Ch.7 - Covalent Bonding and Electron-Dot Structures

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.7 - Covalent Bonding and Electron-Dot Structures

Problem 93

Chapter 7, Problem 93

Four different structures (a), (b), (c), and (d) can be drawn for compounds named dibromobenzene, but only three different compounds actually exist. Explain.(a)
(b)
(c)
(d)

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Identify the positions of the bromine atoms in each of the four structures (a), (b), (c), and (d).

Recognize that the benzene ring has six carbon atoms, and the positions of the substituents can be described as ortho (adjacent), meta (one carbon apart), or para (opposite).

Determine the relative positions of the bromine atoms in each structure: (a) ortho, (b) meta, (c) meta, and (d) para.

Notice that structures (b) and (c) are actually the same compound because they both have bromine atoms in the meta positions, making them indistinguishable from each other.

Conclude that only three distinct compounds exist: ortho-dibromobenzene, meta-dibromobenzene, and para-dibromobenzene.

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Key Concepts

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

Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but different structural arrangements. In the case of dibromobenzene, the different positions of the bromine atoms on the benzene ring lead to distinct isomers, which can exhibit different physical and chemical properties.

Symmetry and Chirality

Symmetry plays a crucial role in determining the existence of distinct compounds. In dibromobenzene, certain arrangements of bromine atoms may lead to symmetrical structures that are indistinguishable from one another, resulting in fewer unique compounds than the number of possible structures. Chirality, or lack thereof, can also influence the number of isomers.

Resonance Structures

Resonance structures are different ways of representing the same molecule, where the actual structure is a hybrid of these forms. For dibromobenzene, while multiple resonance structures can be drawn, they may not correspond to unique compounds if they represent the same arrangement of atoms. Understanding resonance helps clarify why only three distinct compounds exist despite four possible structures.

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Resonance Structures

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(a) Add double bonds where needed to draw a complete electron-dot structure.

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Textbook Question

Benzene has the following structural formula.