42 E - Organic Chemistry (Mcmurry)

Question

2-Chloro-2-methylpropane reacts with water in three steps to yield 2-methyl-2-propanol. The first step is slower than the second, which in turn is much slower than the third. The reaction takes place slowly at room temperature, and the equilibrium constant is approximately 1.

1. Give approximate values for${\mathbf{\Delta G}}^{\pm }$ and ${\mathbf{\Delta G}}^{{}^o}$that are consistent with the above information.
2. Draw an energy diagram for the reaction, labeling all points of interest and making sure that the relative energy levels on the diagram are consistent with the information given.

Step-by-Step Solution

Answer

a. The ${\text{ΔG}}^{\text{±}}$value for the Ist step is approximately 80 kJ/mol. The ${\text{ΔG}}^{\text{±}}$values for the second and third steps are smaller, i.e. 60 kJ/mol for step 2 and 40 kJ/mol for step 3. The value of ${\text{ΔG}}^{\text{±}}$ is approximately zero.

 

b. An energy diagram for the reaction is,

Step 1: Energy Diagram

An energy diagram is a graph between energy and reaction progress which shows the intermediates and transition states. This diagram shows the activation energy change in going from reactant to product.

 

Step 2: The approximate values for ΔG ± and ΔG o

c. In the first step, the${\text{ΔG}}^{\text{±}}$ is approximately 80 kJ/mol because the reaction occurs slowly at room temperature. The${\text{ΔG}}^{\text{±}}$ values are smaller in the second and third steps, i.e. 60 kJ/mol for step 2 and 40 kJ/mol for step 3. The value of ${\text{ΔG}}^{\text{±}}$ is approximately zero because is closer to 1.

            The transition states in the reaction 

Step 3: An energy diagram for the reaction

d. An energy diagram for the given reaction shows the three transition states. The first transition state has greater ${\text{ΔG}}^{\text{±}}$value, and then comes the second transition state, which is lesser than the first. The third transition state has very small ${\text{ΔG}}^{\text{±}}$ value. Thus, the energy diagram with two intermediate formations is shown below,

                   An energy diagram for the reaction