August 31, 2020
I’m useful in the lab if you don’t mind my smell.
What molecule am I?

Pyridine is a colorless liquid with a foul odor and several hazardous properties. In the late 1840s, physician/chemist Thomas Anderson at the University of Edinburgh produced several liquids by heating animal bones to high temperatures. One of these turned out to be pyridine, which he named after the Greek word pyr (fire).

About 20 years later, chemists Wilhelm (aka Guglielmo) Körner at the University of Milan (Italy) and James Dewar at the University of Cambridge (UK), working separately, elucidated the structure of pyridine. The two chemists were friends, but they disagreed about which of them was the first to conceive of the structure. An interesting account of this episode is given by Alan J. Rocke at Case Western Reserve University (Cleveland) in a 1988 article.

In 1881, German chemist Arthur Rudolf Hantzsch at the University of Leipzig (Germany) developed a cumbersome, low-yielding synthesis of pyridine. Later, in 1924, Russian chemist Aleksei Chichibabin came up with an improved method, which is still in use today: a reaction between formaldehyde, acetaldehyde, and ammonia over a transition-metal fluoride catalyst to give dihydropyridine, followed by high-temperature catalytic oxidation to pyridine. Several other methods are also used, including the oxidative dealkylation of alkylpyridines.

Pyridine’s structure is isoelectronic with that of benzene, but its properties are quite different. Pyridine is completely miscible with water, whereas benzene is only slightly soluble. Like all hydrocarbons, benzene is neutral (in the acid–base sense), but because of its nitrogen atom, pyridine is a weak base.

In industry and in the lab, pyridine is used as a reaction solvent, particularly when its basicity is useful, and as a starting material for synthesizing some herbicides, fungicides, and antiseptics. Current worldwide pyridine production is ≈20,000 t/year, valued at about US$600 million.

Pyridine hazard information

Hazard class* Hazard statement
Flammable liquids, category 2 H225—Highly flammable liquid and vapor Chemical Safety Warning
Acute toxicity, oral, category 4 H302—Harmful if swallowed  Chemical Safety Warning
Acute toxicity, dermal, category 4 H312—Harmful in contact with skin Chemical Safety Warning
Skin corrosion/irritation, category 2 H315—Causes skin irritation Chemical Safety Warning
Serious eye damage/eye irritation, category 2A H319—Causes serious eye irritation Chemical Safety Warning
Acute toxicity, inhalation, category 4 H332—Harmful if inhaled Chemical Safety Warning
Carcinogenicity, category 2 H351— Suspected of causing cancer Chemical Safety Warning
Hazardous to the aquatic environment, acute hazard, category 3 H402—Harmful to aquatic life  

 *Globally Harmonized System of Classification and Labeling of Chemicals.
Explanation of pictograms.

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Pyridine fast facts

CAS Reg. No. 110-86-1
Empirical formula C5H5N
Molar mass 79.10 g/mol
Appearance Colorless liquid
Boiling point 115 ºC
Water solubility  Miscible

MOTW update

Firefly luciferin was the Molecule of the Week for October 26, 2015. As its name implies, the molecule is essential for fireflies to emit light. The catalyst for the light-producing reaction is the enzyme luciferase, which may be useful in the quest for testing for COVID-19 infections and for developing drugs to combat the disease. Pei-Yong Shi and colleagues at the University of Texas Medical Branch at Galveston found that the bright light generated by luciferase allows them to visually detect antibodies against the SARS-CoV-2 virus several times faster than other methods. Likewise, the authors expect that luciferase will accelerate the development of therapies and vaccines against the virus. 

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