Organic Reaction: Electrophilic Addition with HBr

This question involves predicting the major product when an alkene reacts with HBr. The key concept is Markovnikov's Rule.

Step 1: Identify the Reactants

The starting material is an alkene (cyclic, with a double bond). The reagent is HBr, which is a source of H⁺ and Br⁻ ions.

Step 2: Understand the Mechanism

HBr adds to alkenes via electrophilic addition. The mechanism occurs in two steps:

Step 2a: Formation of the Carbocation
The electron-rich double bond attacks the electrophilic hydrogen (H⁺) from HBr. This results in the formation of a carbocation intermediate. The stability of this carbocation determines the major product.

Step 2b: Nucleophilic Attack
The bromide ion (Br⁻) then attacks the carbocation, forming the final alkyl bromide product.

Step 3: Apply Markovnikov's Rule

Markovnikov's Rule states: "In the addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon of the double bond that has the greater number of hydrogen atoms."

A more modern way to state this is: The electrophile (H⁺) adds in such a way that it generates the most stable carbocation intermediate.

Carbocation stability follows the order: Tertiary (3°) > Secondary (2°) > Primary (1°).

Step 4: Analyze the Given Alkene

While the exact structure isn't drawn here, the options suggest the alkene is part of a bicyclic system. The critical step is to protonate the double bond to form the most stable carbocation possible. In many bicyclic systems, a bridgehead carbocation is highly unstable (Bredt's rule). Therefore, addition will occur away from the bridgehead to form a more stable secondary carbocation.

Step 5: Predict the Major Product

Based on the stability of the intermediate carbocation (avoiding a bridgehead carbocation), the bromine atom will add to the more substituted carbon of the double bond, following Markovnikov's rule. The correct product is the one where Br is attached to the carbon that was the tertiary carbon in the carbocation intermediate.

Final Answer

The major product is the third option:

Related Topics

Markovnikov's Rule
Electrophilic Addition Reactions
Carbocation Stability
Bredt's Rule
Alkene Reactivity

Key Formulae and Theory

General Reaction:
$R - CH = CH 2 + HBr \to R - CH (Br) - CH 3$

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

Bredt's Rule: A bridgehead carbon of a bicyclic system cannot be sp² hybridized (i.e., cannot be part of a double bond or form a stable carbocation) if the ring system is too small and rigid.