Hofmann Elimination

When a tetraalkylammonium halide (the quaternary salt reached by exhaustive alkylation, section 13.6.2) is heated together with moist silver oxide, it is converted into the corresponding quaternary ammonium HYDROXIDE -- a deliquescent (readily absorbs atmospheric moisture) crystalline solid that is strongly basic, comparable in strength to sodium hydroxide or potassium hydroxide. When this quaternary ammonium hydroxide is then subjected to STRONG heating, it undergoes a beta-elimination reaction to give an alkene -- this specific elimination reaction is called Hofmann elimination. A distinctive and important feature of Hofmann elimination, worth contrasting directly with the more familiar Saytzeff (Zaitsev) elimination rule, is that Hofmann elimination gives the LEAST substituted alkene as its major product (rather than the MOST substituted, more thermodynamically stable alkene that Saytzeff elimination favours) -- this happens because the bulky, positively-charged quaternary ammonium leaving group sterically favours the base removing a hydrogen from the LESS hindered, more accessible beta-carbon. Worked through with a specific example: N,N,N-triethylpropylammonium iodide, heated first with moist Ag2O, gives N,N,N-triethylpropanammonium hydroxide (the iodide simply swapped for hydroxide); this hydroxide, on further strong heating, undergoes Hofmann elimination -- losing water and forming …