Reduction of Functional Groups
In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups. In many ways, the reactivity of aluminium hydride is similar to that of lithium aluminium hydride. Aluminium hydride will reduce aldehydes, ketones, carboxylic acids, anhydrides, acid chlorides, esters, and lactones to their corresponding alcohols. Amides, nitriles, and oximes are reduced to their corresponding amines.
In terms of functional group selectivity, alane differs from other hydride reagents. For example, in the following cyclohexanone reduction, lithium aluminium hydride gives a trans:cis ratio of 1.9 : 1, whereas aluminium hydride gives a trans:cis ratio of 7.3 : 1.
Alane enables the hydroxymethylation of certain ketones, that is the replacement of C-H by C-CH2OH). The ketone itself is not reduced as it is "protected" as its enolate.
Organohalides are reduced slowly or not at all by aluminium hydride. Therefore, reactive functional groups such as carboxylic acids can be reduced in the presence of halides.
Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups.
Aluminium hydride can be used in the reduction of acetals to half protected diols.
Aluminium hydride can also be used in epoxide ring opening reaction as shown below.
The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding.
Aluminium hydride even reduces carbon dioxide to methane under heating:
- 4 AlH3 + 3 CO2 → 3 CH4 + 2 Al2O3
Read more about this topic: Aluminium Hydride, Reactions
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