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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3hc017

Which of the following statements about free radical halogenation of alkanes are correct?

  1. ABromination is more selective than chlorination for tertiary over primary hydrogen
  2. BIodination of methane with is exothermic and proceeds readily because the + step is favourable
  3. CChlorination of optically pure (S)-3-methylhexane at gives racemic 3-chloro-3-methylhexane
  4. DA termination step such as explains the trace of ethane in methane chlorination

Answer: A, C, D

The radical at the stereocentre is planar, so the halogen attacks both faces equally to give a racemate. Br abstraction is endothermic with a late transition state, hence more selective. Radical recombination is the termination step giving ethane. Iodine atoms abstracting H from methane is strongly endothermic, so direct iodination does not proceed.
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