Chapter 9.3, Problem 9.12P

Interpretation Introduction

Interpretation:

The given nucleophile favor ${\text{S}}_{\text{N}}\text{2}$ or ${\text{S}}_{\text{N}}\text{1}$ has to be identified.

Concept Introduction:

The strength of the nucleophile also determines through which reaction the given compound undergoes.  The ${\text{S}}_{\text{N}}\text{2}$ process depends on the strength of nucleophile as the rate depends on it.  Generally, a strong nucleophile will speed up the rate at which the ${\text{S}}_{\text{N}}\text{2}$ reaction occurs, and a weak nucleophile slows the rate of ${\text{S}}_{\text{N}}\text{2}$ reaction.

The rate of ${\text{S}}_{\text{N}}\text{1}$ reaction does not depend upon the nucleophile.  This is because, the rate of ${\text{S}}_{\text{N}}\text{1}$ reaction is purely dependent on the concentration of substrate and not concentration of nucleophile.  Hence, for ${\text{S}}_{\text{N}}\text{1}$ reaction, the strength of nucleophile is an irrelevant one.

In simple words it can be said that,

• Strong nucleophile favors ${\text{S}}_{\text{N}}\text{2}$ reaction.
• Weak nucleophile does not favor ${\text{S}}_{\text{N}}\text{2}$ reaction.

The first important thing that has to be identified was whether the given nucleophile is a strong or weak one.  The strength of the nucleophile depends on many factors and they are charge, polarizability etc.

Presence or absence of a negative charge determines the strength of nucleophile.  If a negative charge is present means, the nucleophile will be strong.  More than this polarizability is an important factor that determines the strength of nucleophile.  The size of the atom is directly related to polarizability.  If the size of the atom is more means, then polarizability will be more resulting in strong nucleophile and vice-versa.

Some of the strong nucleophiles are,

    

Some of the weak nucleophiles are,