Which of the following is the major product formed when $H_{\mathrm{Br}}$ reacts with 2-methyl-2-butene under standard conditions?

• A. 2-bromo-3-methylbutane
• B. 3-bromo-2-methylbutane
• C. 1-bromo-2-methylbutane
• D. 2-bromo-2-methylbutane

Answer: (D)

Verified step by step guidance

1. Identify the type of reaction: The reaction of HBr with an alkene like 2-methyl-2-butene is an electrophilic addition reaction, where the alkene's double bond reacts with HBr to form a bromoalkane.
2. Determine the regiochemistry using Markovnikov's rule: In the addition of HX to an alkene, the hydrogen (H) adds to the carbon with more hydrogens already attached, and the halide (Br) adds to the carbon with fewer hydrogens. This leads to the more stable carbocation intermediate.
3. Draw the carbocation intermediate: Protonation of the double bond occurs first, forming the most stable carbocation. For 2-methyl-2-butene, the carbocation forms at the more substituted carbon (the tertiary carbon) because it is more stable.
4. Add the bromide ion (Br⁻) to the carbocation: The bromide ion attacks the carbocation, resulting in the formation of the bromoalkane at the more substituted carbon.
5. Assign the correct IUPAC name to the product: After addition, identify the position of the bromine and methyl groups on the carbon chain to name the major product correctly, which corresponds to 2-bromo-2-methylbutane.