Question

Why is the product of the addition of `Cl_2` to trans-2-butene a meso compound?

Answer 1

The product is meso because the intermediate involves anti addition to a cyclic chloronium ion.

Explanation:

Both chlorine and bromine react by the same mechanism. Just replace Br by Cl in the diagram below.

(from www.chemstone.net)

In the first step, the alkene attacks a chlorine molecule to form a cyclic chloronium ion IX.

Now a chloride ion attacks from the bottom of the chloronium ion.

It can attack at either position c or d.

If it attacks at position c, the bond to `"Cl"^+` breaks and the product is (`2R,3S`)-2,3-dichlorobutane, X (at top right).

If it attacks at position d, the bond to `"Cl"^+` breaks and the product is (`2S,3R`)-2,3-dichlorobutane, XI (at bottom right).

X and XI are the same meso compound.