Depto. Química Orgánica

HYDROGEN HALIDE ADDITION TO ALKENES

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15/07/2017
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Hydrogen halides easily undergo heterolytic cleavage.
The resulting proton is quite strong a electrophile and attacks the 'pi' cloud of a doble C=C bond, leading to a carbocation (the least unstable!!!) that colapses with the halide counterion.
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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 is, Markovnikov studied the reaction on a large series of olefins and stablished his famous rule. 
Markovnikov's rule
: The proton of the hydrogen halide bonds to the least substituted carbon.
Image Vladimir Vasilyevich Markovnikov , also spelled as Markownikoff, (December 22, 1838 – February 11, 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.
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Consider now that the attacking species is the radical halide. Do you understand why the regiochemistry changes?
The peroxide makes the mechanism proceed through radicals instead of carbocations:
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The peroxide homolytically splits into two oxyalkyl radicals that quickly react abstracting  the hydrogen from HBr and forming the Br· radical.
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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 patway is that of the lowest possible energy.