The Stability of Carbonium Ions

Me—O^-CHj <- Me— 6 =CH,

leading to carbonium ions characteristically stabilised by delocalisa­

tion.

An interesting example of a carbonium ion being more stable than

might have been envisaged is the one that can arise from j3-phenylethyl

©

derivatives, PhCH 2 CH 2 Y. The carbonium ion, PhCH 2 CH 2 ,

supposedly involved, would not be markedly stabilised by the phenyl

group on the j8-carbon atom for this is too far away to exert any

marked inductive effect and def^calisation via its IT orbitals is prevented

by the intervening saturated carbon atom. It has, therefore, been

suggested on this and other evidence that the caffionium ion involved

is actually a bridged structure, a phenonium iofl •*

e

that can stabilise itself by delocalisation via the ir orbitals of the

aromatic nucleus.

The most stable carbonium ion of all is derived from cyclohepta-

trienyl (tropylium) bromide which ostensibly has the structure:

• &

It is found, however, to be highly water-soluble, yielding bromide

ions and the evidence is overwhelming that it produces, in solution,

where the charge-spreading may take place through the intervention
of suitably placed it orbitals, e.g.

CHr^CH^-CH 3 «- CH 2 —CH=CH 2