Antimarkonikov's addition of HBr is not observed in (A) Propene (B) But-1-ene (C) but-2-ene (D) Pent-2-ene

Question:

Antimarkonikov's addition of HBr is not observed in 

(A) Propene 

(B) But-1-ene 

(C) but-2-ene 

(D) Pent-2-ene 

Answer: option (C) but-2-ene

Explanation:

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), the addition of HBr 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-CH3. 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-CH3. 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)- But-2-ene: The structure of 2-Butene is CH3-CH=CH-CH3. This is a symmetrical alkene and the Anti- Markovnikov's rule will not be observed in 2-Butene. 

(D)- pent-2-ene The structure of 2-Pentene is CH3-CH=CH-CH2-CH3. 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, option (C) "But-2-ene" is the correct answer.