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Reduction of carboxylic acids to primary alcohols using LiAlH4

Description: Addition of lithium aluminum hydride [LiAlH4]  to carboxylic acids leads to the formation of primary alcohols (after addition of acid)
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Notes: Generally at least 2 equivalents of LiAlH4 are used for this reaction. The identity of the acid used for the workup is not crucial (H3O+ is fine)

Examples:
 

Notes:

Mechanism:

LiAlH4 is a basic reagent and carboxylic acids are (wait for it…) acidic. So the first reaction is simple deprotonation of the carboxylic acid to give the carboxylate (Step 1, arrows A and B). Next, 1,2-addition of hydride from a second equivalent of LiAlH4 leads to a tetrahedral intermediate (Step 2, arrows C and D) which then performs a 1,2-elimination (Step 3, arrows E and F) to give an aldehyde. The aldehyde is then reduced by a third equivalent of LiAlH4 to give the alkoxide (Step 4, arrows G and H) which is then protonated in the workup step (Step 5, arrows I and J) to give the primary alcohol.

Notes: 

  • Although Li2O is shown as the leaving group in step 3 it is also reasonable to draw an aluminum salt (OAlH2) as the leaving group instead.
  • Also, since 3 equivalents of AlH3 are floating around it is reasonable to show them as the reducing agents for the aldehyde.

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