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):

RCH₂COOCH₃ → RCH₂C(OH)Ph₂

The secondary alcohol is then dehy­drated using ethanoic anhydride (CH₃COOCOCH₃) to give an alkene:

RCH₂C(OH)Ph₂ → RCH=CPh₂

The alkene is oxidized with chromic acid:

RCH=CPh₂ → RCOOH + Ph₂CO

The result is conversion of an acid RCH₂COOH to the lower acid RCOOH.