Selective engineering in O-alkylation of m-cresol with benzyl chloride using liquid–liquid–liquid phase transfer catalysis

Abstract

Selective O-alkylation or etherification of phenolic substances is attractive to make pharmaceuticals and fine chemicals. Liquid–liquid phase transfer catalysis (L–L PTC) is used to make such ethers but it always leads to byproduct formation and wastage of catalyst. On the contrary, liquid–liquid–liquid (L–L–L) PTC is an excellent strategy for waste reduction and improving profitability, in which a catalyst-rich middle (or third) phase is formed between aqueous and organic phases. The reaction takes place in the middle phase, which intensifies the rates and offers better selectivity including catalyst reusability unlike the bi-liquid PTC. Selective O-benzylation of m-cresol was studied with benzyl chloride by using L–L–L PTC at 50 °C. It gives 100% selectivity to 3-benzyloxytoluene. The catalyst-rich third phase was recycled many times, thereby leading to profitability and waste minimization. A kinetic model is developed and validated against experimental data. The reaction follows a pseudo-first order rate law. The studies were also extended to the benzylation of o- and p-cresol. The energy of activation for the benzylation of o-, m- and p-cresols were found to be 12.84, 10.00 and 12.12 kcal/mol, respectively. The results are novel.