Concept:Thionyl chloride (\( \text{SOCl}_2 \)) is the premier, preferred reagent for converting carboxylic acids into acid chlorides because the byproducts are gaseous and immediately leave the reaction mixture, driving it to completion.
Formula:$$ \text{CH}_3\text{COOH} + \text{SOCl}_2 \longrightarrow \text{CH}_3\text{COCl} + \text{SO}_2\uparrow + \text{HCl}\uparrow $$
Solution:- Acetic acid reacts with \( \text{SOCl}_2 \) to form acetyl chloride.
- The gas evolution (\( \text{SO}_2 \) and \( \text{HCl} \)) makes the product incredibly easy to purify without complex distillation.
Why other options are incorrect:Aqueous \( \text{HCl} \) does not readily substitute the \( -\text{OH} \) of a carboxylic acid because the leaving group (\(\text{OH}^-\)) is poor. Options C and D are not halogenating agents.
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