a result would change the 100% transmit-
tance linea~-The cell was then dried with air in preparation
for the next determination. At the end of the kinetic run

the transmittance for the initial concentration was corrected
for dilution due to the addition of the four ml of silver
nitrate solution.

The per cent transmittance was determined by measur—
ing the distance between the 0% base line and the 100% line
(determined for each time-concentration determination) and
dividing this value by the distance between the 0% base line
and the tip of the transmittance peak and then multiplying
by 100.

The results of the kinetic determinations at equal
molar concentrations of the reactants on a second order plot
are shown in Graphs 5, 4, and 5 for determination at 10, 21,
and 510, respectively. There are a number of conclusions that
can be drawn from these plots. First, that as the reaction
temperature was increased, the results became more eratic,
especially at 510. Second, the plots for the determinations

at 10 and 210 are very comparable.

41

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42

 

 

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p—
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-CH
p-Cl\ P 3‘
p-OCHg
ioJ
i l L 1 l l
I l I l l I
0 1 2 5 4 5 6

t(sec) x 10"3

Graph 5. Second Order Kinetic Plots for the Reactions
of p—Substituted Phenylchloroformate with
Silver Nitrate (Equal Molar Concentrations)
at 10°.

45

 

 

 

o__”_

t(sec) x 10‘3

Graph 4. Second Order Kinetic Plot for the Reaction of
p-Substituted Phenylchloroformates with Silver
Nitrate (Equal Molar Concentration) at 21°.

50‘

 

Graph 5.

44

 

l
I

t(sec) x 10"3

Second Order Kinetic Plots for the Reactions
of p—Substituted Phenylchloroformates with
Silver Nitrate (Equal Molar Concentrations)
at 51°.

45

The most important conclusion to be drawn is that
there is no simple overall reaction order. However, if
only the first 20% of each of the kinetic determinations is
examined it is noted that these portions of the plots are
straight lines and follow a second order rate law.
Additionally, it is observed that in the initial 20% of the
reaction there is a definite increase in the reaction rate
in going from p-methoxy (+1) to p—nitro (-I) phenylchloro-
formate. Thus, it is emphasized that all of the following
data were determined from the initial 20% of the kinetic
determinations and that all subsequent discussion deals with
the initial rates (20% completion) and are not related to
overall rates of reaction. This is followed even in the
cases of p-methyl and p-phenylphenylchloroformate reactions,
both of which followed a second order plot ¢t>90% completion
at 10 and 21°.

Possibly the abnormalities were due to the auto-
catalytic effects of one of the products formed during the
reaction or to a surface area effect. To examine these
possibilities experimentally kinetic determinations were
conducted with; product added at the initiation of the
reaction, freshly precipitated silver chloride added at the
initiation of the reaction, and finally glass wool added to
the reaction flask. All three variables were determined to
have no effect on the rate of the reaction. However, these

observations do not rule out any effect which an unstable

46

intermediate could have on the reaction rate. The effect

of water in the reaction was also studied experimentally.

Water effectively decreased the rate of the reaction prob-
ably due to the formation of a heterogeneous mixture (the

chloroformate separates from the reaction solution on the

addition of water) (Graph 7).

Pseudo first order reaction rates for the p-substi-
tuted phenylchloroformates with silver nitrate were de-
termined by flooding the reaction with a five-fold excess of
silver nitrate at 21°. In this case first order plots gave
good straight lines for 80% of the reaction and then started
to drOp off. As in the previous cases the rate constant and
subsequent data were determined for the initial rates only.
The fact that pseudo first order kinetics were obtained
under these reaction conditions indicates that the silver
nitrate must take part in the rate determining step, of these
rearrangement reactions.

Collective examination of the second order and pseudo
first order kinetic data indicates that the first step in
the reaction sequence is rate determining.

The activation parameters indicate that the sub-
stituent on the benzene ring has a large effect on the over-
all rate of the reaction. The energy of activation varies
from 8.0 for p-nitro to 18.8 kcal/mole for p—methoxy phenyl-

chloroformate.

47

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49

Table IV. Initial Rate-Constants and Activation Parameters
for the Reaction of p-Substituted Phenylchloro-
formates with Silver Nitrate

 

 

 

p- Temp k4 x 103 Ea AS
Substituent °C fl/mole sec kcal/mole e.u.
Hydrogen 10 1.12
21 5.79 18.8 —6.7
51 5.01
Methoxy 10 0.54
21 1.27 20.1 -5.1
51 5.95
Methyl 10 0.72
21 2.16 19.6 -6.1
51 8.24
Phenyl 10 1.40
21 5.62 19.0 -6.6
51 14.82
Bromo 10 1.85
21 5.81 16.8 -15.7
51 14.74
Chloro 10 2.68
21 6.62 14.6 -20.9
51 16.54
Nitro 10 16.41
21 25.52 8.0 -40.7
51 44.41

 

The negative entropy (AS*) values indicate that there
is a decrease in the freedom of the reactants with a very
large AS* difference between the p-methoxy (AS* = -5.1)

*
and p-nitro (AS = —40.7) phenylchloroformate.

50

Table V. Initial Pseudo First Order Rate Constants for the
Reaction of p-Substituted Phenylchloroformates
with a Five—fold Excess of Silver Nitrate at 21°

 

 

Compound k; x 104/sec
p-Methoxyphenylchloroformate 6.57
p-Methylphenylchloroformate 7.54
p-Phenylphenylchloroformate 12.48
Phenylchloroformate 15.01
p-Bromophenylchloroformate 14.17
p-Chlorophenylchloroformate 21.28
p-Nitrophenylchloroformate 65.66

 

The rate of the reaction in the presence of pyridine
was also examined” As shown in Graph 8, the rate is indeed
enhanced but the overall shape of the curve is not changed
indicating that whatever causes the abnormalities of the
second order plot is not affected by the presence of pyridine
in the reaction medium.

From the physical data an isokinetic temperature for
the reaction was determined by plotting AS* versus AH*, see
Graph 9. The isokinetic temperature determined was 52.00
which is 210 above the highest temperature used in the
kinetic determinations. Thus, subsequent data for the
Hammett relationship should be valid.

A Hammett plot of the reaction rates at 10, 21, and
510 (equal molar concentration) give a reasonably straight
line when the p—é substituent values are plotted versus
log (ii). The values for p obtained were positive (100,

1.5017; 21°, 1.1482; 31°, 0.9211)(see Graphs 1o, 11, 1?).

51

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52

     
  

 

 

-4O 6
-50..
As*
e.v.
-20_.
m = T
* * * p—Br
AH = mAS + AF
*
AH* = 3.252 x 102 as + 2.074 x 10
—10_J , ,
F Isokinetic temperature T = 525.2:K
T = 52.0 C
P'CHs
G p-OCH3
O 1 i i i j i i
8 10 12 14 16 18 20

AH* x 10’3 cal/mole

Graph 9. Graph of AS* versus AH* for the determination
of the Isokinetic Temperature for the Reaction
of p-Substituted Phenylchloroformates with
Silver Nitrate (Equal Molar Concentrations).

55

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57

These values indicate that a negative charge is being
developed in the transition state and-since the p's are
relatively large a negative charge of some magnitude is be-
ing developed. In reaction with a five-fold excess of silver
nitrate, a p value of 0.9383 was obtained, again indicating
a negative charge was formed in the transition state.
Conclusions based on the experimental evidence de-
termined in an investigation of the rearrangements of aryl—
chloroformates in reaction with silver nitrate in acto—
nitrile as a reaction media are:
a) the major products are 2—nitro-4—substituted
phenols;
b) a free radical mechanism is not operative;
c) an intermolecular mechanism is not operative;
d) the reaction rate is enhanced by the addition of
pyridine to the reaction media;
e) the initial reaction rate is second order for
equal molar concentrations of silver nitrate
and aryl chloroformate;
f) the initial rate is pseudo first order in the
presence of excess silver nitrate;
g) the entropy change, AS, is negative, and
h) the value of p in the Hammett relationship
is positive.
These observations suggest the mechanism for the rearrange-

ment reaction of p-substituted phenylchloroformate with

58

silver nitrate in acetonitrile is a two step sequence,

0- E- c1 0— 5— -ON02
+ AgNOs —l—> + A9121
R/ R
o . /0 OH
_. /
\ , £1) 4 H r"
R N\ R R
d 0 N02

with step one as being rate determining,

EXPERIMENTAL

I. Reagents

A. Acetonitrile

 

Acetonitrile was distilled from calcium hydride
through a two-foot column packed with fine glass helices.
The fraction boiling in the range 81.1-81.5 was collected

and stored over calcium hydride until use.

B. d-Methylstyrene

 

Eastman's Reagent Grade d—methylstyrene was frac—
tionated through a glass helic-packed column and the
material boiling in the range from 164.5-165.5 was used

in the present investigation.

C. Galvinoxyl

 

Obtained from Sheldon Lande, and used as received.

D. Phenylchlorothiolformate

 

This compound was obtained from the Aldrich Chemi-

cal Company and was used as obtained.

E. Standard Silver Nitrate

 

Standard silver nitrate was prepared by weighing
the required quantity of salt directly (Baker Certified
Reagent) and dissolving it to volume in Acetonitrile.

59

60

F. Standard Tetrabutylammonium Hydroxide
Standard tetrabutylammonium hydroxide~(0.0489N)

was obtained from Professor K. G. Stone, of this department.

G. Silver Acetate

 

This material was obtained from the K & K Laborator-

ies and was used as obtained.

H. Silver Trifluoroacetate
This compound was obtained from the K & K Laborator—

ies and used as received.

II. Preparation of Aryl Chloroformates

A. Typical Preparation

 

In a typical preparation of an aryl chloroformate
a liter three-necked flask fitted with a drOpping funnel,
dry ice-acetone reflux condenser (methanolic potassium
hydroxide and water traps connected to the outlet), liquid
addition tube, magnetic stirrer, and immersed in a dry ice—
acetone cooling bath, was charged with 325 ml. of benzene
and 62.07 g. (0.50 mole) of p-methoxyphenol. Through the
addition tube, 67.8 ml. of liquid phosgene was cautiously
added to the benzene solution of the phenol. The addition
tube was then replaced with a glass stopper. To the vigor—
ously stirred reaction mixture 66.92 ml. (0.05 mole) of

N,N-dimethylaniline was added from the dropping funnel

61

while maintaining the reaction temperature between 0 and 50.
During the addition of each drop of N,N—dimethylaniline,

the reaction solution underwent a color change from light
blue to light green. On the addition of the final milliliter
of the amine the reaction solution changed to a yellow
colored slush. The reaction mixture was set aside for 10
minutes and 50 ml. of water was cautiously added, since a
violent reaction took place if the water was added too
rapidly. The benzene layer was separated; washed with dilute
hydrochloric acid, dilute sodium hydroxide, and finally with
water after which it was dried over anhydrous magnesium
sulfate. The benzene was removed in a rotary evaporator

and the crude product was distilled in vacuo to obtain 89.9
g. (96.5%) of clear colorless p-methoxyphenylchloroformate
boiling at 490/0.27 mm. and having a refractive index of

n19 1.5257. (See Table VI for the other arylchloroformates

D

prepared in the course of this investigation.)

B. Attempted Preparation of 2,6,-Di—t-Butylphenyl-
chloroformate

 

 

An attempt to prepare 2,6—di—t—butylphenylchloro-
formate by the procedure described was unsuccessful. The
starting phenol was recovered in 99%.

A variety of reaction conditions were employed un—
successfully. These included higher concentration of phos-
gene or amine, no solvent, higher reaction temperature

(15-200), and various combinations of these. Aqueous sodium

62

 

 

 

 

 

 

mm.ma ma.sa oe.m aa.m ms.mm am.mm Hmnucmm21n
mm.ma ma.sa Hm.m ae.m oo.am am.mm Hmnunmm21a
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sm.om m».om mm.a aa.a mo.sm mm.mm Hmcwan>:6621m
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mmmwamcd HmucwEmHm

 

 

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.H> GHQNE

65

hydroxide-was-tried in place of the N,N-dimethylaniline
as a base but to no avail. The steric hinderance of the two
t-butyl groups ortho to the hydroxyl group evidently is

great enough to prevent reaction with phosgene.

III. Product Analyses

 

A. Reaction of Naphthyl and p:Substituted Phenyl-
chloroformates with Silver Nitrate

 

Using the apparatus illustrated in Diagram 1, a
solution (precooled to 00) containing 11.77 g. (0.05 mole)
of p—bromophenylchloroformate in 150 ml. of acetonitrile
was added to a solution (precooled to 00) containing 8.50
g. (0.05 mole) of silver nitrate in 150 ml. of acetonitrile.
The temperature of the solution was maintained at 00 while
a continuous stream of nitrogen was passed through the
apparatus throughout the entire reaction period. A half-
hour after initiation of the reaction a considerable quan—
tity of silver chloride had precipitated and the reaction
solution had turned a light yellow in color. After four
hours of reaction the solution had turned a dark orange in
color and after twenty hours the reaction was complete as
evidenced by the absence of a precipitate when chloride ion
was added to an aliquot of the clear reaction solution.

The silver chloride was removed by filtration, washed with
two 20 ml. portions of cold acetonitrile and then with two

10 ml. portions of cold anhydrous ether, dried at 1050 for

64

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65

two hours and weighed (7.10 g. 99.0%). The combined aceto-
nitrile waShings were added to the original filtrate and the
acetonitrile was removed in a rotary evaporator. The resi-
due was dissolved in approximately 150 ml. of anhydrous
ethyl ether, decolorized with Norite A, and the volume of
the ether solution was concentrated to 50 ml. Cooling the
concentrated solution in an ice bath yielded 7.8 g. of
yellow colored crystals; reduction of the mother liquor to
15 ml. and cooling yielded a second quantity of the crystal-
line solid (2.8 g.). The total yield of 2—nitro-4—bromo—
phenol obtained was 10.60 g. (97.7%) melting at 88-890.
Literature value (19): melting point 890.

After setting the dry ice traps aside at room
temperature for one hour the tared carbon dioxide absorption
tube, packed with a 2:1 mixture of ascarite and anhydrous
magnesium perchlorate, was reweighed and the gain in weight
(2.2 g., 98.5%) of the tube was taken as equivalent to the
weight of the carbon dioxide evolved during the reaction.

The results for all of the naphthyl and p—substituted
phenylchloroformates reaction with silver nitrate are tabu-
lated in Table II.

B. Reaction of the Hindered Phenylchloro-
formates with Silver Nitrate

 

 

1. Reaction of 2,6-dimethylphenylchloro-
formate with silver nitrate

 

Using the apparatus illustrated in Diagram 1, 9.23

g. (0.05 mole) of 2,6—dimethylphenylchloroformate dissolved

66

in 25 ml. of acetonitrile was added to a solution contain—
ing 8.50 g. (0.05-mole) of silver nitrate in 50 ml. of
acetonitrile. Five minutes after initiation of the reaction
the solution had become dark yellow in color, silver chloride
had started to precipitate and the reaction mixture had to
be cooled by immersion in an ice bath to maintain the re-
action temperature below 300. After 14 hours of reaction
the silver chloride was removed by filtration, washed with
two 20 ml. portions of acetonitrile and then two 10 ml.
portions of anhydrous ether, dried at 1050 for 90 minutes
and weighed (7.12 g., 99.2%). The combined acetonitrile
washings were added to the filtrate and it was decolorized.
On cooling it yielded yellow colored crystals of product
which on recrystallization from methylene bromide gave 5.23
g. (62.6%) of light yellow colored 4—nitro—2,6-dimethylphenol
melting at 167.5-168.3O. A mixed melting point with an
authentic sample of 4—nitro-2,6-dimethylphenol gave a melt—
ing point of 167—1680. Literature value (25): melting
point 169-1700.

Concentration of the mother liquor yielded 0.78 g.
of dark red colored crystals which decomposed at 2240.
This material was identified as 3,5,3',5'—tetramethyl-
diphenequinone-(4,4') (24).

The carbon dioxide evolved during the reaction was
determined as described for the p-substituted chloroformates.

The yield was 83%.

67
2. Reaction of 2,6-diisopropylphenyl-
chloroformate with silver nitrate

Using the experimental procedure described for the
2,6—dimethylphenylchloroformate 8.9 g. (0.05 mole) of
2,6-diisopropylphenylchloroformate was allowed to interact
with 8.50 g. (0.05 mole) of silver nitrate. Following the
product isolation previously described a dark reddish—
yellow colored solution was obtained. This was diluted to
125 ml. in a volumetric flask with acetonitrile, and a 10
ml. aliquot was chromatogramed on an acid-washed alumnia
column using acetonitrile as the elutant. The initial band
which came off the column was dark yellow in color and on
evaporation of the solvent yielded 8.2 g., equivalent to a
73% yield of 4-nitro-2,6—diisopropylphenol which melted at
1120. Literature value (28): melting point 1120. A sec-
ond band was obtained which yielded a dark red compound
(1.2 g.) This material decomposed at 2450, and was identi-
fied as 3,5,3',5'-tetraiSOpropyldipheanuinone-(4,4').
The yield of silver chloride and carbon dioxide in this
reaction were 96.0% and 74.1%, respectively.

3. Reaction of 2,4,6-trimethylphenylchloro—

formate with silver nitrate

Using the procedure already described 20.00 g.
(0.10 mole) of 2,4,6-trimethylphenylchloroformate was
allowed to interact with 17.12 g. (0.10 mole) of silver

nitrate in 300 ml. of acetonitrile as a reaction media.

68

After the reaction mixture had been set aside for 24 hours
at room temperature, it was heated to 300 and held at this
temperature, for an additional 24 hours resulting in the
formation of a dark red solution containing silver chloride
and a red crystalline product. Filtration of the solution
gave a mixture of silver chloride and red crystals plus a
dark red filtrate. The red crystals were insoluble in the
common organic solvents, and the silver chloride was sepa-
rated from the red crystals with ammonium hydroxide. The
red crystalline solid was then washed with water and ace-
tone yielding 4.73 g. of material which decomposed at
224-2300. It was identified as 3,5,3',5'-tetramethylstilbene-
quinone-(4,4'). Literature value (26): melting point
224-2260.

Calc'd for C18H1802: C, 81.17; H, 6.81.

Found: C, 81.16; H, 6.66

The red filtrate was subjected to column chromato-
graphic analysis on an acid—washed alumnia column. Forty
distinct bands appeared on the column. None of these bands

were identified.

C. Reaction of Phenylchlorothiolformate
with Silver Nitrate

 

 

The apparatus illustrated in Diagram 1, and
the experimental procedure as described for the p-substituted
phenylchloroformates were used in this reaction. The sole

”product" obtained was a black untracable amphorous solid

69

that failed to react with acetic anhydride indicating the
absence of any phenolic monomer. In subsequent reaction

of phenylchlorothioformate with silver nitrate)acetic an-
hydride was added after the reaction had proceded for 30
minutes and 60 minutes. However, in each case only a

black tarry material was formed from which no aceylated
phenol could be obtained. In all cases the silver chloride
and carbon dioxide was isolated in yields between 95-98%

and 70—85%, respectively.

D. Trapping Experiments

 

1. Reaction of p-phenylphenylchloroformate
with silver nitrate in the presence of
sodiumAp—methoxyphenoxide

 

 

To 4.5 g. (0.025 mole) of silver nitrate in 150 ml.
of acetonitrile, precooled to 00, in a 250 ml. flask fitted
with a reflux condenser, drying tube, cooling bath, and
magnetic stirrer were added a solution containing 5.82 g.
(0.025 mole) of p-phenylphenylchloroformate in 50 ml. of
acetonitrile and a second solution containing 3.65 g.

(0.025 mole) of sodium p-methoxyphenoxide in 25 ml. of

acetonitrile. A black precipitate formed immediately in the
reaction mixture; simultaneously the reaction solution under-
went a color change from light green to yellow. The mixture
was allowed to warm to room temperature. On being set aside

for 12 hours, with stirring, the black precipitate turned

70

to a light yellow colored precipitate. Product isolation
from the reaction mixture was then conducted in the same
manner as for the reactions of other p-substituted phenyl-
chloroformates. The yield of 4-phenyl-2—nitrophenol obtained
was 5.23 g. (97.3%) melting at 65.5-660. Literature value
(17): melting point 660. For the investigation, additional
rearrangement reactions with the organic base as a catalyst.
the same experimental procedure was used as described except
that the sodium p—methoxyphenoxide was added 10 minutes and
60 minutes after initiation of the reaction with yields of
98.3% and 97.9%, respectively, being obtained of the ex-

pected product.

2. Reaction of p—phenylphenylchloro-
formate with silver nitrate in the
presence of p-methoxyphenol

 

 

The experimental procedure described above was used
for this trapping experiment. To 11.63 g. (0.05 mole) of
p-phenylphenylchloroformate in 100 ml. of acetonitrile was
added a solution containing 8.50 g. (0.05 mole) of silver
nitrate in 200 ml. of acetonitrile. After the reaction had
proceded for 10 minutes, a solution containing 12.4 g.
(0.10 mole) of p-methoxyphenol in 50 ml. of acetonitrile
was added to the reaction mixture. Following the usual
product isolation procedure 10.01 g. (93.1%) of 2-nitro-4-
phenylphenol melting at 65—660 was obtained. Literature

value (17): melting point 66°.

71

3. Reaction of p-phenylphenylchloro-
formate with silver nitrate in the
presence of a-methylstyrene and
galvinoxyl

 

The same procedures, compound and molar ratios of
reactant as described above for the reaction in the
presence of p-methoxyphenol were employed in this study of
a possible free radical mechanism being operative in these
rearrangements. The yield of 2-nitro-4-phenylphenol in the
presence of a-methylstyrene and galvinoxyl was 95.1% and
96.3%, respectively.

B. Reaction of p-Substituted Phenylchloroformates
with Silver Salts Other than Silver Nitrate

1. Reaction of p-substituted phenyl-
chloroformates with silver acetate

To 0.25 mole of p-substituted phenylchloroformate
in 25 ml. of acetonitrile was added 0.25 mole of silver
acetate in 25 ml. of acetonitrile. A1precipitate was
immediately formed. The reaction mixture was set aside for
24 hours with constant stirring and product isolation was
then carried out in the same manner as for reactions with
silver nitrate and the arylchloroformates. In all cases

only non-nitrated phenol was obtained (see Table III).

2. Reaction of p-phenylphenylchloro-
formate with silver trifluoroacetate

To 1.36 g. (0.0058 mole) of phenylphenylchloro-
formate in 25 ml. of acetonitrile was added 1.28 9. (0.0058

mole) of silver trifluoroacetate in 25 ml. of acetonitrile

72

whereupon an immediate precipitation of silver chloride
occurred. After the reaction had been set aside for 24
hours, the silver chloride was removed by filtration, washed
with acetonitrile and ether, dried at 1060 for 2 hours and
weighed (0.75 g., 94%). The acetonitrile washings were
added to the filtrate, and the acetonitrile solvent was
evaporated in a rotary evaporator leaving a white solid melt-
ing at 95-1050. On recrystallization from a 2:1 ether-pentane
mixture 1.51 g. (98%) of white crystals melting at 98 to 1030
was obtained. The compound was identified as 2-hydroxy-5-
phenyl-trifluoromethyl ketone. This compound decomppses on
standing.

Calc'd for C14H9F302: C, 63.16; H, 3.14; F, 21.41.

Found: C, 63.46; H, 3.15; F, 22.01.

F. Reaction of p-Methoxyphenylchloroformate
with Silver Nitrate in the Presence of
Pyridine

 

To a solution containing 0.1085 mole of p-methoxy-

phenylchloroformate in 50 ml. of acetonitrile was added
0.1085 mole of silver nitrate and 0.01 ml. of pyridine.
A rapid evolution of carbon dioxide occurred after approxi-
mately 50 seconds. The reaction was set aside for 2 hours
and then product isolation in the usual manner. The yields
of silver chloride and 2-nitro-4-methoxyphenol was 98.9%

and 83.4%, respectively.

73

IV. Kinetics of the Reactions of p—Substituted
Phenylchloroformates with Silver Nitrate

The rates of the reaction were determined by measur-
ing the rate of disappearance of the chloroformate absorption

band at approximately 1777 cm-1

employing a Unicam S. P. 200
recording infrared spectrophotometer in conjunction with a
variable temperature I. R. cell, equipped with silver chloride
windows, and apparatus as illustrated in Diagram 2. This
method was chosen after initial attempts to use conductio-
metric, amperometric, and carbon dioxide evolution methods
proved unfeasible in their attempted application to this
system.

The following experimental procedure was used in a
typical kinetic determination. A 50 ml. aliquot of a standard
p-substituted phenylchloroformate was added to the reaction
flask and magnetically stirred. The solution was then allow-
ed to reach bath temperature which was controlled to.:1-_0.01O
with a thryratron electronic relay. The solution was then
injected with a 4 ml. aliquot of a standardized silver nitrate
solution. At predetermined time intervals, samples were
filtered, withdrawn from the reaction and injected into the
sample cell. The carbonyl absorption of the chloroformate
was scanned and the time noted at which the minimum trans-
mittance occurred. The sample was withdrawn from the cell

with a hypodermic syringe and the cell was washed with

acetonitrile. Acetonitrile was injected into the cell and

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75

the 100% point determined; the 100% transmittance point had
to be determined after each sample was examined since silver
chloride precipitated onto the cell windows during the
measurements and thus affected the 100% point. In prepara-
tion for the determination of the next point, the cell was
dried by forcing dry air through it. In this manner the rate
of the disappearance of the chloroformate peak was followed.
See Table I, page 6, for the absorption of the carbonyl
band of the aryl chloroformates.

The second order rate constants were calculated from

equation (1)

_ = kpt + —— (1)

(where k is the second order rate constant, C is the concen-
tration in liters/mole, and t is the time in sec.) by
plotting the reciprical of the concentration versus time

and using the least squares method to determine the slope
(kg) and intercept (CO). The first order constants were

determined from equation (2) by plotting the logarithm
ln C =-k1t + 1n CO (2)

of the concentration versus time and using the method of
least squares to determine the slope (gké53) and the inter—
cept (log CO). It was previously established that the

absorbance of the various aryl chloroformates varies linearly

76

with concentration throughout the concentration range em-
ployed in this investigation.

The energy of activation, E was calculated using

al

equation (3)

k : se-Ea/RT (3)

where k is the rate constant, Ea is the experimental acti-
vation energy in calories, R is the gas constant per mole

(1.987 calories/OK), and s is the fre uency factor ( re-
g P

exponential factor). Taking the logarithm of both sides (4)
—E
=
log k log 5 + 2T305RT (4)

and plotting log k versus 1/T, the energy of activation was
obtained by multiplying the slope of the line, which was
determined by the method of least squares, by —2.303R.
The least squares method also gave the value for the
logarithm of the frequency factor, 5, which is the intercept
for equation (4).

The entropy of activation, AS*, was calculated

from the Eyring Equation (5)

I *-
k = e1;1 T eAS /R eEa/RT (5)

where k is the rate constant, k' is Boltzmann's constant,

*-
T is the absolute temperature, h is Planck's constant, AS

a

is the entropy difference between the initial and activated

77

states in entropy units, Ea is the experimental activation
energy in calories, R is the gas constant in calories/OK,
and e is the base of the natural logarithm system.

Combining equations (1) and (5) results in equation
(6)

«X.
s = ele eAS /R (6)

Taking the logarithms of both sides and rearranging gives

*

AS

R ln S - R ln (ek't/h) (7)

*

therefore at T = 210 AS 4.5765 (log s - 13.2218) (8)

The entropy of activation was obtained by solving
equation (8) by substituting the values of log 5 that had
been previously determined (see the calculations for Ea).

The enthalpy, AH*, was calculated from equation (9)
.x.
AH = E - RT (9)

by substituting the appropriate values for Ea, R, and T.
The isokinetic temperature was determined from
equation (10)

«X.

AH* = TAS* + AF (10)

by a plot of AH* versus 68* using the least squares method

to determine the slope which is the isokinetic temperature.

Rho (p) of the Hammett relation was determined from

equation (11) by plotting the log (ii) versus é>again using
log (fii) = 6 p (11)

the least squares method to determine the slope.

78

V. Miscellaneous Preparations

 

A. Preparation of Sodium p-Methoxyphenoxide

To a solution containing 6.45 g. (0.16 mole) of
sodium hydroxide in 100 ml. of water was added 20 g. (0.16
mole) of p-methoxyphenol. The resulting solution was
evaporated to dryness under high vacuum to obtain 21.9 g.

(0.15 mole) of light brown colored product.

B. Preparation of p—Methylphenylcarbamate

 

An excess of anhydrous ammonia gas was passed into
a solution 1.00 g. (0.0059 mole) of p—methylphenylchloro—
formate in 50 ml. of anhydrous ethyl ether until precipi-
tation ceased. The reaction mixture was washed twice with
water; the ether layer was separated, dried over anhydrous
sodium sulfate and the ether distilled under vacuum leaving
a fluffy white residue. This, on recrystallizing from
petroleum ether, yielded 0.80 g. (90.4%) of a white product
melting at 155-1560. Literature value (27): melting point

1540.

SUMMARY

1. Four previously undescribed aryl chloroformates,
p—bromophenyl, p—methoxyphenyl, 2,6-dii50propylphenyl, and
2,4,6—trimethylphenyl, were prepared and characterized.

2. The rates and products of the reactions of seven
pksubstituted phenylchloroformates with silver nitrate were
studied. The products found were 2-nitro-4—substituted
phenols. The initial rates of the reactions were found to
be second order at equal molar concentrations of reactants
and first order in the presence of excess silver nitrate.
The Hammett equation was found to be applicable to these
reactions with a positive rho (p) value. This suggests that
the rate determining step is the first step; that is, the
initial reaction of the chloroformate with silver nitrate
to form a nitratocarbamate.

3. A product study was conducted on the reaction of
silver nitrate with phenylchloroformates substituted in both
ortho positions and both ortho positions plus the para
position. The major product obtained when the ortho posi—
tions were blocked was p—nitro compound. When all three
positions (ortho and para) were blocked, a dimer quinoid

product was obtained.

79

80

4. The major products from the reaction of a- and
S—naphthylchloroformates with silver nitrate were found to
be 2-nitro—1-naphthol and 1—nitro—2-naphthol, respectively.

5. The reactions of some p—substituted phenylchloro-
formates with silver acetate and silver trifluoroacetate
were also examined. With silver trifluoroacetate the
products were substituted phenyl trifluoromethyl ketones.

6. In addition the reaction of phenylchlorothiol-

formate with silver nitrate was investigated.

/“

.L.

2.

10.

11.

12.

13.

14.

15.

16.

17.

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82

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