Question

Find product X from following chemical reaction:
 $ Ph-CH-C(C{{H}_{3}})-C{{H}_{3}}+HBr\xrightarrow{peroxide}(X) $
(A) $ Ph-CH(Br)-CH(C{{H}_{3}})-C{{H}_{3}} $
(B) $ Ph-C{{H}_{2}}-(Br)C(C{{H}_{3}})-C{{H}_{3}} $
(C) $ Ph-C{{H}_{2}}-CH(C{{H}_{3}})-C{{H}_{2}}-Br $
(D) $ Ph-CH=C(C{{H}_{3}})-C{{H}_{2}}-Br $

Answer 1

Hint :According to Markownikoff’s rule, when an unsymmetrical reagent is added to a carbon-carbon double bond of an alkene, the negative part of the reagent is added to more substituted doubly bonded carbon atom and the positive part of the reagent is added to less substituted doubly bonded carbon atom.

Complete Step By Step Answer:
The chemical formula of $ 3-phenylpropene $ is $ Ph-CH-C(C{{H}_{3}})-C{{H}_{3}} $ or $ {{C}_{6}}{{H}_{5}}C{{H}_{2}}CH=C{{H}_{2}}. $ It has an aliphatic part containing 3 carbon atoms and an aromatic part containing six carbon atoms. The aromatic part is a benzene ring. The aliphatic part is a three carbon atom chain containing a carbon-carbon double bond. Let us look at the reaction;
Proton from $ HBr $ is donated to doubly bonded carbon atoms to form less stable secondary carbocation.
 $ {{C}_{6}}{{H}_{5}}C{{H}_{2}}CH=C{{H}_{2}}+{{H}^{+}}\to {{C}_{6}}{{H}_{5}}\text{C}{{H}_{2}}C{{H}^{+}}+\text{C}{{H}_{3}} $
Less stable, secondary carbocation rearranges to more stable benzylic carbocation via $ 1,2-hydride $ shift. More stable benzylic carbocation then accepts a bromide ion to form $ 1-\text{ }bromo-1-phenylethane $
Thus, the complete chemical reaction is given as; $ {{C}_{6}}{{H}_{5}}-CH-C(C{{H}_{3}})-C{{H}_{3}}+HBr\xrightarrow{peroxide}{{C}_{6}}{{H}_{5}}-C{{H}_{2}}-(Br)C(C{{H}_{3}})-C{{H}_{3}} $
Therefore, the correct answer is option B.

Note :
The order of stability of carbocations is $ benzylic>tertiary>secondary>primary. $ but if peroxides are used, then 3-phenylpropene on reaction will give as the major product. In the presence of peroxides, anti Markownikoff’s rule is followed. According to peroxide effect when an unsymmetrical reagent is added to a carbon-carbon double bond of an alkene, in presence of peroxide, the negative part of the reagent is added to less substituted doubly bonded carbon atom and the positive part of the reagent is added to more substitute doubly bonded carbon atom.