A Guidebook to Mechanism in Organic Chemistry

Classification of Reagents

etc.

(XXXVII)

will tend to be most readily attacked by positively charged ions such

as PbN 2 e,^he diazonium cation, or by other species which, though

not actually ions themselv«, possess an atom or centre which is

electron-deficient, for example the sulphur atom in sulphur trioxide:

Oe

A.

*Azo-coupling (p. 112) or sulphonation (p. 108) takes place on a
carbon atom of the nucleus rather than on oxygen because of the
charge-transfer from oxygen to«carbon that can take place as shown
above and because of the greater stability of the carbon rather than the
oxygen-substituted products. mm
Conversely, an electron-deficient centre, such as the carbon atom
in methyl chloride (XXJ|VIIi)

H

\«+ »-

• H—C-*-Cl
  /
  H
  (XXXVIII)

will tend to be most readily attacked by negatively charged ions such

as ®OH, eCN, etc., or by other species which, though not actually

ions themselves, possess an atom or centre which is electron-rich, for

example the nitrogen atom in ammonia or amines, H 3 N: or R 3 N:.

It must be emphasised that only a slightly unsymmetrical distribution

of electrons is required for a reaction's course to be dominated: the

presence of a full-blown charge on a reactant certainly helps matters

along but is far.from being essential. Indeed the requisite unsymmetri­

cal charge distribution may be induced by the mutual polarisation of

o

readily react. An electron-rich compound, such as phenoxide ion,

(XXXVII)