Question

Question #bb29f

Answer 1

No, we can't conclude that the cis form is always more stable than the trans form. It's just the opposite!

Explanation:

Your premise is incorrect. We DON'T know that a greater boiling point means greater stability.

The two are not related.

Boiling points depend on the strengths of intermolecular forces.

Molecular stability depends on the strengths of intramolecular forces.

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.

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

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

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.

Molecular Stability

Cis alkenes are less stable than trans alkenes because of steric interference — the repulsive van der Waals forces between the electron clouds of the groups.

The heats of hydrogenation of cis- and trans-but-2-ene are about 119 and 115 kJ/mol.

Thus, the cis isomer is less stable than the trans isomer.

Conclusion

Cis alkenes are less stable than trans alkenes, but they have higher boiling points.