A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

The addition of HCN is base-catalysed indicating that the rate-deter­
mining step of the reaction is attack by eCN. The process is com­
pleted by reaction with HCN if the reaction is being carried out in
liquid HCN but by reaction with H 2 0 if in aqueous solution. This
reaction has provided a great deal of the kinetic data on the addition
of anions to carbonyl compounds, while the addition of bisulphite
has afforded much evidence on the relative steric effect, on such addi­
tion, of groups attached to the carbonyl carbon atom. There is evidence
that the effective attacking agent in the formation^£ bisulphite
derivatives is actually the more powerfully nucleophilic SOsse even
under conditions in which its concentration relative to HSOse is very
small. As is expected, the relative ease of addition, and stability of the
derivative once formed, is considerably less with ketones than with
aldehydes.
Halide ion will add to a )>C=0 group in the presence of acid, but
the equilibrium is so readily reversible that the resultant 1,1-halo-
hydrin cannot be isolated. If the reaction is carried out in alcohol,
however, the a-halogeno-ether so produced may be isolated provided
the solution is first neutralised:
H
«• OH OH
/"V He ® Cl© / He /e
H,C=M5 HjC—OH H 2 C ^=a= H 2 C
\ \
ci ci
H
OMe ®OMe

-H.O

/ -H® / MeOH

H 2 C / H 2 C , H 2 C

\ \

e

\

Cl Cl Cl

i-Chloromethyl ether

(v) Amine derivatives

Reaction with NH 3 , R-NH 2 or, more specifically, HO-NH 2 ,

NH 2 CONHNH 2 and PhN*L;NH 2 is the classical method by

which liquid aldehydes and ketones are characterised.

There is spectroscopic and other evidence that in the formation of

oximes, semicarbazones and probably phenylhydrazones, attack of

the nucleophile, R-NH 2 , on the carbonyl compound, to form the

adduct (V), is rapid and is followed by rate-determining, acid-catalysed