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

Jul 27, 2015

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 ${\text{Cl}}^{+}$ breaks and the product is ($2 R , 3 S$)-2,3-dichlorobutane, X (at top right).

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

X and XI are the same meso compound.