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21.3: Reactions with Acid Chlorides and Esters

  • Page ID
    216882
  • The mechanism of action of hydride reductions on acid chlorides and esters (carboxyl groups) is similar to that taking place with carbonyl compounds, except that acid chlorides and esters have a leaving group (–Cl and –OR). So the reaction does not stop at formation of the alkoxide ion as a tetrahedral intermediate, but keeps going with an internal nucleophilic displacement of the leaving group. The direct outcome of this process is formation of the corresponding carbonyl compound (aldehyde or ketone), which may or may not undergo further reduction to alcohol, depending on the nature of the reagents used and reaction conditions. The following mechanism illustrates this concept. For simplicity, only the hydride ion is shown.

    clipboard_ea1de80561b303e63c7e7de740f2c20da.png

    If a full reactivity reducing agent such as LiAlH4 is used, the reaction does not stop at the aldehyde stage, since the carbonyl carbon of the aldehyde can be attacked by another hydride equivalent. This results in formation of the primary alcohol (after hydrolysis of the alkoxide ion) as the final product.

    clipboard_ef603681b0c1ee3ceac58f974051a2ed7.png

    The net reaction then is:

    clipboard_e56e5c7c398f4da0c6341b9b9bb168fec.png

    The reaction with an ester is similar, but the leaving group is different (R’O ). Can you draw the mechanism that leads to formation of the products shown?

    clipboard_e99c779517df351f8c4834e63cb147201.png

    Notice that with both (and all) carboxyl groups, hydride reductions lead to formation of primary alcohols only. There is no possibility of forming secondary alcohols by this method because the carboxyl group is at the end of the carbon chain, or else the chain gets broken so that the carboxyl carbon ends up at the end of a chain in the final product.

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