Experiment 5, page 1

EXPERIMENT 5:
REDUCTIONS WITH LITHIUM ALUMINIUM HYDRIDE

Aims of the experiment
To demonstrate the control of reactivity in LiAlH4reductions possible by altering the mode of
addition, by reducing cinnamaldehyde eitherto cinnamyl alcohol OR dihydrocinnamyl alcohol.

Techniques used/learned:
Handling of pyrophoric and moisture sensitive reagents (LiAlH4); vacuum distillation.

Introduction
Lithium aluminium hydride is an excellent reagent for the reduction and hydrolysis of certain polar
groups. For example, –COCl, -CO2H, –CO2Et, and –CHO are reduced to –CH2OH; >CO to
>CHOH; –CH=NOH and –CONH2to –CH2NH2. Provided the correct solvent is used, primary
and secondary halides can be reduced to hydrocarbons. Carbon-carbon double and triple bonds are
generally unaffected except when they are αβ- to a polar group.

1

The reductions often proceed at room temperature or below and are usually rapid and free from
side reactions. The compound to be reduced is added slowly (by continuous extraction if it is
sparingly soluble) to an excess of the reagent suspended or dissolved in an ether.
Selective reduction of polar groups in the presence of other reducible functions can frequently be
achieved by an inverse addition method: the reagent is added slowly to the substance to be
reduced, so that the reagent is never present in excess. Thus by inverse addition –C≡N can be
reduced to –CH=NH (normal addition gives –CH2NH2) and cinnamaldehyde to cinnamyl alcohol
(normal addition gives hydrocinnamyl alcohol).

Selective reduction of functional groups can also be achieved by chemical modification of the
reagent, e.g. as LiAlH(OBut)3, or by the use of the related complex hydrides, lithium and sodium
borohydrides. Useful modifications in the properties of lithium aluminium hydride is also effected
with aluminium chloride and other Lewis acids.

O

OH

OH

Department of Chemistry, Imperial College

Third Year Advanced Practical Organic Chemistry