Organic chemistry topic heterocyclic compounds

1. Heterocyclic
Compounds
Lecture 18

2. Homocyclic Compunds
• Homocyclic compounds are molecules that are, or contain, ring structures that consist only
of carbon atoms within the ring. An example is benzene
• Benzene is a homocyclic compound of six carbon atoms bonded together in a hexagonal
ring, with one hydrogen atom bonded to each of the six carbons

3. Heterocyclic Compounds
• Heterocyclic compounds are rings containing at least one non-carbon atom in the
ring.
• An example is heterocyclic amines which are six-membered rings of five carbons
and one nitrogen atom
• Heterocyclic compounds have an immense range of properties and uses and even
constitute their own branch of chemistry (heterocyclic chemistry).

4. Classification
• Non-Aromatic :
• Examples are Pyrrolidine and Piperidine
• Aromatic:
• These heterocyclic compounds have aromatic ring
• Example is pyridine

5. • Non-aromatic heterocyclic compounds have same chemical and physical
properties essentially because of the particular hetero atom
• Aromatic heterocyclic compounds differ in physical and chemical properties.
• Due to this reason, the focus is on the aromatic heterocyclic compounds

6. Classification of Aromatic Heterocyclic
Compounds on the basis of Electronic
Structure
• Five membered rings
• Six membered rings

7. Polycyclic Aromatic Heterocyclic Compounds:
• Heterocyclic aromatic compounds can be polycyclic as well. A benzene ring
and a pyridine ring, for example, can share a common side in two different
ways
• One way gives a compound called quinoline; the other gives isoquinoline

8. Nomenclature

9. Importance of Heterocyclic Compounds
• Used as solvents
• Found in the biomolecules such as RNA and DNA
• Used as drugs for the treatment of different types of diseases; For instance,
Quinolones as antimalarial drug; Lactams as antibiotics
• Rings in which the heteroatom is oxygen, nitrogen, or sulfur rank as both the
most common and the most important

10. Medicinally important heterocyclic compounds
• Pyrrole
• Furan
• Thiophene
• Pyridine
• Pyrimidine
• Pyrazine

11. Huckle’s rule (4n+2)
• Hückel’s rule can be extended to heterocyclic aromatic compounds
• Unshared electron pairs of the heteroatom may be used as electrons as
necessary to satisfy the 4n+2 rule
• In pyrrole, the unshared pair belonging to nitrogen must be added to the
four electrons of the two double bonds in order to meet the six--electron
requirement. The nitrogen of pyrrole is sp2 -hybridized and the pair of
electrons occupies a p orbital where both electrons can participate in the
aromatic system

12. Pyrrole
• Very Important five-membered nitrogen heterocycle
• Colorless, Highly refractive liquid that can turn dark on exposure to air
• Present in many naturally occurring compounds such as bone oil, coal-tar
• It is basic unit of porphyrin system that is present in hemoglobin and
chlorophyll

13. Furan
• Furan is a colourless mobile liquid
• It has an odour of chloroform. It is insoluble in water but is very soluble in
alcohol and ether. It produces an intense green colour on pine shavings
moistened with hydrochloric acid
• Furan is of little use but some of its derivatives (furfural) are of commercial
importance

14. THIOPHENE
• Thiophene is a colorless mobile liquid, having benzene like odor
• It is insoluble in water but dissolves in organic solvents
• It is comparatively stable to oxidation
• Thiophene also has alternate double and single bond structure and the
aromatic character in it is very pronounced as compared to furan and pyrrole