(235g) Solvent Topography and Steric Factors: Effects on Crystal Morphology

Effects of hindrance resulting from solvent topography on the resultant crystal morphology were examined via cooling crystallization of various carboxylic acids in isomeric butyl- and pentyl- alcohols. These isomeric solvents were selected in such a way that the major distinguishing factor was the position of the hydroxyl functional group along the carbon chain. Our experiments show that the magnitude of hindrance is related to the degree of branching at the substituted carbon, with hindrance increasing in the order of 1º < 2º < 3º alcohols. The resulting crystals displayed a trend of low, intermediate, and high aspect ratios corresponding to 1º, 2º, and 3º alcohols, respectively. In particular, 3º alcohols tend to yield significantly different crystal morphologies compared to 1º and 2º alcohols. Hence, the assertion that polar solvents, such as alcohols, always yield low aspect ratio plate-like crystals is incorrect and that the position of hydroxyl functional group plays a major role in enhancing or limiting solute-solvent hydrogen bonding interactions. Infrared spectroscopic analysis revealed that the intermolecular hydrogen-bonding ability of 1º alcohols > 2º alcohols > 3º alcohols, and this helps explain morphological trends observed at the macroscopic level.