Question

cis- 2- butene $ \xrightarrow[{{\text{peroxide}}}]{{{\text{HBr}}}} $ product; Product of the reaction is:
(A) Racemic
(B) Diastereomers
(C) Meso
(D) E and Z isomer

Answer 1

Hint: To answer this question, you must recall the reactions of an alkene with various reagents and the mechanisms followed. Also you must recall the types of isomerism shown in an organic compound. There are broadly two types of isomerism, that is stereoisomerism and structural isomerism. The reaction proceeds through Anti-Markovnikov rule.

Complete step by step solution
When hydrogen bromide is reacted with an alkene in presence of a peroxide, the reaction proceeds by the Kharasch effect or the peroxide effect or the Anti- Markovnikov’s mechanism. Since the given alkene is a symmetrical alkene, the product obtained would be the same as that produced during Markovnikov's reaction. The only difference is that the reaction proceeds by free radical mechanism.
The product obtained on the addition of hydrogen bromide over cis- 2- butene is 2- bromo- butane. The carbon atom on which the bromine atom is attached is bonded to 4 different groups, i.e. it is a chiral carbon atom. Thus, the given compound can show stereoisomerism.
Two different conformations are possible for this carbon atom that are, R and S configurations. Both are produced in equal amounts. Hence a racemic mixture is obtained.
The correct answer is A.

Note
Geometrical isomerism and conformational isomerism are both types of stereoisomerism. In stereoisomerism, the compounds have similar molecular formula and structural formula but differ in the arrangement of the atoms in the space around the central atom they are bonded to. In structural isomerism, the compounds have the same molecular formula differ in the structural formula that is the bonds formed.