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Ester hydrolysis-Acid and base catalyzed hydrolysis of ester, Practice Problems and FAQs

Soaps are an integral part to maintain the good health and hygiene of individuals. Soaps are necessary for cleaning dirt and oil from items, especially the skin. Soaps are commonly used for bathing, cleaning, and other home tasks. Soap is now an essential everyday item and finds its importance in everyday life. But, have you ever wondered how is soap made? Soaps are made by base catalysed hydrolysis of ester. So lets understand what is ester hydrolysis. Under what conditions does it occur?

Table of contents

  • Introduction
  • What are ester?
  • Hydrolysis of Ester
  • Acid-catalyzed hydrolysis of ester
  • Saponification: (Base- catalyzed hydrolysis of ester)
  • Practice problems
  • Frequently Asked Questions-FAQs

Introduction

The phrase hydrolysis is made up of two words: hydro, which means water, and lysis, which means breakdown. In simple words, hydrolysis is defined as a reaction in which water is used to break particular bonds in a chemical. Usually, the compounds that are broken down with the help of water are complex molecules. This is the counterpart of condensation, which results in the formation of a large molecule from two small molecules. Lets study in detail about ester hydrolysis.

What are Esters?

Esters are presented as derivatives of carboxylic acids. The '-OH' of '-COOH' (carboxylic acid functional group) is replaced with 'OR' in these compounds. As a result, the functional group of the molecule has the structure R-COO-R'. Thus, the structure of the functional group of the compound is R-COO-R’. Here, R stands for alkyl/aryl group; this group can be either identical or different. Esters can undergo a wide set of reactions, including reactions with Grignard’s reagent and hydrolysis reactions.

Hydrolysis of Ester

Ester contains a carbonyl carbon atom which is a weak electrophile. This electrophile attracts the nucleophile, OH- from the water and brings about the hydrolysis of the ester. The breakdown of the bond causes one fragment of ester to react with the hydrogen of water and form alcohol, whereas the other carbonyl part will react with the hydroxyl group of the water and form a carboxylic acid. Very few ester molecules will be able to undergo hydrolysis under normal conditions, so the only way to enhance the rate of the reaction is by adding acid or base catalyst.

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1. Acid-catalyzed hydrolysis of ester

Acid-catalyzed hydrolysis of the ester is a reversible reaction and follows nucleophilic substitution mechanism.


The mechanism of the above mentioned reaction is given below.

Mechanism:

The carbonyl carbon is transformed to a powerful electrophile in the first step of acid-catalyzed ester hydrolysis by protonation of the carbonyl group's oxygen atom. This hydrogen atom is donated by the acid catalyst.

The water molecule will now attack the carbonyl carbon atom by donating the oxygen’s lone pair of electrons and forming a bond. This bond formation between water and carbonyl carbon will cause the double bonded electrons to be transferred to the carbonyl group’s oxygen. This step is called nucleophilic addition.

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Proton is transferred from the O of H2O to O of -OR . Now, O of -OH comes back to form C=O bond followed by the removal of R-OH.

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Thus, final deprotonation occurs, to form carboxylic acid.

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2. Base-catalyzed hydrolysis of ester (Saponification)

Saponification refers to the base-catalyzed hydrolysis of an ester, which is commonly employed in the production of soap from triacylglycerol (ester).

In this case, the base's hydroxide ions (catalyst-sodium hydroxide) attack the ester's carbonyl group.

Example-


Mechanism of basic hydrolysis of ester:

First involves the nucleophilic attack of OH-on electrophilic carbonyl carbon to give tetrahedral anionic intermediate.

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Negative charge on O comes back and form C=O followed by removal of R20-, given corresponding carboxylic acid. 

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Deprotonation of carboxylic acid occurs under basic condition to give carboxylate anion. 

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Due to the three carboxyl groups in triacylglycerol, three molecules of sodium hydroxide are needed to react with one molecule of triacylglycerol.

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At the end of the reaction, sodium salts of fatty acids (carboxylic acid) are obtained with glycerol (alcohol).

Applications of saponification:

1. Producing hard and soft soaps

Soaps generated through saponification have different properties. As a result, they are divided into two categories: hard and soft soaps. Because of the alkali employed in their synthesis, the soaps in these categories are frequently distinct from one another. Hard soaps are ones that contain NaOH as a main ingredient. In hard water, which contains magnesium and calcium salts, hard soaps are also beneficial. Soft soaps are the one that are derived from KOH.

2. Fire extinguishers

The use of wet fire extinguishers can put out fires initiated by cooking oils and fats. Saponification reactions are used in these types of fire extinguishers. Saponification is the process of converting highly flammable liquids into non-combustible soaps.

Practice problems

Q 1. What will be the product of the acidic hydrolysis of the following ester?

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a. C6H5-COO18H +C2H5OH
b. C6H5-COOH +C2H5O18H
c. Both A and B
d. None of the above

Answer: A

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 Q 2. The nucleophile involved in acid-catalyzed hydrolysis of the ester is:

a. H2O
b. OH-
c. ROH
d. None of the above

Answer: A
The nucleophile involved in acid-catalyzed hydrolysis of the ester is H2O. The water molecule attacks the carbonyl carbon atom by donating the oxygen’s lone pair of electrons and forming a bond. 

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Q 3. Acid-catalyzed hydrolysis is an example of 

a. Nucleophilic addition reaction
b. Electrophilic addition reaction
c. Nucleophilic substitution reaction 
d. Electrophilic substitution reaction

Answer: C
It is a nucleophilic substitution reaction as water act as a nucleophile, by attacking the carbonyl carbon of ester. 

Q 4. What will be the product of the basic hydrolysis of the following ester?

CH3COOC2H5 + NaO18H ?

a. CH3COO18H +C2H5OH
b. CH3CO(O18)-Na+ +C2H5OH
c. CH3COOH +C2H5O18H
d. CH3OONa++C2H5O18H

Answer: B

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Frequently Asked Questions-FAQ

Q 1. What is the application of saponification in daily life?
Answer:
Hydrolysis of esters in the presence of potassium or sodium hydroxide forms the product of potassium or sodium salts of fatty acid, which is used to prepare soaps. Potassium hydroxide is used in the preparation of soft soaps, whereas sodium hydroxide is used in the preparation of hard soaps.

Fatty acid salts are also found in wet fire extinguishers.

Q 2. Out of acidic and basic hydrolysis of ester, which will be faster?
Answer: 
Acidic hydrolysis will be faster because -OH leaves as good leaving group i.e. H2Ounlike in basic hydrolysis where it leaves as OH- which is a poor leaving group.

Q 3. What is the role of H3O+ at the end of the basic hydrolysis of ester?
Answer: 
Sodium salt of carboxylic acid is obtained on basic hydrolysis of any given ester. In order to get carboxylic acid as the final product, acidic workup is done.

Q 4. How can we make acidic hydrolysis of ester irreversible reaction?
Answer: 
The products of acidic hydrolysis of ester are carboxylic acid and alcohol. Removing either one of them would make the reaction move forward according to Le chatelier's principle. 

Related Topics

Nucleophilic Substitution Reaction

Addition of grignard's reagent on carbonyl compounds

Electrophilic Addition Reactions-Electrophilic addition reactions on alkenes

Acid derivatives - Structure and chemical properties

Electrophilic Addition Reactions-Electrophilic addition reactions on Alkynes

Structure, physical properties, chemical properties and uses of carboxylic acid


 

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