Heterocyclic Compounds – Vocabulary Review

A vocabulary set covering fundamental definitions, naming prefixes/suffixes, aromaticity rules, and representative monocyclic, bicyclic, and tricyclic heterocycles discussed in the lecture notes.

Carbocyclic compound

A cyclic organic molecule whose ring is composed exclusively of carbon atoms (also called a homocyclic compound).

Alicyclic compound

A non-aromatic carbocycle; its ring is strained and readily undergoes ring-opening reactions (e.g., cyclopropane).

Aromatic carbocycle

A highly stable carbocyclic ring that obeys the rules of aromaticity and behaves like benzene (e.g., naphthalene).

Heterocyclic compound

A cyclic organic molecule whose ring contains at least one atom other than carbon (commonly N, O, or S).

Heteroatom

The non-carbon atom (N, O, S, etc.) incorporated into a heterocyclic ring.

Alicyclic-type heterocycle

An unstable, non-aromatic heterocycle (e.g., cyclic imines, lactones) that readily undergoes ring opening.

Aromatic-type heterocycle

A heterocycle that is planar, conjugated, follows Hückel’s (4n+2) π rule, and prefers substitution over addition (e.g., pyridine, thiophene).

Hückel’s rule

The aromaticity criterion requiring a planar, cyclic, conjugated system with (4n + 2) π-electrons.

Aza-

Nomenclature prefix indicating nitrogen as the heteroatom in a ring.

Oxa-

Nomenclature prefix indicating oxygen as the heteroatom in a ring.

Thia-

Nomenclature prefix indicating sulfur as the heteroatom in a ring.

Dihydro-

Prefix showing a ring is reduced by two hydrogens (partially saturated).

Tetrahydro-

Prefix showing a ring is reduced by four hydrogens (partially saturated).

Perhydro-

Prefix showing a ring is fully saturated with hydrogen atoms.

-idine / ‑olidine

Suffixes used for fully saturated five-membered heterocycles (e.g., pyrrolidine).

Aziridine

A three-membered saturated nitrogen heterocycle (C₂H₅N).

Azirine

A three-membered unsaturated nitrogen heterocycle containing a C=N double bond.

Oxirane (ethylene oxide)

A three-membered saturated oxygen heterocycle; industrial epoxide.

Oxirene

A hypothetical or highly unstable three-membered unsaturated oxygen heterocycle.

Azete

A four-membered unsaturated nitrogen heterocycle (C₃H₅N).

Azetidine

A four-membered saturated nitrogen heterocycle (C₃H₇N).

Oxetane

A four-membered saturated oxygen heterocycle (C₃H₆O).

Pyrrole

A five-membered aromatic ring containing one nitrogen atom.

Furan

A five-membered aromatic ring containing one oxygen atom.

Thiophene

A five-membered aromatic ring containing one sulfur atom.

Pyrazole

A five-membered aromatic ring with two adjacent nitrogen atoms (1,2-diazole).

Imidazole

A five-membered aromatic ring with non-adjacent nitrogens at positions 1 and 3.

Oxazole

A five-membered aromatic ring containing one oxygen and one nitrogen atom.

Thiazole

A five-membered aromatic ring containing one sulfur and one nitrogen atom.

Pyridine

A six-membered aromatic ring containing one nitrogen atom (C₅H₅N).

4H-Pyran

A six-membered heterocycle with one oxygen; unsaturated but non-aromatic in its 4H form.

Pyridazine

A six-membered aromatic ring with nitrogens at positions 1 and 2.

Pyrimidine

A six-membered aromatic ring with nitrogens at positions 1 and 3.

1,3,5-Triazine

A six-membered aromatic ring containing three nitrogen atoms at positions 1,3,5.

Azepine

A seven-membered unsaturated heterocycle containing one nitrogen atom.

Oxepin

A seven-membered unsaturated heterocycle containing one oxygen atom.

Indole

A bicyclic system: benzene fused to pyrrole; parent of many natural products (e.g., tryptophan).

Benzimidazole

Benzene fused to imidazole; common pharmacophore in drugs.

Benzothiazole

Benzene fused to thiazole; found in dyes and vulcanization accelerators.

Quinoline

Benzene fused to pyridine (benzo[b]pyridine); a basic heteroarene.

Isoquinoline

Structural isomer of quinoline where the nitrogen occupies position 2 of the fused bicyclic system.

Quinazoline

Benzopyrimidine in which benzene is fused to a 1,3-diazine ring.

Carbazole

Tricyclic compound consisting of two benzene rings fused to a central pyrrole ring.

Acridine

Tricyclic heteroarene with a central nitrogen-containing ring flanked by two benzenes.

Phenothiazine

Tricyclic compound with two benzenes fused to a central thiazine ring; core of several antipsychotics.

1,2,3-Triazole

Five-membered aromatic ring with nitrogens at positions 1,2,3; key click-chemistry motif.

1,3,4-Thiadiazole

Five-membered aromatic ring containing two nitrogens and one sulfur at positions 1,3,4.

Isoxazole

Five-membered aromatic ring with adjacent oxygen and nitrogen atoms (1,2-oxazole).

Isothiazole

Five-membered aromatic ring with adjacent sulfur and nitrogen atoms (1,2-thiazole).

Pyrazine

Six-membered aromatic ring with nitrogens at positions 1 and 4 (1,4-diazine).

Cinnoline

Bicyclic system: benzo-fused 1,2-diazine (benzopyridazine).

Quinoxazine

Benzodiazine in which benzene is fused to a 1,4-diazine ring (pyrazine).

Coumarin

Benzopyrone with a lactone structure; fragrant compound in tonka beans.

Chromone

Benzopyran-4-one scaffold; key in flavonoid chemistry.

Purine

Fused bicyclic heteroarene consisting of an imidazole ring fused to a pyrimidine; basis of nucleic acids.

Xanthine

Purine derivative (2,6-dihydroxypurine); metabolic intermediate in caffeine breakdown.

Phenoxazine

Tricyclic system where an oxygen and nitrogen bridge two benzene rings.

Phenazine

Tricyclic system with two nitrogens in a central diazine ring fused to two benzenes.

Dibenzazepine

Three-ring scaffold with a seven-membered central azepine fused to two benzenes (e.g., carbamazepine core).

Benzodiazepine

Fused benzene–diazepine (seven-membered ring with two nitrogens); core of many anxiolytics.