Tuesday, February 5, 2008

Ch.28 Carboxylic Acid - Ester Hydrolysis

Ester hydrolysis

Hydrolysis of Esters

Esters are hydrolysed water and the process is accelerated by dilute mineral acids (HCL or H2SO4) or alkalies.

Ethyl acetate + water  gives Acetic acid + Ethyl alcohol (in the presence of dilute mineral acid)

Ethyl acetate + Sodium hydroxide (alkali)  Sodium acetate + Ethyl alcohol

Acidic hydrolysis is reversible.

Alkaline hydrolysis is irreversible because resonance stabilized carboxylate (acetate ion is formed and it has very little tendency to react with an alcohol.

Hydrolysis of esters by alkalies is known as saponification. Saponification leads to the formation of soaps. It is a very rapid reaction as alkali acts as catalyst and also as a reactant.

Mechanism of alkaline hydrolysis of esters.

Step I. the nucleophile, OH ¯ ion from the alkali attacks the carboxyl carbon to form an intermediate

Step II the intermediate, then loses a molecule of ethoxide ion to form acetic acid .

Step III. Ethoxide ion then removes a proton from acetic acid and thus acetate ion is formed.

Step IV. This acetate ion combines with Na+ to form Sodium acetate.

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