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

In the reaction between an acyl halide and an alcohol the thermodynamically favoured products are the free carboxylic acid and the alkyl halide. The initial reaction is, generally, the formation of an ester and HHal. When the alcohol is very prone to yield an alkyl cation upon protonation by HHal, formed H2O exhibited a superior reactivity and competed successfully with the alcohol for the acyl halide making, therefore, ester formation practically confined to a triggering role. But, in those cases where the cation is less easily formed, ester formation was favoured and, consequently, became the necessary elementary step towards alkyl halide formation. This final product, on the other hand, might be extremely slow to form in an SN2 reaction between the protonated ester function and the halide ion. In these instances, therefore, as 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 situation above outlined, is given by alpha-hydroxy-alpha-phenylbenzeneacetic acid (2y), which appears to undergo direct chlorine-hydroxyl interchange 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 conditions. Some 28 different substrates were tested for reaction with AcCl (1a), whereas the action of eight acyl halides (1) against (RS)-alpha-methylbenzenemethanol (2n) and alpha-phenylbenzenemethanol (2p), as well as the effect of five different solvents on the reaction between two alcohols (2p and 2-methyl-2-propanol, 2c) with 1a, were observed.