The reaction of propene with HBr in the presence of ROOR...

The reaction of propene with HBr in the presence of ROOR (peroxide) proceeds through which of the following most stable intermediates? (a) $\mathrm{CH}_{3}-\dot{\mathrm{CH}}-\mathrm{CH}_{3}$ (b) $\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2} \mathrm{Br}$ (c) (d) $\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}$

The reaction of propene with HBr in the presence of ROOR (peroxide) proceeds through which of the following most stable intermediates?
(a) $\mathrm{CH}_{3}-\dot{\mathrm{CH}}-\mathrm{CH}_{3}$
(b) $\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2} \mathrm{Br}$
(c)
(d) $\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}$

Key Concepts

Free Radical Mechanism

In the presence of peroxides, the addition of HBr to alkenes such as propene proceeds via a free radical mechanism rather than the typical ionic pathway. This mechanism involves the generation of radicals, initiation by peroxide decomposition, and a chain propagation sequence where radicals add to the alkene, forming new radical intermediates which then react with HBr to yield the final product.

Radical Stability

The stability of radical intermediates is a key factor in determining the course of a free radical reaction. Secondary radicals, like the isopropyl radical, are more stable than primary radicals due to greater hyperconjugation and inductive stabilization. This increased stability directs the reaction to preferentially form the more stable radical intermediate, influencing the regiochemistry of the product.

Anti-Markovnikov Addition

Anti-Markovnikov addition refers to the regiochemical outcome observed in free radical addition reactions where the hydrogen atom of HBr adds to the more substituted carbon atom of the alkene. This results in the formation of a radical that is positioned on the less substituted carbon, which, due to radical stability considerations, ultimately leads to the final product that is opposite to what is predicted by the traditional Markovnikov rule.

Peroxide Effect

The peroxide effect is the phenomenon where the presence of organic peroxides causes the reaction mechanism to shift from an ionic to a free radical pathway. Peroxides decompose to form initiating radicals which start the chain reaction, ultimately leading to anti-Markovnikov products by favoring the formation of more stable radical intermediates. This effect is central to understanding the reactivity and product distribution in the addition of HBr to alkenes under radical conditions.

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