Odd–even chain packing, molecular and thermal models for some long chain sodium(I) n-alkanoates

Abstract

A homologous series of sodium(I) n-alkanoates, NaCnH2n−1O2, with chain lengths n=8–18, inclusive, have been synthesized and their structural and thermal properties investigated via Fourier Transform Infrared and Solid State 13C NMR spectroscopies, X-ray powder diffraction, Thermogravimetry, Differential Scanning Calorimetry, Polarizing light microscopy and variable temperature Infrared spectroscopy. The measurements show that metal-carboxylate coordination is via asymmetric chelating bidentate bonding with extensive carboxyl group inter-molecular interactions in which four oxygen atoms are bonded tetrahedrally to a sodium atom. Furthermore, the compounds crystallize in a monoclinic crystal system with the hydrocarbon chains in the fully extended all-trans conformation, advancing along the c-axis. Moreover, the chains are packed as tilted (θ∼63°), non-overlapping, tail-to-tail lamellar bilayers that are not in the same plane, within a lamellar. Though these compounds are nearly isostructural, there are subtle differences in the packing of the hydrocarbon chains in the crystal lattice, resulting in odd–even alternation in the terminal methyl group asymmetric stretching vibration and chemical shift. These differences arise from the relative vertical distances between hydrocarbon planes within the lamellar; such that, for odd-chain compounds, larger inter-planar distances result in less efficient packing in the crystal lattice and hence, lower inter-planar van der Waals interactions between hydrocarbon chains.Thermal traces, for all compounds, show several partially reversible solid–solid pre-melting transitions associated with different degrees of gauche conformers in the alkyl chains. The reversible gauche-trans isomerism, of the methylene groups, is kinetically controlled; hence, super-cooling of the melt and other transitions, are observed for all compounds. The kinetics of chain reversion follow the exponential law of nucleation, though complicated by competing processes. Thermogravimetric data show that all compounds decompose at temperatures in excess of 690K; therefore, free radical thermal cracking of the hydrocarbon chains, in conjunction with decarboxylation is proposed for their non-oxidative degradation mechanism.