A Guidebook to Mechanism in Organic Chemistry

Cannizzaro Reaction

Hydride ion, He, is transferred from aluminium isopropoxide to the
ketone (VIII) via a cyclic transition state and an equilibrium thereby
set up between this pair on the one hand and the mixed alkoxide (IX)
plus acetone on the other. That there is indeed such a specific transfer
of hydrogen may be demonstrated by using (Me 2 CDO) 3 Al when
deuterium becomes incorporated in the a-position of the resultant
carbinol, R CD(OH) R'.
Acetone is the lowest boiling species in the system, so by distilling
the mixture the equilibrium is displaced to the right, the secondary
alcohol (X)^8liig freed from the alkoxide (IX) by the excess isopro-
panol present. Because the establishment of this equilibrium is the
crucial stage, the reaction is, naturally, very highly specific in its action
and ^G^C^, -C=C-, NOs, etc., undergo no reduction. The
reaction may be reversed, RCH(OH)R'—>RCO-R', by use of
aluminium t-butoxide and a large excess of acetone to displace the
equilibrium to the left.
(c) Cannizzaro reaction: The disproportionation of aldehydes lack­
ing any a-hydrogen atoms (i.e. PhCHO, CH 2 0 and R 3 CCHO)
to acid anion and primary alcohol in the presence of concentrated
alkali, is also a hydride transfer reaction. In its simplest form the
reaction rate « [PhCHO]^2 [eOH] and the rea«ien is believed to
follow the course: ^

HO!) HQ OH O 9

Ph—C±0 Ph—C^OB + C—Ph -> Ph—C=0 + H—C—Ph

H (HJ^H H (XI)

I

O^9 HO

Ph—0=0 + H—C—Ph

(XII) H

Rapid, reversible addition of EOH to/one molecule of aldehyde

results in transfer of hydride ion to a second; this is almost certainly

the rate-determifling step of the.reaction. The acid and alkoxide ion

(XI) so obtained then become involved in a proton exchange to yield

the more stable pair, alcohol and acid anion (XII), the latter, unlike

the alkoxide ion, being able to stabilise itself by delocalisation of its