Hydrolysis of esters to carboxylic acids with aqueous acid
Description: When an ester is treated with aqueous acid (and heat) it is converted into a carboxylic acid
Notes: This reaction is an equilibrium and is the opposite of Fischer esterification. To ensure this reaction goes to the carboxylic acid, water is generally used as solvent.
Acid is often written as H3O(+) or H2SO4/H2O
Notes: The byproduct of all of these reactions is the alcohol. Note the last example which shows a cyclic reactant to a linear product – these types of reactions often give students problems!
Mechanisms: This reaction follows the same 5-step sequence as its reverse reaction, the Fischer esterification. Protonation of the carbonyl oxygen (Step 1, arrows A and B) makes the carbonyl carbon a much better electrophile. It undergoes 1,,2-addition by water (Step 2, arrows C and D) whereupon the proton from water is transferred to the alcohol oxygen (Step 3, arrows E and F). This makes this group a much better leaving group, and the alcohol is then displaced in an elimination reaction (Step 4, arrows G and H) to give the protonated carboxylic acid, which is then deprotonated (Step 5, arrows I and J).
Notes: All of these steps are actually in equilibrium. This reaction proceeds the way it does because water is present in large excess (it’s the solvent)
There are other ways of reasonably showing proton transfer (i.e. Step 3) and also several other species that could act as bases besides TsO(–) here is just the counter-ion for H3O(+) and is not crucial for the reaction itself.