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Experiment 5, page 1

Department of Chemistry, Imperial College
Third Year Advanced Practical Organic Chemistry

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.

Introduction¹
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 ab- to a polar group.

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(OBu^t)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.