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

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Answer 1 · 2017-09-17T17:40:04

Usually, the cis isomer has stronger intermolecular forces.

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

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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.