Polymorphism in Long-Chain N-Alkylammonium Halides

Abstract

The structures and properties of long-chain organic molecules provide a fascinating glimpse into polymorphism and the packing forces that operate between molecules in the solid state. These molecules are roughly cylindrical in shape and normally stack in layers consisting of densely packed molecules, all in parallel, much like pencils in a box.Two factors influence their molecular packing in a crystal lattice – the forces between the end groups in a layer and the van der Waals interactions between the alkyl chains in a layer. Both forces contribute to the lattice energy, and their relative strengths determine the crystal structure. Most long-chain compounds exhibit polymorphism because of their conformational flexibility and the weak intermolecular forces directing the packing.

A study of the structural properties of primary n-alkylammonium halides enabled us to evaluate the relative importance of intermolecular forces of the two types as the polar ends of the layers form strong hydrogen-bonded networks that dominate the packing on crystallization. Here we illustrate trends in the crystal chemistry of the n-alkylammonium halides, and the investigative methods, using as examples n-octadecylammonium chloride,C₁₈H₃₇NH⁺ ₃ Cl^–, and their related bromides and iodides