THE REACTION BETWEEN ACYL HALIDES AND ALCOHOLS - ALKYL HALIDE VS ESTER FORMATION

In the reaction between an acyl halide and an alcohol the thermodynami cally favoured products are the free carboxylic acid and the alkyl hal ide. The initial reaction is, generally, the formation of an ester and HHal. When the alcohol is very prone to yield an alkyl cation upon pr otonation by HHal, formed H2O exhibited a superior reactivity and comp eted successfully with the alcohol for the acyl halide making, therefo re, ester formation practically confined to a triggering role. But, in those cases where the cation is less easily formed, ester formation w as favoured and, consequently, became the necessary elementary step to wards alkyl halide formation. This final product, on the other hand, m ight be extremely slow to form in an SN2 reaction between the protonat ed ester function and the halide ion. In these instances, therefore, a s well as in the cases when a basic solvent competes for the proton of HHal, the ester is the final product. A notable exception of the situ ation above outlined, is given by alpha-hydroxy-alpha-phenylbenzeneace tic acid (2y), which appears to undergo direct chlorine-hydroxyl inter change through a quaternary intermediate (E), in the end collapsing to alpha-chloro-alpha-phenyl-benzeneacetic acid (4y). Different systems were compared using CH2Cl2 as a solvent under strictly similar conditi ons. Some 28 different substrates were tested for reaction with AcCl ( 1a), whereas the action of eight acyl halides (1) against (RS)-alpha-m ethylbenzenemethanol (2n) and alpha-phenylbenzenemethanol (2p), as wel l as the effect of five different solvents on the reaction between two alcohols (2p and 2-methyl-2-propanol, 2c) with 1a, were observed.