Difference Between Pyridine and Pyrimidine

The key difference between pyridine and pyrimidine is that pyridine structure resembles the structure of benzene with one methyl group replaced by a nitrogen atom, whereas pyrimidine structure, though it too resembles the structure of benzene, has two methyl groups replaced by nitrogen atoms.

Pyridine and pyrimidine are organic compounds. These are named as heterocyclic organic compounds because they are cyclic structures having two different types of atoms making the ring. These ring structures contain carbon and nitrogen atoms.

CONTENTS

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

What is Pyridine?

Pyridine is a heterocyclic organic compound having the chemical formula C₅H₅N. The structure of this compound resembles the structure of benzene, with one methyl group replaced by a nitrogen atom. Concerning the properties, pyridine is a weak alkaline and exists in the liquid state; it occurs as a viscous liquid. Furthermore, it is colorless and has a distinctive fish-like odor. Also, this liquid is water-soluble and is highly flammable

Moreover, pyridine is diamagnetic. The structure of the molecule is a hexagon. C-N bond is shorter than C-C bonds. When considering the crystallization of pyridine, it crystallizes in the orthorhombic crystal system. However, the pyridine molecule is an electron-deficient structure because of the presence of more electronegative nitrogen atom. Therefore, it tends to undergo electrophilic aromatic substitution reactions. Another reason for this ability is the presence of a lone electron pair on the nitrogen atom.

When considering the applications of pyridine, it is mainly useful as a component in pesticide, as a polar-basic solvent, as the Karl Fischer reagent in organic synthesis, etc.

What is Pyrimidine?

Pyrimidine is an aromatic heterocyclic compound having the chemical formula C₄H₄N₂. This compound has nitrogen atoms in 1 and 3 positions. It is a nitrogen base that includes the three major nitrogen bases of DNA: cytosine, thymine, and uracil. The molar mass of this compound is 80 g/mol.

In pyrimidine, the pi-electron density is low because of the presence of heteroatoms in the ring. Thus, it enables the compound to undergo nucleophilic aromatic substitution. Besides, the compound is basic due to the presence of a lone electron pair on the nitrogen atom.

What is the Difference Between Pyridine and Pyrimidine?

Both the pyridine and pyrimidine are heterocyclic organic compounds. However, the key difference between pyridine and pyrimidine is that the pyridine structure resembles the structure of benzene with one methyl group replaced by a nitrogen atom, but though the pyrimidine structure too resembles the structure of benzene, it has two methyl groups replaced by nitrogen atoms. Thus, the chemical formula of pyridine is C₅H₅N, while the chemical formula of pyrimidine is C₄H₄N₂. And, the molar mass of pyridine is 79 g/mol, while the molar mass of pyrimidine is 80 g/mol. Besides, the pyrimidine molecule is more pi-electron deficient than pyridine because the presence of two nitrogen atoms lowers the number of pi electrons present in the ring.

Moreover, pyrimidine is more basic than pyridine. Here, the basicity is determined by the lone electron pairs on nitrogen atoms in this molecule. Since pyrimidine has two nitrogen atoms, it is comparatively more basic.

Below infographic summarizes these differences between pyridine and pyrimidine.

Summary – Pyridine vs. Pyrimidine

Both the pyridine and pyrimidine are heterocyclic organic compounds, and their structures resemble the structure of benzene. However, the key difference between pyridine and pyrimidine is that pyridine has one methyl group replaced by a nitrogen atom in the benzene ring, whereas pyrimidine has two methyl groups replaced by nitrogen atoms.

Reference

1. “Pyridine.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 23 Nov. 2018, Available here.
2. “Pyridine.” Wikipedia, Wikimedia Foundation, 29 Nov. 2019, Available here.
3. “Pyrimidine.” Wikipedia, Wikimedia Foundation, 18 Nov. 2019, Available here.

Image Courtesy:

1. “Pyridine numbers” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia
2. “Pyrimidine 2D numbers” By Jynto (talk) – Own work. The source code of this SVG is valid. This structural formula was created with ChemDraw (Public Domain) via Commons Wikimedia