The concept of kinetic versus thermodynamic control of reaction products has frequently been employe — Organic Chemistry Chemistry Question
Kinetics and Thermodynamics
The concept of kinetic versus thermodynamic control of reaction products has frequently been employed in organic synthesis to direct product formation, for example, in sulfonation, Diels–Alder, isomerization and addition reactions. Here the interconversion of two different products can be achieved competitively by controlling the reaction conditions. It is normally represented as a concurrent reaction scheme, as shown below for the case where the reaction of A proceeds competitively toward products B and C.
k1
---⟶
B A
<-----
k-1
k2
---⟶
A C
<-----
k-2
The energy profile for the reaction is depicted in the Figure below.
[VISUAL]
The rate constants are as follows: k1 = 1, k–1 = 0.01, k2 = 0.1, and k–2 = 0.0005 min–1. Estimate the product ratio B/C in the first 4 minutes of the reaction.
Model Answer
In the beginning 4 min of the reaction,
d[B]/dt = k1 * [A] (1)
d[C]/dt = k2 * [A] (2)
Dividing (1) by (2) gives:
d[B]/d[C] = k1 / k2
Thus:
B/C = k1 / k2 = 1 / 0.1 = 10
Using the same rate constants, estimate the product ratio B/C when the reaction time exceeds 4 days.
Model Answer
When the reaction is complete, the system reaches thermal equilibrium.
[B]/[A] = k1 / k-1
[C]/[A] = k2 / k-2
Thus:
[B]/[C] = (k1 * k-2) / (k2 * k-1) = (1 * 0.0005) / (0.1 * 0.01) = 0.0005 / 0.001 = 1/2
B is called the kinetic–controlled product, while C is called the thermodynamic–controlled product. Which reaction process is favoured when the temperature of the system increases?
Model Answer
A → C (thermodynamically controlled reaction process) is favoured when temperature increases. The system will reach thermal equilibrium more rapidly.