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

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Answer 1 · 2015-07-27T20:04:49

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

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

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.