A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient N and O Atoms

In the two latter cases it is an acyl carbonium ion that may be
momentarily formed, in contrast to the alkyl ones that we have seen
so far. This acyl carbonium ion reacts with water to yield the corres­
ponding fatty acid and regenerates a proton:

R-CO + HiO ^ RCOOH ^ RC0 2 H + H®

H

The reversal of this, and of the second reaction above, are involved
in some examples of the acid-catalysed formation of an ester from the
corresponding acid and alcohol (p. 187). •

(iii) Decomposition

The most common example is the decomposition of a diazonium
salt, R-N,®:

[R_N^N «- R_N=N| -> R®+N 8

This may be observed with both aromatic and aliphatic diazonium
compounds but, under suitable conditions, these may also undergo
decomposition to yield free radicals (p. 255).
The catalysis of^i number of nucleophilic displacement reactions
of hSItdes by Ag® is due to 'electrophilic pull' on the halogen atom by
the heavy metal cation:

Agffi + Br—R -> AgBr+R®

The presence of Ag® may thus have the effect of inducing a shift in

mechanistic type from SJV2 to SNl but the kinetic picture is often

complicated by the fact that the precipitated silver halide may itself

act as a heterogeneous catalyst for the displacement reaction.

It should be emphasised that the methods of formation of car­

bonium ions considered above are not intended to constitute a

definitive list.

THE STABILITY OF CARBONIUM IONS

The major factor influencing the stability of carbonium ions is that

the more the positive charge may be shared among nearby atoms the

greater will be the stability of the ion. This is particularly marked