Organic Chemistry

21.10 Aromatic Six-Membered-Ring Heterocycles

Pyridine

When one of the carbons of a benzene ring is replaced by a nitrogen, the resulting

compound is called pyridine.

The pyridinium ion is a stronger acid than a typical ammonium ion because the

acidic hydrogen of a pyridinium ion is attached to an hybridized nitrogen, which is

more electronegative than an hybridized nitrogen (Section 6.9).

Pyridine is a tertiary amine, so it undergoes reactions characteristic of tertiary amines.

For example, pyridine undergoes reactions with alkyl halides (Section 10.4), and it

reacts with hydrogen peroxide to form an N-oxide (Section 21.7).

PROBLEM 17 SOLVED

Will an amide be formed from the reaction of an acyl chloride with an aqueous solution of

pyridine? Explain your answer.

N +

+ CH 3 I

N

CH 3

I−

N +

+ HO OH

N

OH + HO−

+N

O−

+ H 2 O

pKa = 0.79

N-methylpyridinium iodide

pyridine-N-oxide

SN 2

+ H+

+ N

N

H H

+ H+

+ NH

N

H

pyridinium ion

pyridine

piperidinium ion piperidine

pKa = 5.16

pKa = 11.12

sp^2

sp^3

sp^3

sp^2

N

N

pyridine orbital structure of pyridine

these

electrons are

in an sp^2 orbital

perpendicular

(^4) to the p orbitals
3
1
6 2
5
902 CHAPTER 21 More About Amines • Heterocyclic Compounds
Tutorial:
Lone-pair electrons on
nitrogen heterocycles
3-D Molecule:
Pyridine