Chapter 16, Problem 16.86P

Interpretation Introduction

(a)

Interpretation:

Why the given ${\text{S}}_{\text{N}}\text{2}$ reaction would require isotopically labeled compounds it to be explained.

Concept introduction:

An ${\text{S}}_{\text{N}}\text{2}$ reaction involves substitution of a leaving group by another nucleophile in a single step mechanism. The incoming nucleophile starts forming a bond to the electrophilic carbon while the bond to the leaving group is still not completely broken. There is no intermediate involved in this reaction as both reactants are involved in only the elementary step.

Interpretation Introduction

(b)

Interpretation:

How mass spectrometry will help in answering the question from part (a) is to be explained.

Concept introduction:

If the nucleophile and the leaving group are the same, using an isotopically labeled reactant helps in establishing if the reaction actually occurs. Use of isotopically labeled reactants (say the nucleophile) will lead to a small but measurable difference in the molecular mass of the reactant and the product. This difference can be determined using mass spectrometry as it is capable of detecting molecular mass differences of 1 amu.