Reaction Mechanism of Phase-Transfer Catalysis Initiated by Hydroxide Ion: Effect of Alkalinity

摘要

The reaction of 2,4,6-tribromophenol (ArOH) and #alpha#-bromo-o-xylene (RBr) catalyzed by tetrabutylammonium bromide (QBr) was carried out in an alkaline solution of KOH/organic solvent two-phase medium. A peculiar phenomenon, in which the rate of a PTC/OH reaction does not change monotonously with the concentration of alkaline compound, is observed. This result comes from the fact that the concentration of the active intermediate ArOQ is affected by the concentration of alkaline compound added due to dissociation of ArOH and salting out effect. Not only the polarity of organic solvent, but also the Lewis base of the organic solvent affected by the concentration of alkaline compound influences the reaction rate. The dielectric constants and the Lewis base of organic solvents in affecting the solvation are used to explain how the reaction rate is influenced by the solvents. The concentration of tetrabutylammonium 2,4,6-tribromophenoxide (ArOQ) remains constant throughout the path of reaction. The resistance of mass transfer of ArOQ between the two phases is negligible compared to the slower rate of organic-phase reaction.