Which 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 264 kJ/mol and 347 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 primary effect in this regard
The answer is B.
Solution
A $\sigma$ bond is formed by head-on (axial) overlap of orbitals, which is more extensive than the sideways overlap that forms a $\pi$ bond. So a $\sigma$ bond is stronger than a $\pi$ bond; true. The C–C $\sigma$ bond energy is about 347 kJ/mol. The C=C double bond is about 611 kJ/mol, so the extra $\pi$ bond contributes only about 264 kJ/mol. So the $\sigma$ bond energy is about 347 kJ/mol and the $\pi$ bond energy about 264 kJ/mol. The statement giving them the other way round (264 for $\sigma$ and 347 for $\pi$) is not correct. Free rotation is possible about a $\sigma$ bond (cylindrically symmetric) but not about a $\pi$ bond, because rotation would break the sideways overlap; true. The $\sigma$ bond fixes the direction (bond axis) between the carbon atoms, and the $\pi$ bond simply lies above and below this axis; true. So the incorrect statement is the one about bond energies.
ConceptRelative strength of sigma and pi bonds
Sigma and pi bonds in past papers
3 questions from this chapter have appeared across 3 exam years.
