Question

Toluene on oxidation with chromyl chloride produces: (a) Benzaldehyde (b) Benzoic acid (c) Acetophenone (d) Benzyl alcohol

Short answer

(a) Benzaldehyde

Step-by-step solution

Understand the reagent

Chromyl chloride ( ext{CrO}_2 ext{Cl}_2) is a strong oxidizing agent that is used in the Etard reaction to oxidize toluene to benzaldehyde. It selectively oxidizes the methyl group (\(-CH_3\)) in the presence of other groups to form an aldehyde.

Identify the reaction type

The oxidation of toluene using chromyl chloride is specifically the Etard reaction. In this reaction, toluene's methyl group is oxidized to form a formyl group, leading to the formation of benzaldehyde.

Predict the product

In an Etard reaction, toluene ( ext{C}_6 ext{H}_5 ext{CH}_3) is oxidized to form benzaldehyde ( ext{C}_6 ext{H}_5 ext{CHO}). The methyl group is oxidized stepwise to the aldehyde by chromyl chloride without further oxidation to a carboxylic acid or other derivatives.

Select the correct option

Given the options, (a) Benzaldehyde is the correct product of the oxidation of toluene with chromyl chloride during the Etard reaction.

Key concepts

Oxidation of Toluene

Toluene, a simple aromatic hydrocarbon, undergoes oxidation in various ways depending on the reagents used. In the Etard reaction, toluene is specifically oxidized using chromyl chloride. The oxidation process targets the methyl group attached to the benzene ring. Here, the oxidation doesn't involve breaking the aromatic ring but instead modifies the methyl group.
The transformation occurs as the methyl group ( (-CH_3) ) is converted into a formyl group ( (-CHO) ), thus forming benzaldehyde. This precise conversion is one of the primary steps to understand when dealing with the mechanism of toluene oxidation in this context. The end product, benzaldehyde, remains an essential compound in both academic and industrial chemistry.
Understanding this specific oxidation is crucial since not all oxidizing agents oxidize toluene to the same level of oxidation product. For instance:

• Strong oxidizing agents may further oxidize to form benzoic acid.
• The Etard reaction stops at the aldehyde stage, ensuring further transformations don't occur.

This careful control of the oxidation level is imperative in chemical processes.

Chromyl Chloride

Chromyl chloride, with the chemical formula (CrO_2Cl_2) , is a uniquely strong oxidizing agent. It is specifically employed in the Etard reaction due to its selective action. It can oxidize the methyl group in toluene to form an aldehyde without affecting other functional groups on the aromatic ring.
Chromyl chloride has the ability to distinguish between potential sites of oxidation. It targets only the methylene (-CH_2-) group of toluene, ensuring controlled oxidation. This selectivity is due to the chromium-oxygen bond nature in (CrO_2Cl_2) , which readily participates in oxidation reactions.

• Chromyl chloride is specifically chosen for its ability to limit oxidation to the aldehyde level.
• Other reagents might over-oxidize, leading to unintended products like benzoic acid.

In laboratory settings, carefully controlling the reaction condition, including the reagent and temperature, ensures chromyl chloride performs its role effectively.

Formation of Benzaldehyde

The formation of benzaldehyde is the principal goal of the Etard reaction when oxidizing toluene. Benzaldehyde, one of the simplest aromatic aldehydes, features a formyl group directly attached to a benzene ring.
In the reaction mechanism, chromyl chloride attacks the methyl group of toluene. This oxidative attack converts it stepwise into a formyl group, leading to the creation of benzaldehyde. Importantly, the process halts at this point without further oxidation.
The strategic formation step involves:

• Oxidation through the chromyl chloride, ensuring the stoppage at the aldehyde stage.
• Careful monitoring of conditions to avoid over-oxidation to benzoic acid.

Benzaldehyde is valuable in various industry sectors like fragrances and flavorings. Its selective synthesis via the Etard reaction underlines the importance of understanding both reagent behavior and reaction conditions.