A Guidebook to Mechanism in Organic Chemistry

7

ADDITION TO GARBON-OXYGEN DOUBLE

BONDS

THE structure of the carbonyl group in aldehydes and ketones is, to

judge from its reactions, not entirely adequately reji^sented by

. e e
y>C=Ot nor by the obvious alternative )>C—O. The reality lies
somewhere between them

C±6 -> yc-O i.e. >C^O

the electrons of the n bond joining carbon to oxygen being drawn

towards oxygen on account of the greater electronegativity of the

latter. This would imply that the characteristic reaction of the car­

bonyl group would be a nucleophilic attack on carbon by an anion,

Ye (e.g. eCN) or an electron-rich species, and such is, indeed, found

to be the case (c/.^Wdition to )>C=C<^ which is usually initiated

by X^*r an electronfleficient complex). Reaction could, of course,

equally well be initiated by attack of an electrophile,on the oxygen

atom of the ^>C=0 group, but this is usually of significance only

where the electrophile is H® (or a Lewis acid),, amounting to acid-

catalysis of the subsequent addition of a nucleophile:

EFFECT OF pH
It might be expected that carbonyl addition reactions would be
powerfully acid-catalysed, for after attack on oxygen by a proton the
carbon atom will become considerably more positive and hence
readier to react with a nucleophile:

v H© \ffl

Y=O^">C-OH

Though this is true, most of the anions that are used»as adducts are
derived from weak acids so that as the solution becomes more acid
their dissociation is suppressed, leading to a drop in [Ye], e.g. eCN
->HCN. Where the nucleophile is not an anion,* e.g. R-NH 2 , a