Polarity of the acid chain of esters and transesterification activity of acid catalysts

The effect of polarity of the esters on the transesterification reaction rate has been investigated. The effect was studied in homogeneous and heterogeneous catalysts. The polarity of different ethyl alkanoate esters was varied by (i) increasing the number of carbon atoms in the acid alkyl chain of the esters and (ii) introducing Br and hydroxy substituents at the end of the acid chain of ethyl hexanoate. Polarity was determined through the lambda(max) of the UV-Vis spectrum of the betaine dye dissolved in the investigated esters (ET(30) scale). The transesterification reaction was carried out with methanol and by using sulfuric acid and a Dowex DR2030 sulfonic resin as homogeneous and heterogeneous catalysts, respectively. it was observed that, in addition to steric hindrance, the polarity of the ester chain has an effect on the reaction rate of the heterogeneous acid catalysts. It is proposed that the positive or negative effect of the polarity is due to repulsive or attractive interactions of the ester chain with the polar groups of the resin and/or with the methanol molecules present in the pores. A very positive effect is found in heterogeneous acid catalysis if H-bonds can stabilize the active intermediate participating in the rate determining step. The attractive or repulsive interactions are absent in the homogeneous case. (C) 2008 Elsevier Inc. All rights reserved.