Reaction Analysis: Electrophilic Addition to Alkene

The given reaction shows an alkene undergoing electrophilic addition with HBr. The alkene is unsymmetrical (has different groups attached to each carbon of the double bond), so we must use Markovnikov's rule to predict the major product.

Step 1: Identify the Alkene

The reactant is 3-methylbut-1-ene. Its structure is: CH₂=CH-CH(CH₃)₂.

Let's label the carbons of the double bond for clarity:

• Carbon 1 (less substituted): CH₂=
• Carbon 2 (more substituted): -CH-CH(CH₃)₂

Step 2: Apply Markovnikov's Rule

Markovnikov's rule states that in the addition of a protic acid (like HBr) to an unsymmetrical alkene, the hydrogen atom (H⁺) adds to the carbon of the double bond that has the greater number of hydrogen atoms (i.e., the less substituted carbon).

In our alkene:

• Carbon 1 has 2 hydrogens.
• Carbon 2 has 1 hydrogen.

Therefore, the H⁺ from HBr will add to Carbon 1. The Br^- ion will then add to the more stable carbocation intermediate that forms on Carbon 2.

Step 3: Analyze the Carbocation Stability

After H⁺ adds to Carbon 1, a carbocation forms on Carbon 2. Let's examine its structure: ${\mathrm{CH}}_3-\stackrel{+}{C}-\mathrm{CH}({\mathrm{CH}}_3{)}_2$

This is a secondary carbocation. However, it is stabilized by hyperconjugation from the three alkyl groups attached to it. This makes it a stable intermediate. There is no possibility for a rearrangement to form an even more stable tertiary carbocation in this specific molecule.

Step 4: Predict the Final Product

The bromide ion (Br^-) attacks the carbocation, forming the final alkyl bromide. The major product is 2-bromo-2-methylbutane.

Its structure is: CH₃-C(Br)(CH₃)-CH₃ or (CH₃)₂C(Br)CH₃. This is a tertiary alkyl halide.

Final Answer

The major product is the tertiary bromide: 2-bromo-2-methylbutane.

This corresponds to the third option in the list of possible answers.

Related Topics & Formulae

Key Concept: Markovnikov's Rule

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: Electrophilic Addition

General Reaction:

Carbocation Stability Order

The stability of the carbocation intermediate is the driving force behind Markovnikov's rule. The order of stability is:

Methyl < Primary (1°) < Secondary (2°) < Tertiary (3°) < Resonance stabilized (e.g., allylic, benzylic)

This stability is due to:

• Hyperconjugation: The overlap of the sigma bonds of adjacent alkyl groups with the empty p-orbital of the carbocation.
• Inductive Effect: Alkyl groups are electron-donating, which helps stabilize the positive charge.

Anti-Markovnikov Addition

In the presence of peroxides (ROOR), HBr adds to alkenes in an anti-Markovnikov fashion. This is due to a free radical mechanism, not an electrophilic one. This exception only applies to HBr; HCl and HI do not show this behavior.