Lecture note- 3
Organic Chemistry CHE 502
HETEROCYCLIC COMPOUNDS
Co-Coordinator – Dr. Shalini Singh
DEPARTMENT OF CHEMISTRY
UTTARAKHAND OPEN UNIVERSITY
, UNIT 4: HETEROCYCLIC COMPOUNDS- I
CONTENTS
4.1 Objectives
4.2 Introduction
4.3 Classification of heterocyclic compounds
4.4 Nomenclature of heterocyclic compounds
4.5 Molecular orbital picture
4.6 Structure and aromaticity of pyrrole, furan, thiophene and pyridine
4.7 Methods of synthesis properties and chemical reactions of Pyrrole, Furan, Thiophene and
Pyridine
4.8 Comparison of basicity of Pyridine, Piperidine and Pyrrole
4.9 Summary
4.10 Terminal Question
4.1 OBJECTIVES
In this unit learner will be able to
• Know about the most important simple heterocyclic ring systems containing heteroatom
and their systems of nomenclature and numbering.
• Understand and discuss the reactivity and stability of hetero aromatic compounds.
• Study the important synthetic routes and reactivity for five and six member hetero
aromatic compounds.
• Understand the important physical and chemical properties of five and six member hetero
aromatic compounds.
• Know about the applications of these hetero aromatic compounds in the synthesis of
important industrial and pharmaceutical compounds
,4.2 INTRODUCTION
Heterocyclic compound is the class of cyclic organic compounds those having at least one hetero
atom (i.e. atom other than carbon) in the cyclic ring system. The most common heteroatoms are
nitrogen (N), oxygen (O) and sulphur (S). Heterocyclic compounds are frequently abundant in
plants and animal products; and they are one of the important constituent of almost one half of
the natural organic compounds known. Alkaloids, natural dyes, drugs, proteins, enzymes etc. are
the some important class of natural heterocyclic compounds. Heterocyclic compounds can be
easily classified based on their electronic structure. Heterocyclic compounds are primarily
classified as saturated and unsaturated. The saturated heterocyclic compounds behave like the
acyclic derivatives with modified steric properties. Piperidine and tetrehydrofuran are the
conventional amines and ethers of this category. However, unsaturated heterocyclic compounds
of 5- and 6- member rings have been studied extensively because of their unstrained nature. The
unstrained unsaturated heterocyclic compounds include Pyridine, Thiophene, Pyrrole, Furan and
their benzo fused derivatives. Quinoline, Isoquinoline, Indole, Benzothiophene, and Benzofuran
are some important example of benzo fused heterocycles. Heterocyclic compounds have a wide
application in pharmaceuticals, agrochemicals and veterinary products. Many heterocyclic
compounds are very useful and essential for human life. Various compounds such as hormones,
alkaloids antibiotic, essential amino acids, hemoglobin, vitamins, dyestuffs and pigments have
heterocyclic structure.
In the present unit, students would be able to learn about the common five and six membered
heterocyclic compounds, such as Pyrrole, Furan, Thiophene, Pyridine and Piperidine etc.
4.3 CLASSIFICATION OF HETEROCYCLIC COMPOUNDS
Based on the structural and electronic arrangement the heterocyclic compounds may be classified
into two categories.
i. Aliphatic heterocyclic compounds
ii. Aromatic heterocyclic compounds
The aliphatic heterocyclic compounds are the cyclic amines, cyclic amides, cyclic ethers and
cyclic thioethers. Aliphatic heterocycles those do not contain double bonds are called saturated
, heterocycles. The properties of aliphatic heterocycles are mainly affected by the ring strain.
Examples of aliphatic heterocyclic compounds are shown in figure 1.
HN O S O NH S
Aziridine Ethylene oxide Thiirane Oxetane Azetidine Thietane
O NH
O
O O NH
O
Tetrahydrofuran 1,2-dioxane 1,4-dioxane Pyrrolidine Piperidine
(THF)
Figure 1. Examples of aliphatic heterocyclic compounds
However, aromatic heterocyclic compounds are analogous of benzene. The aromatic heterocyclic
compounds also follow the Huckel’s rule. According to Huckel’s rule an aromatic compounds
must be cyclic in nature with planar geometry due to conjugate double bonds and must have
(4n+2)π electrons. Examples of aromatic heterocyclic compounds are shown in figure 2.
O NH S O
N
N
H
Furan Pyrrole Thiophene Pyridine Indole Benzofuran
H
N N HN O NH
N
N N
N
Carbazole Quinoline Isoquinoline Imidazole Oxazole Pyrazole
N N N
N N
N
N N
H
Pyridazine Pyrimidine Purine
Figure 2. Examples of aromatic heterocyclic compounds
A hetero cyclic ring may comprise of three or more than three atoms, which may be saturated or
unsaturated. Also heterocyclic ring may contain more than one heteroatom which may be either
similar or different.
Organic Chemistry CHE 502
HETEROCYCLIC COMPOUNDS
Co-Coordinator – Dr. Shalini Singh
DEPARTMENT OF CHEMISTRY
UTTARAKHAND OPEN UNIVERSITY
, UNIT 4: HETEROCYCLIC COMPOUNDS- I
CONTENTS
4.1 Objectives
4.2 Introduction
4.3 Classification of heterocyclic compounds
4.4 Nomenclature of heterocyclic compounds
4.5 Molecular orbital picture
4.6 Structure and aromaticity of pyrrole, furan, thiophene and pyridine
4.7 Methods of synthesis properties and chemical reactions of Pyrrole, Furan, Thiophene and
Pyridine
4.8 Comparison of basicity of Pyridine, Piperidine and Pyrrole
4.9 Summary
4.10 Terminal Question
4.1 OBJECTIVES
In this unit learner will be able to
• Know about the most important simple heterocyclic ring systems containing heteroatom
and their systems of nomenclature and numbering.
• Understand and discuss the reactivity and stability of hetero aromatic compounds.
• Study the important synthetic routes and reactivity for five and six member hetero
aromatic compounds.
• Understand the important physical and chemical properties of five and six member hetero
aromatic compounds.
• Know about the applications of these hetero aromatic compounds in the synthesis of
important industrial and pharmaceutical compounds
,4.2 INTRODUCTION
Heterocyclic compound is the class of cyclic organic compounds those having at least one hetero
atom (i.e. atom other than carbon) in the cyclic ring system. The most common heteroatoms are
nitrogen (N), oxygen (O) and sulphur (S). Heterocyclic compounds are frequently abundant in
plants and animal products; and they are one of the important constituent of almost one half of
the natural organic compounds known. Alkaloids, natural dyes, drugs, proteins, enzymes etc. are
the some important class of natural heterocyclic compounds. Heterocyclic compounds can be
easily classified based on their electronic structure. Heterocyclic compounds are primarily
classified as saturated and unsaturated. The saturated heterocyclic compounds behave like the
acyclic derivatives with modified steric properties. Piperidine and tetrehydrofuran are the
conventional amines and ethers of this category. However, unsaturated heterocyclic compounds
of 5- and 6- member rings have been studied extensively because of their unstrained nature. The
unstrained unsaturated heterocyclic compounds include Pyridine, Thiophene, Pyrrole, Furan and
their benzo fused derivatives. Quinoline, Isoquinoline, Indole, Benzothiophene, and Benzofuran
are some important example of benzo fused heterocycles. Heterocyclic compounds have a wide
application in pharmaceuticals, agrochemicals and veterinary products. Many heterocyclic
compounds are very useful and essential for human life. Various compounds such as hormones,
alkaloids antibiotic, essential amino acids, hemoglobin, vitamins, dyestuffs and pigments have
heterocyclic structure.
In the present unit, students would be able to learn about the common five and six membered
heterocyclic compounds, such as Pyrrole, Furan, Thiophene, Pyridine and Piperidine etc.
4.3 CLASSIFICATION OF HETEROCYCLIC COMPOUNDS
Based on the structural and electronic arrangement the heterocyclic compounds may be classified
into two categories.
i. Aliphatic heterocyclic compounds
ii. Aromatic heterocyclic compounds
The aliphatic heterocyclic compounds are the cyclic amines, cyclic amides, cyclic ethers and
cyclic thioethers. Aliphatic heterocycles those do not contain double bonds are called saturated
, heterocycles. The properties of aliphatic heterocycles are mainly affected by the ring strain.
Examples of aliphatic heterocyclic compounds are shown in figure 1.
HN O S O NH S
Aziridine Ethylene oxide Thiirane Oxetane Azetidine Thietane
O NH
O
O O NH
O
Tetrahydrofuran 1,2-dioxane 1,4-dioxane Pyrrolidine Piperidine
(THF)
Figure 1. Examples of aliphatic heterocyclic compounds
However, aromatic heterocyclic compounds are analogous of benzene. The aromatic heterocyclic
compounds also follow the Huckel’s rule. According to Huckel’s rule an aromatic compounds
must be cyclic in nature with planar geometry due to conjugate double bonds and must have
(4n+2)π electrons. Examples of aromatic heterocyclic compounds are shown in figure 2.
O NH S O
N
N
H
Furan Pyrrole Thiophene Pyridine Indole Benzofuran
H
N N HN O NH
N
N N
N
Carbazole Quinoline Isoquinoline Imidazole Oxazole Pyrazole
N N N
N N
N
N N
H
Pyridazine Pyrimidine Purine
Figure 2. Examples of aromatic heterocyclic compounds
A hetero cyclic ring may comprise of three or more than three atoms, which may be saturated or
unsaturated. Also heterocyclic ring may contain more than one heteroatom which may be either
similar or different.
