Kinetics: Acid-catalyzed Iodination of Acetone

Computer-interfaced Experiments - Absorbance Measurement

Kinetics
Acid-catalyzed Iodination of Acetone
Pseudo Zero Order Reaction

Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch

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

German version

Chemicals:
acetone > 99.5 % (M = 58.08 g / mol, d = 0.783 g /mL)
0.05 M iodine solution
0.5 M sulfuric acid

Apparatus and glass wares:
magnetic stirrer hotplate
magnetic stirring bar
stirring bar remover
crystallizing dish d = 190 mm, h = 90 mm (for water bath)
6 beakers 100 mL
contact thermometer
thermometer 0 - 50 °C (resolution: 0.1 °C)
2 burettes 25 mL
3 volumetric pipettes 2 mL
volumetric pipette 4 mL
volumetric pipette 5 mL
3 graduated cylinders 50 mL
washing bottle with dist. water
pipet bulbs
photometer fitted with a recorder output: Spectronic 20 Bausch & Lomb
test tube cuvettes (Spectronic)
disposal container

Hazards and safety precautions:

Iodine is toxic - may be fatal if swallowed or inhaled. Harmful by inhalation and through skin absorption. Readily absorbed through skin. Very destructive of mucous membranes and upper respiratory tract, eyes and skin. Severe irritant.

Acetone is highly flammable. Irritating to eyes.

Safety glasses and nitrile gloves required. The experiment should be carried out in a fume cupboard!

Theoretical background:

Reaction mechanism

The reaction of acetone with iodine producing iodine acetone, represents in the acid aqueous medium, (like other ketone halogenations) a irreversible redox reaction, catalyzed by protons. In a first step (reaction 1) acetone (A) is protonated producing AH⁺:

Reaction 1 (fast)

AH⁺ is formed only with in very small concentrations. In the reverse reaction AH⁺ decomposes to regenerate hydrogen ion and acetone (keto- or enol-form).

Reaction 2 (slow)

Reaction 1 is like many other protonation-deprotonation reactions relatively fast. However, the formation of the enol (reaction step 2) takes place slowly. Since the H ⁺ taken up by the acetone is regenerated by the formation of enol, a hydrogen catalyzed keto-enol-isomerization reaction takes place. Iodine reacts with enol irreversibly to form iodoacetone (AI) and hydroiodic acid (HI):

Reaction 3 (fast)

Since this reaction is very fast (k₃ >> k₂), the enol formed in reaction step 2 is immediately converted. The second reaction step is rate-determining. The formed enol reacts in the third step immediately with iodine to form iodine acetone. When all the iodine has been consumed the reaction will stop.

Kinetic equations (Download)

Experimental procedure:

50 mL of the reaction solution should have have the following proton- and iodine concentrations: c_H = 0.08 mol · L^-1, c_I2 = 0.004 mol · L^-1, c_Ac = 2.7 mol · L^-1

Calculation of the appropriate volumes (Download PDF file)

Fig. 1: Experiment set-up

7 mL of the 0.5 molecular sulfuric acid are pipetted into a graduated 50 mL measuring cylinder and from a burette the calculated volume of acetone is added. Afterwards the acid acetone solution is accurately made up to a volume of 46 mL with dist. water. Now the solution is poured into a beaker. 4 mL of the 0.05 molecular iodine solution are pipetted into a second beaker. The top of the beaker is covered with a piece of parafilm foil. The plastic film is firmly fastened around the mouth of the beaker to prevent the release of iodine into the air.

The two beakers are placed in a water bath, in which a contact thermometer is immersed. The level of the water bath should be well above the solution level in the beakers. A thermometer (resolution: 0.1 °C) is dipping into the acidic aqueous acetone solution. The two solutions are allowed to equilibrate in the constant-temperature water bath (this usually takes about 10 minutes).

When the solutions have reached thermal equilibrium the temperature is read to the nearest 0.1 °C. Now the acidic acetone solution is poured into the beaker containing the iodine solution.

A cuvette is filled about 3/4 full with the reaction solution. If necessary the outside of the cuvette is wiped to dry. The cuvette covered with a piece of Parafilm is inverted 2-3 times to ensure a proper mixing. Immediately the cuvette is placed into the sample compartment of the photometer and the sensing software is started. The measuring interval is 1 second.

The experiments are carried out at three different temperatures in the range from 20 °C to 40 °C.

The in-situ determination of the reaction rate on the basis of a continuous logging of photometrical data is allowed in rapid reactions (small change in temperature during te reaction).

During the thermal equilibration period the Calibration of the photometer and the matching of the program 'Multimeter' takes place.

Spectronic 20 (Download) features an analog output on the bottom of the instrument. The analog output of the photometer is connected to the input B of the INTERFACE.

The photometer has been designed so that when it displays 100 % transmittance, the analog signal at its output connector is 1 VDC; when the instrument displays 0 % transmittance, the output voltage is 0 VDC.

Calibration of the photometer: Using the wavelength control knob a wavelength is set to 490 nm. After the photometer has been zeroed, a cuvette is filled with 2 ml of iodine solution mixed with 2 ml of acetone. The cuvette, covered mith a bit of Parafilm, is inverted 2-3 times and placed into the sample compartment. When the transmittance maximum has been reached, the instrument is adjusted so that the meter needle reads 100% transmittance (zero absorbance).

Matching of the program 'Multimeter':
- In the program 'Measuring and Evaluating' the subprogram 'Multimeter' is activated and under the menu item <F3> 'Select measur. quantities' ® 'Reselect channel B' the quantity 'Voltage DC' is selected.
- After switching to <F4>'Automatic/Param./Select formula' the measuring interval is set to 10 s.
- Under the menu item 'Enter formula' T is entered. T stands for 'Physical symbol' . After the 'No. decimal places' has been set to 4 , the beginning of the formula
T (n, t, U) = is completed by entering U·100.

program

Fig. 2: Matching of the program

Mesurement: After thermal equilibrium has been established the reaction temperature is read to the nearest 0.1 °C. Afterwards the acetone solution is poured smoothly and rapidly to the iodine solution. A clean, dry cuvette is filled about 3/4 full with the reaction solution. If necessary the outside of the cuvette is wiped to dry. The cuvette, covered with a bit of Parafilm is placed immediately into the light-tight sample compartment of the photometer. Simultaneously the sensing software is started by pressing <F1>.

Before the data are stored, by means of <F7 >'Select representation'® 'Display' transmittance T is selected for the x-axis and time t for the y1-axis. Under 'Select graph options' is being ensured that the data points are displayed as crosses.

The remainder of the reaction solution is poured immediately into the disposal container.

Data analysis:

A direct comparison of the measurements is allowed in an overlay mode activated by switching to <F8> 'Disc operations'®'Multigraph on' . The desired series of measurements are selected individually, in order to represent them together in the main menu by activating <F6>'Evaluate in graph'. Condition for a successful overlay is a identical labelling of the axes. Fig. 3 shows the overlay of three series of measurements. By pressing <Alt> <F4> and <F4> and by entering the appropriate number of the graph, a fitted smooth curve is drawn through the data points.

transmittance

Fig. 3: Multigraph screen - temperature effect
1: 22.3 °C 2: 28.8 °C 3: 36.4 °C

The transmittance values of the individual series of measurements are converted into absorbance values.

By switching the program to <F4> 'Select formula' a submenu is opened, allowing to enter a formula. A is entered. A stands for 'Physical symbol' . After the 'No. decimal places ' has been set to 4, the beginning of the formula A (n, l, U) = is completed (according to A = 2 - logU·100) by entering -lnU/ln10 (Fig. 4)

program

Fig. 4: Matching of the program

Under <F7>'Select representation' the x-axis is labelled with A and the y1-axis with t.

The plot of t versus A provides a straight line up to a reaction conversion of 100%.

Under the menu item <F9> the linear portion of the graph is highlighted in the graph cursor mode:
- By appropriate positioning of the cursor and by means of the key combinations <Ctrl> <®> and <Ctrl ><¬> the start and end point of the portion (that is to be highlighted) can be located.
- By striking the function key <F2> a best-fit straight line is applied to the linear portion of the graph. The interception of this straight line with the y-axis determines the value of the time t, at which the reaction is completed. By pressing <Alt> <F2> and by entering the appropriate graph number the appropriate value for y (x = 0) is calculated (Fig. 5).

absorption

Fig. 5: Plot of time vs absorbance - determination of the reaction endpoint
1: 22.3 °C 2: 28.8 °C 3: 36.4°C

Fig. 5 shows, that the change in the concentration is proportional to the time. The reaction rate is constant. The reaction is pseudo zero order .

Determination of the rate constant k according to equation (21) Kinetic equations (Download PDF file):

k = 0.004 / (0.08 · 2.7 · t)

Measurement	T [ °C ]	t [ s ]	k [ L · mol^-1 · s^-1 ]
1	22.3	434	0.000042669
2	28.8	257	0.000072057
3	36.4	137	0.000135172

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 plots according to the ARRHENIUS and EYRING equation will be generated (Fig. 6).

Tab. 2: Calculation of the activation parameters

Arrhenius and Eyring

Fig. 6: ARRHENIUS (1) and EYRING plot (2)

References:
Computer-Interfaced Experiments Kinetics: Acid-catalyzed Iodination of Acetone - Test for a Pseudo Zero Order Reaction
Kinetics of the Iodination of Acetone
Bromination of Acetone

Index of CASSY Experiments