Pyridine

Pyridine is a basic heterocyclic organic compound. It has the chemical formula \(C_5H_5N\). Pyridine is structurally related to benzene, with one methine group i.e., =CH- is replaced by a nitrogen atom. Pyridine is highly flammable. It is a water-miscible liquid with an unpleasant fish-like smell and colourless. Historically, pyridine was made from coal tar. Today pyridine is produced on the scale of 20,000 tons per year worldwide.

The pyridine has three resonance structures. Pyridine as basic as it is more fundamental than pyrrole. In pyridine and pyrrole, lone pairs of electrons on N are different. Therefore, a \(H^+\) ion or a Lewis acid can be conveniently transferred to the lone pair of electrons on the N atom in pyridine.

Properties of Pyridine

Pyridine and its derivatives are stable and unreactive liquids. It has a strong penetrating odour that is unpleasant. Pyridine is the hydrogen derivative of this ring. This ring is benzene in which one CH- is replaced by a nitrogen atom. The structure of pyridine is analogous to benzene. Though relatable by replacement of CH by N.

• The molecular weight or molar mass of pyridine is 79.1 gram per mole. The density of pyridine is 982 kilogram per meter cube. Also, the boiling point of pyridine is \(115^oC\) and the melting point of pyridine is \(-41.6^oC\).

Physical Properties of Pyridine

• The molecular electric dipole moment is 2.2 debyes.
• Pyridine is diamagnetic. Pyridine has a diamagnetic susceptibility of \(-48.7 \times 10^{-6} cm^3 mol^{-1}\).
• The standard enthalpy of formation of pyridine is \(100.2 kJ mol^{-1}\) in the liquid phase and 140.4 kJ·mol⁻¹ in the gas phase. At \(25^oC\) pyridine has a viscosity of 0.88 mPa/s. At \(25^oC\) thermal conductivity of pyridine is \(0.166 W m{-1} K{-1}\).
• The enthalpy of vaporization of pyridine is \(35.09 kJ mol^{-1}\) at the boiling point and normal pressure. Also, the enthalpy of fusion of pyridine is \(8.28 kJ mol^{-1}\) at the melting point.
• Some critical parameters of pyridine are temperature 620 K, pressure 6.70 MPa, and volume \(229 cm^3 mol{-1}\). Pyridine ring forms a \(C_5N\) hexagon. The optical absorption spectrum of pyridine in hexane has three bands at the wavelengths of 195 nm, 251 nm and 270 nm.

Chemical Properties of Pyridine

• Due to the presence of the electronegative nitrogen in the pyridine ring, the molecule is relatively electron deficient. That’s why it enters less readily into electrophilic aromatic substitution reactions than benzene derivatives. Pyridine is more prone to nucleophilic substitution as it is evidenced by the ease of metalation by strong organometallic bases.
• With electrophiles, electrophilic substitution takes place where pyridine shows aromatic properties. With nucleophiles, pyridine reacts at 2 and 4 positions and behaves similar to imines and carbonyls. The reaction with Lewi’s acids results in the addition to the nitrogen atom of pyridine. It is similar to the reactivity of tertiary amines.
• The nitrogen centre of pyridine is a basic lone pair of electrons. Protonation of pyridine gives pyridinium ion i.e. \(C_5H_5NH^+\). The \(pK_a\) of the conjugate acid is 5.25. The structures of pyridine and pyridinium are almost the same. The pyridinium cation is isoelectronic to benzene. Pyridinium p-toluenesulfonate is an illustrative pyridinium salt. This salt is produced by treating pyridine with p-toluenesulfonic acid. Pyridine undergoes N-centred alkylation, acylation, and N-oxidation.

Uses of Pyridine \(C_5H_5N\)

The uses of Pyridine are:

• In the chemical industries as an important raw material.
• As an antiseptic in dental care products.
• As a solvent that is suitable for dehalogenation.
• In pharmaceuticals.
• For antifreeze mixtures as a denaturant.
• As a sulfonating agent.
• As a reducing agent.
• In dyes and paints.
• As a disinfectant.
• In coordination chemistry as a ligand.

FAQs on Pyridine

Question 1:  Is pyridine an aromatic compound?
Answer: Pyridine is a substance with no colour. Pyridine is related to the aromatic compound benzene. The nitrogen atom in pyridine creates a dipole in the molecule, this dipole moment makes it unlikely that an electrophilic reaction will occur. At the meta position, nucleophilic reactions also occur to ensure that a positive charge is not on the nitrogen.

Question 2: Is pyridine a good Nucleophile?
Answer: Pyridine is a natural carbonyl group nucleophile. It is often in use as a catalyst in acylation reactions. The nitrogen atom in pyridine is nucleophilic and that’s the reason why it is difficult to deallocate the lone pair of electrons on nitrogen across the ring.