A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems •

attacking nucleophile has been formed (Swl), (b) simultaneously with
the formation of the bond to the attacking nucleophile (S^2), and
(c) after the bond to the attacking nucleophile has been formed
(Sjy2 (aromatic)).
The reason for the activating effect of electron-withdrawing groups,
especially N0 2 , on nuclear halogen atoms is their ability to stabilise
intermediates such as (XXXI) by delocalisation; it would therefore
be expected that nitro-groups would be most effective when o- and p-
to the substituent to be replaced, for in the m-position they can only
assist in spreading the charge via their inductive effects. The presence
of nitro-groups in the 2-, 4- and 6-positions in picryT chloride,
(OgNVCelVCI, thus confers almost acid chloride reactivity on the
halogen, their effect is so pronounced. 2- and 4-, but not 3-, halogeno-
pyridines also undergo ready replacement reactions for exactly the
same reasons (the electron-withdrawing group here being the
heterocyclic nitrogen atom); they do, indeed, resemble the corres­
ponding o- and /»-nitrohalogenobenzenes though the activation of the
halogen is slightly less than in the latter.
If nitro-groups are to stabilise, and so assist in the formation of,
intermediates such as (XXXI) the p orbitals on the nitrogen atom
of the NOa group must be able to become parallel to those on the
adjacent nuclear carTJFJn atom. For this to happen the oxygen atoms
attacflMto nitrogen mflst also lie in or near the plane of the nucleus. If
such atoms are forced out of this plane by steric factors, the N0 2 group
becomes a much less effective activator as only its inductive effect can
then operate. Thus the bromine in (XXXV) is replaced much more
slowly than in /*-nitrobromobenzene (XXXVI) because the o-methyl
groups in the former prevent the oxygen atoms of the nitro-group
from becoming coplanar with the nucleus and so inhibit the
withdrawal of electrons from it by the mesomeric effect (cf. p. 23):

Me I

©

O t

\©/

N

I

O

Me

O e O

Br

(XXXV)

Br

(XXXVI)