Lu Le Laboratory: Saponification and Hydrolysis-Carboxylic acid-Lu Le Laboratory

Sunday, April 7, 2013

Saponification and Hydrolysis-Carboxylic acid-Lu Le Laboratory

All the derivatives of carboxylic acids can undergo hydrolysis (cleavage by water) to yield the carboxylic acids themselves. These hydrolysis reaction can be carried out under either acidic or alkaline conditions. For example:

In the case of an ester, the acid-catalyzed hydrolysis is reversible; therefore, a large excess of water is used to drive the reaction to the carboxylic acid and alcohol side. The acidic hydrolysis of the other derivatives is not reversible.

Saponification: Alkaline Hydrolysis of an Ester

The alkaline hydrolysis of an ester is called saponification. This reaction is irreversible and thus gives a better yield of a carboxylic acid and an alcohol than does acidic hydrolysis of an ester.

The saponification reaction itself yields a carboxylate ion and an alcohol. If a free carboxylic acid is the desired product, the reaction mixture must be acidified after the saponification is complete. The intermediate carboxylate is rarely isolated.

Saponification:

RCO₂R’ + OH^-→ RCO₂^-+ R’OH

Acidification:

RCO₂^- + H⁺ → RCO₂H

Saponification is an example of a nucleophilic acyl substitution, and proceeds in two step: an addition, followed by an elimination. Step 1 is the nucleophilic attack of OH^- on the partially positive carboxyl carbon.

Step two is the elimination of the alcohol, which occurs in two stages: loss of an alkoxide ion and a concurrent acid-base reaction, resulting in the observed products, an alcohol and a carboxylate ion.

Mechanism:

Chemicals:

1. Hydrochloric acid: 12M, 2~4 mL

2. Methyl benzoate: 1.4 g

3. Sodium hydroxide: 1.2 g

Procedure

Saponification

1. Weight 1.4 g of methyl benzoate into a 50mL round-bottom flask.

2. Add a solution of 1.2g sodium hydroxide in 15mL of water.

3. Add a stir bar, fit the flask with a reflux condenser, and heat the mixture under reflux until it becomes homogeneus (20~40 minutes).

It becomes homogeneous

Acidification

1. Cool the solution to room temperature in an ice bath, transfer it to an conic flask, and add concentrated hydrochloric acid dropise the mixture is acidic to pH paper or litmus paper.