# Which stereoisomer of 3-hexene forms a meso compound when it reacts with Br_2 ?

May 21, 2015

trans-Hex-3-ene forms a meso compound when it reacts with bromine.

The bromination of an alkene involves the formation of an intermediate cyclic bromonium ion, followed by the anti-addition of a bromide ion to form the product.

The rigidity of the ring controls the stereochemistry of the reaction.

This means that the bromination of a trans alkene gives a meso dibromide.

For example, the bromination of trans-but-2-ene yields a meso compound.

Similarly, the bromination of hex-3-ene gives meso-2,3-dibromohexane as the product.

Attack of ${\text{Br}}^{-}$ at C-3 gives ($3 S , 4 R$)-3,4-dibromohexane.

Attack of ${\text{Br}}^{-}$ at C-4 gives ($3 R , 4 S$)-3,4-dibromohexane.

These two structures are superimposable on each other, so the product is a meso compound.