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
trans-2-hexene
cis-2-hexene
1-hexene
cis-3-hexene
trans-3-hexene
1 Answer
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
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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.
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For Cis hex-3-ene the final products are two stereo isomers