Question

The double bonds are stronger and shorter than single bonds. For example, a C−C single bond has an average bond energy of 347 kJ/mole while a C=C double bond has an average bond energy of 611 kJ/mole. Use valence bond theory to explain why a double bond is not simply twice as strong as a single bond.

Answer 1

Bond energy of C=C is not simply double the bond energy of C-C

Explain:

C=C, here C atom is sp² hybridized. So, it has one sigma type bond and one pi-type bond. Sigma bond is formed by face-to-face overlapping between two C-sp² orbitals. On the other hand, pi-bonds are formed by side-wise overlapping of two C-sp² orbitals. Now, bonds that are formed by face-to-face overlaping of orbitals are more stronger, hence requires more energy to break. Pi-bonds are weeker than sigma bonds and having less bond energy than sigma bond.

Thats why C=C (1 sigma and 1 pi bond) bond energy is not exactly double the bond energy of C-C bond.