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.
