Synthesis of Phenylacetic Acid in the Reaction of Benzyl Alcohol with Carbon Monoxide Catalyzed by Rhodium Complex

Abstract

Synthetic and kinetic invstigations have been carried out on the formation of phenylacetic acid in the carbonylation of benzyl alcohol catalyzed by RhC133H 20 in the presence of benzyl iodide. The carbonylation was carried out at 110∼160°C and under 10∼50 kg/cm' CO in the liquid phase using the autoclave equipped with a high-pressure gas reservoir, and the reaction products were analyzed by gas chromatography; gas chromatography-mass spectrometry, NMR-, and IR-spectrometries.
The produc t distributions in the carbonylation were remarkably affected by various reaction factors, such as the amounts of RhCl₃⋅3H₂O and benzyl iodide, the concentration of benzyl alcohol, partial pressure of CO, reaction temperature, and reaction time, the optimum condilions for the synthesis of phenylacetic acid being- benzyl alctthol: RhCl₃⋅3H₂O: benzyl iodide: solvent = 1.0: 6.3 × 1.1 × 1 0': 8×10^-1 (molar ratio), ca.30 kg/cm², ca.140°C, and ca.90 min, respectively. Tile conversion I of benzyl alcohol and the selectivity to phenylacetic acid under these conditions lead to 100 mol% and 94 mol%, respectively. In the initial stage of the carbonylation, diberizyl ether (DBE) and benzyl phenylacetate (BPA) were mainly produced, which were gradually converted to phenylacetic acid. Toluene and bibenzyl were formed in small amounts as the by-products. The kinetics of the carbonylation was studied by measuring the rates of CO consumption under the different reaction conditions. The rate equation was obtained as rate =k[Rh complex] [benzyl iodide]. The rate-determining step in this carbonylation is thus the oxidative addition of benzyl iodide to Rh ( I ) complex as in the case of the carbonylation of CH₃OH proposed by Forster. The fact that the carbonylation of a binary mixture of benzyl alcohol and CH₃OH gave predominantly the carbonylation products of benzyl alcohol indicates that benzyl alcohol was more easily carbonylated than CH₃OH. Although the rate of the carbonylation of benzyl alcohol was considerably faster than that in the case of CH₃OH, the active catalyst species, [Rh (C0)₂I₂]-, may be the same in both case s, and the catalytic cycle conducted by the species is proposed.