On the crystal structures and melting point alternation of the n-alkyl carboxylic acids

The crystal structures of the complete series of n-alkyl carboxylic acids from hexanoic to pentadecanoic acid have been determined following in situ crystallisation. The structures reveal that the melting point alternation across the series is correlated with alternating crystal density. All even acids crystallise from the liquid as the C modi. cation, while the odd acids exhibit a transition from the C' to the C modification between undecanoic and tridecanoic acid. In each structure, molecules form hydrogen-bonded dimers arranged into bilayers, with a rectangular packing arrangement in the plane perpendicular to the dimer long axes. The packing density within bilayers is comparable in each case and the alternating crystal density can be attributed solely to alternating packing density between bilayers. The carboxyl groups are identically disposed in all structures, but adjacent dimers are offset to differing degrees parallel to their long axes to reduce in-plane O...O repulsions, giving rise to angles between n-alkyl chains. The extent of the lateral offset diminishes in general across the series as the n-alkyl chains exert an increasing driving force towards parallel alignment. The systematic alternation in the packing density between bilayers arises as a result of different orientations adopted by the terminal C-C bonds with respect to the methyl group interface. Several of the key structural features may be rationalised using a simple two-dimensional model that describes the packing of modified parallelograms and trapezoids, representing the even and odd acids, respectively. Extension of the model into three dimensions permits complete rationalisation of the structures.