Among the following, the number of reaction(s) that produce(s) benzaldehyde is :
This question asks you to identify which reactions produce benzaldehyde (C6H5CHO). Benzaldehyde is an aromatic aldehyde. Let's analyze each reaction step by step:
Reaction (i): This is the Etard reaction. Chromyl chloride (CrO2Cl2) oxidizes toluene (C6H5CH3) to benzaldehyde. The methyl group (-CH3) is selectively oxidized to an aldehyde group (-CHO).
Conclusion:
Reaction (ii): This reaction uses Carbon tetrachloride (CCl4) and Anhydrous AlCl3 with benzene. This is a form of the Gattermann-Koch reaction, where a formyl group (-CHO) is introduced onto the benzene ring using CO and HCl. The reagents given (CCl4 and AlCl3) can generate the required electrophile (e.g., [CCl3]+), but the typical product is benzotrichloride (C6H5CCl3), not benzaldehyde. Hydrolysis of benzotrichloride yields benzoic acid (C6H5COOH), not benzaldehyde.
Conclusion:
Reaction (iii): This is the Rosenmund reduction. An acyl chloride (benzoyl chloride, C6H5COCl) is reduced to an aldehyde using hydrogen gas (H2) over a palladium catalyst poisoned with barium sulfate (Pd/BaSO4). This reaction selectively stops at the aldehyde stage.
Conclusion:
Reaction (iv): Benzene reacts with Hydrogen cyanide (HCN) and Anhydrous AlCl3. This is another variation of the Gattermann reaction. Here, HCN and HCl (generated in situ) are used to introduce the -CHO group. The mechanism involves the formation of an imine intermediate, which on hydrolysis gives benzaldehyde.
Conclusion:
Final Answer: Three reactions produce benzaldehyde: (i), (iii), and (iv).
Key Reactions for Aldehyde Synthesis:
Important Distinction: The Gattermann-Koch reaction (using CO, HCl, AlCl3) and the reaction with CCl4/AlCl3 (which gives carboxylic acid after hydrolysis) are different and must not be confused.
In the given transformation, which of the following is the most appropriate reagent ?
In the following reaction, the major product is:
