HX Addition (X = halogen)

Hydrogen halides easily undergo heterolytic cleavage. The resulting proton is a strong electrophile and attacks the π cloud of a C=C double bond, leading to a carbocation (the least unstable!!!) which collapses with the halide counterion.

Animation of the addition of HX to propene

Addition of HX to an alkene: electrophilic attack of the proton on the double bond and nucleophilic attack of the halide on the carbocation

Energy diagram of the addition of HCl to propene: the pathway through the secondary carbocation (TS2) is lower than that of the primary one (TS1) and leads to 2-chloropropane

Any of the four hydrogen halides (HI, HBr, HCl or HF) gives the addition reaction in a regioselective way:
The formed product depends on the relative stability of the possible intermediate carbocations. In short, the initial protonation produces the least unstable carbocation.

Long before chemists knew what a carbocation was, Markovnikov studied the reaction on a large series of olefins and established his famous rule. Markovnikov's rule: the proton of the hydrogen halide bonds to the least substituted carbon.

Vladimir Vasilyevich Markovnikov (also spelled Markownikoff; 22 December 1838 – 11 February 1904), was a Russian chemist.

But, what happens if we add a peroxide to the reaction medium? The regiochemistry of the reaction changes. The mechanism of the reaction should thus be different.

HBr with 1-butene: without peroxides it gives 2-bromobutane (Markovnikov, via carbocations); with peroxides (ROOR), 1-bromobutane (anti-Markovnikov, via radicals)

Consider now that the attacking species is the halogen radical. Do you understand why the regiochemistry changes?

The peroxide makes the mechanism proceed through radicals instead of carbocations:

Initiation steps: the peroxide breaks homolytically (ΔH° = +35 kcal/mol) and the RO· radicals abstract the hydrogen of HBr to give Br· (ΔH° = −34 kcal/mol)

The peroxide homolytically splits into two oxyalkyl radicals that quickly react abstracting the hydrogen from HBr and forming the Br· radical.

Propagation steps: Br· adds to 1-butene to give the secondary radical, which abstracts a hydrogen from HBr to give 1-bromobutane and Br· (ΔH° = −11 kcal/mol)

Now Br·, NOT H·, attacks the olefin rendering the least unstable secondary C· radical. This is the key to the different regiochemistry:
Without peroxides the reaction proceeds through carbocations;
with peroxides the attacking species is Br· and the reaction goes via radicals. In both cases the reaction pathway is that of the lowest possible energy.