A Guidebook to Mechanism in Organic Chemistry

4

CARBONIUM IONS, ELECTRON-DEFICIENT

N AND O ATOMS AND THEIR REACTIONS

REFERENCE has already been made in the last chapter to the gener­
ation of carbonium ions as intermediates in displacement reactions
at a saturated carbon atom, e.g. the hydrolysis of an alkyl halide
that take place via the SN\ mechanism. Carbonium ions are, however,
fairly widespread in occurrence and although their existence is
normally only transient, they are of considerable importance in a wide
variety of chemical reactions.

METHODS OF FORMATION OF CARBONIUM IONS

(i) Direct ionisation

This has already been commented on«in the last chapter, e.g.

It should be emphasised however that a highly polar, ion-solvating
medium is usually necessary and that it is ionisation (i.e. the formation
of an ion pair) rather than dissociation that may actually be taking
place.
The question of how the relative stability, and consequent ease of
formation, of carbonium ions is influenced by their structure will be
discussed below (p. 82).

(ii) Protonation

This may, for instance, occur directly by addition to an unsaturated
linkage, e.g. in the acid-catalysed hydration of defines (p. 143):

-CH=CH- -CH,—CH- v=± -CH,—CH -CH,—CH-

• Me,CCl ->• MeaC® + CIe
  PhCH,Cl ->• PhCH,® + Cl^9
  CH,=CHCH,CI -» CH,=CHCH,® + Cle
  MeOCH,Cl ->• MeOCH,® + Cl®

H.O

OH