Question

Question:(a) What reaction conditions are needed to convert (R)-2-ethyl-2-methyloxirane to (R)-2-methylbutane-1,2-diol?

(b) What reaction conditions are needed to convert (R)-2-ethyl-2-methyloxirane to (S)-2-methylbutane-1,2-diol?

Short answer

Answer

$$\begin{gathered} \left[1\right]\mathrm{NaOH}/{\mathrm{H}}_2\mathrm{O} \\ \left[2\right]{\mathrm{H}}_2{\mathrm{SO}}_4/{\mathrm{H}}_2\mathrm{O} \end{gathered}$$

Step-by-step solution

Step-by-Step Solution Step 1: Base-catalyzed epoxide ring-opening

Epoxide ring opening can be both acid-catalyzed and base-catalyzed in nature. The base-catalyzed ring-opening follows the pathway.

In such cases, in which the epoxide is asymmetric, the nucleophile attacks at the less substituted carbon center.

The general reaction is shown below:

Base-catalyzed epoxide ring-opening

Acid-catalyzed epoxide ring-opening

The base-catalyzed ring-opening follows the ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$pathway. The nucleophile attacks at the more substituted carbon center.

The general reaction is shown below:

Acid-catalyzed epoxide ring-opening

Determination of reaction conditions

[1] The conversion of (R)-2-ethyl-2-methyloxirane to (R)-2-methylbutane-1,2-diol would require the reaction pathway to be $\left[{\mathrm{S}}_{\mathrm{N}}2\right]$in nature, so that the nucleophile attacks at the less substituted carbon center.

The reaction conditions are shown below:

Base-catalyzed epoxide ring-opening of (R)-2-ethyl-2-methyloxirane

[2] The ${\mathrm{S}}_{\mathrm{N}}1$ pathway would lead to the change in conformation of the product from (R) to (S). The reaction condition is shown below:

Acid-catalyzed epoxide ring-opening of (R)-2-ethyl-2-methyloxirane