Q.23: Meso 2,3-Dibromobutane Synthesis

Meso 2,3-dibromobutane forms through electrophilic addition of Br₂ to (E)-2-butene. This reaction proceeds via anti addition stereospecifically, yielding the meso stereoisomer due to the trans geometry of the alkene. Option (B) is correct, as confirmed by standard organic chemistry mechanisms.​

Option Analysis

Option (A): Electrophilic addition of HBr to (E)-1-bromobut-2-ene (BrCH₂-CH=CH-CH₃, trans) follows Markovnikov’s rule, adding H to C3 and Br to C2, producing 2,3-dibromobutane (BrCH₂-CHBr-CH₂-CH₃). This lacks the required symmetry at C2-C3 for the meso form and yields constitutional isomers instead.​

Option (B): Br₂ adds to (E)-2-butene (trans-CH₃CH=CHCH₃) via bromonium ion intermediate, enforcing anti addition. From the trans alkene, Br atoms add oppositely relative to the original methyl groups, forming the (2R,3S)-2,3-dibromobutane meso compound with a plane of symmetry.​

Option (C): Br₂ addition to (Z)-2-butene (cis-CH₃CH=CHCH₃) also anti, but cis geometry yields the racemic pair: (2R,3R) and (2S,3S)-2,3-dibromobutane enantiomers, not meso.​

Option (D): Nucleophilic addition of Br₂ does not occur with alkenes; Br₂ acts as electrophile. (Z)-1-bromobut-2-ene addition would anyway mismatch the symmetric meso product structure.​

Meso 2,3-dibromobutane synthesis hinges on stereospecific anti addition of Br₂ to alkenes, a key topic for CSIR NET organic chemistry. The meso 2,3-dibromobutane synthesis from (E)-2-butene demonstrates how trans alkene geometry produces the achiral meso diastereomer through bromonium ion mechanism.​

Reaction Mechanism

Br₂ approaches the alkene π bond, forming a three-membered bromonium ion. Bromide attacks from the opposite face (anti addition), ensuring stereospecificity. For (E)-2-butene, this yields the symmetric (2R,3S)-meso-2,3-dibromobutane with identical substituents and a mirror plane.​

• Bromonium formation: Top face bridges C2-C3.

• Br⁻ attack: Bottom face opens ring at either carbon (enantiotopic).

• Result: Single meso product, no racemization.​

Why Not Other Alkenes?

Cis-(Z)-2-butene gives rac-(2R,3R)/(2S,3S) due to mirrored threo addition. Crotyl bromides like 1-bromobut-2-ene produce 1,2-dibromobutane regioisomers, not symmetric 2,3-dibromobutane.​

This distinction clarifies CSIR NET questions on stereoselective halogenation.​