Home » Uncategorized » The curious case of the carbon – carbon bond strengths

The curious case of the carbon – carbon bond strengths

Readers of previous posts will (hopefully) remember that the covalent bond is defined as the electrostatic force of attraction between the localized shared pair of electrons and the 2 positively charged nucleus.

Since electrostatic forces of attraction depend on the difference in charges, a double bond is going to be stronger than a single bond and a triple bond is going to be stronger than a double bond. For instance, MgO has a higher m.p. as compared to NaF because there is a greater difference in the charges between the Mg2+ and O2– ions as compared the Na+ and F ions, hence the electrostatic forces of attraction between the Mg2+ and O2– ions are stronger and require more energy to break, resulting in higher m.p.

The same holds true for covalent bonds. Thus, the C ≡ C bond is stronger than the C = C bond. In turn, the C = C bond is stronger than the C – C bond. See diagram below.

post5d1

However, closer inspection of the bond energies indicates that the increase in bond strength is non-linear in nature. In fact, it is increasing at a decreasing rate!

post5d2

Tell me why!

As the number of electrons in the same space increases, there would be more repulsion between the electrons (since like charges repel), resulting in a lower than proportionate increase in the bond strength as the number of bonds increases.

Note: JC students should know that the “proper explanation” should include a discussion about the sigma, pi bonds and their different degrees of overlap of the electron cloud. However, the basic idea remains the same. There is less overlap for pi bonds because electrons are arranged in such a way so as to minimize repulsion. Even then, the 2 pi bonds are not of equal strength, suggesting that there remains some degree of electron repulsion.

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