An acyl chloride can be synthesized by reacting a carboxylic acid with thionyl chloride ($\text{SOCl}_2$).

To elaborate, the preparation of an acyl chloride is classified as a type of acylation reaction and is a common procedure in organic chemistry. The general reaction can be represented as follows:

In this equation, $R$ denotes any alkyl or aryl group.

The reaction commences with the carboxylic acid ($\text{RCOOH}$) interacting with thionyl chloride. Thionyl chloride acts as a potent dehydrating agent, effectively removing water from the carboxylic acid and facilitating the formation of an acyl chloride ($\text{RCOCl}$).

Typically, this reaction is conducted under reflux conditions. This involves heating the reaction mixture to its boiling point while condensing the vapors back into the flask. The reflux setup ensures that the reaction proceeds to completion without the loss of volatile components.

The by-products generated during this reaction are sulfur dioxide ($\text{SO}_2$) and hydrogen chloride ($\text{HCl}$), both of which are gaseous and can be readily expelled from the reaction mixture. The resulting acyl chloride is usually purified through distillation.

It is crucial to perform this reaction within a fume hood due to the release of toxic and corrosive gases. Additionally, thionyl chloride is hazardous and can cause severe burns, necessitating careful handling.

In conclusion, the synthesis of an acyl chloride involves the reaction of a carboxylic acid with thionyl chloride under reflux conditions. This straightforward process yields an acyl chloride, which serves as a valuable intermediate in various organic reactions.