Pyridine – 6-membered Heteroaromatic Ring: Detailed Explanation

Q: Is pyridine a base?

It is a weak base with K b value of 2.3 × 10-9

Table of Contents

What is Pyridine?

Pyridine is one of the six-membered aromatic heterocyclic compounds with the chemical formula C₅H₅N. It has a similar structure to benzene, in which one methine group is substituted by a nitrogen atom. It is a colorless to yellow liquid that is extremely flammable, mildly alkaline, and water-soluble, with a strong, unpleasant fish-like odor.

Structure of Pyridine

Pyridine is characterized as an aromatic compound due to its characteristics. It is flat, with bond angles of 120^o, and the four carbon-carbon bonds are all the same length, as are the two carbon-nitrogen bonds. It has a resonance energy of 23 kcal/mol based on its heat of combustion. It can be described as a hybrid of Kekule structures I and II. Structure III will be applied to represent it, with the circle indicating the aromatic sextet.

Source of pyridine compounds

It is found in coal tar. Along with it are a number of methylpyridines, the most important of which are the monomethyl compounds known as picolines. Oxidation of picolines yields pyridine carboxylic acids.

Properties of Pyridine

Pyridine, also known as Azine, has the chemical formula C₅H₅N.
The nitrogen atom in pyridine gets protonated due to its electronegative nature.
It has a molecular weight of 79.1g/mol and a density of 982 kg/m².
The boiling point of pyridine is 115^oC and the melting point is -41.6^oC.

Basicity of Pyridine

Pyridine is a weak base, with K_b = 2.3 × 10^-9. The basic nature of it is because of the unshared pair of electrons in one of the s orbitals of nitrogen.
It is much stronger than pyrrole but much weaker than an aliphatic amine.
Two sp² orbitals of nitrogen overlap with the sp² orbitals of two carbons on either side. The unhybridized orbital forms part of the delocalized electron cloud. The third sp² orbital of nitrogen is unshared and is available for sharing with acids.

Reactions of Pyridine

The ring of azine undergoes a substitution reaction, both electrophilic and nucleophilic.

Nucleophilic Substitution in Pyridine

Due to the decrease in electron density of ring carbon atoms, they becomes susceptible to nucleophilic attack. Since positions 2- and – are very much electron deficient than position-3, nucleophilic substitution occurs readily at positions 2- and 4-, the 2-position being more preferred.

Attack at position 2 furnishes a carbanion which is a resonance hybrid of structures I, II, and III as shown above.

The most important example of a nucleophilic substitution reaction is the Chichibabin reaction.

Electrophilic Substitution in Pyridine

It resembles a highly deactivated benzene derivative in terms of electrophilic substitution. It only undergoes nitration, sulfonation, and halogenation at extremely high temperatures, and it does not undergo the Friedel-Crafts reaction at all.

Substitution occurs mainly at the 3- (or β-) position. For example;