Q.The addition of HBr to 1-butene gives a mixture of products A, B and C
(A) CH3-CHBr-CH2-CH3 (2-bromobutane)
(B) CH3-CHBr-CH2-CH3 (2-bromobutane)
(C) CH3-CH2-CH2-CH2-Br (1-bromobutane)
The mixture consists of

The key idea is Markovnikov vs Anti-Markovnikov addition. HBr adds to an unsymmetrical alkene like 1-butene in two ways depending on the reaction conditions.

1. In the absence of peroxides, HBr follows Markovnikov’s rule: the H attaches to the less substituted carbon (C-1) and Br to the more substituted carbon (C-2), giving 2-bromobutane as the major product.
2. In the presence of peroxides, the reaction follows anti-Markovnikov addition: Br attaches to the less substituted carbon (C-1), giving 1-bromobutane as the major product. …

1. Identify the reaction and the rule.

   The addition of HBr to an unsymmetrical alkene like 1-butene ($\ce{CH2=CH-CH2-CH3}$) is governed by Markovnikov’s rule: the hydrogen adds to the carbon with more hydrogens already, and the bromine adds to the carbon with fewer hydrogens. This gives the more stable carbocation intermediate.

2. Draw the two possible carbocations.

   Protonation of the double bond can occur in two ways:

   • H⁺ adds to C-1 (terminal carbon): forms a secondary carbocation at C-2: $\ce{CH3-CH+-CH2-CH3}$ (more stable).
   • H⁺ adds to C-2: forms a primary carbocation at C-1: $\ce{CH2+-CH2-CH2-CH3}$ (less stable).

   The secondary carbocation is about 25 kJ/mol more stable, so it forms much faster and dominates the product.

3. What does each carbocation give?

   • The secondary carbocation captures Br⁻ to give 2-bromobutane: $\ce{CH3-CHBr-CH2-CH3}$.
   • The primary carbocation captures Br⁻ to give 1-bromobutane: $\ce{CH3-CH2-CH2-CH2-Br}$.

   So under normal (ionic, peroxide-free) conditions, 2-bromobutane is the major product and 1-bromobutane is the minor product.

4. Why are A and B the same compound?

   Look at the problem: both A and B are written as $\ce{CH3-CHBr-CH2-CH3}$ (2-bromobutane). They are chemically identical — the same molecule. The question lists them separately only to test whether you notice this. So A and B together represent the major product, and C is the minor product. …