Question

Benzaldehyde is obtained as a product in reactions (A) \(\mathrm{Ph}-\mathrm{CH}_{3}+\mathrm{CrO}_{2} \mathrm{Cl}_{2} \stackrel{\mathrm{CS}_{2}}{\longrightarrow} \stackrel{\mathrm{H}_{2} \mathrm{O}^{\oplus}}{\longrightarrow}\) (B) \(\mathrm{Ph}-\mathrm{H}+\mathrm{CO}+\mathrm{HCl} \frac{\mathrm{Anhy}, \mathrm{ACl}_{\mathrm{J}}}{\mathrm{CuCl}}\) (C) \(\mathrm{Ph}-\mathrm{CH}_{3}+\mathrm{CrO}_{3}+\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O} \longrightarrow{\mathrm{H}_{3} \mathrm{O}^{\circ}}{\longrightarrow}\) (D) \(\mathrm{Ph}-\mathrm{COCl}+\mathrm{H}_{2} / \mathrm{Pd} \stackrel{\mathrm{BaSO}_{4}}{\longrightarrow}\)

Short answer

Benzaldehyde is obtained as a product in reactions (A), (B), and (D). Reaction (A) involves the chlorination of toluene and subsequent hydrolysis to form benzaldehyde. Reaction (B) is the Gattermann-Koch reaction, in which benzene reacts with carbon monoxide and hydrogen chloride in the presence of a catalyst to form benzaldehyde. Reaction (D) involves the hydrogenation of benzoyl chloride to form benzaldehyde. Reaction (C), however, does not produce benzaldehyde.

Step-by-step solution

Understand what benzaldehyde is

Benzaldehyde is an organic compound with the chemical formula C6H5CHO. It is mainly known for its almond-like aroma. In these exercises, its chemical formula is denoted as "Ph" (Phenyl) which represents the phenyl group -C6H5 and the rest of benzaldehyde formula CHO.

Analyze reaction (A)

In reaction (A), we have the following reaction mechanism: 1. \(Ph-CH_3\) (Toluene) reacts with \(CrO_2Cl_2\) in presence of \(CS_2\) solvent to form benzylidene chloride \(Ph-CHCl\), through chlorination of the benzylic carbon. 2. \(Ph-CHCl\) reacts with aqueous acid \(H_2O^+\) to form benzaldehyde (Ph-CHO) through hydrolysis. This reaction produces benzaldehyde as a product.

Analyze reaction (B)

In reaction (B), we have the Gattermann-Koch reaction, in which: 1. \(Ph-H\) (Benzene) reacts with \(CO\) (carbon monoxide) and \(HCl\) (hydrogen chloride) in the presence of cuprous chloride catalyst (\(CuCl\)) and aluminum chloride (\(Anhy\), \(AlCl_3\)), to form benzaldehyde (Ph-CHO). This reaction also produces benzaldehyde as a product.

Analyze reaction (C)

In reaction (C), we have: 1. \(Ph-CH_3\) (Toluene) reacts with chromic anhydride (\(CrO_3\)) and acetic anhydride \((CH_3CO)_2O\) forming benzophenone \(Ph-C(=O)-Ph\). This is Jones Oxidation. 2. \(Ph-C(=O)-Ph\) reacts with hydronium ion, but it doesn't convert benzophenone to benzaldehyde. Thus, this reaction does not produce benzaldehyde.

Analyze reaction (D)

In reaction (D), we have: 1. \(Ph-COCl\) (benzoyl chloride) reacts with \(H_2\) (hydrogen) in the presence of the \(Pd\)-catalyst supported on \(BaSO_4\) to form benzaldehyde (Ph-CHO) through hydrogenation. This reaction produces benzaldehyde as a product.

Conclusion

From the given reactions, reactions (A), (B), and (D) produce benzaldehyde as a product, while reaction (C) does not.

Key concepts

Organic Chemistry and Benzaldehyde Synthesis

Organic chemistry is a branch of chemistry that focuses on the study of the structure, properties, and reactions of organic compounds, which contain carbon. In the context of benzaldehyde synthesis, understanding the fundamental concepts of organic reactions is crucial. Benzaldehyde itself is an aromatic aldehyde famous for its almond-like fragrance, and it is commonly used in flavoring, perfumes, and as a precursor to various other organic compounds.

To synthesize benzaldehyde, students must familiarize themselves with various organic reactions and mechanisms. As seen in the solutions provided for the exercise, different reactants and catalysts can lead to the production of benzaldehyde through a variety of pathways. For instance, the use of a chlorinating agent followed by hydrolysis, or the Gattermann-Koch reaction which involves a reaction of benzene with carbon monoxide and hydrogen chloride in the presence of catalysts.

When encountering benzaldehyde synthesis in the curriculum, it's essential for students to be adept at recognizing functional groups, understanding reaction conditions, and predicting the outcome of reactions. Mastery of these topics not only aids in problem-solving but also is pivotal for deeper comprehension of organic chemistry principles.

Chemical Reactions in Benzaldehyde Production

The production of benzaldehyde can be achieved through various chemical reactions, each involving different reactants and conditions. Some of the key reactions include chlorination, hydrolysis, the Gattermann-Koch reaction, oxidation, and hydrogenation. These reactions are prime examples of how organic chemists convert one organic molecule into another with more useful or desired properties.

In reaction (A), the process starts with chlorination, where toluene is converted into a chlorinated intermediate. Subsequent hydrolysis results in the formation of benzaldehyde. The Gattermann-Koch reaction (B) illustrates the use of carbon monoxide as a carbon source in a carbonylation reaction to construct the aldehyde group directly onto the benzene ring. Jones Oxidation (C) typically converts primary alcohols into carboxylic acids and secondary alcohols into ketones but does not give benzaldehyde from benzophenone. Lastly, the hydrogenation process (D) demonstrates the reduction of a carbonyl chloride to an aldehyde.

These reactions underscore the versatility and creativity needed in organic synthesis. By knowing the behavior of different chemical species under various conditions, students can predict and strategize the synthesis of target molecules like benzaldehyde.

JEE Main and Advanced: Organic Chemistry in Competitive Exams

For students preparing for the Joint Entrance Examination (JEE) Main and Advanced, organic chemistry is a significant portion of the chemistry syllabus. It demands a strong understanding of concepts, mechanisms, and the ability to apply this knowledge to problem-solving. Exercises like benzaldehyde synthesis are typical in the type of questions that may appear in these exams.

Competitive exams often challenge students to identify the correct reactants, reagents, and conditions necessary to carry out a particular reaction. The ability to analyze and choose the correct sequence of reactions, as demonstrated in the exercise on benzaldehyde synthesis, is key. Strategies for tackling these questions include understanding reaction mechanisms, memorizing functional group transformations, and being familiar with named reactions, such as the Gattermann-Koch reaction mentioned earlier.

Incorporating these problem-solving tactics during exam preparation can significantly enhance a student's performance. Moreover, understanding the underlying concepts deeply not only prepares students for JEE but also lays a solid foundation for future studies in chemistry and related fields.