Nucleophilic Substitution at a Saturated Carbon Atom
ionisation is largely recovered in the energy of solvation of the ions so
formed. The cation (I) in which the carbon atom carries a positive
charge is a carbonium ion and during its formation the initially
tetrahedral carbon atom collapses to a more stable planar state in
which the three methyl groups are as far apart as they can get; attack
by eOH or solvent can then take place from either side. If this assump
tion of a planar state is inhibited by steric or other factors (cf. p. 65),
the carbonium ion will be formed only with difficulty if at all, i.e.
ionisation, and hence reaction by the SNl mechanism, may then not
take place. ^
Kinetics alone can, in some cases^oe an insufficient'guide as to
which mechanism is being followed, unless the reaction is investigated
under more than one set of conditions. Thus where the solvent can
act as a nucleophilic reagent, e.g. H 20 under S^2 conditions:
Rate oc [R-Hai][H,0].
But as [HaO] is effectively constant, the rate becomes proportional tc
[R-Hal] and study of the kinetics in water alone would erroneously
suggest that the reaction was of the SNl type. Such attack by the
solvent, in this case H 2 0, is known as solvolysis.
* EFFECT OF SOLVENT
The*3olvent in which'a reaction is carried out may exert a profound
effect on the mechanism by which such reaction takes place. So far as
the hydrolysis or solvolysis of a given halide is concerned, the more
polar the solvent employed the more likely is the reaction to proceed
via the SN\ rather than the. Sjy2 mode, and such changeovers, as the
solvent is varied, are well known. This change in mechanism is in part
due to a solvent of high dielectric constant promoting ionisation but
also to the fact that ions so produced will become-highly solvated in
suitable solvents, e.g. water. This solvation process is attended by the
liberation of considerable amounts of energy which may go a long way
towards providing the energy necessary for ionisation, which is thus
further promoted. That such solvation effects are of great importance
is confirmed by the fact that though SN\ reactions are not unknown in
the vapour phase, where solvation of ions is naturally impossible, they
are very much less common than those in solution.
For a halide that undergoes hydrolysis by the Sjyl mode in a number
of different solvents, the rate of hydrolysis is observed to increase as
the solvent becomes more polar and/or a better medium for solvating