Heterocyclic compound

A cyclic compound whose ring contains at least one atom other than carbon. Its ring atoms commonly include nitrogen, oxygen, or sulfur.

Cyclic compound: A compound whose atoms form at least one closed chain or ring. Heterocycles are cyclic compounds distinguished by having a noncarbon ring atom.
Pyridine: A six-membered aromatic ring containing one nitrogen atom. Its nitrogen lone pair lies outside the aromatic electron system and can accept a proton.
Purine: A fused nitrogen-containing aromatic ring system that forms the core of several biological molecules. Purine rings occur in adenine, guanine, and many nucleotides.
Carbocyclic compound: A cyclic compound whose ring atoms are all carbon. Replacing a ring carbon with a heteroatom distinguishes heterocycles from carbocycles.
Aromaticity: A property of cyclic, conjugated systems in which delocalized electrons produce unusual stability. Many heterocycles are aromatic, but ring heteroatoms affect electron sharing.
Pyrrole: A five-membered aromatic ring containing one nitrogen atom. Its nitrogen lone pair contributes to aromaticity, making it much less basic than pyridine.
Pyrimidine: A six-membered aromatic ring containing two nitrogen atoms. Pyrimidine rings form the cores of cytosine, thymine, and uracil.
Acyclic compound: A compound whose molecular structure contains no rings. Acyclic compounds lack the closed ring that defines heterocyclic compounds.
Heteroatom: An atom in an organic compound that is not carbon or hydrogen. A heteroatom within the ring is the defining feature of a heterocycle.
Imidazole: A five-membered aromatic ring containing two nitrogen atoms in distinct bonding environments. Its two nitrogens illustrate how different ring positions produce distinct basicity and protonation behavior.

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