AIIMS 2003 Chemistry Bond Dissociation Energy MCQ Question

To determine which compound possesses the C–H bond with the lowest bond dissociation energy (BDE), we need to understand the nature of the C–H bonds in each of the given compounds: toluene, benzene, n-pentane, and 2,2-dimethylpropane.

Explanation of Correct Answer: A) Toluene

1. Structure of Toluene: Toluene (C₆H₅CH₃) consists of a benzene ring (C₆H₆) with a methyl group (CH₃) attached. The C–H bonds in the methyl group are not as strong as the C–H bonds in the aromatic benzene ring due to the stabilization provided by the resonance in benzene.

2. Bond Dissociation Energy (BDE): The BDE is the energy required to break a bond in a molecule, forming free radicals. In toluene, the C–H bond in the methyl group has a lower BDE compared to the C–H bonds in the benzene ring, which are strengthened by resonance.

Comparison with Other Options:

• B) Benzene: The C–H bonds in benzene are particularly strong due to resonance stabilization. The BDE for C–H bonds in benzene is higher (approximately 112 kcal/mol) compared to the C–H bond in the methyl group of toluene. Therefore, benzene does not have the lowest bond dissociation energy.

• C) n-Pentane: n-Pentane (C₅H₁₂) has C–H bonds that are relatively stronger than those in toluene's methyl group. The BDE for n-pentane's C–H bonds is around 98 kcal/mol. While these bonds are weaker than those in benzene, they are stronger than the C–H bond in toluene's methyl group.

• D) 2,2-Dimethylpropane: This compound has a branched structure and features C–H bonds that are also stronger than the C–H bond in toluene. The BDE for the C–H bond in 2,2-dimethylpropane is approximately 97 kcal/mol. Thus, like n-pentane, it does not have the lowest BDE.

Conclusion

The C–H bond in toluene's methyl group is the weakest among the listed compounds due to the absence of resonance stabilization and the relatively higher electron density around the methyl group. The C–H bond dissociation energy in toluene is approximately 105 kcal/mol, which is lower than the BDEs in benzene, n-pentane, and 2,2-dimethylpropane.

Thus, the correct answer is:

A) Toluene - it possesses the C–H bond with the lowest bond dissociation energy among the given options.