Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 112

As shown in Table 7.2 , there are many different possible nucleophiles. There-
fore, there are many different products from nucleophilic substitution.

Table 7.2 Selected Alkyl Halide R-X Nucleophilic Substitutions
Nucleophile

Product Class

Hydroxide

HOR

Alcohol HO

Alkoxide

R 1 OR

Ether

R 1 O

R 1 SR

Thioalkoxide Thioether R 1 S

Cyanide

N CR

Nitrile

N C

Alkynide

R 1 CC R

Alkynes

R 1 CC

R 1 CO 2 R

Carboxylate R 1 CO 2 Ester

H 3 N

Ammonia Ammonium halide

RNH 3 X

FIGURE 7.15
General nucleophilic substitution.

A nucleophilic substitution follows two main mechanisms, depending on the
substrate, nucleophile, and solvent. As Figure 7.13 shows, these are labeled as
S N 1 and S N 2 to show which pathway has been followed.
The difference between these mechanisms is the role of the nucleophile. In an
S N 2 process, the reaction is started by the attack of the nucleophile at the polar-
ized electrophilic carbon. This concerted process goes through a transition state
in which partial bond forming and breaking occurs. Then, this five-centered
state breaks down to give the final substituted product.

The rate of reaction depends on the concentration of both substrate and nucleophile.
In other words, it shows a bimolecular dependence on two different species. There-
fore, this mechanism is described as S N 2 (substitution nucleophilic bimolecular).
The alternative S N 1 process is stepwise. The first step is the heterolysis to give an
intermediate carbocation. This carbocation intermediate reacts with the nucleo-
phile to complete the substitution. The reaction rate depends only on the con-
centration of the substrate and not on the nucleophile. In other words, it shows
a unimolecular dependence on a single species. Therefore, this mechanism is
described as S N 1 (substitution nucleophilic unimolecular).