Question

What is selectivity? Is there any effect of selectivity in the halogenation reaction?

Answer 1

Selectivity is basically the tendency for a particular product to be made. It could be given in various subterminology:

• Regioselectivity - the tendency to form one constitutional isomer over another in a reaction.

This derives from the idea of adding certain atoms to different regions across the same bond. A common example is hydrohalogenation (`"HX"`) of asymmetrical alkenes, yielding the Markovnikov addition product.

• Stereoselectivity - the tendency to form one stereoisomer over another in a reaction.

An `"S"_N2` reaction inverts the stereoconfiguration of a chiral compound, and thus is stereoselective for the opposite stereoisomer (EX: `"R config reactant"` `->` `"S config product"`).

Halogenation of alkenes, i.e. `"X"_2` (`"X" = "Cl", "Br"`) dissolved in `"CH"_2"Cl"_2` or `"CCl"_4`, involves a symmetrical halogen.

After the first `"X"^(+)` cation adds across the double bond to make a halonium `pi` complex (a three-membered `"C"-stackrel(delta^+)("X")-"C"` ring), the `"X"^-` halide would do a back-side attack (usually to the more-substituted carbon).

Having a backside-attack of a `pi` complex like that then necessarily means an anti-addition as we see above (a "frontside-attack" would lead to a syn addition). That allows for formation of stereoisomers if the left half of the alkene is different from the right half.

You also don't get rearrangement after formation of the anti addition product, so no carbocation intermediate actually forms:

Therefore, we find that halogenation is definitely selective.