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?

검증된 단계별 안내

Step 1: Understand the nitration reaction. Nitration involves the addition of a nitro group (-NO₂) to an aromatic ring. This is typically achieved using a mixture of concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄), which generates the nitronium ion (NO₂⁺), the electrophile responsible for the reaction.

Step 2: Analyze the structure of p-xylene. p-Xylene is a benzene ring with two methyl groups (-CH₃) attached at the para positions (1,4 positions). The symmetry of the molecule plays a key role in determining the number of isomers formed during nitration.

Step 3: Predict the positions where the nitro group can be added. In p-xylene, the methyl groups are electron-donating and activate the benzene ring, making the ortho and para positions relative to each methyl group more reactive. However, due to the symmetry of p-xylene, the addition of a nitro group will result in identical products at certain positions.

Step 4: Consider the symmetry of the molecule. Because p-xylene is symmetric, nitration at the ortho position relative to either methyl group will produce the same compound. Similarly, nitration at the para position relative to one methyl group will produce a single unique compound. Thus, only two distinct isomers can be formed.

Step 5: Conclude the number of isomers. Based on the analysis, nitration of p-xylene results in two distinct isomers due to the symmetry of the molecule and the activation effects of the methyl groups.

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주요 개념

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Disubstituted Benzene Isomers

Disubstituted benzene compounds can exist in three distinct isomeric forms: ortho, meta, and para. These terms refer to the relative positions of the substituents on the benzene ring. Understanding these isomers is crucial for predicting the outcomes of chemical reactions, such as nitration, which can yield different products based on the starting isomer.

Nitration Reaction

Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced into an aromatic compound. The 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 product depends on the original substituents on the benzene ring, influencing the number of isomers formed.

Körner’s Absolute Method

Körner’s absolute method is a historical technique used to determine the structure of disubstituted benzene derivatives by introducing a third substituent and analyzing the resulting isomers. By observing how many isomers are produced upon nitration, chemists can infer the original positions of the substituents. This method provides insight into the reactivity and orientation of substituents in aromatic compounds.