Introduction:

LiAlH4 is a reducing agent that is commonly used in organic chemistry to reduce various functional groups such as carbonyl groups (C=O), carboxylic acids (COOH), and esters (COOR). However, the reduction of carbon-carbon double bonds or triple bonds by LiAlH4 is a bit more complicated.

Reduction of Carbon-Carbon Double Bonds:

LiAlH4 can reduce carbon-carbon double bonds (C=C) under certain conditions. However, the reduction of C=C is not straightforward and requires special care to prevent over-reduction. Over-reduction can lead to the formation of alkanes (C-C single bonds) instead of alkenes (C=C double bonds).

The reduction of C=C by LiAlH4 involves the formation of an intermediate alkoxide ion. This intermediate can undergo further reduction to form an alkane if not carefully controlled. Therefore, to prevent over-reduction, a weaker reducing agent such as NaBH4 is often used in combination with LiAlH4.

Reduction of Carbon-Carbon Triple Bonds:

LiAlH4 cannot reduce carbon-carbon triple bonds (C≡C) under normal conditions. This is because the C≡C bond is very strong and stable, and it requires a much stronger reducing agent than LiAlH4 to break it.

However, under certain conditions, LiAlH4 can reduce C≡C to form an alkene (C=C). This is achieved by first converting the C≡C triple bond to a C=C double bond using a suitable reagent such as H2/Pd-C or Lindlar's catalyst. The resulting alkene can then be reduced by LiAlH4 in the same way as described above.

Conclusion:

In summary, LiAlH4 can reduce carbon-carbon double bonds under certain conditions, but it requires special care to prevent over-reduction. LiAlH4 cannot reduce carbon-carbon triple bonds under normal conditions, but it can reduce alkynes (C≡C) to alkenes (C=C) after converting the triple bond to a double bond using a suitable reagent.

Lithium Aluminium Hydride cannot reduce isolated C=C double bonds or triple bonds i.e. it does not reduce alkenes or alkynes to alkanes.