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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 37a
Chapter 16, Problem 37a

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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Step 1: Understand the problem. Körner’s method involves determining the number of isomers formed when a third group (such as a nitro group) is added to a disubstituted benzene derivative. In this case, we are analyzing m-xylene, which has two methyl groups in the meta positions on a benzene ring.
Step 2: Recall the nitration reaction mechanism. Nitration involves the addition of a nitro group (-NO₂) to the benzene ring using a mixture of concentrated HNO₃ and H₂SO₄. The nitro group can attach to any of the available positions on the benzene ring, depending on the substituents already present.
Step 3: Analyze the symmetry of m-xylene. The meta positions of the two methyl groups create a specific pattern of substitution on the benzene ring. This symmetry determines how many unique positions are available for the nitro group to attach, leading to distinct isomers.
Step 4: Determine the possible positions for nitration. In m-xylene, the nitro group can attach to the remaining positions on the benzene ring that are not already occupied by the methyl groups. Consider the ortho, meta, and para positions relative to each methyl group and eliminate duplicates caused by symmetry.
Step 5: Count the number of unique isomers. After analyzing the possible positions for the nitro group and accounting for symmetry, determine how many distinct nitro-m-xylene isomers can be formed. This will give the final answer to the problem.

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Key Concepts

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

Nitration of Aromatic Compounds

Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced into an aromatic ring. This process typically involves the use of a nitrating mixture, such as concentrated nitric acid and sulfuric acid, which generates the nitronium ion (NO2+), the active electrophile. The position of substitution on the aromatic ring depends on the existing substituents, which can direct the incoming nitro group to ortho, meta, or para positions.
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Isomerism in Disubstituted Benzenes

Disubstituted benzenes can exhibit isomerism based on the relative positions of the substituents on the benzene ring. The three main types of isomers are ortho (1,2-), meta (1,3-), and para (1,4-). The specific arrangement of substituents affects the chemical properties and reactivity of the compound, making it essential to identify the correct isomer when discussing reactions such as nitration.
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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 analyzing the number of isomers produced from the nitration of the compound, chemists can deduce the original positions of the substituents. This method was particularly useful before the advent of modern spectroscopic techniques, providing a systematic approach to structural determination.
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Related Practice
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

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.

b. Toluene reacts with bromine in the presence of light to give benzyl bromide. Propose a mechanism for this reaction.

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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.

a. Use resonance forms to show the delocalization (over four carbon atoms) of the unpaired electron of the benzyl radical.

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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

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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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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