On p. 146 Loudon asks “why an sp2-sp3 C-C bond is stronger than an sp3-sp3 C-C bond”. It’s a good question because it might help explain why more substituted alkenes are more stable than their less substituted isomers, a topic that was covered in today’s lecture. Unfortunately, the explanation provided by Loudon strikes me as incorrect.
Loudon makes the (correct) point that “a bond with more s character involves electrons of lower energy than one with less s character” (p. 146). However, he then equates lower electron energy with increased bond strength. This is incorrect.
(An aside: Loudon is not careful enough here in his use of technical terms. When he talks about “the energy of the electrons in the bond”, it would be more accurate to say “the energy of the electrons in the bonding MO”.)
Where does Loudon go wrong? He forgets that MO theory equates bond strength with the BMO orbital stabilization energy, the energy difference between the BMO and the hybrid orbitals. The s character of a BMO is determined by the s character of its component hybrid orbitals. If we find that a BMO is stabilized by enhanced s character, we must necessarily find that the hybrid orbitals are stabilized by enhanced s character too. Since the BMO and hybrids are both stabilized, the orbital stabilization energy and bond strength are unaffected.
A better explanation is to look at hybrid orbital overlap. Although it is far from obvious why this should be so, computer calculations show that the overlap between sp2 and sp3 orbitals is better than the overlap between sp3 and sp3 orbitals. As we know, larger overlap means a larger orbital stabilization energy and a stronger bond.