What is the Difference Between Quinoline and Isoquinoline

Quinoline and isoquinoline are two types of heterocyclic aromatic compounds. Heterocyclic aromatics are molecules with rings containing carbon, nitrogen, oxygen, or sulphur. These rings are flat and stable due to delocalized electrons. 

What is the difference between quinoline and isoquinoline? In quinoline, the nitrogen atom is located at the first position of the fused ring system, while in isoquinoline, the nitrogen atom is located at the second position of the fused ring system.

Key Areas Covered

1. What is Quinoline 
      – Definition, Features 
2. What is Isoquinoline
      – Definition, Features 
3. Similarities Between Quinoline and Isoquinoline
      – Outline of Common Features
4. Difference Between Quinoline and Isoquinoline
      – Comparison of Key Differences
5. FAQ: Quinoline and Isoquinoline
      – Answers to Frequently Asked Questions

Key Terms

Quinoline, Isoquinoline, Heterocyclic 

What is Quinoline

Quinoline is a heterocyclic aromatic compound having a double ring structure with both carbon and nitrogen atoms. Quinoline’s chemical formula is C₉H₇N. Quinoline has a strong odour and is a colourless liquid which turns yellow brown with time. It’s slightly soluble in cold water but readily dissolves in hot water and most organic solvents.

The ring of quinoline is very stable. There are over 200 naturally occurring quinoline alkaloids. Quinoline is also involved in manufacturing various chemicals, including nicotinic acid (vitamin B3) and hydroxyquinoline sulphate (an antiseptic).

In the past, quinoline was produced from coal tar, but now it is made synthetically. Quinoline acts as the precursor for molecules such as dyes, herbicides, and antimicrobials. It is also used in medicine. Quinoline has uses as a solvent and reagent, corrosion inhibitor, and rubber industry.

What are Isoquinoline

Like quinoline, isoquinoline is a heterocyclic aromatic compound. In isoquinoline, the nitrogen atom occupies the second position, compared to the first position in quinoline.  There are over 2500 known isoquinoline alkaloids found in various plants, particularly those belonging to the poppy, barberry, and buttercup families. It is used in pain relief and stimulants and has anticancer properties. It also acts as a precursor in the synthesis of certain pharmaceuticals and agrochemicals.

Similarities Between Quinoline and Isoquinoline

1. Both are heterocyclic aromatic compounds.
2. They serve as foundational structures for a vast array of more complex molecules.
3. Both quinoline and isoquinoline display similar aromatic characteristics.
4. While not highly soluble in cold water, they dissolve readily in hot water and most organic solvents.

Difference Between Quinoline and Isoquinoline

Definition

• Quinoline is a heterocyclic aromatic compound with a fused structure consisting of a benzene ring and a pyridine ring in which the nitrogen atom is at position 1. On the other hand, isoquinoline is a heterocyclic aromatic compound with a fused structure consisting of a benzene ring and a pyridine ring in which the nitrogen atom is at position 2.

Found in

• Quinoline alkaloids are less common in nature compared to isoquinoline alkaloids.

Derived from

• Quinoline alkaloids are derived from the amino acid tryptophan. Isoquinoline alkaloids are derived from the amino acid tyrosine.

Structure

• In quinoline, the nitrogen atom is located at the first position of the fused ring system, while in isoquinoline, the nitrogen atom is located at the second position of the fused ring system.

Boiling Point

• Quinoline has a higher boiling point (around 268 °C) compared to isoquinoline.

Conclusion

Both quinoline and isoquinoline are heterocyclic aromatics. The main difference between quinoline and isoquinoline is that in quinoline, the nitrogen atom is located at the first position of the fused ring system, while in isoquinoline, the nitrogen atom is located at the second position of the fused ring system. These two compounds are involved in different types of reactions and have different uses.

FAQ: Quinoline and Isoquinoline

1. Which is more basic quinoline or isoquinoline?

Quinoline and isoquinoline are both equally basic. Isoquinoline might have a slight edge due to the position of its nitrogen atom, but the difference is minor.

2. Is quinoline a strong or weak base?

Quinoline is a weak base. While it has a lone pair on its nitrogen willing to accept a proton, surrounding atoms and steric effects weaken its basicity compared to simpler amines.

3. How does KMnO4 react with quinoline and isoquinoline?

Potassium permanganate (KMnO4) acts as a strong oxidizing agent, breaking down the aromatic rings of quinoline and isoquinoline. This results in a mixture of smaller molecules like pyridine tricarboxylic acid.

4. What is the reduction of quinoline?

Reduction targets the double bonds in the quinoline ring. This process uses catalysts and reducing agents (e.g., hydrogen, hydrosilanes). The result is a tetrahydroquinoline, where the aromatic ring becomes saturated with hydrogen.

5. Can isoquinoline be reduced?

Yes. Isoquinoline reduction targets its double bonds, just like quinoline. Catalysts and reducing agents (hydrogen, hydrosilanes) are used again. The outcome is a tetrahydroisoquinoline, where the aromatic ring gains hydrogen saturation.

Reference:

1. “Quinoline: A versatile heterocyclic.” Science Direct.
2. “Isoquinoline – An Overview.” Science Direct.

Image Courtesy:

1. “Quinoline chemical structure” By Vaccinationist – Own work (CC BY-SA 4.0) via Commons Wikimedia