Pyridine

Pyridine is a six-membered aromatic ring containing five carbon atoms and one nitrogen atom, with formula C₅H₅N. It is a basic, polar liquid used as a solvent, reagent, and building block.

Aromaticity: The unusual stability and electronic structure of cyclic, conjugated molecules with delocalized π electrons. Pyridine retains aromaticity because its nitrogen lone pair does not supply electrons to the ring.
Pyridine basicity: The tendency of pyridine to accept a proton at its ring nitrogen. Its moderate basicity distinguishes the nitrogen lone pair from the aromatic π electrons.
Pyridine as a solvent: The use of pyridine as a polar, basic solvent in laboratory and industrial chemistry. Its ability to dissolve many organic compounds and neutralize acids suits selected reactions.
Pyrrole: A five-membered aromatic heterocycle containing one nitrogen atom, with formula C₄H₅N. Unlike pyridine, pyrrole uses its nitrogen lone pair as part of the aromatic sextet.
Heterocyclic compound: A cyclic compound whose ring contains at least one atom other than carbon. Pyridine is a foundational example of a nitrogen-containing heterocycle.
Nucleophilic aromatic substitution: A substitution reaction in which a nucleophile replaces a leaving group on an electron-deficient aromatic ring. Pyridine’s electron-poor ring can undergo nucleophilic substitution, especially at the 2- and 4-positions.
Pyridine in acylation: The use of pyridine to promote or mediate reactions that transfer acyl groups. Pyridine can act as a base and, in some reactions, an acyl-transfer catalyst.
Piperidine: A saturated six-membered nitrogen heterocycle with formula C₅H₁₁N. Hydrogenating pyridine removes aromaticity and produces the more strongly basic piperidine ring.
Pyridine nitrogen: The ring nitrogen atom in pyridine, whose lone pair lies outside the aromatic π system. Its available lone pair makes pyridine basic without disrupting aromaticity.
Electrophilic aromatic substitution: A reaction in which an electrophile replaces a hydrogen on an aromatic ring. Pyridine’s electron deficiency makes this reaction harder than it is for benzene.

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