A) Heterocycles with 6 atoms and 6 p electrons.

They are compounds related to benzene in which some of the carbon atoms are replaced by heteroatoms. In general, they are systems with electronic deficit ( p deficient).
We can consider pyridine as one of the most typical systems of this family of heterocycles.
resonance in pyridine 01
Resonant structures of pyridine
resonance in pyridine 02

The more electronegative nitrogen draws the electron cloud towards it, leaving the electron-deficient carbons behind. Therefore, this system is considered pi deficient with respect to benzene. In the following image I represent the average charge densities that each carbon supports, in a benzene all carbons would support a density of 1.

pyridine electron density

The electronegativity of nitrogen causes the retention of the electronic cloud on it, decreasing the aromaticity of the system, as well as its ability to attack electrophiles. However, reactions that proceed with nucleophile attack on the ring are favored.

Eight types of aza compounds derived from benzene are known:

azaarenes 01

[1] Pyridine

[2] Pyridazine

[3] Pyrimidine

[4] Pyrazine

azaarenes 02

[5] 1,2,3-Triazine

[6] 1,2,4-Triazine

[7] 1,3,5-Triazine

[8] 1,2,3,5-Tetrazine

Other heterocycles belonging to this family with a heteroatom other than nitrogen are:

other heteroatoms

[9] Pyrilium cation

[10] Silabenzene

[11] Phosphabenzene

[12] Thiopyrilium cation

[13] Arsabenzene


B) Heterocycles of 5 atoms and 6 pi electrons.

They are compounds with electronic excess or p -surplus. Carbons have a higher charge density than benzene, this makes them very reactive towards electrophiles but drastically decreases their reactivity towards nucleophiles. A typical member of this family is pyrrole.

pyrrole aromaticity

The most representative members of this family of heterocycles are:

heterocyles five members 01

[14] Pyrrole

[15] Thiophene

[16] Furan

[17] Imidazole

[18] Pyrazole

heterocyles five members 02

[19] Oxazole

[20] Isoxazole

[21] Thiazole

[22] Isothiazole


C) Condensed or fused systems

They are made up of several rings, which condense by sharing bonds.
fused heterocycles
[23] Quinoline
[24] Isoquinoline
[25] Indole