Carboxylic Acids and Their Derivatives

Reduction of Lactones

We have seen that esters can be reduced to alcohols by lithium aluminum hydride (LiAlH₄) through two consecutive additions of hydride ions to the carbonyl carbon.

 

 

The mechanism starts with nucleophilic attack of hydride on the ester carbonyl, forming a tetrahedral intermediate. When the carbonyl double bond is restored, the alkoxide group leaves, producing an aldehyde intermediate. Since aldehydes are even more reactive than esters toward hydride reduction, a second hydride rapidly adds to the aldehyde, ultimately giving a primary alcohol after protonation.

 

 

The reduction of lactones follows a very similar mechanism. The key difference is that the alkoxy group is part of the same molecule because lactones are cyclic esters.

So, when the tetrahedral intermediate then collapses, restoring the carbonyl, ring opening occurs rather than expulsion of a separate alkoxide ion.

This produces an open-chain aldehyde containing an alkoxide group elsewhere in the molecule. The aldehyde is then attacked by a second hydride ion, forming another alkoxide intermediate. Finally, aqueous workup protonates both alkoxide groups to give the corresponding diol.

 

 

Overall, lactones are reduced to diols by LiAlH4, and an important feature of this reaction is that the ring opening occurs during the collapse of the tetrahedral intermediate. Unlike ordinary esters, where a separate alkoxide ion leaves, the alkoxy portion of the lactone remains connected to the molecule, leading to the formation of a single open-chain product containing both oxygen atoms.

We have a dedicated post on the reactions of Lactones and Lactams with lots of practice problems here.

 

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