The hydrolysis of an ester is a reaction in which the (-OR) group of the ester is replaced by the hydroxyl (-OH) group from water. This reaction can be catalyzed either by an acid or a base.
Esters are derivatives of carboxylic acid, they are formed by the elimination of a water molecule from the condensation elimination reaction of an acid and alcohol. Esters undergo the nucleophilic substitution reaction when added to water.
Esters are organic compounds with an ester (-COOC-) linkage. These are usually liquids and low melting solids. All methyl and ethyl esters are liquid at room temperature. The boiling points of ester are lower than acid and alcohol, but they are close to those of aldehyde and ketones. Although they are less soluble in water, their solubility further decreases with increasing carbon chain length.
- Esters are used as flavoring agents.
- They are used in many kinds of paint removers, for example, a dilute solution of Ethyl acetate is used as nail polish remover.
- In the form of fats and oils, esters are a part of our daily diet.
Hydrolysis is a type of chemical reaction in which the chemical bonds are broken down by the addition of water, as evident from its name, hydro means water and lysis means to unbind or break down.
Hydrolysis is seen in different types of reactions like hydrolysis of;
- Salts of weak acids and weak bases
- Esters and amides
- ATP (Adenosine triphosphates)
- Metal ions in aqueous solutions
Nucleophilic substitution reaction
It is a type of reaction in which an electron-rich species (nucleophile) attacks the electrophilic carbon resulting in the displacement of leaving group.
Hydrolysis of esters is an example of a nucleophilic substitution reaction.
Hydrolysis of esters
When esters are heated in the presence of a mineral acid e.g. dilute sulphuric acid (H2SO4) or hydrochloric acid (HCl) or a strong alkali such as sodium hydroxide (NaOH), they are hydrolyzed slowly to carboxylic acid and alcoholic groups.
Acid-catalyzed hydrolysis of esters
Esters are hydrolyzed into carboxylic acids and alcohols on heating with water in the presence of mineral acids such as H2SO4 or HCl. The mineral acids act as catalyzing species and this reaction is known as acid-catalyzed hydrolysis of esters.
It is a reversible reaction, and it is just the reverse of acid-catalyzed esterification. However, the conditions are slightly different for the formation of ester or its hydrolysis. For example:
- Esterification is favored in the presence of concentrated acids.
- Ester hydrolysis is favored in the presence of dilute acids.
CH3COOCH3 + H2O 〈HCl〉 ⇌ 〈HCl〉 CH3COOH + CH3OH
Mechanism of acid-catalyzed hydrolysis
- In the first step, the oxygen atom of the carbonyl group is protonated by the mineral acid.
- The positively charged oxygen atom becomes more electronegative, it increases the electrophilicity of the carbonyl carbon. The water molecule attacks the electrophilic carbon, resulting in the formation of a tetrahedral intermediate.
- In the next step, the molecule gets rid of the -OR group. This happens by protonation at the -OR group by an acid.
- Since (-ROH) is a good leaving group, it detaches itself from the molecule. At this point, a protonated carboxylic acid is generated.
- A weak base such as methanol or water is used to deprotonate the carboxylic acid.
Base catalyzed hydrolysis
The hydrolysis of an ester in the presence of a strong alkali such as sodium hydroxide into the sodium salt of carboxylic acid and alcohol is known as base-catalyzed hydrolysis or saponification.
Unlike acid-catalyzed hydrolysis, it is an irreversible reaction. This is because a negatively charged and resonance stabilized carboxylate ion shows less tendency to react with alcohol.
CH3COOCH3 + H2O 〈NaOH〉 → CH3COO–Na+ + CH3OH
Mechanism of base-catalyzed hydrolysis
- In the first step, the hydroxide ion acts as a nucleophile. It attacks the electrophilic carbonyl carbon, resulting in the formation of a tetrahedral intermediate. This step is reversible.
- The negatively charged oxygen atom generates a double bond and it removes a -OR group. It results in the formation of carboxylic acid, this step is irreversible.
- In the last step, the alkoxide ion abstract acidic hydrogen and formed a carboxylate ion and alcohol.
Rate of hydrolysis of esters
There are two methods to increase the rate of hydrolysis of ester:
- Esters are less electrophilic and do not react with water at a pH of 7. One method for increasing the rate of ester hydrolysis is to increase the electrophilicity of the ester. This can be done by using a mineral acid to form a positively charged intermediate which is more reactive. Mineral acids such as HF, HCl, HBr, HI, H2SO4, H3PO4, H3BO3, and HNO3, etc serve the purpose.
- The rate of ester hydrolysis can also be enhanced by increasing the reactivity of a nucleophile. The hydroxide ion should be used instead of water. It means that alkalis are suited as well to increase the rate of hydrolysis. Alkalis like NaOH, KOH, Mg(OH)2, and Ca(OH)2, etc can be used.
Order of acid-catalyzed hydrolysis of esters
Acid-catalyzed hydrolysis of an ester is a first-order reaction. The progress of this reaction is studied by volumetric analysis such as acid-base titration. With the passage of time, the greater quantity of acetic acid is produced, the more and more acid will be used.
Order of base-catalyzed hydrolysis of esters
Base-catalyzed hydrolysis of an ester is a second-order reaction. The progress of this reaction is studied by the volumetric analysis. The amount of NaOH consumed is decreasing every moment.
- Hydrolysis of esters is a slow process because of the two factors:
- Low nucleophilicity of water molecules.
- Stearic hindrance is much effective due to the presence of two oxygen atoms around the electrophilic carbon.
- As the hydrolysis of esters can be driven by acid-catalyzed mechanism, and since the product of this reaction is also acid, this reaction can perform autocatalysis.
- Acid-catalyzed hydrolysis of esters is a first-order reaction while base-catalyzed hydrolysis of esters is second ordered.
- Hydrolysis of esters is a temperature dependent reaction. Increasing the temperature increases its rate, however above a certain temperature the esters boil limiting this temperature dependency is to a certain limit (B.P ≈ 98 ºC for small chain esters).
How do you hydrolyze an ester?
Esters are hydrolyzed into carboxylic acid and alcohol. The hydrolysis of an ester is catalyzed either by mineral acids e.g. hydrochloric acid (HCl) and sulphuric acid (H2SO4) or alkali. e.g potassium hydroxide (KOH) and sodium hydroxide (NaOH).
Acid-catalyzed hydrolysis is the reverse of esterification.
What is the order of hydrolysis of an ester?
Acidic hydrolysis of an ester is a pseudo-first-order reaction while alkaline hydrolysis of an ester is a second-order reaction. The progress of the reaction is studied by the volumetric analysis.
Which catalyst is used in the hydrolysis of an ester?
The mineral acids such as sulphuric acid and hydrochloric acid are used as catalysts in the hydrolysis of an ester. They provide an H+ ion and increase the electrophilicity of the carbonyl carbon.
The base catalysts are also used such as sodium hydroxide. They produce (–OH) ions that are strong nucleophiles and hence accelerate the reaction.
Is ester hydrolysis reversible?
- Acid-catalyzed hydrolysis of an ester is a reversible reaction. It is a reverse of the esterification reaction.
- Base catalyzed hydrolysis of an ester is an irreversible reaction because the products formed during this reaction are alcohol and carboxylate ion which is resonance stabilized and does not readily react back to form esters.
What type of reaction is the hydrolysis of an ester?
Hydrolysis of an ester is a nucleophilic substitution reaction.
Acidic hydrolysis → water acts as a nucleophile that attacks protonated electrophilic carbon center of esters.
Alkaline hydrolysis → alkali (–OH) acts as a nucleophile to attack electrophilic centers of esters.
Are esters soluble in water?
Esters are slightly soluble in water because they have an oxygen atom which makes a hydrogen bond with the hydrogen atom of water.
However, the esters do not have hydrogen directly bonded with a highly electronegative atom i.e. F, O, N to make the H-bond with the oxygen atom of the water molecule so they are less soluble than carboxylic acids.
Why noncatalyzed hydrolysis of esters is a very slow reaction?
In non-catalyzed hydrolysis of esters, water acts as a nucleophile. However, it is a weak nucleophile so that the attack on the carbonyl carbon is very difficult. It requires an acid catalyst that increases the electrophilicity of the carbonyl carbon.
What is the difference between solvolysis and hydrolysis?
Hydrolysis is a chemical reaction in which chemical bonds are broken down in the presence of water whereas solvolysis is a chemical reaction in which chemical bonds are broken down in the presence of solvents. Solvolysis is a vast term that has several types like alcoholysis, hydrolysis, etc.
Why do amides require much harsher conditions for hydrolysis than esters?
In the periodic table, when two atoms are near (small size), their orbitals overlap, and the bonds become strong. The orbital overlap between the C-N bond of amide is better than the C-O bond of ester. So, much harsher conditions are required to break the C-N bond.
What is produced by the hydrolysis of oil?
In the hydrolysis of oil, the water reacts with oil to produce glycerol and fatty acid. This reaction is commercially important because fatty acids are used in soap making process called saponification.
Why ester hydrolysis is often performed base-catalyzed instead of acid-catalyzed?
Alkaline hydrolysis is preferred over acid-catalyzed hydrolysis because it is an irreversible reaction, and one of the products is salt of the carboxylic acid. It is soluble in water and can be easily separated from the reaction mixture.
What is formed when ester reacts with sodium hydroxide?
When ester reacts with sodium hydroxide, it produces alcohol and sodium salt of carboxylic acid. This reaction is known as base-catalyzed hydrolysis of ester or saponification.
What is the method to make an ester?
Different methods are used to make an ester. For example one of the methods for the preparation of ester is esterification. In this method, alcohols are treated with carboxylic acids in the presence of acids to form esters. Fischer esterification is generally the preferred way of esterification.