LiAlH4 Reaction and Mechanism

LiAlH₄ Mechanism

The commonly used reagent for the reduction of aldehydes and ketones are LiAlH₄ or NaBH₄. The reaction proceeds by successive transfer of hydride ions (H⁻) from boron or aluminium to four different carbonyl carbons. As all four of the hydrides are transferred, there are distinct reducing agent used in every step of reduction. In the first step, the reducing agent is AlH₄⁻ (I), while in the second step, the reducing agent is RCH₂OAlH₃− (II).

In the same manner, species [RCH₂O]2AlH₂⁻ (species III) and [RCH₂O]₃AlH⁻ (species IV) will be obtained. Reduction with NaBH₄ is usually done in water or in alcohol which are used as the solvent. The solvent destroys the intermediate alkoxide ion and yields alcohol. If water or an alcohol is not used as the solvent, aqueous acid can be added after the reduction to convert the alkoxide to alcohol.

Lialh4 Mechanism Steps

The lialh₄ mechanism for reduction with lithium aluminium hydride is very similar to that of NaBH4. As LiAlH₄ violently reacts with water and other polar protic solvents to give molecular hydrogen, reduction with LiAlH₄ are done in aprotic solvents like anhydrous ether. Ethyl acetate is added to decompose the aluminium complex. But reduction with NaBH₄ can be carried out in water or ethanol solution.Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah. 

Mechanism of reduction by LiAlH₄:

Aldehydes are reduced to primary alcohols by both LiAlH₄ and NaBH₄ while these reducing agents reduce ketones to secondary alcohols. 

(i)

(ii)

(iii)

(iv)

LiAlH4 can reduce all functional groups of column ‘A’ into Column ‘B’. Generally, it is unable to reduce double bond, which are in conjugation with group the double bond is reduced when a phenyl group is attached to the β−carbon.

Example of LiAlH4

C₆H₅−CH=CH−CHO  C₆H₅CH₂CH₂CH₂– OH

can reduce all the above groups of column ‘A’ into groups of column ‘B’ except, −COOH and −COOR. NaBH₄ is able to reduce the double bonds, which are in conjugation with group.