The effect of ultrasonic irradiation on the specific reaction rate constant in the acid hydrolysis of ethyl acetate

The speed of a chemical reaction is important in determining the efficiency of many industrial chemical reactions. In organic reactions particularly, when there is the possibility of several reactions going on simultaneously, that reaction which is fastest is the one that predominates and often affects the economics of the process.

The object of this investigation was to determine the effect of ultrasonic irradiation on the specific reaction rate of the hydrolysis of an aqueous solution of ethyl acetate in the presence of hydrochloric acid as the catalyst.

The commercially obtained ultrasonic generator was limited to four frequencies - namely, 400, 700, 1000, and 1500 kilocycles. Two thousand volts plate voltage could be applied to the quartz crystal plates provided the plate current did not exceed 360 milliamperes.

A special pyrex glass reactor was designed and constructed in order that isothermal conditions could be maintained within the boundary of the reactor. The period of time the reaction mixture was subjected to ultrasonic irradiation was arbitrarily set at 10 minutes for a given power and frequency at 19 ± 0.1°C and 30 ± 0.1°C respectively. The tests were then repeated using a 20 minute period of irradiation. Individual mixtures were respectively irradiated at 0, 500, 1000, and 1500 volts plate voltage.

The composition and volume of the mixture irradiated was 100 ml of 1.9904 N hydrochloric acid, 90 ml of distilled water free from carbon dioxide, and 10 ml of ethyl acetate making a total volume of 200 ml.

The chemical reaction assumed for the investigation was

CH₃COOC₂H₅ + H₂O + H⁺ ⇋ CH₃COOH + C₂H₅OH + H⁺

The course of the reaction was followed in such a way that the concentrations of the reacting substances were not disturbed. Samples of the mixture were drawn off at intervals and titrated with sodium hydroxide using phenolphthalein as the indicator.

Calculations were based on the difference of the titration at any time t in minutes and the titration at the end of 24 hours. When the reaction was completed, this difference was a measure of the concentration of ethyl acetate remaining unchanged or unhydrolyzed at time t. Each molecule of ethyl acetate that hydrolyzed produced one molecule of acetic acid, and the increase in acidity was a direct measure of the amount of ethyl acetate that had reacted. The amount of hydrochloric acid remained unchanged throughout the experiment.

The concentrations of ethyl acetate calculated from the differences between the titration values and the final titration were arranged in tabular form. A graph was drawn on semilogarithmic paper by plotting the concentration of ethyl acetate, c, against time, t, in minutes and the best straight line through the points.

Using the equation, - log c_A (k / 2,303) t+ constant,

the specific reaction rate, k, was calculated by multiplying the slope of the line by 2.303.

The activation energy, E, was calculated from the values of k at 19 ± 0.1°C and 30 ± 0.1°C using the equation,

E = [2.303 log₁₀ k₂/k x R x T₁ x T₂] / [T₂ - T₁]

where k₁ = specific reaction rate at 19 ± 0.1°C

k₂ = specific reaction rate at 30 ± 0.1°C

R = Constant

T₁ = Absolute temperature for 19 ± 0.1°C

T₂ = Absolute temperature for 30 ± 0.1°C

The results of this investigation showed that the specific reaction rates were increased by ultrasonic irradiation of 400, 700, 1000, 1500 kc in the order of five percent for 20 minutes irradiation over the rates for 10 minutes irradiation.