Which alkenes will result in a meso-form when the alkenes below reacts with molecular bromide (Br_2Br2) cold/dark?

trans-2-hexene
cis-2-hexene
1-hexene
cis-3-hexene
trans-3-hexene

1 Answer
P dilip_k
Apr 23, 2016

trans-3-hexene

Explanation:

Addition of Br atoms across the c=c occurs in anti positions in all cases . But the product molecule formed in case of trans-3-hexene has got a plane of symmetry, which divides the molecule in two similar halves in such a way that one half is mirror image of other.

Logical prooceeding

  • Here the product formed would be a mesomer. So the substrate,alkene must be symmetric across C=CC=C.

  • In the list of hydrocarbons given in the problem there are only two alkenes ( cis-hex-3-ene and trans-hex-3-ene ) which are symmetric across C=CC=C.

  • As explained belowBr_2Br2 molecule causes electrophilic attack on pi - bondπ−bond forming a sigma-σ−complex which in next step is attacked by Br^-Br− ion and ultimately a trans addition occurs.

  • For Trans hex-3-ene the final product is a mesomer.

  • For Cis hex-3-ene the final products are two stereo isomers

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