Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 133

PROGRAM 19 Nucleophilic Substitution
A In the Programs which follow, curly arrows are used to show bond breaking and
making. These arrows will help you follow electron movements and keep a count of
electrons. The arrows also show details of the reaction mechanism.

Study the conversion of an alkyl halide into an alcohol.
H H

Reagent

H CCl H C OH

H H

Product

Substrate

If you compare the product with the substrate, you see that the chlorine ligand is
replaced by a hydroxyl ligand. The process must be a substitution. However, no
information is given about how the reaction occurred.
Use your knowledge of bond polarity (inductive effect) to draw a reasonable
mechanism for the substitution process.

B The C-Cl bond is the most polar bond in the substrate. This means the electro-
philic carbon can be attacked by a nucleophile. By studying the alcohol product, we
can see that hydroxide is the likely reagent (see Program 15E).

OH + Cl CH 3

HO CH 3 Cl

You have just described a nucleophilic substitution. The reaction is classified as
nucleophilic substitution because a nucleophile replaces the chloride leaving group.
Proper choice of the nucleophile can give many different functional classes of product.

All of these substitutions are the simple exchange of single bonded ligands, and there
is no change in the bond order between substrate and product.
Can you suggest some examples of substitution reactions?

C You may have included nucleophiles such as:

CN

Nitriles

RCH 2 CN

HNR' 2

RCH 2 NR' 2 Amines

RCH 2 Cl

OR'

RCH 2 OR' Ethers

Alkyl halide

CCR'

RCH 2 CCR'

Extended alkynes

There are two ways in which the above reactions might occur. The difference depends
on the timing of the departure of the leaving group and the arrival of the nucleophile.