Question

Additions of halogens to alkenes are considered stereospecific reactions. What is the definition of a stereospecific reaction?

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Answer 1

When alkenes are treated with halogens, the reaction proceeds through a stepwise attack of halogens as X⁺ and X^- for a halogen X₂. Thus, after the first attack of a halogen, a three membered ring forms by the bridging of a halogen atom with the two carbons in the double bond called a halonium ion intermediate. The steric hinderance presented by this intermediate prevents the other halogen atom from approaching in the same face thus resulting in an anti-addition product always. Such an anti-addition results in cis-alkenes giving a racemic mixture of two enantiomers which arise by starting attack at two different faces, each generating one enantiomer and such attacks happening at an equal rate for both faces while trans-alkenes give a meso-product as irrespective of the face of attack, there will be a mirror plane after the anti-attack in all symmetrical trans-alkenes and only a single enantiomer in case of unsymmetrical trans-alkenes.

Mechanism:

Stereospecific reactions are those in which the stereochemistry of the product is strictly dictated by that of the substrates. Since the stereochemistry on the alkene here dictates the stereochemistry of the chiral centres generated in the product, addition of halogens to alkenes are considered to be stereospecific.