Dissolving Borax Glass
Date: Fall 2011
Why do cis isomers have lower melting points than their trans isomers?
It is not an absolute rule. It's true much of the time because molecules with the trans arrangement about a double bond pack better. That makes the solid state more stable, requiring more energy to disrupt (melt).
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
This may not always be true, but it may be true for most cis/trans isomers. The reason would have to do with the ability of the structure to sustain the inter-particle attractions required for a solid to form. Understanding this requires an understanding of permanent and temporary dipole-dipole forces. See:
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Update: June 2012