[fc3stud]

Can you explain why the cis isomer of but-2-ene has a higher mp and bp than the trans isomer?  Surely the trans can pack better?  Or does pi bond repulsion play a role?  Many thanks, Andrew Walker.

[wereworm73]

It's the dipole-dipole attractions that make the difference here.  The dipole moments cancel out in the trans isomer but not in the cis.

[Donaldson Tan]

There is pi bond in both isomers, so if there is a pi bond repulsion, it must affect both isomers in the same fashion.

It is clear that the fact that the cis- exhibit higher mp/bp means that the cis- exhibits stronger intermolecular bonding.

The only way this can happen is due to that the cis-isomer must has an overall polarity, thus constituting for strong intermolecular bonding.  Although the trans-isomer can pack better, both isomers essentially share the same number of intermolecular bonding sites. This means although the trans- has a greater density than the cis-isomer, it has a lower mp/bp than the cis-isomer.