Which one of the following statements is not correct for sigma- and pi- bonds formed between two carbon atoms?
- sigma-bond is stronger than a pi-bond.
- Bond energies of sigma- and pi-bonds are of the order of 264 kJ/mol and 347 kJ/mol, respectively
- Free rotation of atoms about a sigma-bond is allowed but not in case of a pi-bond.
- Sigma-bond determines the direction between carbon atoms but a pi-bond has no primary effect in this regard.
Answer
(2) Bond energies of sigma- and pi-bonds are of the order of 264 kJ/mol and 347 kJ/mol, respectively
Solution
As per NCERT, a sigma bond is formed by head-on (axial) overlap, which is larger than the sideways overlap of a pi bond, so a sigma bond is stronger than a pi bond.
The C–C sigma bond energy is ≈ 347 kJ mol-1 and the C=C bond energy is ≈ 611 kJ mol-1, so the pi bond energy is
≈ 611 - 347 = 264 kJ mol-1
Thus the values in statement (2) are interchanged (sigma ≈ 347 kJ/mol and pi ≈ 264 kJ/mol), so statement (2) is not correct.
Statements (1), (3) and (4) are correct: rotation about a pi bond would break the sideways overlap, and the direction of the C–C bond (bond axis) is fixed by the sigma bond.
Answer: (2)