Sommelet–Hauser rearrangement
The Sommelet–Hauser rearrangement (named after M. Sommelet and Charles R. Hauser) is a rearrangement reaction of certain benzyl quaternary ammonium salts. The reagent is sodium amide or another alkali metal amide and the reaction product a "N","N"-dialkylbenzylamine with a new alkyl group in the aromatic ortho position. For example, benzyltrimethylammonium iodide, [(CHCH)N(CH)]I, rearranges in the presence of sodium amide to yield the "o"-methyl derivative of "N","N"-dimethylbenzylamine. Mechanism. The benzylic methylene proton is acidic and deprotonation takes place to produce the benzylic ylide (1). This ylide is in equilibrium with a second ylide that is formed by  deprotonation of one of the ammonium methyl groups (2). Though the second ylide is present in much smaller amounts, it undergoes a 2,3-sigmatropic rearrangement because it is more reactive than the first one and subsequent aromatization to form the final product (3). The Stevens rearrangement is a competing reaction.