aromatic heterocyles
1. They are cyclic compounds with conjugated double bonds.
2. Flat, there can be no sp3 hybridized atoms.
3. Hückel's rule: number of electrons p: 4n +2 with n = 0,1,2......
If it complies with rule 4n it is antiaromatic (especially unstable). In case of not complying
the first two conditions we say that it is non-aromatic.

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

There are several criteria to determine the degree of aromaticity of a heterocycle:

A) Link lengths.

A heterocycle is all the more aromatic the smaller the difference between the lengths of the different bonds that make up the ring. Benzene has the same length in all its carbon-carbon bonds, which makes it the most aromatic compound. The heterocycles present differences in the bond distances, being less aromatic the greater these differences are.

distances bond aromaticity heterocycles

All C-C bonds in benzene measure 1.395 A, an intermediate bond distance between single (1.48 A) and double (1.34 A) bonds. In furan there is a notable difference between the length of the bonds, indicating a lower aromaticity than in pyrrole. Pyridine, on the other hand, has very similar C-C bond distances between them and similar to those of benzene, which shows greater aromaticity than pyrrole or furan.

6-Membered heteroaromatics possessing -OH or -SH groups in 2,4-positions preferentially exist in solution in the keto or thioketo forms. In solution, compounds with amino groups in 2,4-positions preferentially exist in the amino form, rather than as an imino tautomer.

heterocycle tautomerism 01

[1] 2-Hydroxypyridine

[2] 2-Pyridone

[3] 4-Hydroxypyridine

[4] 4-Pyridone

[5] 2-aminopyridine

In this section we are going to study the influence of annular stresses on the properties of heterocycles.

A) Angular stress in small heterocycles (3 and 4 members).

The natural bond angles of a sp3 carbon are 109.5º, however in three-membered cycles this angle decreases to 60o, which produces enormous tension. To alleviate this tension, the bonds between carbons are no longer straight, bending, giving rise to curved bonds called "banana bonds". The angles formed by these bonds are 104º, a more tolerable discrepancy.

non-aromatic heterocycles 01


Hybridization Bond angle


The phenomenon by which the axial conformation of a substituted heterocycle in position 2 is stabilized is called anomeric effect. For example, 2-methoxyoxane presents a 27:73 ratio in favor of the conformation with methoxide in axial position.

anomeric effect 01