Question

Which of the following alkenes does not undergo anti-Markovnikov addition of HBr?

• A. Propene
• B. 1-Butene
• C. 2-Butene
• D. 3-Methyl-2-Pentene

Hint:

In the presence of peroxides, the addition of HBr to alkenes follows the anti-Markovnikov rule, where the hydrogen atom attaches to the carbon with fewer alkyl groups.

Answer 1

The Correct Option is C

Anti-Markovnikov addition occurs when HBr adds to an alkene such that the hydrogen atom attaches to the carbon atom with the fewest alkyl groups, resulting in the formation of the less stable carbocation. This addition typically occurs with peroxides (as in the case of the anti-Markovnikov mechanism).
Step 1: Propene (C$_3$H$_6$) undergoes anti-Markovnikov addition of HBr in the presence of peroxides. In this reaction, the hydrogen adds to the carbon with fewer alkyl groups, and the bromine adds to the carbon with more alkyl groups.
Step 2: 1-Butene (C$_4$H$_8$) also undergoes anti-Markovnikov addition of HBr in the presence of peroxides, similar to propene. 
Step 3: 2-Butene (C$_4$H$_8$) undergoes Markovnikov addition of HBr, where the bromine atom attaches to the carbon with more alkyl groups, and the hydrogen attaches to the carbon with fewer alkyl groups. Therefore, 2-Butene does not undergo anti-Markovnikov addition. 
Step 4: 3-Methyl-2-pentene (C$_6$H$_{12}$) undergoes Markovnikov addition and does not follow the anti-Markovnikov addition rule. 
Thus, the correct answer is 2-Butene, which does not undergo anti-Markovnikov addition.