Ch. 16 - Aromatic Compounds

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 16 - Aromatic Compounds

Problem 43b

Chapter 16, Problem 43b

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.

b. Predict which nitrogen atoms are basic.

Verified step by step guidance

Examine the structures of the pyrimidine and purine bases provided in the image. Pyrimidine bases (cytosine and uracil) have a single six-membered ring, while purine bases (guanine and adenine) have a fused bicyclic structure consisting of a six-membered ring and a five-membered ring.

Identify the nitrogen atoms in each base. Nitrogen atoms in heterocyclic rings can act as basic sites if they have a lone pair of electrons that is not involved in resonance or aromaticity.

For pyrimidine bases (cytosine and uracil), focus on the nitrogen atoms in the six-membered ring. The nitrogen atoms that are not directly bonded to hydrogen or involved in resonance stabilization are likely to be basic.

For purine bases (guanine and adenine), analyze the nitrogen atoms in both the six-membered and five-membered rings. Nitrogen atoms with lone pairs that are not delocalized into the aromatic system are potential basic sites.

Predict the basicity of each nitrogen atom based on its electronic environment. Nitrogen atoms with lone pairs that are readily available for protonation (not involved in resonance or aromatic stabilization) are considered basic.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Basicity of Nitrogen Atoms

Basicity in organic chemistry refers to the ability of a compound to accept protons (H+ ions). In the context of nitrogen atoms in heterocyclic bases, basicity is influenced by the availability of a lone pair of electrons on the nitrogen atom, which can bond with protons. The nitrogen atoms in purines and pyrimidines can vary in their basicity depending on their position within the ring structure and the presence of adjacent electronegative atoms.

Heterocyclic Compounds

Heterocyclic compounds are cyclic structures that contain at least one atom that is not carbon, such as nitrogen, oxygen, or sulfur. In RNA, the heterocyclic bases (cytosine, uracil, guanine, and adenine) are crucial for the structure and function of nucleic acids. The presence of nitrogen in these bases contributes to their chemical properties, including their ability to form hydrogen bonds and participate in base pairing.

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 structure. Understanding the structural differences between these bases is essential for predicting their chemical behavior, including which nitrogen atoms are likely to be basic and how they interact in RNA.

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Nitrogenous Bases Example 1

Related Practice

Textbook Question

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.

c. Do any of these bases have easily formed tautomers that are aromatic? (Consider moving a proton from nitrogen to a carbonyl group to form a phenolic derivative.)

Textbook Question

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.

a. Determine which rings of these bases are aromatic.

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Textbook Question

An unknown compound gives the following mass, IR, and NMR spectra. Propose a structure, and show how it is consistent with the spectra. Show the fragmentations that give the prominent peaks at m/z 127 and 155 in the mass spectrum.

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Textbook Question

How would you convert the following compounds to aromatic compounds?

(d)

(e)

(f)

Textbook Question

Consider the following compound, which has been synthesized and characterized:

a. Assuming this molecule is entirely conjugated, do you expect it to be aromatic, antiaromatic, or nonaromatic?

b. Why was this molecule synthesized with three tert-butyl substituents? Why not make the unsubstituted compound and study it instead?

Textbook Question

How would you convert the following compounds to aromatic compounds?

(a)

(b)

(c)

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