Hydrolysis of Ester - Acid Hydrolysis of Ester
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.
FAQs on Ester Hydrolysis
1. What is the hydrolysis of an ester as per the CBSE Class 12 syllabus?
Ester hydrolysis is a chemical reaction where an ester molecule is split into its constituent parts—a carboxylic acid and an alcohol—by reacting with water. This process is typically very slow and is therefore catalysed by either a dilute acid or a dilute alkali (base).
2. What are the two main types of ester hydrolysis?
The two primary methods for hydrolysing an ester are:
- Acid-Catalysed Hydrolysis: The ester is heated with an excess of water containing a strong acid catalyst (like dilute H₂SO₄). This reaction is reversible.
- Alkaline Hydrolysis (Saponification): The ester is heated with a strong base, such as sodium hydroxide (NaOH). This reaction is irreversible and yields a carboxylate salt and an alcohol.
3. How is ester hydrolysis carried out in an acidic medium?
In an acidic medium, ester hydrolysis is performed by heating the ester under reflux with a dilute mineral acid like dilute hydrochloric acid (HCl) or sulphuric acid (H₂SO₄). The reaction is an equilibrium process. For example, when ethyl ethanoate is hydrolysed, it produces ethanoic acid and ethanol. To push the equilibrium towards the products, a large excess of water (from the dilute acid) is used.
4. What is saponification, and why is it also called alkaline hydrolysis of esters?
Saponification is the chemical term for the hydrolysis of an ester using a base. It is called alkaline hydrolysis because it uses an alkali, such as sodium hydroxide (NaOH). The term 'saponification' literally means "soap making" (from Latin *sapon*, meaning soap), as this exact reaction is used to produce soap by hydrolysing fats and oils (which are large esters).
5. Why is alkaline hydrolysis of an ester considered irreversible, while acid hydrolysis is reversible?
The key difference lies in the stability of the products formed. In alkaline hydrolysis, the carboxylic acid produced immediately reacts with the alkali to form a stable carboxylate salt (R-COO⁻). This salt is resonance-stabilised and shows no tendency to react with the alcohol, making the reaction a one-way process. In contrast, acid hydrolysis is an equilibrium between the ester, water, carboxylic acid, and alcohol, allowing the reverse reaction (esterification) to occur simultaneously.
6. What is the step-by-step mechanism for the acid-catalysed hydrolysis of an ester?
The acid-catalysed hydrolysis of an ester, such as ethyl ethanoate, follows these steps:
- Step 1: Protonation: The carbonyl oxygen of the ester is protonated by the acid catalyst, making the carbonyl carbon more electrophilic.
- Step 2: Nucleophilic Attack: A water molecule acts as a nucleophile and attacks the highly electrophilic carbonyl carbon.
- Step 3: Proton Transfer: A proton is transferred from the oxonium ion to the -OR' group (ethoxy group), turning it into a good leaving group (-OHR').
- Step 4: Elimination: The alcohol molecule (ethanol) is eliminated, and a double bond reforms between the carbon and the remaining oxygen.
- Step 5: Deprotonation: The proton on the carbonyl oxygen is removed, regenerating the acid catalyst and forming the final carboxylic acid product.
7. What is the most significant real-world application of ester hydrolysis?
The most significant industrial application of ester hydrolysis is the manufacture of soap. This process, known as saponification, involves the alkaline hydrolysis of triglycerides (esters of fatty acids and glycerol) found in animal fats and vegetable oils. Heating these fats and oils with a strong base like sodium hydroxide breaks them down into glycerol (an alcohol) and the sodium salts of fatty acids, which are the primary chemical components of soap.
8. How does ester hydrolysis fundamentally differ from esterification?
Ester hydrolysis and esterification are essentially reverse processes. The key differences are:
- Reaction Type: Hydrolysis is a decomposition reaction that splits an ester using water. Esterification is a condensation reaction that combines a carboxylic acid and an alcohol, removing water.
- Reactants vs. Products: The products of hydrolysis (carboxylic acid and alcohol) are the reactants for esterification, and vice-versa.
- Reaction Conditions: Hydrolysis is favoured by an excess of water (e.g., dilute acid). Esterification is favoured by the removal of water, often using a concentrated acid as both a catalyst and a dehydrating agent.
