Question

Which alkenes will result in a meso-form when the alkenes below reacts with molecular bromide (`Br_2`) cold/dark?

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

Answer 1

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=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=C`.

• As explained below`Br_2` molecule causes electrophilic attack on `pi - bond` forming a `sigma-`complex which in next step is attacked by `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