When HBr adds to (CH3)2C = CH2 under radical conditions, two...

When HBr adds to $(CH_3)_2C = CH_2$ under radical conditions, two radicals are possible products in the first step of chain propagation. Draw the structure of both radicals and indicate which one is
formed. Then draw the preferred product from HBr addition under radical conditions.

Key Concepts

Regiochemistry in Radical Additions (Anti-Markovnikov Addition)

Under radical conditions, the addition of HBr to alkenes follows an anti-Markovnikov pattern. This occurs because the more stable radical intermediate is formed at the more substituted carbon, which then leads to the product where the bromine atom attaches to the less substituted carbon. This regiochemical outcome contrasts with ionic addition mechanisms that typically follow Markovnikov's rule.

Chain Propagation Steps

The chain propagation steps are critical in radical reactions. During propagation, a radical adds to the double bond to form a new radical intermediate, and then this intermediate reacts with HBr to abstract a bromine atom, regenerating another radical. The competition between different radical intermediates, based on their stability, controls which propagation pathway is favored and, consequently, which product is predominantly formed.

Radical Chain Mechanism

This concept involves reactions that proceed through initiation, propagation, and termination steps using radical intermediates. In the context of HBr addition under radical conditions, the reaction starts with the formation of radicals, typically generated by heat or light, which then participate in chain propagation steps by adding to the double bond and abstracting atoms from reagents to form new radicals, ultimately leading to the final product.

Radical Stability

Radical stability is a key factor determining the course of a radical reaction. Radicals that are more substituted, particularly tertiary radicals, are generally more stable than secondary or primary radicals due to hyperconjugation and inductive effects. This stability influences which radical intermediate is favored during the reaction, ultimately directing the regioselectivity of the product formation.

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