[Chemistry Class Notes] Ester Hydrolysis Pdf for Exam

Hydrolyzing esters – splitting or dividing them into carboxylic acids (or their salts) and alcohols with the help of water, dilute alkali or dilute acid. It begins by looking at the hydrolysis of simple esters like ethyl ethanoate and goes on to look at hydrolyzing larger, more complicated ones to make soap.

Hydrolyzing Simple Esters

What is Hydrolysis?

Precisely, hydrolysis is a reaction with water. That is just what occurs when esters are hydrolyzed with the help of water or by dilute acids such as dilute hydrochloric acid.

The alkaline hydrolysis of esters basically include reaction with hydroxide ions, but the overall result is the same that it is taken together with the other two.

 

Hydrolysis Using Water or Dilute Acid

The reaction with clean water is so very slow that it is never used. The reaction is catalyzed by dilute acid, and so the ester is heated under reflux with a dilute acid like dilute sulphuric acid or dilute hydrochloric acid.

Here are two common examples of hydrolysis using an acid catalyst.

First, hydrolyzing ethyl ethanoate:

And then hydrolyzing methyl propanoate:

Remember that the reactions are alterable reversible. To create the hydrolysis as complete as possible, you would have to use plenty of water. The water comes from the dilute acid, and so you would blend the ester with an extra of dilute acid.

Note:  These reactions are just the reverse of those used to make an ester from a carboxylic acid and an alcohol. The only difference in that situation is that you use a concentrated acid as the catalyst. To get as much ester as possible, you wouldn’t add any water or else you would favor the hydrolysis reaction. The process for the acid hydrolysis of esters is enclosed in the catalysis section of this site. 

 

Hydrolysis Using Dilute Alkali

This is the normal way of hydrolyzing esters. The ester is heated under reflux with a dilute alkali such as sodium hydroxide solution.

There are two big benefits of doing this rather than using a dilute acid. The reactions are one-way rather than reversible, and the products are easier to split.

Taking the similar esters as above, however using a sodium hydroxide solution rather than a dilute acid: 

First, hydrolyzing ethyl ethanoate using sodium hydroxide solution:

. . . and then hydrolyzing methyl propanoate in a similar way:

Remember that you get the sodium salt made rather than the carboxylic acid itself.

This mixture is comparatively easy to divide. Given you use an extra of sodium hydroxide solution, there won’t be any ester left. 

The alcohol produced can be distilled off. That’s easy!

If you want the acid rather than its salt, all you have to do is to add an extra of a strong acid like dilute sulphuric acid or dilute hydrochloric acid to the mixture left after the first distillation.

The mixture is flooded with hydrogen ions. These are selected by the ethanoate ions (or propanoate ions) existing in the salts to make ethanoic acid (or propanoic acid, etc). Because these are weak acids, once they are combined together with the hydrogen ions, they tend to stay combined.

The carboxylic acid can now be distilled off.

 

Hydrolyzing Complicated Esters to Make Soap

These next big deals with the alkaline hydrolysis (with the help of sodium hydroxide solution) of the big esters are seen in animal and vegetable fats and oils.

If the big esters existing in vegetable or animal oils and fats are heated with high concentrated sodium hydroxide solution precisely the similar reaction happens as with the simple esters.

A salt of a carboxylic acid is produced – in this case, the sodium salt of a big acid such as octadecanoic acid (stearic acid). These salts are the significant ingredients of soap – the ones that do the cleaning.

Alcohol is also made – in this case, the more complicated alcohol, propane-1, 2, 3-triol (glycerol).

Because of its relationship with soap making, the alkaline hydrolysis of esters is sometimes recognized as saponification.