Odd-even effect in melting properties of 12 alkane-α,ω-diamides

Fusion and solid-to-solid transitions of a homologous series of 12 linear alkane-α,ω-diamides H2NCO-(CH2)(n-2)-CONH2, where n = (2 to 12 and 14), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion of even terms decreased from T fus ∼ 572 K to about 460 K, whereas those of odd terms remained substantially constant at about 450 K. Solid-to-solid transitions were also detected for oxamide, malonamide, succinamide, adipamide, suberamide, and dodecanediamide. Regular odd-even alternation was displayed by the temperature, enthalpy, and entropy of fusion values, terms with even number of carbon atoms showing higher values than odd terms. This behaviour was attributed to different crystal packing allowing consonance between hydrogen bonding and dispersive interaction in even terms, which are characterised by multilayer structure, whereas in odd terms a strained three-dimensional network results in looser packing. Parallel alternation of densities in solid alkane-α,ω-diamides supports this interpretation. Comparison was made with literature values for temperatures, enthalpies, and entropies of fusion of isoelectronic linear alkanes, dicarboxylic acids, and alkyldiamines