Barbier-Wieland degradation

The stepwise degradation of a carboxylic acid to the next lower homologue. First the ester is converted into a tertiary alcohol using a Grignard reagent (PhMgX) and acid (HX):

The secondary alcohol is then dehydrated using ethanoic anhydride (CH₃COOCOCH₃) to give an alkene:
The alkene is oxidized with chromic acid:
The result is conversion of an acid RCH₂COOH to the lower acid RCOOH.

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