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. The Barbier–Wieland degradation was discovered by H. Wieland in 1912 and independently by P. Barbier in 1913.