The bromohydrin shown (single stereoisomer) is treated with concentrated HBr. The reaction proceeds through the neighbouring bromine (bromonium ion) with overall retention at the carbon bearing OH, and the bromide then opens the bromonium at either carbon. What is the dibromide product?

ARacemic 2,3-dibromobutane (a pair of enantiomers)
BMeso-2,3-dibromobutane only
CA single chiral enantiomer of 2,3-dibromobutane
DAn equimolar mixture of meso and racemic dibromides

Answer: B

The compound is (2R,3S)-3-bromobutan-2-ol (erythro). Neighbouring bromine displaces water from the back side, and bromide reopens the bridged ion from the back side: net retention. Replacing OH by Br in the same SMILES gives C[C@@H](Br)[C@H](C)Br, which RDKit identifies as meso (mirror-symmetric). Because the bromonium from the erythro isomer is meso-like, attack at either carbon gives the same meso 2,3-dibromobutane.

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN1 rearrangement and neighbouring group participation in drawn substrates

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The bromohydrin shown (single stereoisomer) is treated with concentrated HBr. The reaction proceeds through the neighbouring bromine (bromonium ion) with overall retention at the carbon bearing OH, and the bromide then opens the bromonium at either carbon. What is the dibromide product?

Answer: B

More questions on SN1 rearrangement and neighbouring group participation in drawn substrates