Consider the following compound (X) [figure] The most stable and least stable carbon radicals, respectively, produced by homolytic cleavage of corresponding C-H bond are :
- III, II
- II, IV
- I, IV
- II, I
Answer
(4) II, I
Solution
Homolysis of each marked C–H bond gives the following radicals:
I: H-C C- is formed by breaking the sp C–H bond, so the odd electron sits in an sp orbital (50% s-character, held very close to the nucleus) and cannot be delocalised least stable.
II: HC C- CH- is a propargylic radical; the odd electron is delocalised over the triple bond by resonance HC C- CH-R H C=C=CH-R most stable.
III: a 3° alkyl radical, stabilised only by hyperconjugation and +I effect.
IV: a 1° alkyl radical — less stable than the 3° radical III, but far more stable than the alkynyl radical I.
Stability order: II > III > IV > I
Most stable = II, least stable = I.
Answer: (4)