A Guidebook to Mechanism in Organic Chemistry

Electrophilic Substitution of Other Aromatic Species

atom because of the latter's being more electronegative than carbon.
This is reflected in the observed dipole moment of pyridine

,1 = 2-30 GO O ,* = 3-9D

(XXIV) (XXV)

and the compound would therefore be expected to have a deactivated

nucleus towards electrophilic substitution (cf. nitrobenzene (XXV)).

The deactivation of the^ucleus is considerably increased on electro­

philic attack, for the positive charge introduced on nitrogen by pro­

tonation, or by direct attack on it of the substituting electrophile,

withdraws electrons much more strongly:

H

In fact electrophilic substitution is extremely difficult, sulphonation,

for example, requiring twenty-four hours heating with oleum at 230°.

Substitution takes place at the /^-position (m- to the electron-with­

drawing centre), the explanation being similar to that already

discussed for nitrobenzene (p. 121).

Pyrrole (XXVI) also has delocalised n orbitals but nitrogen has

here had to contribute two electrons so becoming virtually non-basic

(p. 56) and the dipole moment is found to be in the opposite direction

to that of pyridine:

/I = 1-8 D i- 9

(XXVI)

It is thus referred to as a n excessive heterocycle as compared with

pyridine which is a n deficient one. It behaves like a reactive benzene

derivative, e.g. aniline, and electrophilic substitution is very easy.