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

1 Answer
Jul 27, 2015

Answer:

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.

Trans
(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.