Kinetics: Alkaline Hydrolysis of Esters

Computer-Interfaced Experiments - Conductivity Measurement

Kinetics
Alkaline Hydrolysis of Esters - Second Order Reaction

Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch

Datalogging and data analysis using the Program "Conductivity" and the Analog-Digital-Converter
CASSY-E - LEYBOLD DIDACTIC

German version

Chemicals:
ethyl formate (m.w. = 74.0 g / mol, d = 0.92 g / mL
ethyl acetate (m.w. = 88.1 g / mol, d = 0.9 g / mL)
ethyl propionate (m.w. = 102.13 g / mol, d = 0.89 g / mL)
ethyl butyrate (m.w. = 116.16 g / mol, d = 0.88 g / mL)
0.02 M sodium hydroxide solution

Apparatus and glass wares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish d = 190 mm, h = 90 mm (for water bath)
100 mL round bottom flask with center neck NS 29/32 and 2 angled side necks 14/23
contact thermometer
thermometer 0 - 50 °C (resolution: 0.1 °C)
conductivity measuring cell
micropipette
volumetric pipette 50 mL
pipette bulb

Hazards and safety precautions:

Ethyl formate is extremely flammable. Eye, skin and respiratory system irritant. May act as a narcotic.
Ethyl acetate is highly flammable. Harmful if swallowed in quantity. Vapours may cause drowsiness.
Ethyl propionate is highly flammable. Irritating to eyes, respiratory system ans skin.
Ethyl butyrate is flammable. May act as an irritant.

Safety glasses and gloves must be worn. Good ventilation required.

Theoretical background:

The reaction rate constant for the alkaline hydrolysis of esters such as ethyl acetate may be determined conductometrically, since the stoichiometry of a typical reaction is

As the reaction proceeds, hydroxide ions are consumed and acetate ions are produced.

The conductance of an ion depends on its ionic mobility, which in turn is determined by the size of the ion. Since the conductance of the large acetate ion is less then that of the smaller, more mobile hydroxide ion, the conductivity of the reaction solution decreases as the alkaline ester hydrolysis proceeds. Hence, the progress the reaction may be monitored by following the change in the electrical conductance of the reaction mixture with time. The rate constants will be determined at three temperatures, so that the activation energy for the reaction may be obtained from a plot of lnk versus 1/T.

Kinetic equations (Download PDF file)

Experimental procedure:

The conductivity measuring cell is connected to the conductivity box plugged at input A of CASSY INTERFACE.

A three-necked round bottom flask is fitted with an internal thermometer, a conductivity meassuring cell and a stopper. 100 mL of 0.02 M sodium hydroxide solution (0.002 mol) are pipetted into the flask placed in a water bath. The platinized electrode surfaces of the conductivity measuring cell and the tip of the thermometer need to be totally submerged in the reaction solution. The positions of the thermometer and the conductivity probe are adjusted so that they are not struck by the stirring bar (Fig. 1).

Using a hotplate stirrer and a contact thermometer the water in the water bath is warmed up to the desired temperature (40 - 60° C). The ester hydrolysis experiments are carried out at three different temperatures. Ethyl formate is hydrolyzed at different temperatures between 5 °C and 15 °C. A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath.

The level of the water bath should be well above the solution level in the reaction vessel.

The hydroxide solution is allowed to equilibrate in the constant-temperature water bath.

Fig. 1: Experiment set-up

After thermostating for about 15 minutes, the sodium hydroxide solution will come to the temperature of the waterbath. The temperature should be controlled as precisely as possible.

When thermal equilibrium has been reached, the program 'Conductivity' is started and the subprogram 'Kinetics' is activated. In order to configure the measurement, the appropriate settings must be done in consideration of the following menu instructions:

· <F3>'Calibrate conductivity meter'
· <F2>'Select measuring range'

In order to calibrate the measuring cell the cell constant (1.05 cm^-1) and the appropriate reaction temperature are to be entered. The reaction temperature is read from the internal thermometer to the nearest 0.1 °C. The measuring range is restricted to 0 .. 0.2 mS/cm and the recording time is set to 600 s. A recording time of 90 seconds is sufficient for the hydrolysis of ethyl formate.

Using a micropipette 0.004 mol of the ester (0.322 mL ethyl formate or 0.39 mL ethyl acetate or 0.46 mL ethyl propionate or 0.53 mL ethyl butyrate) are added to the sodium hydroxide solution while vigorously stirring. Immediately the sensing software is started by pressing the function key <F1 >.

The change in the conductance of the reaction mixture is displayed on the screen.

The series of measurements are stored.

Data analyis:

Data analysis using the program "Conductivity / Kinetics"

A direct comparison of the measurements is allowed in an overlay mode that can be activated by switching to <F8> 'Disc operations'®'Overlay on'>. The desired series of measurements are selected individually, in order to represent them together in the main menu under <F6>'Evaluate in graph'. Condition for a successful overlay is an identical label of the axes.

Fig. 2: Multigraph screen hydrolysis of ethyl propionate overlay of the conductivity curves
(In the overlay the temperature data are rounded.)

According to equation (15) Kinetic equations (Download PDF file) a new plot is selected for the determination of rate constants.

In the subprogram 'Kinetics' the program 'Conductivity' provides a formula editor, which allows the integration of the measured conductivity values into a formula. With the aid of the instruction <F4>'Select formula' a submenu is opened allowing to enter a formula. y is set for 'Physical symbol'. After setting the 'No. decimal places' the beginning of the formula y(n,t,æ,T)= is completed by entering

ln(0.5 · (æ_o- æ_¥) / (æ- æ_¥) + 0.5) · 50)

where æ_o is the initial conductance and center> æ_¥ the conductance at completion of the reaction. The corresponding values are found in the data table by activating the menu point <F5>'Output measured values' . The multiplication factor 50 considers x_o (i.e. the molarity of the used sodium hydoxide solution) in equation (15) Kinetic equations (Download PDF file).

The program is switched to < F7>'Select representation' ®'Representation' and 'Graph order: 0' is entered. Now the main menu is opened by activating <F6>'Evaluate in graph'.

The plot of y versus t gives a straight line up to the point at which approx. 80% of the limiting reactant has been consumed.

Switching to the graph cursor mode with the function key <F9> allows to highlight the linear portion of the graph: The determination of start and end point of the graph portion that is to be highlighted, takes place by appropriate cursor positioning and by means of the key combinations <Ctrl> <®> and <Ctrl><¬> . Striking the function key <F2 > a 'best-fit' straight line is drawn through the linear portion of the curve. The slope of the straight line is calculated by pressing <Alt ><F2 > (Fig. 3). The slope corresponds to the rate constant k.

Fig. 3: Hydrolysis of ethyl propionate (T = 48.3 °C) determination of the rate constants k
y = ln(0.5 · (k_o - k_¥) / (k - k_¥) + 0.5) · 50

For all three measurements a plot of y versus t is made.

After switching to < F8>'Disc operations'®'Overlay on' the three series of measurements are loaded individually, in order to represent them together in the main menu by activating <F6> 'Evaluate in graph'. The overlay mode permits the determination of the activation energy. As already shown, a straight line is applied to the linear portion of each curve. Both the rate constant and the reaction temperature are automatically saved with each measurement file. After pressing the function key <F1> the numbers of two straight lines are entered. The program calculates the activation energy according to equation (6) Arrhenius Equation .

In the case under consideration (Fig. 4) , a comparison of all three measurement series (1 / 2: 44 kJ · mol ^-1, 1 / 3: 46 kJ · mol^-1, 2 / 3: 47 kJ · mol^-1) results in an Arrhenius activation energy E_a of 45.6 kJ · mol^-1 .

Fig. 4: Hydrolysis of ethyl propionate determination of the activation energy
y = ln(0.5 · (k_o - k_¥) / (k - k_¥) + 0.5) · 50

Also for the other reaction temperatures the rate constants are determined.

	48.3 [ °C ]	52.1 [ °C ]	58.4 [ °C ]
k [ L · mol^-1 · s^-1 ]	0.352	0.431	0.601

Tab. 1: Rate constants k

If the reaction temperatures and the corresponding rate constants are entered into the table of the Excel file Activation parameters (Download), then all activation parameters (Tab. 2) will be calculated and the ARRHENIUS and EYRING plot will be generated (Fig. 5).

Tab. 2: Calculation of the activation parameters

Arrhenius and Eyring

Fig. 5: ARRHENIUS (1) and EYRING plot (2)

Data analysis using Excel - determination of the rate constants and the activation parameters

Using the calculation program Microsoft Excel the data analysis will convince more. The axes in the diagram "conductivity curves" can be optimized (Fig. 6). The hiding of an excess of data using a dialog module programmed in Visual Basic permits the plot with point markers. Thus the linear portion of the graphs can be seen clearly (Fig. 7).
However, a comparison between the data analysis using the CASSY program "Conductivity / Kinetics" and the data evaluation carried out with Microsoft Excel shows a good agreement.

spread aheet

Tab. 3: Hydrolysis of ethyl propionate data sheet
k(t), conversion according to y = ln(0.5 · (k_o-k_¥) / (k-k_¥) + 0.5) · 50

Fig. 6: Hydrolysis of ethyl propionate conductivity curves
1: 48.3 °C 2: 52.1 °C 3: 58.4 °C

Fig. 7: Hydrolysis of ethyl propionate - second order plot determination of the rate constants k
1: 48.3 °C 2: 52.1 °C 3: 58.4 °C
y = ln(0.5 · ( k_o-k_¥) / ( k-k_¥) + 0.5) · 50

	48.3 [ °C ]	52.1 [ °C ]	58.4 [ °C ]
k [ L · mol^-1 · s^-1 ]	0.3500	0.4281	0.5985

Tab. 4: Rate constant k

Tab. 5: Calculation of the activation parameters

Arrhenius and Eyring

Fig. 8: ARRHENIUS (1) and EYRING plot (2)

	ethyl formate	ethyl acetate	ethyl propionate	ethyl butyrate
E_a [ kJ · mol^-1 ]	40.2	42.2	47.1	48.9
lnA	19.05	14.98	16.58	16.57
DH ^‡ [ kJ · mol^-1 ]	37.8	39.6	44.4	46.2
DS ^‡ [ J · mol^-1· K^-1 ]	- 95	- 129	- 116	- 116
DG ^‡ [ kJ · mol^-1 ] bei 303.65 K	66.6	78.8	79.6	81.4

Tab. 6: Alkaline hydrolysis of esters Activation parameters

Discussion:

The carbon atom of the ester carbonyl group has a significant amount of positive character. Due to the difference in electronegativity between carbon and oxygen, both the pi bond and the sigma bond of the carbonyl are distorted. The pi bond has a resonance dipole too, which gives the carbon atom a positive charge. Also the alkoxy group is more electronegatively charged than the carbon atom and thus it also contributes to the positive nature of the carbonyl carbon atom.

The decreasing reactivity in the order ethyl formate > ethyl acetate > ethyl propionate > ethyl butyrate can be explained by the electron-donating inductive ability of the alkyl groups. The + I effect increases from methyl to butyl. Due to the positive inductive effect of alkyl groups the electron density on the C atom of the carbonyl function is increased. The higher the electron density at the ester carbonyl carbon the more hindered is the nucleophilic approach to the reaction center. However the differences in the reactivity might be particularly caused by steric hindrance. The more bulky the alkyl groups, the more strongly the reaction center is shielded (Fig. 9).

Fig. 9: Ethyl acetate Space filling model

References:
Georg Schmeer Alkaline Hydrolysis of Trifluoro acetic acid ethyl ester with LiOH (+ 1 water) - Animation - Computer-Interfaced Experiments Kinetics: Alkaline Hydrolysis of Ethyl Acetate - Second Order Reaction -
Computer-Interfaced Experiments Kinetics: Hydrolysis of Methyl Formate using an acidic Ion Exchanger - First Order Reactio
Microscale Projection Experiments Reactivity of Aromatic Esters

Index of CASSY Experiments