A Guidebook to Mechanism in Organic Chemistry

• Nucleophilic Attack on Aromatic Species

®N

ci

EtO®

0000

Q

®N

OEt

e/ \

O O

(XXXI)

kinetic stuSies in a number of examples supporting the bimolecularity
of the reaction. That an actual intermediate such as (XXXI) is
formed, unlike aliphatic bimolecular nucleophilic substitution where
the bond to the leaving group is being broken as that to the entering
group is being formed, is shown by the fact that chlorides and bro­
mides react in a number of cases at essentially the same rate. The
breakage of the carbon-halogen bond can thus not be involved in the
rate-determining step for a C—Cl bond is more difficult to break than
an analogous C—Br one and the chloride would, a priori, be expected
to react more slowly than the bromide.
Confirmation of the formation of such an intermediate is provided
by the actual isolation of the same species (X^fHII) from the action
of ®OEt on 2,4,6-trinitroanisole (XXXIII) .and eOMe.a^*,4,f>
trinitrophenetole (XXXIV):

It is also found that acidification of the reaction mixture obtained

from either substrate yields exactfy the same-proportion of (XXXIII)

and (XXXIV)?

We have thus now encountered nucleophilic displacement reactions

in which the bond to the leaving group is broken (a) before that to the

(ii) (0

0 ,N;

NO,