9.3.1 Nucleophilic Substitution (S$_N$) Reactions of Alkyl Halides

Definition of Nucleophilic Substitution Reaction

A nucleophilic substitution reaction (or S${}_N$ reaction) is a reaction in which a halogen atom of an alkyl halide is substituted or replaced by a strong nucleophile.

The general representation of such reactions is:

$\text{R-X}+{\text{Nu}}^{-}\to \text{R-Nu}+{\text{X}}^{-}$

Components of a Nucleophilic Substitution Reaction

1. Substrate Molecule

The alkyl halide molecule (R-X) on which a nucleophile attacks is called the substrate molecule. The carbon atom directly bonded to the halogen is often referred to as the electrophilic carbon due to the polarity of the C-X bond.

2. Nucleophile

A nucleophile is a species having a lone pair of electrons that it can donate to an electrophilic carbon of an alkyl halide. Nucleophiles can be neutral (e.g., H${}_2$O, NH${}_3$) or negatively charged (e.g., HO${}^{-}$, Cl${}^{-}$).

Examples of Common Nucleophiles:

• Poor Leaving Groups: OH${}^{-}$, OR, and NH${}_2�BLOCK2�\text{tertiary}(3^{\circ })>\text{secondary}(2^{\circ })>\text{primary}(1^{\circ })>\text{methyl}�BLOCK3�R-X\underset{\text{Slow, Rate-determining}}{\overset{\text{Ionization}}{\rightleftharpoons }}{R}^{+}+X^{-}�BLOCK4�R^{+}+N{u}^{-}\stackrel{\text{Fast}}{\to }R-Nu�BLOCK5�\text{Rate}=k[R-X]�BLOCK6{�}^{-}$.