Question

You are a co-op student working in a research lab tasked to make 3-chloro-2,2-dimethylbutane from 3,3-dimethylbutan-2-ol. You used the procedure learned in Experiment 5 of your CHEM 266L course but are surprised when 2-chloro-2,3-dimethylbutane is obtained as the major product, while 3-chloro-2,2dimethylbutane is formed as the minor product.

a) Draw the overall reaction showing the starting material, reaction conditions and both the major and minor products (indicate which product is major/minor).
b) Draw a comparative energy diagram showing the formation of both major and minor product from the starting materials to explain how the energy differences lead to these experimental results.
c) How would you modify the reaction conditions to make 3-chloro-2,2-dimethylbutane the major product?

Answer 1

a) The overall reaction for the formation of the major product, 2-chloro-2,3-dimethylbutane, from 3,3-dimethylbutan-2-ol involves chlorination under acidic conditions. The major product is 2-chloro-2,3-dimethylbutane, and the minor product is 3-chloro-2,2-dimethylbutane.

b) The comparative energy diagram shows that the formation of the major product has a lower activation energy compared to the minor product. This energy difference favors the major product's formation under the given reaction conditions.

c) To enhance the formation of 3-chloro-2,2-dimethylbutane as the major product, one could adjust the reaction conditions. Specifically, increasing the temperature or altering the concentration of reactants can influence the relative rates of competing reactions, favoring the desired product.

Step-by-step explanation:

In the chlorination of 3,3-dimethylbutan-2-ol, the major product, 2-chloro-2,3-dimethylbutane, is formed due to the more stable tertiary carbocation intermediate that arises during the reaction. This stability is reflected in the lower activation energy for the major product compared to the minor product. The higher stability of the tertiary carbocation is a result of hyperconjugation and steric factors, making it more favorable.

In contrast, the formation of 3-chloro-2,2-dimethylbutane involves the less stable secondary carbocation intermediate, leading to a higher activation energy. This difference in activation energies dictates the product distribution observed experimentally, with the major product favored due to its lower energy pathway.

To shift the equilibrium towards the formation of 3-chloro-2,2-dimethylbutane as the major product, one can adjust the reaction conditions. Increasing the temperature or optimizing the concentration of reactants can influence the kinetics of the reaction, potentially favoring the desired product by altering the ratio of competing transition states.