Equation for carboxylic acids and their derivatives

Question

Write an equation for the reaction of PCl5 with ethanoic acid:

 

and 

 

How might testing pH followed by the reaction with PCl5 be useful in an
analysis of an unknown organic compound?

Answer 1

CH3C(O)OH + PCl₅ -> CH3C(O)Cl + P(O)Cl₃ + HCl

 

(PCl₅ turns carboxylic acids into acid chlorides)

 

The pKa of a carboxylic acid is 5, and the pKa of one of the byproducts, HCl, is -7. During this reaction, the pH will drop significantly as more HCl is formed.