Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: fc3stud on May 10, 2006, 05:31:06 PM
-
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.
-
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.
-
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.