抄録
Kinetic studies were carried out for C-methylation of hydroquinone and N-methylation of aniline in supercritical methanol without catalyst at a temperature of 350°C and a methanol density of 0.237 g/cm3. Effects of acid, substituent, and reactant concentration on the methylation rates were explored in order to elucidate the reaction mechanism. We found that the reactions are considerably retarded by addition of Br?nsted acids (HNO3, HCl, H3PO4, and H2SO4). The acid effect indicates that the methylations in supercritical methanol cannot be explained by the conventional acid-catalyzed mechanism, the so-called Friedel-Crafts process. Comparison of the reactivities of phenol and N,N-dimethylaniline suggests that the phenolic OH group plays an essential role in the C-methylation. The importance of phenolic OH is supported by the high product selectivity of ortho-methylated phenols, which was 10 times larger than that of para-methylated one. A new pseudo-intramolecular reaction mechanism is thus proposed for the C-methylation of phenol and hydroquinone: the methylation is catalyzed by a proton formed by the dissociation of phenolic OH of its own molecule. The mechanism is compared with that of the catalystless alkylation of phenol by 2-propanol in supercritical water proposed by Sato et al. We also report some curious phenomena found in the N-methylation of aniline: a high selectivity of N-methylaniline relative to the N,N-dimethylaniline, no C-methylation into toluidines, and an acceleration of the reaction by base (KOH and LiOH), suggesting the difference in the mechanism from the conventional one.
