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Ch. 17 - Reactions of Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 17 - Reactions of Aromatic CompoundsProblem 65
Chapter 17, Problem 65

(a) Draw the three isomers of benzenedicarboxylic acid.
(b) The isomers have melting points of 210 °C, 343 °C, and 427 °C. Nitration of the isomers at all possible positions was once used to determine their structures. The isomer that melts at 210 °C gives two mononitro isomers. The isomer that melts at 343 °C gives three mononitro isomers. The isomer that melts at 427 °C gives only one mononitro isomer. Show which isomer has which melting point.

Verified step by step guidance
1
Step 1: Understand the structure of benzenedicarboxylic acid. It consists of a benzene ring with two carboxylic acid (-COOH) groups attached. The three isomers differ in the relative positions of the carboxylic acid groups on the benzene ring: ortho (1,2-), meta (1,3-), and para (1,4-). Draw these three isomers.
Step 2: Analyze the nitration process. Nitration involves substituting a nitro group (-NO₂) onto the benzene ring. The number of possible mononitro isomers depends on the symmetry of the benzenedicarboxylic acid isomer. For example, ortho (1,2-) has two distinct positions for nitration, meta (1,3-) has three distinct positions, and para (1,4-) has only one distinct position due to symmetry.
Step 3: Match the melting points to the isomers based on the nitration results. The isomer that melts at 210 °C gives two mononitro isomers, indicating it is the ortho isomer (1,2-). The isomer that melts at 343 °C gives three mononitro isomers, indicating it is the meta isomer (1,3-). The isomer that melts at 427 °C gives only one mononitro isomer, indicating it is the para isomer (1,4-).
Step 4: Confirm the reasoning by considering the symmetry of each isomer. The ortho isomer (1,2-) has asymmetry, allowing two distinct nitration positions. The meta isomer (1,3-) has less symmetry, allowing three distinct nitration positions. The para isomer (1,4-) is highly symmetrical, allowing only one distinct nitration position.
Step 5: Summarize the findings. The ortho isomer (1,2-) corresponds to the melting point of 210 °C, the meta isomer (1,3-) corresponds to the melting point of 343 °C, and the para isomer (1,4-) corresponds to the melting point of 427 °C. Ensure the drawings of the isomers clearly show the positions of the carboxylic acid groups and the possible nitration sites.

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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 or spatial orientations. In the case of benzenedicarboxylic acid, isomers can differ in the position of the carboxylic acid groups on the benzene ring, leading to distinct physical and chemical properties, such as melting points.
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Nitration

Nitration is a chemical reaction that introduces a nitro group (-NO2) into an organic compound, typically through electrophilic aromatic substitution. The position where nitration occurs on the benzene ring is influenced by the substituents already present, which can direct the incoming nitro group to specific locations, thus helping to differentiate between isomers based on the number of possible mononitro products.
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Melting Point

The melting point is the temperature at which a solid becomes a liquid, and it is a key physical property used to characterize substances. In the context of the isomers of benzenedicarboxylic acid, the melting points provide insight into the stability and intermolecular interactions of each isomer, which can be influenced by factors such as hydrogen bonding and molecular symmetry.
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Related Practice
Textbook Question

The most common selective herbicide for killing broadleaf weeds is 2,4-dichlorophenoxyacetic acid (2,4-D). Show how you would synthesize 2,4-D from benzene, chloroacetic acid (ClCH2COOH), and any necessary reagents and solvents.

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Textbook Question

Furan undergoes electrophilic aromatic substitution more readily than benzene; mild reagents and conditions are sufficient. For example, furan reacts with bromine to give 2-bromofuran.

a. Propose mechanisms for the bromination of furan at the 2-position and at the 3-position. Draw the resonance forms of each sigma complex, and compare their stabilities.

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Textbook Question

Furan undergoes electrophilic aromatic substitution more readily than benzene; mild reagents and conditions are sufficient. For example, furan reacts with bromine to give 2-bromofuran.

b. Explain why furan undergoes bromination (and other electrophilic aromatic substitutions) primarily at the 2-position.

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Textbook Question

In Chapter 14, we saw that Agent Orange contains (2,4,5-trichlorophenoxy) acetic acid, called 2,4,5-T. This compound is synthesized by the partial reaction of 1,2,4,5-tetrachlorobenzene with sodium hydroxide, followed by reaction with sodium chloroacetate, ClCH2CO2Na.

a. Draw the structures of these compounds, and write equations for these reactions.

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Textbook Question

Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result:

a. Explain the change in the product ratios when the temperature is increased.

b. Predict what will happen when the product mixture from the reaction at 0 °C is heated to 100 °C.

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Textbook Question

Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result:

c. Because the SO3H group can be added to a benzene ring and removed later, it is sometimes called a blocking group. Show how 2,6-dibromotoluene can be made from toluene using sulfonation and desulfonation as intermediate steps in the synthesis.

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