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Ch. 16 - Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 16 - Aromatic CompoundsProblem 37c
Chapter 16, Problem 37c

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed.

c. A turn-of-the-century chemist isolated an aromatic compound of molecular formula C6H4Br2. He carefully nitrated this compound and purified three isomers of formula C6H3Br2NO2. Propose structures for the original compound and the three nitrated derivatives

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Step 1: Understand Körner’s method. Körner’s method involves adding a third substituent to a disubstituted benzene derivative and analyzing the number of isomers formed. The number of isomers provides insight into the relative positions of the original substituents (ortho, meta, or para).
Step 2: Analyze the molecular formula of the original compound, C6H4Br2. This indicates a disubstituted benzene ring with two bromine atoms. The positions of these bromine atoms (ortho, meta, or para) will determine the number of nitrated isomers formed.
Step 3: Consider the nitration reaction. Nitration involves the addition of a nitro group (-NO2) to the benzene ring using HNO3 and H2SO4. The positions of the bromine atoms influence the possible positions for the nitro group, leading to different isomers.
Step 4: Interpret the result of three nitrated isomers (C6H3Br2NO2). The formation of three isomers suggests that the original compound is the meta isomer of dibromobenzene. In the meta isomer, the bromine atoms are positioned such that nitration can occur at three distinct positions on the ring.
Step 5: Propose structures. The original compound is meta-dibromobenzene. The three nitrated derivatives are: (1) 3-bromo-5-nitro-bromobenzene, (2) 3-bromo-4-nitro-bromobenzene, and (3) 3-bromo-2-nitro-bromobenzene. These structures correspond to the three possible positions for the nitro group relative to the bromine substituents.

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Key Concepts

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

Disubstituted Benzene Derivatives

Disubstituted benzene derivatives are aromatic compounds that have two substituents attached to the benzene ring. The position of these substituents can significantly affect the chemical properties and reactivity of the compound. The three main positional isomers are ortho (1,2-), meta (1,3-), and para (1,4-), which differ in the relative positions of the substituents on the benzene ring.
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Körner’s Absolute Method

Körner’s absolute method is a classical technique used to determine the structure of disubstituted benzene derivatives by introducing a third substituent, often a nitro group. By observing the number of isomers produced upon nitration, chemists can infer the original positions of the substituents. This method relies on the principle that different isomers will yield different nitration products based on their spatial arrangement.
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Nitration of Aromatic Compounds

Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced into an aromatic compound. This reaction typically involves the use of a nitrating mixture, such as concentrated nitric acid and sulfuric acid. The position of the nitro group in the resulting isomers depends on the existing substituents on the benzene ring, which can direct the electrophilic attack to ortho, meta, or para positions.
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Related Practice
Textbook Question

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed.

a. How many isomers are formed by nitration of m-xylene?

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Textbook Question

For each NMR spectrum, propose a structure consistent with the spectrum and the additional information provided.

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a. Elemental analysis shows the molecular formula to be C8H7OCl. The IR spectrum shows a moderate absorption at 1602 cm–1 and a strong absorption at 1690 cm–1.

b. The mass spectrum shows a double molecular ion of ratio 1:1 at m/z 184 and 186.

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Textbook Question

Biphenyl has the following structure.

a. Is biphenyl a (fused) polynuclear aromatic hydrocarbon?

b. How many pi electrons are there in the two aromatic rings of biphenyl? How does this number compare with that for naphthalene?

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Textbook Question

The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

c. Which of the following reactions will have the faster rate and give the better yield? Use a drawing of the transition state to explain your answer.

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Textbook Question

Recall (Section 16-10) that two positions of anthracene sometimes react more like polyenes than like aromatic compounds.

b. The Diels–Alder reaction of anthracene with maleic anhydride is a common organic lab experiment. Predict the product of this Diels–Alder reaction.

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Textbook Question

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed.

b. How many isomers are formed by nitration of p-xylene?

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