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

Question

Maya Allen · Accepted Answer

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.

#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.

Carter Allen · Answer

undefined Acetyl chloride or acetic anhydride are used instead of acetic acid in preparing esters of acetic acid because they are more reactive towards alcohol and result in faster esterification reactions. Additionally, the byproducts formed during the reaction are either gaseous or easily removed, simplifying the purification process. This makes acetyl chloride and acetic anhydride more efficient and practical reagents for ester synthesis compared to acetic acid.