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Ch.7 - Covalent Bonding and Electron-Dot Structures
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) Resonance structure (a) of dibromobenzene showing bromine atoms in one position.
(b) Resonance structure (b) of dibromobenzene with bromine atoms in a different position.
(c) Resonance structure (c) of dibromobenzene illustrating another arrangement of bromine atoms.
(d) Resonance structure (d) of dibromobenzene depicting a fourth arrangement of bromine atoms.

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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.
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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.
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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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Related Practice
Textbook Question
Benzene has the following structural formula.

(b) Which statement best describes the carbon–carbon bonds in benzene? (i) Three carbon–carbon bonds are longer and weaker than the other three carbon–carbon bonds. (ii) All six carbon–carbon bonds are identical, and their length and strength are between a double and single bond. (iii) The length of carbon–carbon double bond switches back and forth between the length of a double and a single bond.
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Textbook Question
Draw three resonance structures for sulfur tetroxide, SO4, whose connections are shown below. (This is a neutral mol-ecule; it is not a sulfate ion.) Assign formal charges to the atoms in each structure.
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Textbook Question
Some mothballs used when storing clothes are made of naphthalene (C10H8), which has the following incomplete structure.

(a) Add double bonds where needed to draw a complete electron-dot structure.
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Textbook Question
Use the following information plus the data given in Tables 6.2 and 6.3 to calculate the second electron affinity, Eea2, of oxygen. Is the O2-ion stable in the gas phase? Why is it stable in solid MgO? Heat of sublimation for Mg1s2 = +147.7 kJ/mol Bond dissociation energy for O21g2 = +498.4 kJ/mol Eea1 for O1g2 = -141.0 kJ/mol Net energy change for formation of MgO(s) from its elements = -601.7 kJ/mol
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Textbook Question

Assign formal charges to the atoms in the following structures. Which of the two do you think is the more important contributor to the resonance hybrid?(a)(b)

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Textbook Question
Draw the resonance structure indicated by the curved arrows. Assign formal charges, and evaluate which of the two structures is a larger contributor to the resonance hybrid.

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