A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient N and O Atoms

a carbonium ion that is so stable that its reaction with water, alcohols,
etc., is quite slow. The reason for this quite outstanding stability is
that in the tropylium cation, the seven-membered ring possesses six
w electrons, which can spread themselves over the seven carbon
atoms in delocalised orbitals (cf. benzene, p. 9), thereby conferring
on the ion quasi-aromatic stability:

Br^0

The simple alkyl carbonium ions have already been seen (p. 62)

to follow the stability sequence

Me 3 C > Me 2 CH > MeCH, > CH,

due to the fact that increasing substitution of the carbonium ion

carbon atom by methyl groups results in increasing delocalisation of

the charge by both inductive and hyperconjugative effects. But stabil­

isation here and in other cases requires that the carbonium ion should

be planar, for it is drily in this state that effective delocalisation can

occumAs planarity fe departed from or its attainment inhibited,

instability of the ion, with consequent difficulty in its initial forma­

tion, increases very rapidly. This has already been seen in the extreme

inertness of 1-bromotriptycene, where inability to assume a planar

state prevents formation of a carbonium ion with consequent inert­

ness to Sjyl attack (p. 65).

This great preference for the planar state, if at all possible, effec­

tively settles the question of the stereochemistry of simple carbonium

ions.

TYPES OF REACTION UNDERGONE BY CARBONIUM IONS

Essentially, carbonium ions can undergo three main types of reac­

tion:

(a) combination with a nucleophile,

(b) elimination of a proton, t

(c) rearrangement of structure.

It should be noted that (c) will result in a further carbonium ion

which may then undergo (a) or (b) before a stable product is obtained.