The ribonucleosides that make up ribonucleic acid (RNA) are composed of D-ribose (a sugar) and four heterocyclic “bases.” The general structure of a ribonucleoside is shown here.

The four heterocyclic bases are cytosine, uracil, guanine, and adenine. Cytosine and uracil are called pyrimidine bases because their structures resemble pyrimidine. Guanine and adenine are called purine bases because their structures resemble purine.

a. Determine which rings of these bases are aromatic.

Verified step by step guidance

1. Step 1: Recall the criteria for aromaticity. A molecule is aromatic if it satisfies Huckel's rule, which states that the molecule must be cyclic, planar, fully conjugated (all atoms in the ring must have p-orbitals), and contain (4n + 2) π-electrons, where n is a non-negative integer.
2. Step 2: Analyze the pyrimidine ring structure. Pyrimidine is a six-membered ring with alternating double bonds and nitrogen atoms. It is cyclic, planar, fully conjugated, and contains 6 π-electrons (4n + 2, where n = 1). Therefore, pyrimidine is aromatic.
3. Step 3: Analyze the purine ring structure. Purine consists of a fused bicyclic system with a six-membered ring and a five-membered ring. Both rings are cyclic, planar, fully conjugated, and together contain 10 π-electrons (4n + 2, where n = 2). Therefore, purine is aromatic.
4. Step 4: Examine cytosine and uracil, which are derivatives of pyrimidine. Both retain the aromatic pyrimidine ring structure, meaning they are aromatic as well.
5. Step 5: Examine guanine and adenine, which are derivatives of purine. Both retain the aromatic purine ring structure, meaning they are aromatic as well.

Key Concepts

Aromaticity

Aromaticity is a property of cyclic compounds that exhibit resonance stability due to the delocalization of π-electrons across the ring structure. For a compound to be considered aromatic, it must follow Hückel's rule, which states that it should have a planar structure and contain a specific number of π-electrons (4n + 2, where n is a non-negative integer). This stability contributes to the unique chemical behavior of aromatic compounds.

Pyrimidine and Purine Bases

Pyrimidine and purine bases are two classes of nitrogenous bases found in nucleic acids. Pyrimidines, which include cytosine and uracil, have a single six-membered ring structure, while purines, such as guanine and adenine, consist of a fused double-ring system containing both a six-membered and a five-membered ring. Understanding their structures is essential for analyzing their aromatic properties.

Resonance Structures

Resonance structures are different ways of representing the same molecule that illustrate the delocalization of electrons. In the context of aromatic compounds, resonance contributes to the stability and reactivity of the molecule. Identifying resonance structures helps in determining the aromatic nature of the rings in nucleobases, as it shows how electrons are distributed across the structure.