2.3 Aldehydes by Reduction of Acyl Chlorides, Esters and Nitriles

  Aldehydes and Ketones

    1.0 Introduction
    2.0 Methods of Preparation
    3.0 Physical Properties
    4.0 Relative Reactivities of Carbonyl Compounds
    5.0 Addition of Carbon Nucleophiles
    6.0 Haloform Reactions
    7.0 Aldol Condensations
    8.0 Claisen Condensation
    9.0 Intramolecular Claisen Condensation
    10.0 Cannizzaro Reaction
    11.0 Reformatsky Reaction
    12.0 Addition of Nitrogen Nucleophiles
    13.0 Addition of Oxygen Nucleophile
    14.0 Addition of Sulphur Nucleophile
    15.0 Oxidation of Aldehydes And Ketones
    16.0 Reduction of Aldehyde and Ketones
    17.0 Other Reactions Of Aldehydes And Ketones

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2.3 Aldehydes by Reduction of Acyl Chlorides, Esters and Nitriles

Theoretically, it ought to be possible to prepare aldehydes by reduction of carboxylic acids. In practice, it is not possible, because the reagent normally used to reduce carboxylic acid directly is $LiAl{H_4}$and when any carboxylic acid is treated with $LAH$, it is reduced all the way to 1° alcohol. This happens because $LAH$ is a powerful reducing agent and aldehydes are very readily reduced. Any aldehydes that might be formed in the reaction mixture is reduced by the $LAH$ to 1° alcohol.

The secret of success here is not to use a carboxylic acid itself, but use a derivative of carboxylic acid that is more readily reduced, and to use an aluminium hydride derivative that is less reactive than $LAH$. We shall study derivative of carboxylic acid, but suffice it to say here that acyl chlorides, esters and nitrile are all easily prepared from carboxylic acid, and they all are more easily reduced. Two derivative of aluminium hydride that are less reactive than $LAH$ (in part because they are much more sterically hindered and therefore, have difficulty in transferring hydride ion) are lithium tri-tert-butoxyaluminium hydride and diisobutyl aluminium hydride $(DIBAL-H)$

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Lithium tri-tert-butoxyaluminium hydride

The following scheme summarises how these reagents are used to synthesise aldehydes from acid derivatives.