Concept: Markovnikov's Rule in Alkene Addition Reactions

This question involves predicting the major product when an alkene undergoes hydrohalogenation (addition of HBr). The key concept is Markovnikov's Rule, which states that when an unsymmetrical alkene reacts with an unsymmetrical reagent (like HBr), the hydrogen atom adds to the carbon with the greater number of hydrogen atoms, and the halogen adds to the carbon with fewer hydrogen atoms.

Step-by-Step Solution:

Step 1: Analyze the Reactant

The reactant is an unsymmetrical alkene: 2-methylpropene (isobutylene). Its structure is:

$C H_{3} - C (C H_{3}) = C_{H} H_{2}$

More clearly: The double bond is between a tertiary carbon (with one H) and a carbon with two H atoms.

Step 2: Apply Markovnikov's Rule

When HBr adds across the double bond:

The H⁺ (electrophile) will add to the less substituted carbon (the carbon with more hydrogen atoms).
The Br^- (nucleophile) will add to the more substituted carbon (the carbon with fewer hydrogen atoms), which is the tertiary carbon.

This results in the formation of a tertiary alkyl halide, which is the most stable carbocation intermediate possible.

Step 3: Predict the Major Product

The major product is 2-bromo-2-methylpropane (tert-butyl bromide). Its structure is:

$B r - C (C H_{3})_{3}$

This corresponds to the option: Structure of 2-bromo-2-methylpropane

Final Answer: The major product is Structure of 2-bromo-2-methylpropane (2-bromo-2-methylpropane).

Related Topics:

Electrophilic Addition Reactions
Carbocation Stability (Primary < Secondary < Tertiary)
Anti-Markovnikov Addition (Peroxide Effect)
Hydration of Alkenes

Key Formulae and Theory:

Markovnikov's Rule: "The rich get richer." In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon that already has more hydrogens.

Mechanism:

Step 1: Electrophilic Attack to form Carbocation $R {-CH=CH}_{2} + H^{+} \to R {-C}_{+} H - {CH}_{3}$

Step 2: Nucleophilic Attack $R {-C}_{+} H - {CH}_{3} + {Br}^{-} \to R {-CHBr-CH}_{3}$

Carbocation Stability Order: ${CH}_{3} +^{<1°} < {2°}^{<} 3°$