The effect of positional and geometrical isomerism on the dilatometric properties of some octadecenoic acids

Summary Four isomeric octadecenoic acids, oleic, elaidic, petroselinic, and petroselaidic, were prepared and their expansibility determined. The melting dilation of each acid was calculated and found to increase in the order oleic, petroselinic, petroselaidic, and elaidic. In each instance the melting dilation of the trans acid was greater than that of its cis isomer. Of the two cis acids studied, the melting dilation was less for the acid with the double bond farther from the carboxyl end of the carbon chain. The trans acids did not follow this pattern. At temperatures above 52°C., at which all acids were liquid, the absolute specific volumes of the acids were very nearly equal whereas at temperatures below −7°C, at which all were in the solid state, the specific volumes of petroselinic and elaidic acids were at variance with the specific volumes of oleic and petroselaidic acids. This variance must be attributed to differences in crystal packing. Oleic acid clearly showed the two polymorphic forms previously recognized and, in addition, apparently reversibly transformed at about −5°C., indicating the possible existence of a third polymorphic form. These transformations of oleic acid occurred regardless of tempering, and without visual melting. The other three acids did not exhibit polymorphism under the conditions employed in the dilatometric measurements.