Melting-points of the meta- and para-isomers of anisylpinacolone

The generally higher melting-point of a para-disubstituted benzene relative to the corresponding meta-isomer has been ascribed to the fact that, being more symmetrical, it can pack more tightly. Exceptionally, it was observed that whereas m-anisylpinacolone melts at 58.0 degreesC, the para-isomer melts lower at 39.5 degreesC. In this work we have attempted to understand this apparent anomaly. The crystal structures of both isomers were determined and the packing analysed. Energy calculations of the static structures and molecular dynamics (MD) simulations at temperatures just below the respective melting-points were performed. The structure analyses indicate that the intermolecular contacts are comparably weak in the two cases, and do not appear to be the direct cause of the melting-point difference. Thermal motion analysis, packing energies and MD simulations on minicrystals indicate the importance of both enthalpic and entropic factors in the melting behaviour of the two isomers. The higher melting point of the meta-isomer could originate from both a smaller DeltaS(f) and higher DeltaH(f) relative to the para-isomer. Copyright (C) 2000 John Wiley & Sons, Ltd.