Projection Experiment: Reactivity of Aromatic Esters

Demonstration of Microscale Projection Experiments - Chemie en miniature

Reactivity of Aromatic Esters

Objectives: Ester Hydrolysis, Nucleophilic Acyl Substitution, Substituent Effects

Peter Keusch

German version

Chemicals:
10 mmol ethyl 4-nitrobenzoate / 70 % aqueous solution of DMF (1.954 g / 10 mL)
10 mmol ethyl benzoate / 70 % aqueous solution of DMF (1.43 mL / 10 mL)
10 mmol methyl 4-methoxybenzoate / 70 % aqueous solution of DMF (1.66 g / 10 mL)
phenolphthalein 1 % in ethanol (Merck)
1 N NaOH

Experimental procedure:

The temperature of the water in the cuvette is approx. 70 °C (water bath).

Three test tubes are set up as described in the following table.

Test tube 1 1 mL of solution of ethyl 4-nitrobenzoate
Test tube 2 1 mL of solution of ethyl benzoate
Test tube 3 1 mL of solution of methyl 4-methoxybenzoate

The ester solutions are mixed with 1 drop of alcoholic phenolphthalein and 1 drop of NaOH.

Results:

The violet color of the solutions turns yellow.

Test tube 1 immediately
Test tube 2 after 1 minute
Test tube 3 after 2 minutes

Photo 1

Photo 2

Discussion:

· The three aromatic carboxylic acid esters are present in the same concentration. They are allowed to react with NaOH at same temperature. The endpoint of the reaction - complete consumption of the NaOH caused by the ester excess (Molar ratio: benzoic acid ester / NaOH = 33 : 1) - is indicated by the color change of the indicator.

· The reaction rate decreases in the order ethyl 4-nitrobenzoate > ethyl benzoate > methyl 4-methoxybenzoate.

· The attack of the nucleophile on the electrophilic carbon of the carbonyl group represents the rate determining step of the ester hydrolysis.

· The addition-elimination-mechanism is in agreement with the substituent dependence. The NO₂ group is electron withdrawing by both inductive and resonance effects. The CH₃O group has a strong electron-donating resonance effect which outweighs a weaker electron-withdrawing inductive effect. Electron donating groups make the carbonyl carbon less susceptible to the attack by a nucleophile. The more positive the carbonyl carbon the more reactive will be the ester.

General experimental instructions and index of experiments