Hydrolysis of Ethyl Acetate

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BACKGROUND

Aim: Hydrolysis of Ethyl Acetate.

Complete hydrolysis can be rapidly obtained, if the ester is boiled under reflux with a dilute aqueous solution of either an alkali, such as sodium hydroxide, or of a strong inorganic acid, such as sulphuric or hydrochloric acid.

In the former case, the sodium hydroxide neutralises the acetic acid as fast as it is formed, and so promotes the hydrolysis: in the latter case, the dilute inorganic acid catalyses the hydrolysis, which in the presence of a large excess of water goes practically to completion.

Since the sodium salts of some of the higher fatty acids are soaps, the general process of alkaline hydrolysis of esters is frequently called as saponification.1

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Hydrolysis of Ethyl Acetate

Mechanism of the base catalyzed hydrolysis of esters:

In first step hydroxide nucleophiles attacks at the electrophilic C of the ester C=O, breaking the π bond and creating the tetrahedral intermediate. In second step the intermediate collapses, reforming the C=O results in the loss of the leaving group the alkoxide, RO, leading to the carboxylic acid. 

In third step an acid / base reaction occurring with very rapid equilibrium where the alkoxide RO functions as a base deprotonating the carboxylic acid, RCO2H, (an acidic work up would allow the carboxylic acid to be obtained from the reaction).1

Hydrolysis of Ethyl Acetate

Mechanism of the acid catalyzed hydrolysis of esters: 

In first step activation of the ester occurs. Protonation of the ester carbonyl makes it more electrophilic. In second step the water O functions as the nucleophile attacking the electrophilic C in the C=O, with the electrons moving towards the oxonium ion, creating the tetrahedral intermediate. In third step acid/base reaction occuring.

Deprotonate the oxygen that came from the water molecule to neutralise the charge. In fourth step the acid/base reaction needs to make the -OCH3 leave, but need to convert it into a good leaving group first by protonation. In fifth step the electrons of an adjacent oxygen help “push out” the leaving group, a neutral methanol molecule.

In sixth step acid/base reaction occuring. Deprotonation of the oxonium ion reveals the carbonyl C=O in the carboxylic acid product and regenerates the acid catalyst.2

Hydrolysis of Ethyl Acetate

REQUIREMENTS

Reagents:         

  • Ethyl Acetate
  • 10% Sodium hydroxide
  • Dilute sulphuric acid

Apparatus:       

  • Round-bottomed flask
  • Reflux water-condenser set with knee tube
  • Porcelain fragments

PROCEDURE

5 ml of ethyl acetate is placed in a 100 ml round-bottomed flask, and about 50 ml of 10% sodium hydroxide solution is added, with some porcelain fragments. The flask is fitted with a reflux water-condenser, and the mixture is boiled gently over a wire gauze for 30 minutes.

The condenser is disconnected, and it is fitted by means of a bent delivery-tube (or “knee-tube”) to the flask for direct distillation. The liquid is reheated, and the first 10 ml. of distillate is collected, which will consist of a dilute aqueous solution of ethanol.

The presence of ethanol is confirmed by the iodoform test (To 0.5 ml of ethanol 3 ml of 10% KI solution and 10 ml of NaOCl solution are added. Warmed gently, fine yellow crystals of CHI3 separate. Isopropanol gives CHI3 in the cold. Pure methanol and the other alcohols in this section don’t give the reaction).

The residual liquid in the flask is a dilute alkaline solution of sodium acetate. Dilute sulphuric acid is added to get the acetic acid until the solution is definitely acid to litmus and then distilled off to about 20 ml. On this aqueous distillate the tests for acetic acid is performed.

(General Tests of carboxylic acid: 1. Soluble in NaOH solution. 2. Soluble in Na2CO3 solution with the evolution of CO2. 3. Heating the acids or their salts with soda-lime eliminates the carboxyl group, volatile products being often detectable. 4. With alcohols and sulphuric acid give esters which frequently have characteristic odours. 5. The neutral salts of many acids treated with ferric chloride give colorations or precipitates.)

CONCLUSION

From the above tests it is concluded that ethyl acetate hydrolyses into ethanol and ethanoic acid with a faster rate due to the catalystic effect of NaOH (base) solution, unless the process takes hours to complete the reaction.

REFERENCES

  1. Practical organic chemistry by Frederick George Mann and Bernard Charles Saunders: Page- 99.
  2. Techniques and Experiments for Organic Chemistry by Addison Ault, sixth edition; Page-291.

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