Question Details

Among the given compounds I-III, the correct order of bond dissociation energy of C–H bond marked with * is:


Options

A

II > I > III

B

I > II > III

C

III > II > I

D

II > III > I

Show Answer

Correct Answer :

Option A

II > I > III

II > I > III

Solution :

The correct option is II > I > III.

To determine the correct order of the bond dissociation energy (BDE) of the C–H bonds marked with an asterisk (*), we must analyze the hybridization of the carbon atom involved in each marked bond:
1. Compound I: The marked hydrogen is attached to a carbon atom in the benzene ring. This carbon atom is sp2 hybridized.
2. Compound II: The marked hydrogen is attached to the terminal carbon of the ethynyl (alkyne) group. This carbon atom is sp hybridized.
3. Compound III: The marked hydrogen is attached to the saturated methylene carbon of the cyclopropene ring. This carbon atom is sp3 hybridized.

The bond dissociation energy of a C–H bond is directly related to the s-character of the hybridized orbital of the carbon atom. A higher percentage of s-character means that the bonding electrons are held closer to the carbon nucleus, resulting in a shorter, stronger bond that requires more energy to cleave homolytically.

The s-character for different hybridization states is as follows:

sp hybridization=50% s-character
sp2 hybridization=33.3% s-character
sp3 hybridization=25% s-character

Since the bond strength and dissociation energy increase with increasing s-character, the general order of bond dissociation energy for C–H bonds is:
C(sp)-H>C(sp2)-H>C(sp3)-H

By comparing the marked C–H bonds in the given compounds:
- Compound II involves a C(sp)-H bond (highest s-character, strongest bond).
- Compound I involves a C(sp2)-H bond.
- Compound III involves a C(sp3)-H bond (lowest s-character, weakest bond).

Thus, the correct order of bond dissociation energy is II > I > III.

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