Select the kinetic and thermodynamic products of the reaction shown.
The Correct Answer and Explanation is:
Of course. Here are the kinetic and thermodynamic products for the reaction shown, along with a detailed explanation.
Correct Answer:
The reaction of 4-methylidenecyclohex-1-ene with HBr produces two major products: a kinetic product and a thermodynamic product.
- The Kinetic Product is 4-bromo-4-methylcyclohex-1-ene.
- The Thermodynamic Product is 1-bromo-4-methylcyclohex-2-ene.
Explanation:
This reaction is an electrophilic addition of HBr to a conjugated diene system. The outcome is governed by the principles of kinetic and thermodynamic control.
Step 1: Formation of the Carbocation Intermediate
The first step is the addition of a proton (H⁺) from HBr to one of the double bonds. Protonation occurs in a way that generates the most stable carbocation intermediate. The starting molecule has two double bonds. Protonation of the exocyclic (outside the ring) double bond at the terminal methylene (CH₂) carbon forms a tertiary allylic carbocation. This intermediate is particularly stable because the positive charge is on a tertiary carbon and is also stabilized by resonance with the adjacent double bond in the ring.
Step 2: Resonance and Nucleophilic Attack
This tertiary allylic carbocation exists as a resonance hybrid of two major structures. In one structure, the positive charge is on the tertiary carbon (C4). In the other, the charge is delocalized to a secondary carbon at the other end of the conjugated system (C1).
The Kinetic Product (1,2-Addition):
The kinetic product is the one that forms the fastest. The resonance structure with the charge on the tertiary carbon is the more stable contributor to the hybrid, meaning this carbon bears a greater partial positive charge. The bromide ion (Br⁻) is therefore more likely to attack this position (C4) due to a lower activation energy barrier. This rapid attack results in the 1,2-addition product, 4-bromo-4-methylcyclohex-1-ene. This product forms more quickly, especially at lower temperatures.
The Thermodynamic Product (1,4-Addition):
The thermodynamic product is the most stable product. When the bromide ion attacks the other end of the allylic system (C1), it forms the 1,4-addition product, 1-bromo-4-methylcyclohex-2-ene. To determine which product is more stable, one must examine the stability of the alkene within the product. The kinetic product contains a disubstituted double bond. In contrast, the thermodynamic product contains a more stable trisubstituted double bond. According to Zaitsev’s rule, more highly substituted alkenes are more stable. Therefore, 1-bromo-4-methylcyclohex-2-ene is the thermodynamically favored product, as it is lower in overall energy. This product is favored under conditions that allow the reaction to reach equilibrium, such as higher temperatures or longer reaction times.thumb_upthumb_down
