6.2.2.3 Reduction

The attack of hydride ion on the carbonyl group, i.e., the reduction of aldehydes and ketones may be seen as nucleophilic addition but the actual pathway is quite complicated.  Lithium aluminium hydride is a very powerful reducing agent and easily reduces both aldehydes and ketones to their corresponding alcohols.  The reaction involves irreversible hydride transfer to the carbonyl compound to form an aluminium complex (1) which then further reacts with another three molecules of ketone to form the complex (2) which on treatment with aprotic solvent gives the product.

A much weaker reducing agent is sodium borohydride which may be used in protic solvents. It is less reactive than lithium aluminium hydride thereby allowing more selective reductions. The pathway for reduction is however similar to that of lithium aluminium hydride (Scheme 12).

Scheme 12

There are variety of reducing agents can be derived from these hydride ion donors. Several reducing agents are known tailor made to suit the purpose of selective reduction of one group while keeping the other unaffected.

In the above mentioned reduction protocols, the transfer of hydride ion is irreversible but a reversible hydride transfer to a carbonyl compound is also possible. This reaction known as Meerwein-Pondorf-Verley reduction is similar to the previously mentioned Oppenauer oxidation protocol for preparation of ketones.