The ‘toluene-carrier’ technique has been used for the determination of the C—Br bond dissociation energies in the substituted benzyl bromides:
p
-,
m
- and
o
-xylyl bromides;
p
-,
m
- and
o
-chlorobenzyl bromides;
p
- and
m
-bromobenzyl bromides;
p
- and
m
-nitrobenzyl bromides; and
p
- and
m
-nitrilebenzyl bromides. The rate-determining step of the decompositions of all these compounds is represented by the unimolecular dissociation processes (
s
)
Ph
s
. CH
2
. Br →
Ph
s
. CH
2
• + Br, (
s
) where
Ph
s
. CH
2
. Br refers to the substituted benzyl bromide. Assuming that the frequency factor of the decomposition of each benzyl bromide is equal to the frequency factor of reaction (
u
)
Ph
. CH
2
. Br →
Ph
. CH
2
• + Br, (
u
) the differences in activation energies between
E
u
and
E
s
were calculated using the relation
E
u
─
E
s
=
RT
In (
k
s
/
k
u
); (I)
k
s
and
k
u
denote the unimolecular rate constants of reactions (
s
) and (
u
) respectively. Since
E
s
and
E
u
are equal to the C—Br bond dissociation energies in the substituted benzyl bromides and benzyl bromide itself, equation (I) yields the differences, ∆
D’
s, between
D
(
Ph
. CH
2
—Br) and the values for
D
(
Ph
s
. CH
2
—Br). The calculated differences in the C—Br bond dissociation energies are listed below: substituted ∆
D
substituted ∆
D
benzyl bromides (kcal. /mole) benzyl bromides (kcal. /mole)
o
-chloro 0·9
m
-methyl 0-0
m
-chloro 0·1
p
-methyl 1·4
p
-chloro 0·4
m
-nitro 2·1
m
-bromo 0·3
p
-nitro 1·1
p
-bromo 0·3
m
-nitrile 1·4
o
-methyl 2·0
p
-nitrile 0·7 The significance of these findings is discussed, and the effect of substitution on a bond energy is contrasted with the effect of ionic reactions.
