Inversion of the melting point alternation in n-alkyl carboxylic acids by co-crystallization with pyrazine

The structures of 2 ∶ 1 co-crystals of the n-alkylcarboxylic acids with pyrazine have been determined for methanoic acid to tridecanoic acid. For methanoic acid through to hexanoic acid, the co-crystal structures exhibit considerable variation. For heptanoic acid and longer acids, the structures display regularity, and the co-crystals exhibit an alternation in their melting points that is opposite to that in the n-alkylcarboxylic acids themselves: the melting points of the odd co-crystals are systematically higher than those of the evens. The melting points are directly correlated with the crystal density, which is systematically lower in the even co-crystals on account of reduced packing efficiency at the methyl-group interface between layers in the structures. The arrangement of the methyl groups is governed by a consistent arrangement of pyrazine moieties within the layers.