Industrial and Laboratory Nitrations

2 The Mechanism of Aromatic Nitration Reactions LEON

M.

STOCK

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 18, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch002

Department of Chemistry, University of Chicago, Chicago, Ill. 60637

Evidence developed i n l a r g e measure by Ingold, Hughes, and t h e i r students provided strong support f o r the view that the nitration r e a c t i o n s of aromatic compounds proceeded v i a the ni tronium i o n under a v a r i e t y of c o n d i t i o n s i n c l u d i n g , f o r example, nitration by nitric a c i d i n a c e t o n i t r i l e , n i t r a t i o n by nitric a c i d i n a c e t i c anhydride, and n i t r a t i o n by nitric a c i d i n mixed acid. Each new i n v e s t i g a t i o n of the r e a c t i o n y i e l d e d addi 1,2

Mechanism I HONO NO+

+ H

+

>

+ ArH

HO 2

+

+

> ArN0

NO 2

+

H

+

t i o n a l evidence f o r t h i s idea. Ingold and h i s a s s o c i a t e s used t h i s r e a c t i o n t o e s t a b l i s h the r e a c t i v i t y f o r a great v a r i e t y of aromatic compounds. They observed that the r e l a t i v e rates and isomer d i s t r i b u t i o n s were e s s e n t i a l l y independent of the condi t i o n s used i n the n i t r a t i o n r e a c t i o n . To illustrate, under the c o n d i t i o n s of these various n i t r a t i o n reactions, chlorobenzene reacts l e s s r a p i d l y than benzene whereas toluene reacts more r a p i d l y than benzene. Knowledge of the isomer d i s t r i b u t i o n s enables the c a l c u l a t i o n of the statistically corrected partial rate f a c t o r s , k /k = o , k /k = m , and k ^ / k ^ p . Typical 1

o

H

f

m

H

f

f

r e s u l t s are presented i n Table I. The same r e a c t i v i t y p a t t e r n f o r the a c t i v a t e d toluene and the d e a c t i v a t e d chlorobenzene i s g e n e r a l l y observed i n e l e c t r o p h i l i c substitution reactions. Indeed, the r e s u l t s f o r n i t r a t i o n obey a l i n e a r f r e e energy r e l a t i o n s h i p based on σ as shown i n F i g u r e 1. This r e l a t i o n s h i p does not d i f f e r from the r e l a t i o n s h i p s f o r other e l e c t r o p h i l i c s u b s t i t u t i o n reactions as shown f o r nonc a t a l y t i c bromination and f o r the a c i d - c a t a l y z e d bromination r e a c t i o n with hypobromous a c i d i n aqueous dioxane i n F i g u r e s 2 and 3. +

4

48

In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

Mechanism

of Aromatic

Nitration

Reactions


STOCK



INDUSTRIAL AND LABORATORY NITRATIONS


Mechanism

of Aromatic

Nitration

Reactions


STOCK


INDUSTRIAL AND LABORATORY

52

Table

I.

NITRATIONS

P a r t i a l Rate F a c t o r s f o r the N i t r a t i o n of Toluene and Chlorobenzene. a

Compound

Relative Rate k/k B


Toluene

21

Chlorobenzene 0. 031

a

Isomer D i s t r i b u t i o n o-

m-

E"

61.7

1.9

36.4

29.6

0.9

69.5

P a r t i a l Rate F a c t o r

38.9

1.3

45.8

0. 028 0.00084 0.130

N i t r i c a c i d i n nitromethane at 25°, Ref. 3.

In summary, a l l the e a r l y k i n e t i c and s p e c t r o s c o p i c e v i dence support the c o n t e n t i o n that the n i t r o n i u m i o n i s the e f f e c t i v e e l e c t r o p h i l i c agent. Moreover, the r a t e data f o r the n i t r a t i o n r e a c t i o n presented i n Table I and i l l u s t r a t e d i n F i g ure 1 are r e p r e s e n t a t i v e of the normal r e a c t i v i t y p a t t e r n observed f o r many other e l e c t r o p h i l i c s u b s t i t u t i o n r e a c t i o n s as shown by the r e s u l t s d i s p l a y e d i n F i g u r e s 2 and 3. Nitronium

Salt Nitration-Mixing Control

In 1961, Olah, Kuhn, and F l o o d reported that the n i t r a t i o n r e a c t i o n s with n i t r o n i u m s a l t s such as nitronium t e t r a f l u o r o borate and hexafluorophosphate were q u i t e anomalous. When benzene and toluene were reacted c o m p e t i t i v e l y with these n i t r o n i u m s a l t s i n s u l f o l a n e , a n a l y s i s of the products revealed that t o l u ene was only about 1.5-fold more r e a c t i v e than benzene. Indeed, n e i t h e r m-xylene nor mesitylene were importantly more r e a c t i v e than benzene under these c o n d i t i o n s . The p a r t i a l r a t e f a c t o r s 6

7

c a l c u l a t e d from these r e s u l t s are presented i n Table I I . Two immediate problems were recognized. F i r s t , a l l the p r i o r work had l e d t o the c o n c l u s i o n t h a t the n i t r a t i o n r e a c t i o n occurred v i a the n i t r o n i u m i o n . Y e t , when the n i t r o n i u m i o n was


2.

STOCK

Mechanism

of Aromatic

Nitration

Reactions

53

Table I I . Results f o r the Competitive N i t r a t i o n of Toluene and Benzene with Nitronium T e t r a f l u o r o b o r a t e i n S u l f o l a n e *


P a r t i a l Rate F a c t o r

Isomer D i s t r i b u t i o n

Relative Rate

Compound

o-

m-

p-

Toluene

1.67

65.4

2.8

31.8

3.2

0.14

3.2

Chlorobenzene

0.14

22. 1

0.7

76.6

0. 093

0. 002

0. 64

Ref.

6.

used as the reagent, q u i t e d i f f e r e n t p a r t i a l r a t e f a c t o r s were obtained. Second, although the rate data f o r chlorobenzene are not anomalous, the r e a c t i v i t y p a t t e r n f o r toluene i s d i s t i n c t l y abnormal with m l e s s than one and w i t h the r e l a t i v e r e a c t i v i t y of the other more h i g h l y a l k y l a t e d benzenes much l e s s than expected. Ingold, Lee, and Wei have d i s c u s s e d the c o n d i t i o n s under which the competition method may not y i e l d accurate r e l a t i v e r a t e d a t a . ~ * Lee i n a c a r e f u l a n a l y s i s of the problem p o i n t e d out t h a t the r e l a t i v e r e a c t i v i t i e s d e r i v e d from competition experiments are i n v a l i d when s i g n i f i c a n t r e a c t i o n occurs before the reagents are adequately mixed. T o l g y e s i examined t h i s f e a t u r e of the n i t r o n i u m s a l t n i t r a t i o n r e a c t i o n . He reported that the r a t e of mixing of the reagents i n f l u e n c e d the p r o p o r t i o n s of nitrobenzene and n i t r o t o l u e n e produced i n the competitive n i t r a t i o n r e a c t i o n s w i t h the n i t r o n i u m s a l t s . Olah and Overchuck disputed these o b s e r v a t i o n s . They argued that i m p u r i t i e s i n the s o l v e n t and reagents i n f l u e n c e d T o l g y e s i ' s observations. The controversy between T o l g y e s i and Olah was r e s o l v e d by Ridd and h i s students i n a study of the n i t r a t i o n of bibenzyl. ~ 6 B i b e n z y l was s e l e c t e d f o r study because the i n t r o d u c t i o n of a n i t r o group i n t o one r i n g has a n e g l i g i b l e i n f l u e n c e M e

9

1

1 2

1 3

1 4

1

C

6 5

H

C H

2

C H

C

H

C

H

2 6 5 * 6 5

C H

2

C H

C

H

N

2 6 4 °2 -

°2

N C

H

6 4

C H

2

C H

C

H

N

2 6 4 °2

on the r e a c t i v i t y of the other r i n g , i . e. the i n f l u e n c e s of the N C

H

C H

C H

a

n

d

C

H

C H

C H

g

r

o

u

p

s

a

r

e

n

o

t

i m

o r f c

d

i

f

°2 6 4 2. 2" 6 5 2 2~ P antly " ferent. Thus, a s t a t i s t i c a l product d i s t r i b u t i o n i s expected f o r the n i t r a t i o n r e a c t i o n . Ridd t e s t e d t h i s idea by the study of the n i t r a t i o n of excess b i b e n z y l w i t h n i t r i c a c i d i n a c e t i c anhydride. The r e s u l t s shown i n Table I I I r e v e a l that the expected r e s u l t was obtained. In sharp c o n t r a s t , when the n i t r a t i o n r e a c t i o n was c a r r i e d out with n i t r o n i u m t e t r a f l u o r o b o r a t e i n s u l f o l a n e , a d i s t i n c t l y a s t a t i s t i c a l product d i s t r i b u t i o n was obtained w i t h a l a r g e excess of d i n i t r o b e n z y l s , Table I I I . " The change i n the product d i s t r i b u t i o n i s dramatic. The high p r o p o r t i o n of d i n i t r o b i b e n z y l s i n d i c a t e that the mixing r a t e 16

1 4


1 6

54


Table

I I I . Reaction Products f o r the N i t r a t i o n of B i b e n z y l by N i t r i c A c i d i n A c e t i c Anhydride and by Nitronium Te trafluoroborate i n Sulfolane a

Statistical Product R a t i o

Product

56.25


Bibenzyl 2-Nitro

4-Nitro 2,2'-Dinitro

HN0

}

Observed Product R a t i o in Ac O N0 BF in C H S 0 2

43.75 28.6 1.1

} y

2,4'-Dinitro

>·

4,4'-Dinitro

J

6.25

2

4

4

g

2

45.3 20.9

^

c

b

3

9.8 >22. 2

49.5 12. 4^ 14.8' 5.

V77. 8

ι

37.9

2.4 1.6

25. 1

^

Réf. 14-16. In a t y p i c a l experiment 0.04 mole b i b e n z y l was reacted with 0.027 mole of n i t r i c a c i d i n a c e t i c anhydride. S e v e r a l concentrations of the reagents were studied. These res u l t s are f o r b i b e n z y l = 0.25M and the n i t r o n i u m s a l t 0.125M. c

i s comparable with the n i t r a t i o n r a t e f o r the r e a c t i o n of n i t r o nium s a l t s with aromatic compounds i n s u l f o l a n e , nitromethane, and apparently other organic s o l v e n t s . As noted by T o l y g e s i , the simple competition method i s inadequate f o r the d e f i n i t i o n of r e l a t i v e r e a c t i v i t y i n aromatic n i t r a t i o n by nitronium s a l t s . The p a r t i a l r a t e f a c t o r s d e r i v e d from the competitive r a t e data are anomalous because the mixing rates dominate the chemistry. These data are, a c c o r d i n g l y , meaningless f o r the a n a l y s i s of r e a c t i v i t y patterns and f o r the d e f i n i t i o n of m e c h a n i s m s . ' > 2

N i t r i c Acid Nitration-Encounter

16

17

Control

Important new chemistry was obtained i n 1968 by Coombes, Moodie, and S c h o f i e l d and t h e i r a s s o c i a t e s at E x e t e r . They f i r s t i n v e s t i g a t e d the r a t e s of n i t r a t i o n of simple aromatic compounds i n aqueous s u l f u r i c a c i d and aqueous p e r c h l o r i c a c i d . When the r e a c t i o n i s c a r r i e d out i n 60-70% a c i d , with the aromat i c compound at low c o n c e n t r a t i o n , about 10" M, the observed second order r a t e constant f o r the n i t r a t i o n of benzene i s 5.8 χ 10~ 1 mole" s e c " (68.3% s u l f u r i c a c i d ) and 8.3 χ 10" 1 mole" s e c " (61.1% p e r c h l o r i c a c i d ) . I n d i v i d u a l experiments with n i t r i c a c i d i n excess i n d i c a t e that the r e a c t i o n i s f i r s t - o r d e r i n the c o n c e n t r a t i o n of the aromatic compound. V a r i a t i o n s i n the c o n c e n t r a t i o n of n i t r i c a c i d e s t a b l i s h e d that the r e a c t i o n was a l s o f i r s t order i n n i t r i c a c i d . F u r t h e r , they found that 1 8

4

2

1

1

2

1


1

2.

STOCK

Mechanism

of Aromatic

Nitration

55

Reactions

the second order r a t e constant f o r the n i t r a t i o n of benzene (about 10~ M) by excess n i t r i c a c i d i n c r e a s e d as the a c i d s t r e n g t h of the r e a c t i o n medium i n c r e a s e d . The r e s u l t s are sketched i n F i g u r e 4. The changes i n the second-order r a t e constant f o r benzene p a r a l l e l the a n t i c i p a t e d change i n the c o n c e n t r a t i o n of n i t r o nium i o n i n the r e a c t i o n media. It was concluded t h a t the l a r g e r a t e v a r i a t i o n s f o r benzene r e f l e c t e d changes i n the concentra t i o n of n i t r o n i u m i o n i n the r e a c t i o n s o l u t i o n .


4

Rate = k[ArH][HNOJ 3

a , H

+

S c h o f i e l d and h i s a s s o c i a t e s p o i n t e d out, however, that the r e a c t i o n between benzene and the n i t r o n i u m i o n must be a r a t h e r f a s t r e a c t i o n because the c o n c e n t r a t i o n of the n i t r o n i u m i o n i s only about Ι Ο " M i n the aqueous a c i d s o l v e n t s . Indeed, the value estimated f o r k f o r benzene, about 1 0 1 mole" s e c " , i s only about 100-fold l e s s than the estimated r a t e constant f o r a d i f f u s i o n controlled reaction i n s u l f u r i c a c i d . Extension of the i n v e s t i g a t i o n t o other aromatic compounds provided e q u a l l y i n t e r e s t i n g observations. The second-order rate constants f o r aromatic compounds which are l e s s r e a c t i v e than benzene were e s t a b l i s h e d by the study of the r e a c t i o n i n more a c i d i c media. "" The r e a c t i o n rate data determined i n t h i s work are examined i n F i g u r e 5. The r e a c t i v i t y p a t t e r n f o r aromatic compounds which are l e s s r e a c t i v e than benzene i s e n t i r e l y normal. " " ^+ i ^ about -6, i s q u i t e s i m i l a r t o the p value obtained i n e a r l i e r s t u d i e s of the n i t r a t i o n r e a c t i o n i n organic s o l v e n t s as shown i n F i g u r e 1. A l l these r e s u l t s are c o n s i s t e n t w i t h n i t r a t i o n by the same agent, the n i t r o n i u m i o n , i n organic media as w e l l as i n aqueous a c i d . The k i n e t i c behavior of the aromatic compounds which are more r e a c t i v e than benzene have a l s o been examined with care, ^ J As d i s c u s s e d , the observed second-order r a t e constants f o r the n i t r a t i o n of benzene r e q u i r e t h a t the second-order r a t e constant f o r the r e a c t i o n between n i t r o n i u m i o n and benzene approach the d i f f u s i o n l i m i t f o r 60-70% aqueous a c i d . The k i n e t i c r e s u l t s f o r toluene obey the same r a t e law which was found t o be a p p l i cable f o r benzene and the l e s s r e a c t i v e molecules. More recent work by Chapman and S t r a c h a n u s i n g stopped flow techniques e s t a b l i s h e d that the n i t r a t i o n of toluene i n aqueous a c i d p r o ceeds by both f i r s t and second-order paths, Table IV. The r e a c t i o n i s zero-order i n toluene i n the more concen t r a t e d a c i d but f i r s t - o r d e r i n toluene i n the l e s s a c i d i c s o l vents. The r a t e data and the isomer d i s t r i b u t i o n f o r the n i t r a t i o n of toluene under these c o n d i t i o n s d e f i n e the p a r t i a l r a t e f a c t o r s presented i n Table V. 8

6

1

1

2

1 8

19

21

1 9

2 1

v

a

u

e

+

1 9

2 2




10

12

14

-(H +log a R

| 0

16

H 2 0

)

Figure 4. The relationship between the observed second-order rate constant for the nitration of ben zene and the acidity function

Nitration in Aq. Acid

I -0.4

ι 0

ι 0.4 σ

ι 0.8

+

Figure 5. The relationship between the par tial rate factors for nitration in aqueous sul furic acid and σ +


2.

STOCK

Mechanism

Table IV.

of Aromatic

Nitration

57

Reactions

F i r s t and Second-Order Rate Constants f o r the N i t r a t i o n of Toluene i n Aqueous S u l f u r i c A c i d at 25°. Rate Constants


Acidity, % H

first

SO

k ,

sec

, b _ -1 k, 1 mole 2

-1

sec

(170)

C

74.70

0.8

230

77.00

1.8

1700

(1200)

c

78.95

5.0

8300 (6600)

C

80. 10

6.4

81.45

8.3

-

^ i r s t - o r d e r i n n i t r i c acid. order i n toluene. R e f . 18.

First-order in n i t r i c

acid,

c

Table V.

R e l a t i v e Rates and P a r t i a l Rate F a c t o r s f o r the t i o n of Toluene i n 68.3% S u l f u r i c A c i d

Nitra-

a

Isomer D i s t r i b u t i o n

VB

o-

m-

p-

17

60

3

37

k

a

Ref.

P a r t i a l Rate F a c t o r s f

Hf

1.5

38

m

31

23.

The p a r t i a l r a t e f a c t o r s estimated f o r these r e a c t i o n c o n d i t i o n s d i f f e r only modestly from the data shown i n Table I f o r the n i t r a t i o n r e a c t i o n i n non-aqueous organic media. For compounds which are more r e a c t i v e than toluene, the second-order rate constants reach a l i m i t i n g value. This feature of the r e a c t i o n i s i l l u s t r a t e d by the r e s u l t s summarized i n Table VI. These r e s u l t s are t y p i c a l f o r the n i t r a t i o n r e a c t i o n under many c o n d i t i o n s . The second-order r a t e constants f o r aromatic n i t r a t i o n reach a l i m i t i n g value f o r n i t r i c a c i d n i t r a t i o n i n aqueous s u l f o l a n e , n i t r i c a c i d n i t r a t i o n c a t a l y z e d by s u l f u r i c a c i d i n a c e t i c a c i d , n i t r i c a c i d n i t r a t i o n i n aqueous p e r c h l o r i c a c i d , and n i t r i c a c i d n i t r a t i o n i n aqueous n i t r o m e t h a n e . The simplest explanation which can accommodate a l l the f a c t s i s presented as Mechanism II.2,18,22 23


58


Table VI.

R e l a t i v e Rate Constants f o r the N i t r a t i o n of Aromatic Compounds i n 68.3% S u l f u r i c A c i d a

b R e l a t i v e Rate

Compound

1.00


Benzene Toluene

17

o-Xylene

38

ρ-Xylene

38

m-Xylene

38

Mesitylene

36

a

b Ref. 23. The second-order r a t e constant f o r benzene i s 5.8 χ 10" 1 mole" s e c " . 2

1

1

Mechanism II + HNO

k

i Χ

+ Η

> H_0 + KOL -1

k NO* + ArH =-> 2 < τ -2 Ί

ArH N 0 2

+

o

Encounter Complex k ArH Ν 0 2 Λ

+

3

/+

-3

4 — >^ + Base — -

°2

Χ

// V

\ Γ- Ν 0™ + Base H

+

Λ

2

0 2

Mechanism II i s unusual only i n the sense that an encounter complex i s s p e c i f i c a l l y considered as an i n t e r m e d i a t e . The n i t r a t i o n r e a c t i o n does not, under these c o n d i t i o n s , ex h i b i t a primary k i n e t i c i s o t o p e e f f e c t . ^ Consequently, the r a t e of formation of the benzenonium i o n or one of the p r i o r r e a c t i o n s must be the r a t e l i m i t i n g process. The a p p r o p r i a t e r a t e law based on steady s t a t e approximations f o r t h e n i t r o n i u m i o n and the encounter complex i s r e a d i l y derived. 2 4

1

2


2.

STOCK

Mechanism

of Aromatic

Nitration

k kz "

a

H

59

k [ArH] [ H N O l a Η ( k_ ) k k [ArH] +

d

1

ρ«4-^

Reactions

2

J

0

V

2

+

2

3


Chapman and Strachan have shown that the n i t r a t i o n of toluene may be zero-order or f i r s t - o r d e r i n the aromatic compound. The zero-order r e s u l t corresponds t o the r a t e determining formation of the n i t r o n i u m i o n . Rate = k

ΓΗΝ0 l a , 3 +

obsd

H

T h i s r e s u l t i s r e a l i z e d when the c o n d i t i o n k k [ArH] » 2

k_

3

( k g k_ >

x

+

2

is satisfied. On the other hand, when the r e a c t i o n occurs at the d i f f u s i o n l i m i t , as f o r the xylenes, mesitylene and other r e a c t i v e compounds i n aqueous s u l f u r i c a c i d , ' J the r a t e law is 1 8

2 3

1 9

Rate = k . . [ArH] [ H N O l a ^ obsd 3 + H

w i t h a l i m i t i n g value f o r k

k

- l

a

H

Ο

>

:

>

K

2

K

3

C

A

R

H

:

A K o / i

/

I

(

.

K

The r e q u i r e d c o n d i t i o n i s

3

+

K

- 2

)

W

I

T

H

K

3

K

-2

and the formation of the encounter complex i s the r a t e l i m i t i n g step. F i n a l l y , the r a t e data f o r the n i t r a t i o n of toluene, ben zene, and the other l e s s r e a c t i v e aromatic compounds obey a t h i r d r a t e law Rate = k

[HNO_] [ArH]a , 3 JJ+

obsd

k

with k j dependent on the s t r u c t u r e of the aromatic compound. F o r these cases, the r e q u i r e d c o n d i t i o n i s 0

S (

k

- l

a

H

0

> : >

K

2

K

3

C

A

R

H

]

/

(

K

3

+

K

-

2

)

W

I

T

H

K

3

K

In t h i s medium n i t r i c a c i d i s converted t o a c e t y l n i t r a t e and a c e t i c a c i d . E a r l y i n t e r p r e t a t i o n s of the r a t h e r complex k i n e t i c data suggested that the n i t r o n i u m i o n was the e f f e c t i v e a g e n t . Hartshorn and the Exeter group showed that the r e a c t i o n of mesitylene was encounter c o n t r o l l e d . In a more recent study, Marziano, Rees, and Ridd found that the com p l e x k i n e t i c behavior r e s u l t s , at l e a s t i n p a r t , from the s o l vent e f f e c t of v a r i a t i o n s i n the c o n c e n t r a t i o n of the hydrocarbon. They conclude t h a t c a u t i o n i s necessary i n i n t e r p r e t a t i o n of the a v a i l a b l e data but note that s e v e r a l routes f o r the formation of n i t r o n i u m i o n s o l v a t e d by a c e t i c a c i d are compati b l e with the r e s u l t s . Olah and L i n s t u d i e d the boron t r i f l u o r i d e - c a t a l y z e d r e a c t i o n of methyl n i t r a t e with aromatic compounds i n nitromethane. Using competition methods they found t h a t the toluene t o benzene r a t e r a t i o was 25 and that the r a t e r a t i o s f o r the methylbenzenes, Table IX, reached a l i m i t i n g v a l u e , about 1000. Unfortu n a t e l y , k i n e t i c r a t e data were not obtained. However, the l e v e l ed r e a c t i v i t y of the polymethylbenzenes suggests that these com pounds react at the encounter c o n t r o l l e d d i f f u s i o n l i m i t . The r e a c t i v i t y p a t t e r n f o r n i t r a t i o n i n organic s o l v e n t s does not d i f f e r a p p r e c i a b l y from the r e a c t i v i t y p a t t e r n f o r the r e a c t i o n i n h i g h l y a c i d i c , perhaps aqueous, s o l v e n t s . These r e s u l t s suggest that a common intermediate, presumably the n i t r o nium i o n , i s i n v o l v e d i n a l l these r e a c t i o n s . 2 5

2

l i 2

2 6

ϋ

281

Product

Distributions

Aromatic n i t r a t i o n r e a c t i o n s e x h i b i t , under a l l c o n d i t i o n s , important i n t r a m o l e c u l a r s e l e c t i v i t y . The isomer d i s t r i b u t i o n s f o r the r e a c t i o n s of toluene under a v a r i e t y of c o n d i t i o n s , Table VII, i l l u s t r a t e t h i s f e a t u r e .


2.

STOCK

Mechanism

Table VII.

of Aromatic

Isomer D i s t r i b u t i o n o-

m-

61

Reactions

Isomer D i s t r i b u t i o n s i n the N i t r a t i o n of

Conditions

Toluene Ref.

E"

HN0 ,CH N0 ,25°

61.5

3.1

35.4

29

HN0 ,CF C0 H,25°

61.6

2.6

35.8

29

AcONO ,Ac O,30°

58.1

3.7

38.2

29

MeON0 ,BF ,CHgNOg,25°

63.9

3.4

32.7

28a

N0 BF ,C H S0 ,25O

65.4

2.8

31.8

29

N0 BF ,CH Cl ,-65°

56.6

0.6

42.8

30

N0 CF S0 ,CH C1 ,-60°

62. 1

0. 5

37.4

30

3

3

3


Nitration

2

3

2

2

2

2

2

2

2

3

4

4

4

g

2

2

3

2

3

2

2

Het erogeneous.

At ambient temperatures, the n i t r a t i o n of toluene produces the ortho and para isomers dominantly w i t h between 2 and 4% meta product. A s i m i l a r product r a t i o i s obtained when the r e a c t i o n i s c a r r i e d out heterogeneously with mixed n i t r i c and s u l f u r i c acid. However, as expected, g r e a t e r s e l e c t i v i t y i s achieved a t ^ low temperatures as shown by the r e a c t i o n s w i t h n i t r o n i u m s a l t s . The f a c t t h a t the r e a c t i o n y i e l d s v i r t u a l l y the same product d i s t r i b u t i o n under a l l c o n d i t i o n s supports the view that the same reagent, the n i t r o n i u m i o n , i s the e f f e c t i v e agent under a l l conditions. The n i t r a t i o n r e a c t i o n s a l s o e x h i b i t h i g h s e l e c t i v i t y when the r e a c t i o n s proceed at the encounter c o n t r o l l e d d i f f u s i o n l i mit. To i l l u s t r a t e , the n i t r a t i o n of a n i s o l e y i e l d s l i t t l e meta product: the ortho and para isomers are produced almost e x c l u sively. J S i m i l a r l y , the s u b s t i t u t i o n products of m-xylene 1

9

2

8

2- N i t r o a n i s o l e 3- N i t r o a n i s o l e 4- N i t r o a n i s o l e are formed s e l e c t i v e l y w i t h no d e t e c t a b l e 5 - n i t r o - l , 3 - d i m e t h y l benzene. Moodie, S c h o f i e l d , and Weston examined the d i f f u s i o n l i m i t e d n i t r a t i o n of pseudocumene i n aqueous s u l f u r i c a c i d . In a d d i t i o n t o the products of i p s o s u b s t i t u t i o n , they found that the 5 - n i t r o isomer was formed i n preference t o the r e l a t e d 6n i t r o product. 2 9 a

o x



62

2-Nitro-l,3-dimethylbenzene


4-Nitro-l,3-dimethylbenzene

CH„

56%

Ipso Adducts

7%

3-Nitro-l,2,4-trimethylbenzene

33%

5-Ni t ro-1,2,4-1 rimethylb enz ene

4%

6-Ni t rο-1,2 ,4-1rimethylbenz ene

In a l l these cases, the product d i s t r i b u t i o n s r e f l e c t the r e l a t i v e s t a b i l i t y of the bezenonium ions.

H^N0 unstable compared t o

2

ff^^^jl

The product r a t i o s can, i n p r i n c i p l e , be d i c t a t e d by the r e l a t i v e r a t e s of formation of the benzenonium i o n s , k , or by the e q u i l i b r a t i o n of the ions as shown i n Mechanism I I I . g


2.

STOCK

Mechanism

of Aromatic

Nitration

Reactions

63


Mechanism I I I

D i r e c t evidence i s not a v a i l a b l e i n a l l cases. However, the s o l v o l y s i s of 4 - a c e t o x y - l - n i t r o - l , 2 - d i m e t h y l c y c l o h e x a - 2 , 5 diene leads t o the e x c l u s i v e formation of 3 - n i t r o - l , 2 - d i m e t h y l -



64

32

benzene. F u r t h e r rearrangement t o the 4 - n i t r o d e r i v a t i v e does not occur. Therefore, the r a t e of proton removal i s apparently much g r e a t e r than the r a t e of i s o m e r i z a t i o n . Related adducts r e act i n s i m i l a r w a y s . Consequently, i t seems safe t o conclude that the i s o m e r i z a t i o n r e a c t i o n s of the benzenonium ions do not compete w i t h the b a s e - c a t a l y z e d proton a b s t r a c t i o n r e a c t i o n s i n the aqueous a c i d s and that the energy requirements f o r the conv e r s i o n of the encounter complexes t o the benzenonium ions d i c t a t e the product d i s t r i b u t i o n . Energy p r o f i l e s compatible w i t h the k i n e t i c observations and w i t h the product d i s t r i b u t i o n s are shown i n F i g u r e 6. The energy requirements f o r the formation of c e r t a i n benzenonium i o n s , e.g. those ions which are l e s s s t a b l e than 1nitrobenzenonium i o n such as the h a l o d e r i v a t i v e s and 3-methoxy1-nitrobenzenonium i o n and even the more s t a b l e 2-, 3-, and 4methyl-l-nitrobenzenonium i o n s , are l a r g e r than the energy r e quirements f o r the d i f f u s i v e d i s s o c i a t i o n of the encounter complex. These ions are, a c c o r d i n g l y , produced i n k i n e t i c a l l y s i g n i f i c a n t r e a c t i o n s as shown i n F i g u r e 6A. On the other hand, the energy requirements f o r the formation of other more s t a b l e benzenonium i o n s , e.g. the polymethylbenzene d e r i v a t i v e s and the 2and 4-methoxy-l-nitrobenzenonium i o n s , are l e s s than the energy requirements f o r the d i s s o c i a t i o n of the encounter complex. Thus, the conversion of the encounter complex t o the i o n i s not k i n e t i c a l l y important as shown i n F i g u r e 6B. However, the study of the n i t r a t i o n of pseudocumene r e v e a l s that the r e l a t i v e s t a b i l i t i e s of the benzenonium ions determine the product d i s t r i b u tion. Indeed, the a c t i v a t i o n energy d i f f e r e n c e f o r the convers i o n of the encounter complex t o the 5- and 6-nitrobenzenonium i o n s i s about 1.2 k c a l mole." The a c t i v a t i o n energy f o r the conversion of the encounter p a i r t o the benzenonium i o n i s not known. However, t h i s energy b a r r i e r cannot exceed the low a c t i v a t i o n energy f o r the d i s s o c i a t i o n of the encounter p a i r . The s t r u c t u r a l c h a r a c t e r i s t i c s remain important even though the energy b a r r i e r s f o r the formation of the benzenonium i o n s are modest. As yet no r e a l l y unusual, i n t r a m o l e c u l a r p a t t e r n s of r e a c t i v i t y have emerged f o r n i t r a t i o n . Indeed, e s s e n t i a l l y the same product d i s t r i b u t i o n s are obtained when n i t r a t i o n occurs at the d i f f u s i o n l i m i t i n aqueous a c i d s o l v e n t s and when the r e a c t i o n proceeds more slowly i n organic s o l v e n t s . Apparently, the d i f f e r e n c e s i n the energy b a r r i e r s f o r the formation of the benzenonium ions i n these solvent systems d i f f e r only modestly and the same f a c t o r s govern the product d i s t r i b u t i o n even though the r a t e determining steps are d i f f e r e n t .


33

31

1

34

The p a r t i a l r a t e f a c t o r s f o r n i t r a t i o n are compared w i t h r e l a t e d data f o r a c e t y l a t i o n and c h l o r i n a t i o n i n Table V I I I . As already d i s c u s s e d , the product d i s t r i b u t i o n s and the p a r t i a l r a t e f a c t o r s f o r n i t r a t i o n do not depend importantly on the c h a r a c t e r of the s u p p l i e d reagent. A s i m i l a r p a t t e r n of r e a c t i v i t y i s observed f o r F r i e d e l - C r a f t s a c y l a t i o n . In both r e the



2.

STOCK

Mechanism

of Aromatic

Nitration

Reactions

CH

HC

CD

65

3

ί"

3

ΦΗ ° Ν

HN0 +H 3

2

+

Reaction Coordinate Figure 6. A. The energy profile for the meta and para nitra tion of toluene. The benzenonium ions are produced in the rate determining step. The energy difference at the transition state is about 1700 cal. Β. The energy profile for the 5 and 6 nitration of pseudocumene. The encounter complex is pro duced in the rate determining step. The energy difference at the transition state for the formation of the benzenonium ions is about 1200 cal.


66


Table V I I I .

P a r t i a l Rate F a c t o r s f o r the S u b s t i t u t i o n of Toluene a


Reaction

K

T^ B k

P a r t i a l Rate F a c t o r s

!5f

&f

Ref.

Acetylation CH COF,BF 3

3

CH C0C1,A1C1 ,CS 3

3

2

CH C0C1,A1C1 ,C H C1 3

3

2

4

2

130

4.7

4.3

762

35

141

10.5

8.4

808

35

128

4.5

4.8

750

29b

Nitration HNO ,HQAc,H 0 «3 A HN0 ,CH N0

24.5

42

2.5

58

29

21.0

38.9

1.3

45.8

29

AcCN0 ,Ac 0

23.0

40

3.0

51

29

HN0 ,HOAc

28.8

49

2.4

70

29a

17.0

32

1.7

35

29a

28.0

52.1

2.8

58.1

29a

28.0

51.7

2.2

60.1

29a

3

3

2

2

2

3

VA 2

ra

S0

C

HN0 ,H S0 ,C H S0 3

2

4

4

8

2

HN0 ,CF C0 H 3

3

2

Chlorination Cl_,HQAc C 1

2^

C F

C

3 °2

H0C1,HC10

H

4

C1 ,IC1,C H C1 2

2

4

2

Cl ,SnCl ,C H Cl 2

4

2

4

Cl ,FeCl ,CH N0 2

3

3

2

2

AcOCl,HC10 ,HQAc,H 0 4

a

b

2

340

617

5.0

820

29b

464

928

6.5

928

29b

60

134

4.0

82

29b

230

430

(7.8)

b

520

36

165

360

(5.4)

b

270

36

55

110

3.8

100

36

145

306

237

37

T h e r e a c t i o n s were, f o r the most Estimated by a d d i t i v i t y approach.

(3)

b

p a r t , c a r r i e d out at 25°.


2.

STOCK

Mechanism

of Aromatic

Nitration

Reactions

67

a c t i o n s the r e l a t i v e rates and product d i s t r i b u t i o n s f o r the s u b s t i t u t i o n of toluene, f o r example, are n e a r l y constant even though the absolute r e a c t i o n r a t e s vary e n o r m o u s l y . This f e a ture i s w e l l i l l u s t r a t e d by the f a c t that the antimony pentac h l o r i d e - c a t a l y z e d b e n z o y l a t i o n r e a c t i o n proceeds about 1300f o l d more r a p i d l y than the aluminum c h l o r i d e - c a t a l y z e d r e a c t i o n , yet the nroduct d i s t r i b u t i o n s f o r toluene are the s a m e . Also the P values f o r boron t r i f l u o r i d e - c a t a l y z e d a c e t y l a t i o n of toluene w i t h a c e t y l f l u o r i d e and f o r the aluminum c h l o r i d e c a t a l y z e d a c e t y l a t i o n of toluene with a c e t y l c h l o r i d e are very similar. 38

38b

3


f

3

5

In sharp c o n t r a s t , the r e l a t i v e r a t e s , isomer d i s t r i b u t i o n s , and p a r t i a l r a t e f a c t o r s f o r c h l o r i n a t i o n depend importantly on the c h a r a c t e r of the reagent and c a t a l y s t s . The order-of-magn i t u d e d i f f e r e n c e s i n P^ * i n d i c a t e that there are important v a r i a t i o n s i n the energy content of the t r a n s i t i o n s t a t e s l e a d i n g t o the p o s s i b l e benzenonium ions. The i n v a r i a n c e i n the 1

3

transition

δ+ < δ+

state

t r a n s i t i o n state

p a r t i a l r a t e f a c t o r s f o r a c y l a t i o n , on the other hand, can be comfortably i d e n t i f i e d w i t h a r e a c t i o n mechanism i n which the acylonium i o n i s produced i n a p r i o r step and subsequently r e a c t s i n a rate determining process with the aromatic compound, Mechanism IV. The mechanisms f o r n i t r a t i o n and a c e t y l a t i o n , II and IV, are very s i m i l a r . Both r e a c t i o n s proceed v i a an e l e c t r o p h i l i c intermediate with the d i f f e r e n c e s i n product d i s t r i b u t i o n s r e s u l t i n g from the v a r i a t i o n s i n the energy requirements f o r the formation of the various benzenonium ions.


68


Mechanism IV


RCOX

+

MX

0

— - — > RCO

+

+

MX

+

The l a r g e negative p value, -9 f o r a c e t y l a t i o n compared t o -6 f o r n i t r a t i o n , * * r e f l e c t s the g r e a t e r energy requirements f o r the conversion of the encounter p a i r , ArH RCO+, t o the benzenonium ions. There i s no evidence f o r the r a t e determining formation of the encounter complex even f o r the polymethylbenzenes. In summary, the a v a i l a b l e data suggest that t h e product d i s t r i b u t i o n s i n the n i t r a t i o n r e a c t i o n s are d e f i n e d by the k i n e t i c a l l y c o n t r o l l e d r a t e s of formation of the p o s s i b l e benzenonium i o n r e a c t i o n s even though d i f f e r e n t r a t e l i m i t i n g r e a c t i o n s a r e i n v o l v e d i n the o v e r a l l process. The comparison of n i t r a t i o n , a c e t y l a t i o n , and c h l o r i n a t i o n suggests that n i t r a t i o n and a c e t y l ation f o l l o w q u i t e s i m i l a r r e a c t i o n paths. This F r i e d e l - C r a f t s r e a c t i o n i s more s e l e c t i v e than n i t r a t i o n but the d i f f e r e n c e s i n r e a c t i v i t y a r e manifest i n t h e same key step - the formation of the benzenonium ions. 9

3 9

The

Encounter Complex

The e a r l y work on the nitronium s a l t n i t r a t i o n r e a c t i o n s prompted Olah and h i s a s s o c i a t e s t o propose that p i complexes were produced i n the r a t e determining step of the r e a c t i o n . J More r e c e n t l y , Olah and L i n have argued that the r e l a t i v e r a t e s of n i t r a t i o n i n the boron t r i f l u o r i d e - c a t a l y z e d r e a c t i o n of methyl n i t r a t e with aromatic compounds and i n the nitronium s a l t n i t r a t i o n r e a c t i o n s are p a r a l l e l t o the r e l a t i v e s t a b i l i t i e s of 6


4

0

2.

STOCK

Mechanism

of Aromatic

Nitration

69

Reactions


z

p i complexes between the hydrocarbons and hydrogen chloride. ° Rys, Skrabal and Z o l l i n g e r have examined t h i s viewpoint i n considerable detail.17 The r e s u l t s presented i n Table IX i l l u s t r a t e the p a t t e r n of reactivity. Rys, Skrabal, and Z o l l i n g e r conclude, as d i s c u s s e d i n a p r i o r s e c t i o n , t h a t mixing r a t e s render the r e l a t i v e r a t e data f o r n i t r o n i u m s a l t n i t r a t i o n v a l u e l e s s f o r the d e f i n i t i o n of chemical r e a c t i v i t y p a t t e r n s . I t i s a l s o c l e a r that the boron t r i f l u o r i d e - c a t a l y z e d r e a c t i o n s of methyl n i t r a t e with the p o l y methylbenzenes reach a l i m i t i n g value, about 1000, s i g n a l i n g the onset of encounter c o n t r o l as noted p r e v i o u s l y . A c c o r d i n g l y , n e i t h e r set of data can r e l i a b l y be used t o i n f e r the i n v o l v e ment of p i complex intermediates. They a l s o t e s t e d v a r i o u s l i near f r e e energy r e l a t i o n s h i p s and concluded i n each case that there was no r e l i a b l e c o r r e l a t i o n between the r e s u l t s f o r n i t r a t i o n and p i complex s t a b i l i t y . Moodie, S c h o f i e l d and Weston have a l s o commented on t h i s point. They note that the s e l e c t i v i t y observed i n the r e a c t i o n s of the a c t i v a t e d aromatic compounds can be q u i t e r e a d i l y r a t i o n a l i z e d without r e q u i r i n g an a t t r a c t i v e i n t e r a c t i o n i n the encounter c o m p l e x . Inasmuch as the energy b a r r i e r f o r separat i o n of the encounter complex may exceed 3 k c a l m o l e . i n the s u l f u r i c a c i d s o l v e n t s , there i s an ample energy range f o r sel e c t i v i t y i n the formation of the benzenonium i o n . In b r i e f , the a v a i l a b l e data are mute concerning the exi s t e n c e or non-existence of a t t r a c t i v e i n t e r a c t i o n s i n the encounter complex. There i s no s u b s t a n t i v e support f o r the view that these e n t i t i e s should be c h a r a c t e r i z e d as p i complexes. 1 7

31

- 1

Conclusion The c o n t r i b u t i o n s of s e v e r a l groups have provided important i n f o r m a t i o n concerning the mechanism of the n i t r a t i o n r e a c t i o n . It i s now recognized t h a t mixing e f f e c t s render the n i t r o n i u m s a l t n i t r a t i o n data inexact and that d i f f u s i o n processes prevent the a c q u i s i t i o n of meaningful r e l a t i v e r a t e data f o r the r e a c t i v e compounds. These two complications o f f u s c a t e many c h e m i c a l l y s i g n i f i c a n t events. C a r e f u l k i n e t i c s t u d i e s and accurate determinations of the product d i s t r i b u t i o n provide the b a s i s f o r renewed f a i t h i n the general nature of the n i t r o n i u m i o n n i t r a t i o n mechanism, I I , w i t h the i n t e r e s t i n g f e a t u r e of encounter c o n t r o l f o r s u f f i c i e n t l y r e a c t i v e molecules. Acknowledgement It i s a p l e a s u r e t o acknowledge the able t y p i n g e f f o r t of Mrs. Mary Brown and the support of the Block Fund of the U n i v e r s i t y of Chicago i n the p r e p a r a t i o n and p r e s e n t a t i o n of t h i s review.



Réf. 28.

io

X

8.1

Pent amet hylbenzene

°Ref. 6.

X

2.0

8

io

X

2.8

1,2,4,5-Tetramethylbenzene

R e f . 42.

X

1.0

6

io

X

4.2


b

X

2.0

8

Réf. 41.

X

2.0

7

io

X

1.1


X

6.3

io

X

1.9

1,3,5-Trimethylbenzene

X

8

io

X

1.5

2.0


X

2.0

6

io

X

1.6


X

3.2

6

io

X

2.5

p-Xylene

X

1.0

3

io

X

5.1

m-Xylene

X

7.9

5

io

X

3

790

1.00

3

2

3

d

1. 00

2

7

io io

9

io io

Réf. 43.

7

io

8

9

6

io

2.71

2.68 2.74 2.86

1. 9 χ 2.2 χ

2.60

2. 1 χ

956

2.23

914 1.1 χ

1. 65 2.40

295

1. 96

io io

2. 06

285

1.65

3

io

6

1.81

192

1. 51

1. 00

1.75

25.5

2

1. 67

8

6

io

4

1. 00

4

7

16

Relative Stability P i Complex HCl,n-C H

3

2

0

R e l a t i v e S t a b i l i t y Product R a t i o Product R a t i o Nitration Nitration Protonation N 0 B F , C H S 0 CH ON0 ,CH N0 HF,BF

5.3

605

1.00

a

R e l a t i v e Rate Bromination Br_, 85% HOAc

R e l a t i v e Rates, N i t r a t i o n Product R a t i o s , and P i Complex S t a b i l i t y

o-Xylene

Toluene

Benzene

Compound

Table IX.


ο 2

3

w ο

>

2.

STOCK

Mechanism

of Aromatic

Nitration

Reactions

71

Literature Cited


(1) C.K. Ingold, " S t r u c t u r e and Mechanism in Organic Chemistry," Chapter 6, C o r n e l l U n i v e r s i t y Press, 1953. (2) J.G. Hoggett, R.B. Moodie, J.R. Penton, and K. S c h o f i e l d , " N i t r a t i o n and Aromatic R e a c t i v i t y , " Cambridge U n i v e r s i t y Press, 1971. (3) Bird, M.L. and Ingold, C.K., J. Chem. Soc. (1938) 1959. (4) Stock, L.M. and Brown, H.C., Adv. Phys. Org. Chem., (1963), 135. (5) The r e s u l t s f o r these r e a c t i o n s are summarized in r e f . 4. The data f o r the nitration r e a c t i o n were developed, i n l a r g e p a r t , by C.K. Ingold and h i s students. The data f o r a c i d - c a t a l y z e d hypobromous a c i d bromination were obtained by P.B.D. de la Mare and h i s a s s o c i a t e s . (6) Olah, G.A., Kuhn, S.J. and F l o o d , S.Η., J. Amer. Chem. Soc., (1961), 83, 4511. (7) The low r e l a t i v e r a t e value is obtained r e p r o d u c i b l y even when the c o n c e n t r a t i o n s of the reagents are a l t e r e d . This f e a t u r e of the r e s u l t s is d i s c u s s e d in r e f . 2, p. 66, and by Ridd and his a s s o c i a t e s , r e f . 14. (8) F o r a more complete summary, Olah, G.A., Accounts of Chem. Res., (1971), 4, 240. (9) Ingold, C.K. and Shaw, F.R., J. Chem. Soc., (1927), 2918. (10) T.S. Lee, " i n v e s t i g a t i o n of Rates and Mechanisms of Reac t i o n s , " S.L. F r i e s s and A. Weissberger, e d i t o r s , "Technique of Organic Chemistry," Volume V I I I , 1st E d i t i o n , I n t e r s c i e n c e P u b l i s h e r s , Inc., New York (1953), p. 100. (11) Wei, J . , J. C a t a l y s i s , (1964), 3, 299. (12) T o l g y e s i , W.S., Can. J Chem., (1965), 43, 343. (13) Olah, G.A. and Overchuck, N.A., Can. J. Chem., (1965), 43, 3279. (14) C h r i s t y , P.F., Ridd, J.H. and S t e a r s , N.D., J. Chem. Soc. B, (1970), 797. (15) Gastaminza, A. and Ridd, J.Η., J. Chem. Soc., P e r k i n I I , (1972), 813. (16) Ridd, J.H., Accounts Chem. Res., (1971), 4, 248. (17) Rys, P., Skrabal, P. and Z o l l i n g e r , Η., Angew. Chemie Internat. Ed., (1972), 11, 874. (18) Coombes, R.G., Moodie, R.B. and S c h o f i e l d , Κ., J. Chem. Soc. B, (1968), 800. (19) Hoggett, J.Β., Moodie, R.B. and S c h o f i e l d , Κ., J. Chem. Soc. B, (1969), 1 (20) Coombes, R.G., Moodie, R.B. and S c h o f i e l d , Κ., J. Chem. Soc. B, (1969), 52. (21) Coombes, R. G., Crout, D.H.G., Hoggett, J.G., Moodie, R.Β., and K. S c h o f i e l d , J. Chem. Soc. B, (1970), 347. (22) Chapman, J.W. and Strachan, A.N., J. Chem. Soc. Chem. Comm., (1974), 293. (23) (a) Ref. 2, p. 63. (b) Ref. 4, p. 50.


72



(24) The r o l e of encounter c o n t r o l l e d r e a c t i o n s in chemical ki netics has been considered by (a) R.M. Noyes, "Progress in Reaction K i n e t i c s , " Pergamon Press (1961). (b) North, A.M., Quart. Revs., (1966), 20, 421. (25) Coombes, J . Chem. Soc. B, (1969), 1256. (26) Hartshorn, S.R., Moodie, R.Β., S c h o f i e l d , K. and Thompson, M.J., J. Chem. Soc. B, (1971), 2447. (27) Marziano, N.C., Rees, J.H. and Ridd, J.H., J. Chem. Soc., P e r k i n I I , (1974), 600. (28) (a) Olah, G.A. andLin,H.C., J. Amer. Chem. Soc., (1974), 96, 2892, report that 1% of the meta isomer is obtained. (b) Coombes, R.G. reported a t the Symposium that none of this isomer could be detected under the experimental c o n d i t i o n s . The d e t e c t i o n l i m i t was 0.003%. (29) (a) Ref. 2, p. 64. (b) Ref. 4, p. 50. (30) Coon, C.L., Blucher, W.G. and Hill, M.E., J. Org. Chem., (1973), 25, 4243. (31) Moodie, R.Β., S c h o f i e l d , K. and Weston, J.B., J. Chem. Soc. Chem. Comm., (1974), 382. (32) Myhre, P.C., J. Amer. Chem. Soc., (1972), 94, 7921. (33) (a) Hahn, R.C. and Groen, M.B., J. Amer. Chem. Soc., (1973), 95, 6128. (b) F i s c h e r , A. and Greig, C.C., J. Chem. Soc. Chem. Comm., (1974), 50. (c) F i s c h e r , A. and Ramsay, J.Ν., Can. J. Chem., (1974), 52, 3960. (34) I t should be emphasized that t h i s c o n c l u s i o n is based on t h e idea that the rearrangement r e a c t i o n s of the ions are slow r e l a t i v e t o the r a t e of proton l o s s . T h i s c o n d i t i o n may not be s a t i s f i e d under all circumstances. (35) Olah, G.A. and Kobayashi, S., J. Amer. Chem. Soc., (1971), 93, 6964. (36) Stock, L.M, unpublished r e s u l t s . (37) dela Mare, P.B.D., H i l t o n , I.C. and Varma, S., J. Chem. Soc., (1966), 4044. (38) (a) The a c y l a t i o n r e a c t i o n is d i s c u s s e d by G.A. Olah, " F r i e d e l - C r a f t s and Related R e a c t i o n s , " Volume I, p. 91-115 (1963). (b) Jensen, F.R. and Brown, H.C., J . Amer. Chem. Soc., (1958), 80, 3044. (39) Marino, G. and Brown, H.C., J. Amer. Chem. Soc., (1959), 81, 5929. (40) S e v e r a l v a r i a t i o n s on t h i s p r o p o s a l have been advanced by Olah and h i s a s s o c i a t e s . See r e f . 17 for a critical evalua t i o n of the idea. (41) Brown, H.C. and Stock, L.M., J. Amer. Chem. Soc., (1957), 79, 1421. (42) Mackor, E.L., H o f s t r a , A. and vander Waals, J.Η., Trans Fara. Soc., (1958), 54, 66, 187. (43) Brown, H.C. and Brady, J.D., J. Amer. Chem. Soc., (1952), 74, 3570.


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