Does rearrangement occur in Friedel Crafts alkylation or not?
When you conduct the alkylation of benzene with n-propyl chloride, the product you obtain is isopropylbenzene, so a rearrangement occurred by virtue of the fact that the primary carbocation that forms will undergo a 1,2-hydride shift to form a secondary one. Most alkylation ( aromatic ring and alkyl halides ) are conducted in the presence of aluminium chloride which is one of the strongest Lewis acids. The principle is that the alkyl halide reacts with the aluminium chloride to form a salt: a carbocation and a complex ion of tetrachloroaluminate. You can have methyl chloride react in this way because even if it forms an unstable carbocation, it will readily react with the aromatic ring ( which acts as a Lewis base ). You can have methyl carbocation in solution such as in alkylation reactions, but you can’t isolate a salt containing methyl carbocation in solid state. If you had methyl chloride and aluminium chloride in solid state ( in contact with each other, they would not react to form carbocations.