Free radical reactivity

[durden]

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Why is the first rxn markovinikov but not the second?

 

[Impulse155]

Why is the first rxn markovinikov but not the second?

The second the methyl on the benzene is considered "Benzylic". Has mad resonance so its the most likely place a Br will go. Not to mention all the other carbons are vinyl. The first one your making a 3rd degree radical which is more stable than primaries.

 

[sagarkanodiya]

The reactivity of different compounds toward a certain radical is measured in so-called competition experiments. Compounds bearing carbon-hydrogen bonds react with radicals in the order primary < secondary < tertiary < benzyl < allyl reflecting the order in C-H bond dissociation energy (Wikipedia)

My input is Bromine radical would be more stable if it adds to Benzylic position.You can't talk about Markovnikov or Anti-Markovnikov unless you are adding to both sides of double bond.

This is free radical addition reaction.

 

[Impulse155]

The reactivity of different compounds toward a certain radical is measured in so-called competition experiments. Compounds bearing carbon-hydrogen bonds react with radicals in the order primary < secondary < tertiary < benzyl < allyl reflecting the order in C-H bond dissociation energy (Wikipedia)

My input is Bromine radical would be more stable if it adds to Benzylic position.You can't talk about Markovnikov or Anti-Markovnikov unless you are adding to both sides of double bond.

This is free radical addition reaction.

Isn't benzyl favored over allyl? More resonance.

 

[biomolecmed]

Neither of the reactions are Markovnikov. Those reactions are radical halogenations. When you are doing Br2 with light, Br will always go to the most stable radical.
First reaction it went to the tertiary radical. In the second reaction , the best radical is primary Benzylic. You don`t want to create any radical on the carbons of the Benzene ring, because you don`t want to break the aromaticity. Also, yes, Benzylic radical is more stable because of resonance.

Hope this helps.