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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3o3165

Select the correct statement(s) about neighbouring halogen participation (halonium ions).

  1. AFluorine is the best bridging halogen because it is the most electronegative.
  2. B(2S,3R)-3-bromobutan-2-ol with HBr gives a racemic mixture of chiral 2,3-dibromobutanes through the bromonium ion.
  3. CThe geometry requires the participating halogen and the leaving group to be anti to each other.
  4. DA halogen on the adjacent carbon can displace a leaving group by forming a bridged halonium ion, giving net retention after the external nucleophile opens the ring.

Answer: C, D

Anchimeric assistance by halogen proceeds through a three-membered halonium ion formed by backside attack, which requires the halogen to be anti to the leaving group, and a second backside attack by the external halide gives net retention. Bridging ability increases from F to I because the larger, more polarisable halogens donate lone pairs more readily; fluorine is virtually unable to bridge. The (2S,3R) bromohydrin gives the meso dibromide, not a racemate.
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