Acidic and Alkaline Hydrolysis of Esters

Ester hydrolysis gives carboxylic acid ( or a salt of carboxylic acid ) and an alcohol as products. There are two different hydrolysis methods of esters in the organic chemistry. According to the hydrolysis method, carboxylic acid or salt of carboxylic acid is given. However, alcohol is given as a product in both methods (acidic and alkaline).

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Methods of Ester hydrolysis in chemistry

1. Acidic hydrolysis of esters
2. Alkaline hydrolysis of ester / basic hydrolysis

Ester hydrolysis is usually catalyzed by acids or bases (alkalis). Since -O-R group is a much poorer leaving group than halide or -OCOR, water alone cannot hydrolyze most esters. Therefore, we have to use an acid or a base for hydrolysis of ester reaction.

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Which carbon - oxygen bond in the ester molecule is broken during the hydrolysis process?

After the hydrolysis, the existed oxygen atom in the ester is gone towards the alcohol compound. Therefore bond between carbonyl carbon and oxygen atom is broken. See the figure to understand this bond breaking. This is same for both acidic and alkali hydrolysis of esters.

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Acidic hydrolysis of esters

Concentrated sulfuric acid is used for acidic ester hydrolysis. This reaction is done on heating. A carboxylic acid compound and an alcohol compound are given as products. And this reaction is a reversible one. Therefore we cannot get products completely from this reaction.

Concentrated sulfuric acid (H₂SO₄) behaves as a catalyst to increase the reaction rate. Special thing is, H₂SO₄ increases the reaction rate of both forward reaction and backward reaction.

Acidic hydrolysis of Ethyl ethanoate

Ethyl ethanoate reacts with concentrated sulfuric acid to give ethanoic acid and ethanol which are a carboxylic acid and an alcohol respectively.

Acidic hydrolysis of Ethyl propanoate

Ethyl propanoate acidic hydrolysis gives ethanol and propanoic acid. Concentrated H₂SO₄ is used on heating to get products.

Mechanism of acidic hydrolysis

Reaction is catalyzed by acids (H₂ SO₄) by making the carbonyl carbon more positive and therefore more susceptible to attack by the nucleophile. Neucleophiles desire to attack positively charged atoms.

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Alkaline hydrolysis of ester

Esters are hydrolyzed by strong alkalis such as NaOH_(aq) or KOH_(aq). As products, sodium salt of carboxylic acid and an alcohol are given.

But in this reaction, first carboxylic acid and alcohol are given as products. But produced carboxylic acid reacts with strong alkali to form carboxylic acid salt. Therefore carboxylic acid reacts with NaOH or KOH to give sodium salt of carboxylic acid.

Alkaline hydrolysis of Ethyl propanoate

Ethyl propanoate alkali hydrolysis gives ethanol and sodium propanoate. NaOH_(aq) or KOH_(aq) is used as an alkali for ethyl propanoate alkaline hydrolysis.

Mechanism of alkaline hydrolysis

When bases catalyze the hydrolysis, the attacking specie is the more powerful nucleophile OH^-. OH^- is like to attack positively charged atoms. This reaction is called saponification and gives the salt of the acid.

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Catalyst behavior of ester hydrolysis

Now we are going to study about catalyst behavior in ester hydrolysis.

Hydrolysis of esters by Sulfuric acid (H₂SO₄)

Sulfuric acid is a strong, dibasic acid. As products, a carboxylic acid compound and an alcohol compound is given by adding sulfuric acid (H₂SO₄). Concentration of H₂SO₄ acid is not changed during the reaction. Therefore H₂SO₄ behave as a catalyst.

Hydrolysis of esters by Sodium hydroxide (NaOH)

As acidic hydrolysis, first carboxylic acid and alcohol is given. But carboxylic acids have enough acidic strength to react with aqueous NaOH solution. Therefore NaOH is spent during the reaction and will have a lower concentration than the initial concentration of NaOH. Therefore NaOH is not a catalyst in hydrolysis of ester.

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Equilibrium states of acidic hydrolysis of ester

Acidic hydrolysis of ester is an equilibrium reaction. Therefore, that is practicable only when there is a way of shifting the equilibrium to the right to get better yield. Otherwise there is a mixture of several compounds in the final solution.

What is the best way to get alcohol and carboxylic acid by ester hydrolysis?

• As we leant earlier, alkaline hydrolysis completely hydrolysis the ester and gives the sodium salt of carboxylic acid and alcohol. To get the carboxylic acid from sodium salt of carboxylate, add dilute HCl acid.

Industrial applications of ester hydrolysis

Saponification process of soap production industry

In soap production industry, fat is hydrolyzed by aqueous sodium hydroxide to sodium salt and glycerin. Fat is a mixture of ester compound and there are several types of fat which can be used to manufacture soap. This process is called saponification.

These esters in the fat contain long chain of carbon atoms. In the hydrolysis process, following steps are occurred.

• First, carboxylic acid and glycerol are given.
• Then, produced carboxylic acid reacts with NaOH to give the soap.

Saponification can be done under different conditions.

• Cold process
• Hot process

When temperature is high, saponification become more successful and give good yield in a shorter time. Therefore hot process is more useful in industrial application of soap production.

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Esters susceptible to hydrolysis

Esters susceptible to hydrolysis from aqueous acids and aqueous strong alkalis. But given products are different.

The hydrolysis of ethyl ester with hot alkaline

Ester is broken to two parts, carboxylic acid and alcohol. Ethyl part will become ethanol.

How hydrolysis of an ester is different from hydrolysis of alkene?

Esters and alkenes give different products in hydrolysis. Hydrolysis of alkene produce alcohol as the product. Also for hydrolysis, used reaagents are different.

Is ester acidic or basic?

Soluble esters in the water, do not form H⁺ ions or OH^- ions to show acid base characteristics.

Base catalyzed ester hydrolysis

In acidic hydrolysis, concentrated H₂SO₄ is used as a catalyst.

Reaction of ester with aq NaOH

Ester is hydrolyzed from aqueous NaOH and give alcohol and sodium carboxylate as products.

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References - Ester Hydrolysis

MARCH�S ADVANCED ORGANIC CHEMISTRY, REACTIONS, MECHANISMS,AND STRUCTURE 6^th edition, Michael B. Smith, Jerry March, 16-59 Hydrolysis of Carboxylic Esters (p,1400)

Organic Chemistry Reactions for Grade 12 Chemistry