Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problem 57b

Chapter 9, Problem 57b

What orbitals contain the electrons represented as lone pairs in the structures of purine, and pyrimidine?

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Understand the structures of purine and pyrimidine: Purine is a bicyclic aromatic compound with a six-membered pyrimidine ring fused to a five-membered imidazole ring. Pyrimidine is a six-membered aromatic ring containing two nitrogen atoms at positions 1 and 3.

Identify the lone pairs on the nitrogen atoms in purine and pyrimidine: Lone pairs are non-bonding electron pairs located on the nitrogen atoms in these structures. Each nitrogen atom in the aromatic rings contributes one lone pair.

Determine the hybridization of the nitrogen atoms: In aromatic systems like purine and pyrimidine, the nitrogen atoms are sp² hybridized. This hybridization allows the nitrogen atoms to participate in the conjugated π-system of the aromatic ring.

Assign the orbitals for the lone pairs: In sp² hybridized nitrogen atoms, one of the sp² orbitals contains the lone pair of electrons. The other two sp² orbitals are used for sigma bonding, and the unhybridized p orbital is part of the π-system.

Conclude the orbital location of the lone pairs: The lone pairs on the nitrogen atoms in purine and pyrimidine are located in sp² hybrid orbitals, which are oriented in the plane of the aromatic ring and do not participate in the π-system.

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Key Concepts

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

Lone Pairs

Lone pairs are pairs of valence electrons that are not involved in bonding and are localized on a single atom. In organic molecules, they play a crucial role in determining the molecule's geometry and reactivity. Understanding where these lone pairs are located helps in predicting molecular interactions and the overall shape of the molecule.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding and lone pairs. In purines and pyrimidines, the hybridization of carbon and nitrogen atoms influences the arrangement of electrons and the geometry of the molecule. Recognizing the hybridization state is essential for understanding the spatial distribution of lone pairs.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, which is influenced by the presence of lone pairs and bonding pairs of electrons. The VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict the shape of molecules based on electron pair repulsion. This understanding is vital for analyzing the structures of purine and pyrimidine, as it affects their biological function.

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Related Practice

Textbook Question

What orbitals contain the electrons represented as lone pairs in the structures of quinoline, indole, imidazole?

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Textbook Question

Refer to the electrostatic potential maps <IMAGE> to answer the following questions:

c. Why is the center of the electrostatic potential map of benzene more red than the center of the electrostatic potential map of pyridine?

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Textbook Question

What orbital do the lone-pair electrons occupy in each of the following compounds?

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Textbook Question

Which of the following compounds could be protonated without destroying its aromaticity?

Textbook Question

Following the instructions for drawing the energy levels of the molecular orbitals for the compounds shown in [Figure 8.17], draw the energy levels of the molecular orbitals for the cycloheptatrienyl cation. For each compound, show the distribution of the $\pi$ electrons. Which of the compounds are aromatic?

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Textbook Question

Which of the following has delocalized electrons?

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