Why are acetyl chloride or acetic anhydride used instead of acetic acid in preparing esters of acetic acid?

Acetyl chloride and acetic anhydride are often used instead of acetic acid because they are more reactive and give better yields of product.

The Fischer esterification of an acid is an equilibrium reaction.

##underbrace("CH"_3"CO-OH")_color(red)("acetic acid") + underbrace("H-OCH"_2"CH"_3)_color(red)("ethanol") ⇌ underbrace("CH"_3"CO-OCH"_2"CH"_3)_color(red)("ethyl acetate") + "H"_2"O"##

To get a good yield of ester you must use a large excess of acetic acid and/or remove the water as it is formed.

Acetyl chloride and acetic anhydride are both more reactive than acetic acid.

(from www.chembook.co.uk)

The reactions give almost quantitative yield at low temperature and do not need a strong acid catalyst.

##underbrace("CH"_3"CO-Cl")_color(red)("acetyl chloride") + underbrace("H-OCH"_2"CH"_3)_color(red)("ethanol") → underbrace("CH"_3"CO-OCH"_2"CH"_3)_color(red)("ethyl acetate") + "HCl"##

##underbrace("CH"_3"CO-OCOCH"_3)_color(red)("acetic anhydride") + underbrace("H-OCH"_2"CH"_3)_color(red)("ethanol") → underbrace("CH"_3"CO-OCH"_2"CH"_3)_color(red)("ethyl acetate") + "HOCOCH"_3"##

High yields and low temperatures can be important considerations if the alcohol is expensive or temperature-sensitive.