Reaction Analysis: Electrophilic Addition to Conjugated Diene

The given reaction involves a conjugated diene (1,3-butadiene derivative) undergoing electrophilic addition with HBr. The key concept is 1,2-addition vs 1,4-addition under kinetic vs thermodynamic control.

Step 1: Identify the Reactants

The reactant is a substituted 1,3-butadiene: 2-methylpenta-1,3-diene. Its structure is:

CH₂=C(CH₃)–CH=CH–CH₃

The electrophile is HBr (hydrogen bromide).

Step 2: Mechanism of Electrophilic Addition

HBr adds to the conjugated diene via electrophilic addition. The first step is the attack of the electrophile (H⁺) on the diene system.

Formation of Allylic Carbocation Intermediate: The proton (H⁺) can add to either terminal carbon (C1 or C4) of the diene. Adding to C1 (the less substituted end) forms a more stable, tertiary allylic carbocation.

Protonation at C1:

This generates a resonance-stabilized tertiary allylic carbocation.

Step 3: Resonance Structures of the Carbocation

The positive charge is delocalized over two carbons (C2 and C4).

Structure I (Positive charge on C2): ${\mathrm{CH}}^3-\stackrel{+}{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}3$

Structure II (Positive charge on C4): ${\mathrm{CH}}^3-CH-\mathrm{CH}=\stackrel{+}{C}H-\mathrm{CH}3$

Structure I is tertiary, making it the major contributor to the hybrid.

Step 4: Nucleophilic Attack by Br^-

The bromide ion (Br^-) can attack either of the two carbocations (C2 or C4) from the resonance hybrid.

• 1,2-addition: Attack at C2 (the carbon that was originally sp² in the major resonance form). This gives the kinetic product.
• 1,4-addition: Attack at C4. This gives the thermodynamic product, which is more stable due to a more substituted double bond.

At low temperatures, the reaction is under kinetic control and the 1,2-adduct is the major product. However, this reaction is typically conducted at room temperature or above, favoring the thermodynamic product (1,4-adduct).

Step 5: Predict the Major Product

The major product is the more stable 1,4-addition product: 4-bromo-2-methylpent-2-ene.

Its structure is: BrCH₂–C(CH₃)=CH–CH₂CH₃

This matches the structure in the second option.

Final Answer

The major product is the 1,4-addition product: 4-bromo-2-methylpent-2-ene.

Related Topics

• Conjugated Dienes
• Electrophilic Addition Reactions
• Allylic Carbocation Stability
• Resonance Theory
• Kinetic vs Thermodynamic Control
• Markovnikov's Rule

Key Formulae and Theory

General Reaction: Conjugated diene + HX → 1,2-adduct + 1,4-adduct

Stability of Carbocations: Tertiary > Secondary > Primary > Methyl

Resonance Energy: Allylic systems are stabilized by resonance (10-15 kcal/mol).

Kinetic Control: The product that forms fastest (lower activation energy) is favored at low temperatures. This is usually the 1,2-adduct.

Thermodynamic Control: The most stable product is favored at higher temperatures and longer reaction times. This is usually the 1,4-adduct with a more substituted alkene.