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Question: Anti-Markovnikov’s addition of \(HBr\) is not observed in: (A) Propene (B) But-1-ene (C) 2-But...

Anti-Markovnikov’s addition of HBrHBr is not observed in:
(A) Propene
(B) But-1-ene
(C) 2-Butene
(D) 2-Pentene

Explanation

Solution

The Anti-Markovnikov’s addition of HBrHBr is only observed in unsymmetrical alkenes because the unsymmetrical alkenes have a different number of substituents around the double bond.

Complete step by step solution:
The Anti-Markovnikov’s addition is also called the peroxide effect or the Kharasch effect. The Anti-Markovnikov’s rule states that, in the presence of peroxide like benzoyl peroxide (C6H5CO−O−O−COC6H5{{C}_{6}}{{H}_{5}}CO-O-O-CO{{C}_{6}}{{H}_{5}}), the addition of HBrHBr to unsymmetrical alkenes takes place contrary to the Markovnikov’s rule.
In Markovnikov’s rule, the negative part of the addendum gets attached to the carbon atom in the double bond which has a lesser number of a hydrogen atom or more substituted carbon atom. So, in Anti- Markovnikov’s rule, the negative part of the addendum gets attached to the carbon atom in the double bond which has more number of hydrogen atoms.

(A)- Propene
The formula of Propene is CH2=CH−CH3C{{H}_{2}}=CH-C{{H}_{3}}. This is an unsymmetrical alkene. So according to the Anti- Markovnikov’s rule, the negative part will attach the carbon atom with 2 hydrogen atoms.

(B)- But-1-ene
The formula of But-1-ene is CH2=CH−CH2−CH3C{{H}_{2}}=CH-C{{H}_{2}}-C{{H}_{3}}. This is an unsymmetrical alkene. So according to the Anti- Markovnikov’s rule, the negative part will attach the carbon atom with 2 hydrogen atoms.

(C)- 2-Butene
The structure of 2-Butene is CH3−CH=CH−CH3C{{H}_{3}}-CH=CH-C{{H}_{3}}. This is a symmetrical alkene and the Anti- Markovnikov’s rule will not be observed in 2-Butene.

(D)- 2-Pentene
The structure of 2-Pentene is CH3−CH=CH−CH2−CH3C{{H}_{3}}-CH=CH-C{{H}_{2}}-C{{H}_{3}}. This is an unsymmetrical alkene. So according to the Anti- Markovnikov’s rule, the negative part will attach the carbon atom with the methyl group because it is less substituted.

Hence, the correct answer is an option (c)- 2-Butene.

Note: If a reaction is given and we can differentiate whether the reaction will follow Markovnikov’s rule or Anti- Markovnikov’s rule by the presence of peroxide in the catalyst. If peroxide is present in the catalyst then the reaction will follow Anti- Markovnikov’s rule.