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Pyridine

C5H5N organic
Molar Mass
79.101 g/mol
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Properties

StateLiquid
ColorColorless
SolubilityMiscible with water and most organic solvents
Melting Point-41.6°C
Boiling Point115.2°C

About Pyridine

Pyridine (C5H5N, 79.101 g/mol) is the six-membered aromatic heterocycle obtained by replacing one CH of benzene with N. The molecule keeps benzene's 6π aromaticity but the nitrogen lone pair sits in an sp² orbital in the plane of the ring, perpendicular to the π system — that geometric arrangement is what makes pyridine simultaneously a Brønsted base (pKa of pyridinium = 5.25), a Lewis base for metal coordination, a nucleophilic acyl-transfer catalyst, and yet still aromatic and reluctant to undergo electrophilic substitution. The σ-withdrawing nitrogen deactivates the ring relative to benzene by roughly six orders of magnitude, and when SEAr does occur it goes meta (the 3-position) to put positive charge anywhere except adjacent to the already electron-poor nitrogen. Pyridine is distilled industrially from coal-tar light oil at about 1-2% concentration and synthesized at scale from acetaldehyde plus formaldehyde plus ammonia (the Chichibabin synthesis), feeding a global market around 25,000 tonnes annually. The smell is unmistakable — fish-and-old-burnt-coffee at parts-per-million levels — and a few minutes of distillation in an unhooded fume hood will saturate a building.

Where you'll encounter it

If you've ever run an acylation in an organic synthesis lab — protecting an alcohol as an acetate or making an amide from an acid chloride — pyridine was the base on the bench, scavenging the HCl byproduct and activating the acyl chloride through the in-situ-generated acyl pyridinium intermediate. In a process chemistry plant making vitamin B3 (niacin), pyridine isn't the product — it's the scaffold that gets oxidized at the 3-methyl position to install the carboxylic acid that becomes nicotinic acid, the human-nutrition vitamer used to fortify flour worldwide. In an agrochemical formulation lab, pyridine is the parent ring of paraquat and diquat herbicides, of the pyridyl-substituted neonicotinoid insecticides imidacloprid and acetamiprid, and of glyphosate's amino-methyl-phosphonate cousin. And in a coordination chemistry teaching lab, pyridine is the canonical neutral N-donor ligand that fills out the coordination sphere of [Cu(py)4]²⁺ or trans-[PtCl2(py)2] in undergraduate inorganic course problems.

Common Uses

  • Acid-scavenging base and nucleophilic catalyst for acylation, sulfonylation, and silylation reactions
  • Synthetic precursor to nicotinic acid (vitamin B3) via vapor-phase oxidation of 3-methylpyridine
  • Parent ring for paraquat and diquat bipyridyl herbicides and for neonicotinoid insecticides
  • N-donor ligand in coordination chemistry teaching examples and in catalytic asymmetric synthesis
  • Solvent for Karl Fischer water determination titrations and for cellulose modification chemistry
  • Denaturant added to industrial ethanol at 0.5% to make it undrinkable for tax-exempt applications

Safety Information

GHS classification: Flammable liquid Category 2 (H225, flash point 20 °C closed cup), Acute toxicity oral, dermal, and inhalation Category 4 (H302, H312, H332), Skin irritation Category 2 (H315), Eye irritation Category 2 (H319). Occupational exposure limits are low, and the advisory value is several-fold stricter than the enforceable one. The fish-coffee odor has a detection threshold around 0.04 ppm — well below the irritation level — which provides a useful early warning, but olfactory fatigue sets in within minutes so smell is unreliable as long-term protection. Chronic exposure causes liver and kidney damage. Pyridine crosses skin readily, so nitrile (not latex) gloves and full sleeve coverage are mandatory. Vapor density 2.7 — pools in low spots. Distillations and large-scale acylations belong in a fume hood with the sash low and the building HVAC able to handle the breakthrough.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.

Constituent Elements

C — Carbon H — Hydrogen N — Nitrogen

Frequently Asked Questions

What is the molar mass of pyridine?
79.101 g/mol, from 5 × 12.011 (C) + 5 × 1.008 (H) + 14.007 (N). At density 0.982 g/mL, one molar equivalent for a 100 mmol acylation is about 8.1 mL of pyridine — useful arithmetic to remember when you're scaling up from a literature procedure that specifies pyridine in moles instead of volume. The pyridinium chloride salt (pyHCl) is 115.56 g/mol if you need to weigh out the protonated form for buffer preparation.
How does pyridine differ from benzene?
Replacing one CH with N drops the ring's electron density measurably — pyridine's first ionization energy is 9.3 eV versus benzene's 9.2 eV, but the electrostatic potential map shows positive character building up at the carbons ortho and para to nitrogen. That makes pyridine about a million times less reactive toward electrophilic aromatic substitution than benzene, and what little SEAr does occur goes to the 3-position because attack at 2- or 4- positions would put positive charge directly adjacent to the electronegative nitrogen. Conversely, the in-plane nitrogen lone pair makes pyridine a useful Brønsted base (pyridinium pKa 5.25), a Lewis base for metals, and a nucleophilic catalyst for acyl transfer — none of which benzene can do.
Where is the pyridine ring found in biology?
The pyridine ring is the structural core of vitamin B3 (niacin/nicotinic acid), and its amide form nicotinamide is built into the coenzymes NAD⁺ and NADH that ferry hydride between essentially every redox enzyme in metabolism. Vitamin B6 (pyridoxine, pyridoxal, and pyridoxamine) is a hydroxymethyl-substituted pyridine that becomes pyridoxal phosphate, the cofactor for amino-acid transaminases and decarboxylases. Pyridoxal phosphate–dependent enzymes catalyze around 4% of all classified enzyme reactions, which is why a vitamin B6 deficiency manifests across so many metabolic pathways. Beyond cofactors, the pyridine alkaloids include nicotine, anabasine, and the pyridyl-substituted core of natural products like piericidin.