Final answer:
A C≡C triple bond is not simply three times as strong as a C-C single bond because it consists of one σ bond and two π bonds, with the π bonds being inherently weaker due to less orbital overlap. The overall bond strength increases with each additional pi bond, but not in a directly proportional manner.
Step-by-step explanation:
The reason a C≡C triple bond is not simply three times as strong as a C-C single bond can be explained using valence bond theory. In a single bond, two atoms are joined by one pair of electrons using a σ (sigma) bond, which is characterized by a head-on overlap of atomic orbitals and allows for the most overlap between bonding electrons, leading to a strong bond.
In a triple bond, there is one σ bond and two π (pi) bonds. The π bonds result from the side-to-side overlap of p orbitals and are inherently weaker compared to a sigma bond because of less orbital overlap. Therefore, while a triple bond is stronger overall due to the accumulation of one sigma and two pi bonds, the individual pi bonds contribute less to the strength than a sigma bond, resulting in a bond energy that is not exactly three times that of a single bond.
For instance, a C-C single bond averages 347 kJ/mol, and adding a pi bond increases the strength by 267 kJ/mol. A second pi bond adds a further 225 kJ/mol. Clearly, each addition is not simply doubling or tripling the strength of the bond, reflecting the different contributions of sigma and pi bonds.