Fundamental analysis of microwave irradiated liquid–liquid phase transfer catalysis (MILL-PTC): simultaneous measurement of rate and exchange equilibrium constants in selective O-alkylation of p-tert-butylphenol with benzyl chloride

Abstract

Fundamentals of low power microwave irradiated liquid–liquid phase transfer catalysis (MILL-PTC) have been brought out in enhancements in the rates of alkylation of p-tert-butylphenol with benzyl chloride and selectivity towards O-alkylated product. Further, a method was proposed for measuring simultaneously the reaction rate constant and anion exchange equilibrium constant from the same set of data. The reaction was studied with a variety of phase transfer catalysts using conventional heating (CH) as well as low power (40 W) microwave irradiation (MW). The O-alkylated product 1-(1,1-dimethylethyl)-4-phenylmethoxybenzene was selectively formed with tetra-n-butylammonium bromide (TBAB) as the catalyst under both CH and MW conditions. The rates of PTC reaction were greatly accelerated under MW. The activation energy of the reaction under CH and MW was nearly the same but the pre-exponential factor in the Arrhenius equation was increased by orders of magnitude under MW. The Gibbs free energy calculated for the exchange reactions suggested that the entropy of MILL-PTC reaction was greater than that of conventionally heated liquid–liquid PTC reaction.