Acetic acid reacts with ${\mathrm{PCl}}_5$to form:

Acetyl chloride is produced by heating acetic acid with phosphorus pentachloride. The COOH group is changed to the COCl group in this reaction. 
  

${\mathrm{CH}}_3\mathrm{COOH} + {\mathrm{PCl}}_5 \to {\mathrm{CH}}_3\mathrm{COCl}+\mathrm{HCl} +{\mathrm{POCl}}_3$

The general reaction given by carboxylic acid is nucleophilic addition elimination, also known as acyl substitution