Thermotropic mesomorphic changes in racemic and meso lead(II) 9,10-dihydroxyoctadecanoates

Data are presented for the heats and entropies of phase changes for the racemic and meso lead(II) 9,10-dihydroxyoctadecanoates. The total enthalpy change accompanying the transformation crystal → liquid phase is lower for the lead(II) 9,10-dihydroxyoctadecanoates than for the lead(II) octadecanoate. This suggests the solid phase of the former to be more disordered than that of the latter.Optical examination under a polarising microscope suggests the phase sequence: solid I → mesophase → liquid for the racemic isomer, and solid I → solid II → mesophase → liquid for the meso isomer. The mesophase and solid II phase are absent on reheating the racemic and meso soaps respectively. The behaviour is interpreted in terms of subtle structural rearrangement of the liquid phase arising from extensive hydrogen bonding. The texture of the mesophases of the isomers is observed to be different and the mesophase of the meso soap is stable over a wider temperature range than that of its racemic counterpart. The behaviour is indicative of the effect of stereochemical configuration on the thermal properties of the soap.