Why is the melting point of the cis isomers lower?
You might have thought that the same argument would lead to a higher melting point for cis isomers as well, but there is another important factor operating. In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid. Trans isomers pack better than cis isomers. The “U” shape of the cis isomer doesn’t pack as well as the straighter shape of the trans isomer. The poorer packing in the cis isomers means that the intermolecular forces aren’t as effective as they should be and so less energy is needed to melt the molecule – a lower melting point.
