The migration of an alkyl group or a halogen atom in a sulfonic acid derived from a polyalkylbenzene, a halogenated polyalkylbenzene, or a polyhalogenated benzene is known as the Jacobsen reaction. The reaction is nearly always effected by concentrated sulfuric acid. The migrations of the Jacobsen reaction may be divided into two general types, intramolecular in which the migrating group moves from one position to another in the same molecule, and intermolecular in which there is a transfer of one or more groups from one molecule to from one molecule to another. In most cases migration of both types occur simultaneously. An important characteristic of the reaction is the migration of the alkyl groups to vicinal positions.