The kinetics of the solid state transformation of racemic to optically active 1,1′-binaphthyl

Rates of the solid state reaction which thermally converts racemic to optically active 1,1′-binaphthyl (mp 158 °C) have been determined at temperatures between 105 and 135 °C using polycrystalline samples. The presence of a sufficiently uniform distribution of enantiomorphic seed crystals results in a smooth conversion of the racemate to the eutectic crystal forms of binaphthyl. Although it is possible to force the resolution reaction almost to completion at higher temperatures (150 °C) where the melt phase is an intermediate, lower final specific rotations are obtained when only solid phases are involved (below 145 °C) because of the independent nucleation of racemic material from the reacting crystals. Both spontaneous nucleation and reaction rates in the solid state were increased by grinding the initially prepared samples and were decreased by the storage of samples. The Arrhenius activation energy of this phase transformation giving optically active product is ca. 60 kcal/mol in a variety of different samples. This is consistent with a mechanism in which a molecule of 1,1′-binaphthyl attains considerable freedom from the racemic solid in order to interconvert to its enantiomer and add to the growing optically active phase.