Question

Which one of the following statements is not correct for sigma- and pi-bonds formed between two carbon atoms?

• A. Sigma bond is stronger than a pi bond
• B. Bond energies of sigma- and pi-bonds are of the order of $265\text{kJ/mol}$ and $346\text{kJ/mol}$, respectively
• C. Free rotation of atoms about a sigma-bond is allowed but not in case of a pi-bond
• D. Sigma-bond determines the direction between carbon atoms but a pi-bond has no effect in this regard

Answer 1

The correct option is B Bond energies of sigma- and pi-bonds are of the order of $265\text{kJ/mol}$ and $346\text{kJ/mol}$, respectively

a) A sigma bond is stronger than a pi-bond. Because the extent of overlap is greater in sigma bond formation than in pi bond formation as the former involve axial overlap and latter involve lateral overlap.
b) Bond energy of sigma bond is greater than pi bond as sigma bond is stronger than pi bond. Hence, this option is incorrect.
c) Free rotation of atoms about a sigma bond is allowed but not in case of a pi-bond. As bond rotation is hindered in pi bond.
d) A sigma bond determines the direction between carbon atoms, but a pi-bond has no primary effect in this regard. A pi bond is present only when a sigma bond is already present.