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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1189

3-Methylbut-1-ene, , loses its tertiary allylic hydrogen to a bromine radical. Which statement about the radical formed is correct?

  1. AThe radical has no resonance because C-3 is an sp3 carbon
  2. BThe radical is stabilised only by hyperconjugation; the double bond plays no role
  3. CThe unpaired electron is delocalised over C-3 and C-1, and the C-3 centre is further stabilised by hyperconjugation with six alpha hydrogens
  4. DThe unpaired electron is delocalised equally over C-1, C-2 and C-3

Answer: C

The radical has a p orbital at C-3 next to the C=C, giving as the other resonance form. The unpaired electron is therefore shared between C-3 and C-1 (not C-2), and the two methyl groups on C-3 add six alpha hydrogens for hyperconjugation.
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