Quinoline

Quinoline is a heterocyclic organic compound. The chemical name of quinoline is alpha, beta benzopyridine. The molecular formula for quinoline is C9H7N. In the quinoline molecular formula, then C represents the carbon, H represents the hydrogen and the numbers written in subscript represent the number of respective elements. Quinoline is found in coal tar and bore oil.

Structure of Quinoline

Quinoline consists of a benzene ring fused to the alpha-beta-position of the pyridine ring. In the quinoline structure, there are five double bonds present and eleven single bonds are present. The single bonds are sigma bonds formed by the head-on overlapping. The double bond consists of one sigma bond and one pi bond. The pi bonds are formed by the lateral overlapping of the p orbitals.

Preparation Methods of Quinoline

1. Skraup Synthesis Method: 

This method is the most widely used method for the preparation of quinoline. In this method, aniline and glycerol are heated at a high temperature in the presence of sulphuric acid and mild oxidising agents like nitrobenzene or the presence of peroxides like arsenic peroxide. Ferrous sulphate (FeSO4) or boric acid (H3BO3) is generally added to make the reaction less violent because skraup synthesis is a highly exothermic reaction. 

Mechanism of Skraup Synthesis Method:

Step 1: In this step, glycerol undergoes a dehydration reaction in the presence of sulphuric acid to give acrolein. 

Step 2: In this step, the above-formed acrolein reacts with the aniline and forms an intermediate as a product of this reaction. 

Step 3: In this method the cyclization of the intermediate takes place. This cyclization process occurs in the presence of concentrated sulphuric acid to form 1, 2-dihydro quinoline.

Step 4: In this step, the oxidation of 1,2-dihydroquinoline takes place on the reaction with nitrobenzene. On reacting with nitrobenzene, 1,2-dihydroquinoline forms the quinoline as a product.

2. Friedlander Synthesis- 

In this method, ortho- amino benzaldehyde is condensed with acetaldehyde in the presence of sodium hydroxide (NaOH) solution by the cyclisation process. 

3. Doebner- 

Miller Synthesis- Primary aryl amines with free ortho positions can react with the alpha, beta-unsaturated carbonyl compound in the presence of acid to yield quinolines.

4. Knorr Quinoline Synthesis- 

In this reaction, the condensation of aniline takes place with the beta- ketoester at a high temperature to give an anilide intermediate. This formed anilide intermediate undergoes the cyclisation process. The concentrated sulphuric acid is added to the reaction for the dehydration process. The dehydration phenomena lead to the production of 2-hydroxyquinolines.

Properties of Quinoline

Physical Properties of Quinoline

• Quinoline is a colourless liquid chemical.

• The boiling point of the quinoline is 237 degrees celsius.

• Quinoline is composed of a large number of hydrophobic carbon chains that makes it sparingly soluble in water.

• Quinoline is soluble in organic solvents. 

• The nature of quinoline is basic due to the presence of the pyridine ring. In the pyridine ring, the lone pair is present on the nitrogen atom. Therefore, it can donate a lone pair of electrons, hence, basic.

Chemical Properties of Quinoline

Quinoline gives an electrophilic substitution reaction due to the presence of resonance phenomena in the molecule. 

• Quinoline gives a nitration reaction in the presence of concentrated nitric acid and concentrated sulphuric acid.

• Quinoline gives a bromination reaction in the presence of bromine and silver sulphate. 

• Quinoline compound reacts with the potassium permanganate and undergoes the oxidation reaction. In the oxidation process quinoline gets converted into nicotinic acid.

• Quinoline undergoes a reduction reaction in the presence of tin and hydrochloric acid and gets converted into decahydroquinoline.

Two Important derivatives of Quinoline

1. Quinoline yellow: Quinoline yellow is a quinoline derivative. It's used to add colour to stuff. Quinoline yellow is a beta-diketone and aromatic ketone that belongs to the quinoline family. Quinoline has the molecular formula C18H11NO2. Quinoline's chemical name is quinophthalone. Quinoline has a molecular weight of 273.3 g/mol. 

2. Amino Quinoline: Amino quinoline is a derivative of quinoline. The amino group takes the place of the hydrogen in the eighth spot. As a consequence, it's also known as quinoline's eight amino derivative. Amino quinoline has a structure that is identical to 8-hydroxyquinoline. 

SAR of Quinolines

SAR of quinolines stands for the similar structure-activity relationship of quinolines. The study of quinolines is necessary for the comparison of the antimalarial activity. The asymmetry at the third carbon and fourth carbon is not necessary for the antimalarial activity. The substitution of the halogen group at eight carbon positions will increase the antimalarial activity of the drug. The presence of a methoxy group is not necessary for the antimalarial activity.

Quinoline Uses

• Quinoline is an aromatic compound, used mainly for the production of intermediate compounds in the manufacture of other products.

• It is used in agriculture for the raising and farming of animals and growing of crops.

• It is used as an agrochemical.

• Its materials are used in the building process, such as flooring, insulation, caulk, tile, wood, glass, etc.

• It is used as chemicals in cigarettes, or tobacco-related products, or related to the manufacturing of tobacco products.

Did You Know?

• Quinoline is also known as quinoline yellow lake, due to its yellow colour. It is an aluminium salt of quinoline. 

• The alkaloids of quinoline can be derived from rutaceous plants.

• Quinoline is slightly basic.

Conclusion

This is all about Quinoline, its structure, properties, and uses. Learn how this chemical compound is synthesised and used in different processes. Focus on how the synthesis processes differ from each other by considering the steps of the reactions.