Criegee rearrangement

One alkyl substituent migrates from carbon to the adjacent oxygen atom, replacing the carboxylic acid leaving behind a carbocation.

The order of migrationary aptitude is a tert-butyl as the best substituent followed by isopropyl then ethyl and then the methyl group.

From this it is inferred that the migrating group carries a partial positive charge in the transition state leading to the carbocation.

The consecutive Criegee Rearrangement is carried out in an acidic environment and an ester forms from the carbocation.

This opens the way to multiple O-insertion reactions, eventually leading to the orthoester.