A Guidebook to Mechanism in Organic Chemistry

Hydride Ion Reactions

HO HsO HO

R—N: • C > R—N—C > R—N—C—

I FAST

H H HA

(V)

R—N=C<^+H.O
«-^» (VI)
dehydration of the latter to yield the final derivative (VI). If the acidity
of the solution is increased, however, the rate of dehydration is
naturally accelerated and the initial formation of (V) is slowed owing
to increasing conversion of the reactive nucleophile R-NH 2 .into its
©
unreactive conjugate acid, RNH 3 ; initial attack of the nucleophile
on the carbonyl compound may then become the rate-determining
step of the overall reaction. The fact that oxime formation may also be
catalysed by bases at higher pH is due to the dehydration step being
subject to base—as well as acid—catalysis:

^>C=NOH

With ammonia some few aldehydes (e.g. chloral) yield the aldehyde

ammonia, R',CH(OH)-NH 2 , but these derivatives more often

react further to yield polymeric products. With primary amines, the

derivatives obtained from both aldehydes and ketones eliminate

water spontaneously, as above, to yield the Schiff base, e.g. R' • CH=

NR, (VI).

(vi) Hydride ion reactions

(a) LiAlHf reductions: Here the complex hydride ion, AlH 4 e, is

acting as a carrier of hydride ioq» the latter acting as a nucleophile

towards the carbonyl carbon atom:

R,C=^ + AIH 49 -> R»C—O^3 ^ RjC—OH