Question

An isomer ' \(\mathrm{X}\) ' of ethyl benzene whenever oxidised with \(\mathrm{KMnO}_{4}\) gives dicarboxylic acid which form cict anhydride on heating compound ' \(\mathrm{X}\) ' is (A) \(o\)-Xylene (B) \(p\) - Xylene (C) \(m\)-Xylene (D) Mesitylene

Short answer

The compound 'X' is m-Xylene, as it forms a 1,3-dicarboxylic acid when oxidized with \(\mathrm{KMnO}_{4}\), which further forms a six-membered cyclic anhydride upon heating.

Step-by-step solution

Draw the structure of all the isomers given in the options

We begin by drawing the structure of the isomers given: (A) o-Xylene (ortho-Xylene): Two methyl groups are in ortho-position (1,2- substituents). (B) p-Xylene (para-Xylene): Two methyl groups are in para-position (1,4- substituents). (C) m-Xylene (meta-Xylene): Two methyl groups are in meta-position (1,3- substituents). (D) Mesitylene: It is a trisubstituted benzene with three methyl groups located at 1,3,5 positions.

Oxidize the isomers with \(\mathrm{KMnO}_{4}\)

Now, we need to oxidize these isomers with \(\mathrm{KMnO}_{4}\): For a xylene to give a dicarboxylic acid when oxidized, the two methyl groups must be in positions that won't hinder the oxidation process and are available for the formation of the diacid. (A) o-Xylene: The two methyl groups are adjacent, meaning that the oxidation process will take place without any hindrance, forming a 1,2-dicarboxylic acid. (B) p-Xylene: The two methyl groups are in para-positions, allowing the formation of a 1,4-dicarboxylic acid upon oxidation. (C) m-Xylene: The two methyl groups are in meta-positions, giving a 1,3-dicarboxylic acid when oxidized. (D) Mesitylene: The three methyl groups present at 1,3,5 positions may hinder the oxidation process and the formation of dicarboxylic acid.

Check for the formation of cyclic anhydride upon heating

Now we need to see which formed dicarboxylic acid can give cyclic anhydride on heating: (A) o-Xylene: The adjacent carboxylic groups in 1,2-dicarboxylic acid will form a five-membered cyclic anhydride upon heating. (B) p-Xylene: The carboxylic groups in 1,4-dicarboxylic acid are far apart, preventing the formation of a cyclic anhydride. (C) m-Xylene: The carboxylic groups in the 1,3-dicarboxylic acid are in a position to form a six-membered cyclic anhydride upon heating. (D) Mesitylene: As already inferred, mesitylene may not generate a dicarboxylic acid in the first place. From the analysis above, we can conclude that compound 'X' is the isomer that forms cyclic anhydride upon heating. Therefore, the answer is: (C) m-Xylene

Key concepts

Xylene Isomers Oxidation

Understanding the chemical oxidation of xylene isomers is pivotal for grasping how different substituent positions affect chemical reactions. Xylene, a derivative of benzene, has three isomers: ortho-xylene (o-xylene), meta-xylene (m-xylene), and para-xylene (p-xylene), differentiated by the positions of methyl groups on the benzene ring. When oxidized with a strong oxidizing agent like potassium permanganate (KMnO4), these methyl groups are converted to carboxylic acid functions.

The position of the methyl groups is crucial; it determines not only the ease of oxidation but also the type of dicarboxylic acid formed. For instance, in o-xylene, the proximity of the methyl groups allows for straightforward oxidation, resulting in a 1,2-dicarboxylic acid. In the case of m-xylene, which forms a 1,3-dicarboxylic acid, the methyl groups are ideally positioned to undergo oxidation without steric hindrance. Conversely, in p-xylene, the distance between methyl groups leads to the formation of a 1,4-dicarboxylic acid. These variations have significant implications for the resulting compounds' properties and potential to form cyclic anhydrides.

m-Xylene

m-Xylene, the meta isomer of xylene with a 1,3-disubstitution pattern, is unique in its chemical behavior during oxidation. Because the methyl groups are located at the 1 and 3 positions on the benzene ring, oxidation leads to a symmetrical 1,3-dicarboxylic acid. This spacing allows for the possibility of forming cyclic anhydrides, a reaction that occurs under the influence of heat.

The utility of m-xylene in industrial chemistry stems from its reactivity under controlled conditions, producing valuable intermediates for polymer synthesis and other applications. In educational settings, it serves as an excellent example to illustrate regioselective reactions, where the position of substituents significantly influences the outcome of a chemical transformation.

Dicarboxylic Acid Formation

The oxidation of xylene isomers to dicarboxylic acids is an important process in organic chemistry. Dicarboxylic acids contain two carboxyl functional groups, which are introduced during the oxidation of methyl groups in xylene isomers. The reaction with potassium permanganate (KMnO4) is particularly effective, stripping away the methyl hydrogens and adding oxygen to form the acid groups.

Dicarboxylic acids have widespread applications, including the production of polyamides and polyesters, solvents, and adhesives. Their chemical structure allows for the potential formation of cyclic anhydrides, which occurs more readily with certain isomers due to the spatial arrangement of the carboxyl groups, as seen with m-xylene.

Cyclic Anhydride

Cyclic anhydrides are compounds formed by the dehydration of dicarboxylic acids when exposed to heat. This reaction leads to the loss of a water molecule and the creation of a ring structure connecting the previous acid groups. The ability of a dicarboxylic acid to form a cyclic anhydride depends on the distance between the carboxyl groups: typically, five- or six-membered rings can readily form.

m-Xylene, which yields a 1,3-dicarboxylic acid upon oxidation, is ideally suited for cyclic anhydride formation because the resulting six-membered ring is stable and energetically favorable. Cyclic anhydrides themselves are valuable due to their reactivity and are employed in the synthesis of other complex organic compounds, such as pharmaceuticals, agrochemicals, and dyes.