Question

When an aromatic compound undergoes ozonolysis followed by treatment with \(\mathrm{I}_{2} \mathrm{O} / \mathrm{Zn}\), it produces glyoxal, monomethyl glyoxal and dimethyl glyoxal. It could be (1) benzene (2) toluene (3) 0 -xylene (4) m-xylene

Short answer

o-Xylene

Step-by-step solution

- Understand Ozonolysis Products

Identify the compounds produced after ozonolysis. Ozonolysis of an aromatic compound breaks the double bonds of the aromatic ring resulting in aldehydes or ketones.

- Match Products to Given Compounds

Given products are glyoxal, monomethyl glyoxal, and dimethyl glyoxal. Each of these indicates the number of methyl groups attached to the carbonyl compounds formed.

- Determine the Aromatic Compound

Analyze each option: 1. Benzene will not produce any glyoxal derivatives since it has no methyl groups.2. Toluene will produce only monomethyl glyoxal as it has only one methyl group attached.3. o-Xylene will produce glyoxal, monomethyl glyoxal, and dimethyl glyoxal due to the positions of two methyl groups.4. m-Xylene cannot produce the required glyoxal compounds due to the position of its methyl groups.

- Conclusion

Since o-Xylene (orthoxylene) can produce all the given products: glyoxal, monomethyl glyoxal, and dimethyl glyoxal, it must be the correct answer.

Key concepts

Ozonolysis

Ozonolysis is a reaction where ozone (O3) is used to break double bonds in an organic molecule. This reaction is especially useful for studying alkenes and aromatic compounds because it cleaves the double bonds, resulting in smaller molecules that are easier to analyze. In the case of aromatic compounds, ozonolysis disrupts the continuous conjugated system of the benzene ring, yielding products like aldehydes and ketones. The process typically involves two steps: the initial ozonation and a reductive work-up, often involving zinc (Zn) and acetic acid (HOAc) or hydrogen peroxide (H2O2). The nature of the products depends on the substituents attached to the aromatic ring.

Aromatic Compounds

Aromatic compounds are a special class of cyclic hydrocarbons known for their stability and unique electronic configuration. They consist of a planar ring with conjugated double bonds following Huckel's rule, which states that an aromatic ring has 4n+2 pi electrons (where n is a non-negative integer). The most common example is benzene, which has six pi electrons. Students should note that substituents on the benzene ring can significantly alter its reactivity. Methyl groups, for instance, provide electron-donating effects, making the ring more reactive towards electrophilic substitution reactions. Aromatic compounds are vital in both chemistry and industry because they serve as the building blocks for countless chemical compounds.

Glyoxal Derivatives

Glyoxal derivatives are aldehyde compounds derived from ozonolysis of aromatic methyl-substituted compounds. The general structure includes a R-CHO group where 'R' can be various side chains. For example, glyoxal is OHC-CHO with two formyl groups, monomethyl glyoxal is CH3-CHO, and dimethyl glyoxal has two methyl groups attached. These derivatives help in pinpointing the original aromatic compound's structure because each type of glyoxal derivative produced reveals how many and where the methyl groups were originally placed. In ozonolysis, the carbon-carbon bonds in the aromatic ring are broken, and the substituents directly influence the resulting products. For instance, o-xylene produces glyoxal (no methyls), monomethyl glyoxal (one methyl), and dimethyl glyoxal (two methyls).

Aromatic Substitution Patterns

Understanding the substitution patterns in aromatic compounds is crucial for predicting their behavior during reactions like ozonolysis. Substituents can be ortho (adjacent), meta (one carbon in between), or para (opposite) to each other on the benzene ring. These positions significantly influence the reaction outcomes. In the given exercise, o-xylene (with two adjacent methyl groups) can form glyoxal, monomethyl glyoxal, and dimethyl glyoxal upon ozonolysis. Other isomers like m-xylene (with one carbon separating the two methyl groups) will not yield the same products because the relative positions of the methyl groups change the molecular geometry and reactivity. Recognizing these patterns can help students deduce the correct structure of the parent aromatic compound based on the products formed.