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NEET CHEMISTRYMedium

The incorrect statement among the following for sigma- (σ\sigma) and pi- (π\pi) bonds formed between two carbon atoms is:

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 the 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

Step-by-Step Solution

To identify the incorrect statement, we analyze the properties of sigma and \pi bonds based on chemical principles and the provided sources:

  1. Strength: A sigma bond is stronger than a \pi bond because it is formed by the head-on overlap of atomic orbitals, which results in a greater extent of overlapping .
  2. Bond Energies: According to the sources, the bond enthalpy of a carbon-carbon single bond (which is a sigma bond) is approximately 347 kJ/mol . In a double bond, the \pi bond is weaker, with a bond enthalpy of approximately 284 kJ/mol . Therefore, the statement claiming the sigma bond is 264 kJ/mol and the \pi bond is 347 kJ/mol is incorrect because it misstates the values and implies the \pi bond is stronger.
  3. Rotation: Sigma bonds allow for free rotation of atoms around the bond axis because the electron distribution is symmetrical around the internuclear axis . However, rotation about a \pi bond is restricted because it would disrupt the parallel orientation of p-orbitals required for sideways overlap .
  4. Directionality: Sigma bonds are formed by axial overlap along the internuclear axis and determine the geometry and primary direction of the bond between atoms . Pi bonds are formed by lateral overlap and do not primarily determine the direction of the bond .
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