Chemistry - Hydrocarbon Question with Solution | TestHub
Consider the following compound. Some carbon atoms are labelled as 1, 2, 3, 4.
Which C-H bond has lowest dissociation energy?
Options:
Answer:
Solution:
Nature of Carbon-2: In the given structure (3-methylcyclohexene), carbon-2 (C₂) is a tertiary allylic carbon because it is directly adjacent to the C=C double bond and attached to three other carbon atoms.
Radical Stability: Homolytic cleavage of the C₂–H bond forms a tertiary allylic free radical, which is highly stable due to resonance delocalization across the -bond and hyperconjugation from the methyl group.
Bond Dissociation Energy (BDE): The bond dissociation energy is inversely proportional to the stability of the resulting free radical; a more stable radical requires less energy to form.
Conclusion: Because the radical formed at C₂ is the most stable among all positions, the C₂–H bond is the weakest and has the lowest dissociation energy (Option B).
