Fischer Esterification - Lu Lu Laboratory

A Fischer esterification is the reverse reaction of the acid-catalyzed hydrolysis of an ester. The mechanism of the Fischer esterification reaction is a series of protonations and deprotonations., along with an addition step and an elimination step. First, the carbonyl group of the carboxylic acid is protonated bt the acidic catalyst. Next, the weakly nucleophilic alcohol attacks the carbonyl carbon. In the final steps, an OH group is protonated and then elimination as water. A final deprotonation yields the ester.

Mechanism:

Step 1, protonation :

Step 2, addition of alcohol and loss of H⁺ :

Step 3, protonation, elimination, and deprotonation :

    Because the reaction mixture is an equilibrium mixture, the concentration of reactants and products may be used to determine an equilibrium constant, K:

    The equilibrium constants for Fischer esterifications are generally less than 4. From a synthetic standpoint, this means that when a 1:1 molar ration of reactants is used, the equilibrium mixture contains a substantial amount of the starting carboxylic acid. For example, if K = 4 and a 1:1 molar ration of alcohol to carboxylic acid is used, the highest possible yield of an ester can be 67%.

Chemicals:

1.      1-hexanol: 0.25 moles, 12.771g

2.      Acetic acid: 0.25 moles, 15.013g

3.      Concentrated sulfuric acid: 0.5mL

Procedure:

1.      Heat a mixture of 0.25 mol of glacial acetic acid, 0.25 mol of a primary alcohol, and 0.5 mL of concentrated sulfuric acid under reflux for 1.5 hours.

2.      Cool the reaction mixture.

 Concentrated sulfuric acid is the lower layer

3.      Dilute the mixture with an amount of water 3~4 times its volume.

4.      Wash the organic layer with water, with 10~15 mL of saturated sodium bicarbonate solution.

5.      And again wash with water.

6.      Dry the ester with anhydrous Na₂SO₄.

 

7.      Typical yield is about 80%.

 Other classmates' work. Different kinds

 of alcohols make different esters

Experimental Record

Theoretical Weight of 1-Hexyl Acetate	18.026g
Weight of 1-Hexyl Acetate	14.790g
Yield	82.0%
Theoretical nd20℃	1.4092
nd20℃ of the final product	1.4005

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.

2.      Cool the mixture in an ice bath.

3.      Vacuum-filter the product.

4.      Air-dry the solid at least overnight.

5.      Determine the melting point and percent yield.

Experimental Record

Melting Point	118.0 ~ 120.0 ℃
Weight of Benzoic Acid	1.070 g
Theoretical Weight of Benzoic Acid	1.200 g
Yield	89.2 %