Question

Consider the following compound (X):
 
The most stable and least stable carbon radicals, respectively, produced by homolytic cleavage of corresponding C - H bond are:

• A. I, IV
• B. III, II
• C. II, IV
• D. II, and I

Hint:

Radical stability increases with the number of alkyl groups attached to the carbon carrying the radical. The more alkyl groups, the more stable the radical.

Answer 1

The Correct Option is D

Step 1: C-H Bond I Radical Analysis

Cleavage at C-H bond I yields a radical on an sp-hybridized carbon.

\[ \text{H}-\overset{\text{I}}{\text{C}}\equiv\text{C}-\text{CH}_2-\text{CH(CH}_3)_2 \xrightarrow{\text{cleavage}} \cdot\text{C}\equiv\text{C}-\text{CH}_2-\text{CH(CH}_3)_2 \]

This is an alkynyl radical. Its unpaired electron is in an sp orbital (50% s-character), resulting in extreme instability.

Step 2: C-H Bond II Radical Analysis

Cleavage at C-H bond II produces a radical on the carbon adjacent to the triple bond.

\[ \text{HC}\equiv\text{C}-\overset{\text{II}}{\text{CH}_2}-\text{CH(CH}_3)_2 \xrightarrow{\text{cleavage}} \text{HC}\equiv\text{C}-\dot{\text{C}}\text{H}-\text{CH(CH}_3)_2 \]

This secondary (2°) radical is a propargylic radical. Resonance delocalization of the unpaired electron across the C≡C triple bond provides significant stabilization.

\[ \text{HC}\equiv\text{C}-\dot{\text{C}}\text{H}-\text{R} \longleftrightarrow \text{H}\dot{\text{C}}=\text{C}=\text{CH}-\text{R} \]

Step 3: C-H Bond III Radical Analysis

Cleavage at C-H bond III results in a radical on a tertiary carbon.

\[ \text{HC}\equiv\text{C}-\text{CH}_2-\overset{\text{III}}{\text{CH}}(\text{CH}_3)_2 \xrightarrow{\text{cleavage}} \text{HC}\equiv\text{C}-\text{CH}_2-\dot{\text{C}}(\text{CH}_3)_2 \]

This tertiary (3°) radical is stabilized by hyperconjugation with adjacent methyl and methylene groups. Resonance stabilization is absent as it is not directly adjacent to the triple bond.

Step 4: C-H Bond IV Radical Analysis

Cleavage at C-H bond IV (on a methyl group) generates a radical on a primary carbon.

\[ \text{HC}\equiv\text{C}-\text{CH}_2-\text{CH(CH}_3)\overset{\text{IV}}{\text{CH}_3} \xrightarrow{\text{cleavage}} \text{HC}\equiv\text{C}-\text{CH}_2-\text{CH(CH}_3)\dot{\text{C}}\text{H}_2 \]

This primary (1°) radical is stabilized solely by hyperconjugation.

Conclusion:

Most Stable Radical: The propargylic radical (from II) exhibits resonance stabilization, a significantly stronger effect than the hyperconjugation stabilizing the tertiary radical (from III). Consequently, the radical at position II is the most stable.

Least Stable Radical: The alkynyl radical (from I), with its unpaired electron in an sp orbital, is substantially less stable than radicals on sp³-hybridized carbons (II, III, IV). Thus, the radical formed at position I is the least stable.

The most stable and least stable carbon radicals are generated from C-H bond cleavage at positions II and I, respectively.