Question

In the following pairs, which gas the higher boiling point: `cis`- or `trans`-but-2-ene; `cis`- or `trans`-1,2-dichloroethene?

Answer 1

Usually, the cis isomer has stronger intermolecular forces.

Explanation:

For example, the `"C-C"` single bonds in but-2-ene are polar, because `sp^2` carbons are more electronegative than `sp^3` carbons.

Each `"C-C"` bond dipole is about 0.7 D.

Molecules with a net dipole will have stronger intermolecular dipole-dipole attractions than molecules with no net dipole..

The dipole moment of trans-but-2-ene is zero, because the bond dipoles cancel.

However, the dipole moment of cis-but-2-ene is 0.33 D, the vector sum of the two bond dipoles.

Similarly, cis-1,2-dichloroethene has a high dipole moment, but that of trans-1,2-dichloroethene is zero.

In each case, the molecule with the higher dipole-dipole attractions has the higher boiling point.