Reaction: Ketone + peracid (mCPBA, CF3CO3H) → ester (oxygen inserted between C=O and R)

Migratory aptitude (which group migrates): H > 3° alkyl > cyclohexyl > 2° alkyl > phenyl > 1° alkyl > methyl

Product prediction: The more substituted/higher migratory aptitude group migrates → it ends up on the O side of the ester.

Example: CH3CO-C(CH3)3 + mCPBA → CH3CO-O-C(CH3)3 (tert-butyl acetate). tert-Butyl migrates (3° > methyl).

Aldehyde version: RCHO + mCPBA → RCOOH (formate initially, then acid). Hydrogen migrates (highest aptitude).