Kinetic and Thermodynamic Control of Electrophilic Addition Reactions
Kinetic and Thermodynamic Control of Electrophilic Addition Reactions
This Demonstration shows the effect of thermodynamics and kinetics on electrophilic addition reactions producing dienes. The following reaction is considered:
HBr+CH-CH
CH
2
CH
2
−80°C↙↘40°C
|
40°C
⟶
|
The reaction must overcome the activation barrier shown in blue to produce the carbocation -CHCH; the green curve then shows how this can evolve into a 1,2-addition (-CHBr-CH, kinetic product) or a 1,4-addition (-CHCH-CHBr, thermodynamic product) [1]. The 1,2-addition has a lower activation energy than the 1,4-addition—it occurs faster than 1,4-addition, otherwise it can occur only at high temperature, owing to the greater activation energy. The final proportions of the product are determined by the laws of chemical equilibrium and change according to the temperature. A reaction yielding more of the thermodynamic product is said to be under thermodynamic control, while a reaction yielding more kinetic product is under kinetic control [2].
CH
3
—
—
+
CH
2
CH
3
CH
2
CH
3
Reaction progress can be viewed using the "time" slider; the quantities of the products are proportional to the size of the balls. Using the "reactions" buttons, it is possible to choose a reaction and follow its progress. Use the checkbox to visualize the thermal energy of the molecules and compare it to the activation energies.