Pyridine: Structure, Formula & Physical Properties

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about the aromatic compound pyridine. We will explore how its structure makes it different from other aromatic compounds, such as benzene.

Pyridine and Niacin

Niacin is a necessary vitamin. Adequate amounts have been shown to reduce cholesterol and triglycerides, while too little niacin can lead to a disease called pellagra. Pellagra causes skin lesions, swelling, hair loss, weakness, and confusion. Eventually it leads to an enlarged, weakened heart and, within a few years, death. So niacin is essential for our survival.

Pellagra causes skin lesions such as those seen on this man.
Man with Pellagra

The structure of niacin contains a pyridine ring with a carboxylic acid at the meta position. This pyridine structure can be found in many different compounds, including pesticides and medications.

Niacin contains the structure of pyridine, which is the main ring structure with a nitrogen contained in it.
Niacin structure

Pyridine Formula and Structure

The chemical formula for pyridine is: C5H5N. The only way we can have five carbons, a nitrogen, and only five hydrogens is if the carbons and nitrogen form a ring with alternating double bonds. This is exactly what pyridine does.

Pyridine Structure

Pyridine is an aromatic compound with an amine. An aromatic compound is one where all of the pi electrons (electrons that are part of the second lobe being shared in a covalent bond) are shared in a ring. Although there are alternating double bonds, it really could be drawn with one continuous circle of electrons because the double bonds are shared with every atom on the circle.

The double bonds in pyridine are actually shared between all of the atoms in the circle, so they can be drawn as a circle instead of individual bonds.
Pyridine

Aromatic compounds are very stable. Typically, they can only undergo reactions if the end result keeps the aromaticity of the ring. When scientists first encountered aromatic compounds they were baffled by their stability and structure. It took many years to figure them out.

Pyridine differs from benzene, another common aromatic compound, in that one of its carbon atoms is replaced with a nitrogen. The nitrogen creates a tertiary amine which is able to undergo reactions such as alkylation and oxidation. This amine causes a slight dipole on the ring, so it isn't quite as stable as other aromatic compounds such as benzene. This dipole is the effect of more electrons being drawn towards the nitrogen instead of being shared equally with all the atoms. This is due to the fact that nitrogen is more electronegative (electron-loving) than carbon.

Pyridine shares the electrons in the entire circle, but the nitrogen draws more electrons to it, causing a slight dipole to form.
Pyridine structure colored

Pyridine Physical Properties

Pyridine is a colorless liquid. It boils at 115 degrees Celsius (239 F) and freezes at -42 degrees Celsius (-43.6 F). Its density is very close to that of water, at 0.982 g/cm3 (water's density is 1). It is able to mix easily with water and most other organic solvents.

Since the nitrogen on the ring draws electrons to it, there are three different chemical shifts (a way of measuring differences in atoms) on the hydrogens. These chemical shifts shows us that the electron density is different for each of these atoms.

Due to this characteristic, the H Nuclear Magnetic Radiation (H-NMR, a way to measure the chemical shifts) contains three signals. These signals occur at the ortho, meta, and para positions on the molecule. The two ortho positions (the ones right next to the nitrogen) have a chemical shift of 8.6. The two meta positions (the two which are two away from the nitrogen) have a chemical shift of 7.25, and the para position (the one straight across from the nitrogen) has a chemical shift of 7.64.

They hydrogen atoms on pyridine have three distinct chemical shifts.
Pyridine Chemical shift

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