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Why is thionyl chloride useful for making acyl chlorides?

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oxidation of alcohols — carboxylic acid Alcohols may be oxidized to give aldehydes, ketones, and carboxylic acids. The oxidation of organic compounds generally increases the number of bonds from carbon to oxygen, and it may decrease the number of bonds to hydrogen.

Thionyl chloride is considered the most convenient reagent for converting a carboxylic acid into an acyl chloride because of the nature of its reaction byproducts.

When a carboxylic acid reacts with thionyl chloride, the other products generated are hydrogen chloride and sulfur dioxide. Because both of these byproducts are gases, they naturally escape the reaction mixture. This simplifies the process of isolating the resulting acyl chloride, as there are no liquid or solid impurities to separate from the final product.1

While other reagents like phosphorus trichloride ( $P C l_{3}$ ) or phosphorus pentachloride ( $P C l_{5}$ ) can also facilitate this transformation, the ease of removing gaseous byproducts makes thionyl chloride the preferred choice for this synthesis.12