# Reaction Mechanisms: Free Radical Halogenation

In Free Radical halogenation using chlorine and sunlight, why does the chlorine-free radical get attached to the allylic carbon?

Will it be correct to say that it follows the anti-Markovnikov rule?

But if so, there is no $$\ce{H2O2}$$ present...

The substrate can be considered as but-2-ene.

• @zenix but-2-ene – Devaesh May 10 '20 at 7:55
• This is not $\ce{HX}$ addition to an alkene. Therefore, Markovnikov rule does not apply here. You should change your question accordingly. – Mathew Mahindaratne Sep 18 '20 at 17:38
• More specifically, this is not HBr addition in the presence of peroxide (Kharasch reaction). HCl and HI do not work in the Kharasch reaction. – user55119 Sep 18 '20 at 18:06

H. C. Brown has suggested that the halogen atom does add but, at high temperatures, is expelled before the second step of free radical addition can occur$$^1$$.

Addition of halogen atom gives this radical, which falls apart to regenerate the starting material if the temperature is high or if it does not soon encounter a halogen molecule to complete the addition. Hence, low concentration of halogen can be used with high temperature to favour substitution over addition$$^1$$.

The allylic radical, on the other hand, once formed, has little option but to wait for a halogen molecule, however low the halogen concentration$$^1$$.

For the formation of minor product you may look into this question.

To follow the Markovnikov's or anti-Markovnikov's rule, there should be difference of electronegativity between the two atoms. Therefore, I do not understand what you are trying to ask.

Reference

1. Organic Chemistry- Morrison and Boyd (6E); Page 389-390
• The first step of the allylic mechanism should have the half arrow going to the chlorine radical. – Zhe Sep 18 '20 at 17:33
• I would appreciate a reference to H. C. Brown's suggestion. – user55119 Sep 18 '20 at 18:15
• @Zhe It's done now. Can you please verify the linked question? – Apurvium Sep 19 '20 at 2:28
• @Apurvium Sorry, but I don't see a fix, and the linked question has the same problem. The product from step 1 does not correspond with the arrows (the product based on arrows are the allylic radical, hydrogen radical, and chlorine radical, with no new bond formed between chlorine and hydrogen). – Zhe Sep 19 '20 at 10:58