Aromatic heterocycle compounds contain one or more atoms other than carbon as part of the aromatic ring.

Purines and pyrimidines are examples of biological aromatic heterocycles. Aromatic describes any unusually stable ring system that adheres to the following rules:

• The compound is cyclic
• The compound is planar
• The compound is conjugated (has alternating single and multiple bonds, or lone pairs, creating at least on unhybridized p-orbital for each atom in the ring)
• The compound has 4n + 2 (where n is any integer) π electrons (Hückel’s rule).

The most common example of an aromatic compound is benzene. In an aromatic compound, the extra stability is due to the delocalized π electrons, which can travel throughout the entire compound using available molecular orbitals. Delocalization is a characteristic of all aromatic molecules; consequently, aromatic molecules are fairly unreactive.

A heterocycle is a ring containing at least two different elements in its ring. For example, purines and pyrimidines contain nitrogen in their aromatic rings and these nucleic acids have exceptional stability. This helps explain the utility of nucleotides as the molecule for storing genetic information.

Purines and pyrimidines are important biologically relevant aromatic heterocycles as they make up DNA.

A purine such as adenine (A) and guanine (G) is characterized by double-ring structures and a pyrimidine such as cytosine (C) and thymine (T) is characterized by single-ring structures.

Practice Questions

 

Khan Academy

 

MCAT Official Prep (AAMC)

Section Bank C/P Section Passage 2 Question 9

Practice Exam 2 B/B Section Passage 1 Question 4

 

Key Points

• A heterocycle is a ring containing at least one-non carbon atom.

• Aromatic compounds are rings that have resonance bonds that impart extra stability compared to similar structures.

• Purines and pyrimidines are important biological aromatic heterocycles. They make up DNA.

Key Terms

Purine: A heterocyclic aromatic organic compound that consists of a pyrimidine ring fused to an imidazole ring.

Pyrimidine: Simple aromatic compounds composed of carbon and nitrogen atoms in a six-membered ring.

Aromatic: rings that have resonance bonds that impart extra stability compared to similar structures

Hückel’s rule: A rule used to determine if a planar ring molecule would have aromatic properties. If a cyclic, planar molecule has 4n+2π electrons, it is aromatic

Delocalization: A stabilizing force because it spreads energy over a larger area rather than keeping it confined to a small area.

Heterocycle: A ring containing at least one-non carbon atom.