With HBr gives predominantly, for an aryl-substituted allylic/aryl-ether alkene of the type (4-alkoxyphenyl)–CH=CH–CH_3 type system, the stereochemical outcome at the newly formed C–Br and C–H centres is [Kerala CEE 2008]

JEE Main Chemistry question

With HBr gives predominantly, for an aryl-substituted allylic/aryl-ether alkene of the type (4-alkoxyphenyl)–CH=CH–CH type system, the stereochemical outcome at the newly formed C–Br and C–H centres is [Kerala CEE 2008]

(A)the *erythro* (syn) diastereomer, from *syn* addition
(B)the *threo* (anti) diastereomer, from *anti* addition via a bromonium ionCorrect
(C)a racemic 1:1 mixture of both diastereomers
(D)none of these — addition is non-stereospecific

Answer: (B)

Solution

Electrophilic addition of HBr to an alkene proceeds through a bridged bromonium-ion intermediate, which is opened by bromide from the face opposite to the bromonium ring (*anti*/*trans* addition). For a 1,2-disubstituted alkene,

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Question ID VK-142217