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QUESTION

# Why in the presence of peroxides HCl and HI do not give anti-markovnikov addition to alkenes?

"HCl" and "HI" do not give anti-Markovnikov addition to alkenes because some of the steps in the chain reaction are endothermic.

Reaction with "HBr"

The mechanism for anti-Markovnikov addition of "HBr" is:

Step 1. Abstraction of "H" from "HBr" (highly exothermic).

"HO·" + "H-Br" → "HOH" + "·Br"

Step 2. Addition of "Br·" to the alkene (exothermic)

"CH"_3"CH=CH"_2 + "·Br" → "CH"_3stackrel("·")("C")"H-CH"_2"Br"

Step 3. The carbon radical abstracts another "H" from "HBr" (exothermic)

"CH"_3stackrel("·")("C")"H-CH"_2"Br" + "H-Br" → "CH"_3"CH"_2"-CH"_2"Br" +"·Br"

The reaction is favourable because all the steps are exothermic.

Reaction with "HCl"

The reaction with "HCl" is unfavourable because the "H-Cl" bond is much stronger than the "H-Br" bond.

Step 1 is endothermic, and the reaction becomes too slow to be useful.

Reaction with "HI"

Step 1 is favourable, because the "H-I" bond is relatively weak.

But Step 2, addition of "I·" to the alkene, is unfavourable and endothermic because of the bulk of the iodine radical.

Instead, the iodine radicals combine to form "I"_2.