Question

When 2-methylbut-2-ene is treated with hydrogen chloride, the major product formed is ______.

• A. 3-chloro-2-methylbutane
• B. 2-chloro-2-methylbutane
• C. 2-chloro-3-methylbutane
• D. 2-chlorobutane

Hint:

Always analyze the stability of intermediate carbocations during alkene addition reactions. The stability order is $3^\circ > 2^\circ > 1^\circ$. The reaction will universally favor forming the most stable carbocation possible.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to determine the major product of an electrophilic addition reaction where an unsymmetrical alkene (2-methylbut-2-ene) is reacted with a hydrohalic acid (HCl).

Step 2: Key Formula or Approach:
The addition of a hydrogen halide to an unsymmetrical alkene strictly follows Markovnikov's Rule.
The rule dictates that the negative part of the reagent ($Cl^-$) attaches to the carbon atom of the double bond that bears the lesser number of hydrogen atoms. This occurs because the reaction proceeds via the most stable possible carbocation intermediate.

Step 3: Detailed Explanation:
Let's examine the structure of the starting alkene, 2-methylbut-2-ene:
$CH_3 - C(CH_3) = CH - CH_3$
The double bond exists between Carbon-2 and Carbon-3.
- Carbon-2 is bonded to zero hydrogen atoms (it is a fully substituted tertiary carbon).
- Carbon-3 is bonded to one hydrogen atom (it is a secondary carbon).
Step A: Electrophilic attack by the $H^+$ ion. The $\pi$-bond breaks, and the $H^+$ attaches to Carbon-3. This specifically forms a highly stable tertiary ($3^\circ$) carbocation at Carbon-2.
Intermediate: $CH_3 - C^+(CH_3) - CH_2 - CH_3$ (Tertiary carbocation).
Step B: Nucleophilic attack by the chloride ion ($Cl^-$). The chloride ion rapidly attacks the positively charged tertiary carbon.
Final Structure: $CH_3 - C(Cl)(CH_3) - CH_2 - CH_3$
The IUPAC name for this molecule is 2-chloro-2-methylbutane.

Step 4: Final Answer:
The major product is 2-chloro-2-methylbutane, matching option (b).