Carbon-Carbon Bond Energies: Primary vs. Tertiary Carbon

[blueblueblue]

Hi, first time poster.

Quick question:

Why is the bond energy for a bond that contains a more highly substituted (tertiary) carbon weaker than the bond energy of a bond that contains a less substituted (primary) carbon?

Let me clarify with an example. Say Bond A is a single bond between a methyl carbon and a carbon that has a double bond. Another bond (Bond B) also has a single bond but between a tertiary carbon and another carbon that has a double bond. So, both share an sp2-hybridized and an sp3-hybridized carbon. Now, I know that Bond B has a weaker bond energy because it contains the more highly substituted carbon, but I don't know why that is the case. I figured that the carbons pull some of the electron density away from Bond B, therefore, lengthening the bond and decreasing the bond energy. Is that correct?

Much thanks!

Edit: Hmmm. Now that I think about it, I don't know if this question should be posted on this board. Sorry about that.

 

[TeethWhitener]

Alkylation are actually electron donating groups. But the main reason usually boils down to simple steric hindrance. Bulkier groups cause more steric repulsion, weakening bonds. Furthermore, homolytically breaking the hypothetical bond in your examples creates either a methyl radical or an alkyl radical. Alkyl radicals are generally more stable, and for the reason I pointed out initially (electron donation into the half-filled p-orbital stabilizes the reactive species). So there are two thermodynamic effects working in tandem to destabilize tertiary carbon bonds.