Description: Treatment of an acyl halide with diazomethane (CH2N2) followed by heating with water leads to the formation of a new carboxylic acid where the chain is extended by one carbon. This is called the Wolff rearrangement.
Notes: The reaction can be used with acyl bromides or anhydrides.
Notes: Note how in each case we get a new carboxylic acid, but there’s a new CH2 to the left of the carboxylic acid.
Mechanism: Addition of the diazomethane carbon to the carbonyl carbon of the acyl chloride (Step 1, arrows A and B) followed by elimination of chloride ion (Step 2, arrows C and D) leads to formation of a new ketone, which is then deprotonated by chloride ion to give the “diazo ketone” (Step 3, arrows E, F, and G). The next step is rearrangement – formation of a new C-C bond and breaking off of molecular nitrogen (Step 4, arrows H and I) to give a carbocation, with a resonance form (arrow J) to give an intermediate known as a ketene. Then, addition of water to the carbonyl of the ketene (Step 5, arrows K and L) gives an enolate; drawing the resonance form (arrows M and N) and proton transfer to carbon (Step 6, arrows O and P) finally gives the carboxylic acid.
Notes: It’s possible to draw the reaction as occurring through a “carbene” intermediate if the N2 leaves first in step 4. In other words it’s possible to draw it as a stepwise rather than a concerted mechanism. It’s possible to draw other reasonable mechanisms for the proton transfer step (Step 6).