Chapter 10, Problem 10.5P

Interpretation Introduction

(a)

Interpretation:

The compounds from the given list that will readily react with PCl₃ are to be identified, and the products with correct stereochemistry are to be drawn for them.

Concept introduction:

A reaction between an alcohol and phosphorus trihalide replaces the alcoholic $-\text{OH}$ group with the halide via two successive ${\text{S}}_{\text{N}}\text{2}$ reactions.

Since the mechanism of the reaction is ${\text{S}}_{\text{N}}\text{2}$, tertiary alcohols and alcohols in which the $-\text{OH}$ group is attached to an ${\text{sp}}^{\text{2}}$ or $\text{sp}$ hybridized carbon do not react readily.

If the $-\text{OH}$ group is attached to a chiral carbon, the reaction results in an inversion of the configuration at this carbon.

Interpretation Introduction

(b)

Interpretation:

The complete, detailed mechanism for reactions of the two compounds that readily react with ${\text{PCl}}_{\text{3}}$ is to be drawn.

Concept introduction:

The alcoholic group of secondary alcohol is replaced with a chlorine atom when the substrate is treated with ${\text{PCl}}_{\text{3}}$. The reaction is a combination of two ${\text{S}}_{\text{N}}\text{2}$ reactions.

In the first reaction, the alcohol acts as a nucleophile and attacks the electron-deficient phosphorus in ${\text{PCl}}_{\text{3}}$ to replace one of the chlorine atoms. The second reaction, the leaving group of the first reaction acts as a nucleophile to replace the $-{\text{O(H)PCl}}_{\text{2}}$ group from the product of the first reaction. Since the reaction center, in this case, is a chiral carbon, the reaction results in an inversion of the configuration.