Chapter 9, Problem 9.48P

Interpretation Introduction

Interpretation:

A detailed mechanism for the reaction between the given compound and $\text{potassium tert-butoxide in N, N-dimethylformamide}$ is to be drawn and why it gives a single product is to be explained. A detailed mechanism for the reaction of the compound with dilute potassium ethoxide in ethanol is to be drawn, and why a mixture of two products is obtained is to be explained.

Concept introduction:

Alkyl halides undergo elimination when treated with a strong base, particularly at high temperatures. The halogen is eliminated as halide along with a proton from an adjacent carbon. This results in the formation of a double bond between the carbon atoms.

The reaction can occur via two mechanisms, E1 or E2. A strong base at high concentration in a polar aprotic solvent favors the E2 mechanism. A weak base or a strong base at low concentrations in a polar protic solvent favors the E1 mechanism.

The E2 mechanism is a single-step, bimolecular mechanism. The base extracts a proton from a carbon adjacent to the carbon with the halogen attached to it. The $\text{C}-\text{H}$ bond pair moves toward the carbon with the attached halogen to form a $\text{π}$ bond. At the same time, the carbon-halogen bond breaks heterolytically to give the product. Since this is a single-step, concerted action, the proton and the halogen must be in anti to each other. This also results in the formation of just one elimination product.

The E1 mechanism is a two-step, unimolecular mechanism. The leaving group breaks off in the first, rate-determining step, leading to the formation of a carbocation. The base then extracts a proton from an adjacent carbon to give the elimination product. The carbocation formed in the first step can rearrange if it results in the formation of a more stable carbocation. This results in the formation of a mixture of products.