Chapter 10, Problem 10.27P

Interpretation Introduction

(a)

Interpretation:

The complete, detailed mechanism for the given reaction is to be drawn.

Concept introduction:

Terminal alkynes can be prepared from dihaloalkanes (1, 1-dihaloalkanes or 1, 2-dihaloalkanes) by treating them with a strong base via back to back E2 reaction. Three equivalents of the strong base ${\text{H}}_{\text{2}}{\text{N}}^{\text{-}}$ (${\text{NaNH}}_2$) are used to convert the dihalides into a terminal alkyne. The first two equivalents bring about two separate E2 reactions removing an acidic hydrogen and a halogen atom each time. The third equivalent of the base deprotonates the acidic proton on the newly formed terminal alkyne. Acid workup is necessary whenever the reaction conditions are strongly basic. Acid workup replaces that proton.

Interpretation Introduction

(b)

Interpretation:

The complete, detailed mechanism of the given reaction is to be drawn.

Concept introduction:

Alkynes can be prepared from dihaloalkanes (1, 1-dihaloalkanes or 1, 2-dihaloalkanes) by treating them with a strong base via back to back E2 reaction. When dihaloalkanes are treated with a strong base such as potassium hydroxide, a terminal alkyne is formed. This reaction does not require using three equivalents of $\text{KOH}$ as ${\text{OH}}^{-}$ is not strong enough to deprotonate the terminal alkyne. Water or alcohol can be used as a solvent.