Difference Between Quinoline and Isoquinoline

Thekey differencebetween quinoline and isoquinoline is thatin quinolone, thenitrogenatom is in the first position of the ring structure whereas, in isoquinoline, the nitrogen atom is in the second position of the ring structure.

Isoquinoline is a structuralisomerof quinolone. This means both these compounds have the same chemical formula but different connectivity of atoms. Specifically, the position of the nitrogen atom in the ring structure is different in two compounds.

CONTENTS

1.Overview and Key Difference
2.What is Quinoline
3.What is Isoquinoline
4.Side by Side Comparison – Quinoline vs Isoquinoline in Tabular Form
5.Summary

What is Quinoline?

Quinolone is an organic compound with a heterocyclic aromatic ring structure having the chemical formula C₉H₇N. It is a slightly yellowish, oily liquid. This liquid has a strong odour, and it is highlyhygroscopicin nature. Upon exposure to air, this liquid becomes more yellow and then turns brown.

When considering the chemical and physical properties of quinolone, it is slightly soluble in cold water but readily dissolves in hot water and most other organic solvents. E.g. alcohol, ether, carbon disulfide, etc.

Quinolone was first extracted from coal tar, and coal tar still remains the principal source of quinolone. We can synthesize quinolone using simple aniline compounds. There are a number of routes for this synthesis, including combes quinolone synthesis, Conrad-Limpach synthesis, Doebner reaction, etc.

There are many applications of quinolone: in the manufacturing of dyes, as a solvent for resins and terpenes, as a precursor to producing 8-hydroxyquinoline, production of quinolinic acid, production of anti-malarial derivatives, etc.

What is Isoquinoline?

Isoquinoline is an isomer of quinolone having the chemical formula C₉H₇N. It also occurs as an oily liquid which is highly hygroscopic. It has a strong and unpleasant odour. The liquid can turn in to a yellow or brown colour due to the presence of impurities. Upon crystallization, this compound crystallizes as platelets. These platelets have low water solubility, but they dissolve in organic solvents such as ether, ethanol, acetone, diethyl ether, carbon disulfide, etc. It is also soluble in some dilute acids.

First, isoquinoline was extracted from coal tar, via fractional distillation. Isoquinoline is more basic than its structural isomer quinolone. It is due to the difference in the position of the nitrogen atom in these structures. Due to this basic property, we can obtain isoquinoline from coal tar via fractional distillation.

There are many important applications of isoquinoline: as an anaesthetic compound, as antihypertension agents, as antifungal agents, as disinfectants, as vasodilators, etc.

What is the Difference Between Quinoline and Isoquinoline?

Quinolone and isoquinoline are isomers of each other. These structures form via the combination of a benzene ring and a pyridine ring. The key difference between quinoline and isoquinoline is that in quinolone, the nitrogen atom is in the first position of the ring structure, whereas in isoquinoline, the nitrogen atom is in the second position of the ring structure.

There are many applications of quinoline, such as in the manufacturing of dyes, as a solvent for resins and terpenes, as a precursor to producing 8-hydroxyquinoline, production of quinolinic acid, production of anti-malarial derivatives, etc. The applications of isoquinoline include using it as an anaesthetic compound, as antihypertension agents, as antifungal agents, as disinfectants, as vasodilators, etc.

Below infographic summarizes the difference between quinoline and isoquinoline.

Summary – Quinoline vs Isoquinoline

Quinolone and isoquinoline are isomers of each other. These structures form via the combination of a benzene ring and a pyridine ring. The key difference between quinoline and isoquinoline is that in quinolone, the nitrogen atom is in the first position of the ring structure, whereas in isoquinoline, the nitrogen atom is in the second position of the ring structure.

Reference:

1. “Isoquinoline.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine,Available here.
2.“异喹啉。”维基百科,维基媒体基金会, 29 Feb. 2020,Available here.

Image Courtesy:

1. “Quinoline chemical structure” By Vaccinationist – Own work(CC BY-SA 4.0)viaCommons Wikimedia
2.“Isoquinoline numbered” By Ring0 – Own work (Public Domain) viaCommons Wikimedia