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Hydrocarbons

Long Answer Type

151.

Discuss the free radical mechanism of addition of HBr to propene.
Or
Give the mechanism of addition of HBr to propylene in the presence of peroxide.

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152. Write IUPAC names of the products obtained by addition reactions of HBr to hex-1-ene:
(i) in the absence of peroxide and
(ii) in the presence of peroxide.

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153.
The addition of HBr to propene yields 2-broniopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

Formation of 2-Bromopropane: Addition to HBr to propene is an ionic electrophilic addition reaction in which the electrophile, (H⁺) adds to form a more stable 2° carbocation. In the 2nd reaction, the carbocation is rapidly attacked by the nucleophile (Br^-) to form 2-bromopropane.

Formation of 1-Bromopropane:
In the presence of benzoyl peroxide, the addition of HBr to propene involves free radical mechanism in which Br-free radical is obtained by the action of benzoyl peroxide on HBr.

(i) Br radical adds to propene to form the more stable 2° free radical.

(ii) Free radical thus obtained rapidly abstracts a hydrogen atom from HBr to form 1-bromopropane.