Question

Determine which of the heterocyclic amines just shown are not aromatic. Give the reasons for your conclusions.

Short answer

The conditions for a compound to be aromatic are:

• The compounds must be planar
• The compounds must be cyclic
• The compounds must follow the Huckel rule of pi electrons. $\text{4n+2}$: Pyrrole, imidazole, indole, pyridine, 2-methylpyridine, pyrimidine, and purine.

The systems with six pi-electrons are: Pyrrole, imidazole, pyridine, 2-methylpyridine, and pyrimidine.

The systems with ten pi-electrons are indole and purine.

The other nitrogen heterocycles shown are not aromatic as they do not have cyclic planar pi-systems : piperidine, pyrrolidine, aziridine and 1-methylpyrrolidine.

Step-by-step solution

Step-1. Criteria for a compound to be aromatic, non-aromatic and anti-aromatic:

Aromatic and anti-aromatic compounds differ in only one aspect, that is, the number of electrons they have in the conjugated system. All the other criteria being planar, cyclic and fully conjugated are must for a molecule to be aromatic or anti-aromatic. Aromatic compounds have$\text{4n+2}$pi-electrons in the conjugated system whereas, anti-aromatic compounds have $4\mathrm{n}$pi-electrons.

If any othe factors such as planarity, cyclic system, conjugated system does not match, then the compound is said to be non-aromatic.

Step-2. Aromaticity of given heterocyclic amines:

The heterocyclic compounds shown which satisfy the criteria for aromaticity that is, planar, cyclic pi-system and the Huckel number of$\text{4n+2}$pi electrons : Pyrrole, imidazole, indole, pyridine, 2-methylpyridine, pyrimidine, and purine.

The systems with six pi-electrons are: Pyrrole, imidazole, pyridine, 2-methylpyridine, and pyrimidine are aromatic as they satisfy all the conditions for aromaticity.

The systems with ten pi-electrons are indole and purine are aromatic.

The other nitrogen heterocycles shown are not aromatic as they do not have cyclic pi-systems : piperidine, pyrrolidine, aziridine and 1-methylpyrrolidine.