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?

Verified step by step guidance

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

Key Concepts

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.

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.

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.