A Kinetic Study of the Base Catalyzed Cleavage of Diacetone Alcohol Using a Dilatometer

The University of Lethbridge Department of Chemistry & Biochemistry

Chemistry 2740 Laboratory Experiment 2

A KINETIC STUDY OF THE BASE CATALYZED CLEAVAGE OF DIACETONE ALCOHOL USING A DILATOMETER
The decomposition of diacetone alcohol into two molecules of acetone is catalyzed by hydroxide ions and is an example of an aldol condensation in reverse.
O OH OHO 2CH3-C-CH3

CH3-C-CH2-C(CH3)2

The rate of decomposition is first-order with respect to the concentrations of both diacetone alcohol and hydroxide ion: Rate = k[OH-][diacetone alcohol] (1)

However, since hydroxide ion is a catalyst its concentration remains constant during the reaction. The overall reaction appears first-order (i.e. is a “pseudo first order reaction”) and follows the observable rate law Rate = k' [diacetone alcohol] where k' = k [OH-] (2)

Since the overall reaction is first-order we can study the kinetics of the reaction by measuring any property of the system that undergoes a change which is proportional to the extent of reaction. Such a property in this case is the volume of the reaction solution. The effective volume of one molecule of diacetone alcohol is not the same as the effective volume of two molecules of acetone and as a result the total volume of the reaction solution changes as the reaction proceeds. In this case the solution expands although in some reactions it contracts. A simple instrument for measuring volume changes is a dilatometer which consists of a glass bulb to which is attached a tube with a stopcock (for filling the bulb) and also a piece of long capillary tubing. The bulb is filled with reaction solution to the point where liquid just enters the capillary tube and then the stopcock on the filling tube is closed. As the solution expands it does so into the capillary tube causing the meniscus in the tube to rise. By measuring the distance up the capillary tube that the meniscus travels one has a measure of the volume change. One can determine the actual volume change if the crosssectional area of the capillary is known but even that is not necessary in this experiment. Since the position of the meniscus in the capillary column can be measured accurately using a cathetometer, this is a good experiment to test the Guggenheim method for determining first-order rate constants (refer to Appendix A on “First-order Reactions”). In this method readings are generally made at times t0, t1, t2, t3, etc., with each reading Page 2 - 1

Chemistry 2740 Laboratory Experiment 2 taken at a constant, accurately determined time interval after the preceding measurement. The resulting data list is divided into equal halves. For example, if there are 20 readings taken at times t 0 – t19 with corresponding measurements P0 – P19, the data would be divided in two between readings P9 at t 9 and P10 at t10. Next, the differences between the measurements in the two data sets are taken, i.e., P0-P10, P1-P11, P2-P12, etc. Notice that the time interval between each pair of readings is constant. Finally a plot of the natural logarithm of the differences against time, i.e., ln(P0-P10), ln(P1-P11),… vs. t0, t 1,… should yield a straight line of slope -k, the first-order rate constant.

Apparatus Cathetometer, 3 dilatometers, timer. A dilatometer is a device for measuring the expansion (or contraction) of a liquid. Ours is of relatively simple design and was made locally by Luis Delgado from various pieces of glassware. It consists of an expansion bulb to which is attached a fine capillary tube with a narrow and hopefully uniform bore. The expansion tube is connected at the other end to a filling tube through a stopcock. When the stopcock is closed, a solution in the expansion tube can only expand up the capillary tube. The volume of liquid in a capillary or cylinder is given by the cross-sectional area, A, of the cylinder times its length, l (V = A x l). Thus by measuring the travel, Δl, of the liquid up the capillary tube one has a quantity that is proportional to the change in volume of the reaction mixture (Δ V = A x Δl). As a result one can follow first order reactions with a dilatometer and use the first order equation ln [(lo – l∞) / (lt – l∞)]= kt
Stopcock

Capillary tube

Filling tube Expansion bulb

(3)

A Dilatometer

and other equations such as the Guggenheim equation that are derived from it to analyze the results. This assumes that Δl (and therefore Δ V) is proportional to the extent of reaction.

One must be careful with thermostating when using a dilatometer. A dilatometer, after all, is a glorified thermometer and a quite sensitive one at that. Thus the apparatus and the reaction solution must be pre-equilibrated to the temperature of the reaction. The Page 2 - 2

Chemistry 2740 Laboratory Experiment 2 dilatometer is filled by pouring reaction mixture into the filling tube. Try to pour down the centre of the tube and not down the walls of the tube. Also do not fill the filling tube above the level of the water in the water bath because the part of the filling tube above water level will not be well thermostated. Next the reaction mixture must be forced into the expansion bulb by use of a rubber bulb applied to the top opening of the filling tube. Often air bubbles become trapped just below the stopcock. These can be removed by sucking back with the rubber bulb. Continue to add more reaction mixture to the filling tube, as necessary. Force reaction mixture into the expansion bulb until the liquid level reaches the top of the bulb just below the capillary tube. Stop forcing liquid into the bulb and allow the liquid level to rise into the capillary tube as a result of the flow of liquid from the filling tube to the expansion bulb. DO NOT FORCE LIQUID INTO THE CAPILLARY TUBE. Close the stopcock. The dilatometer is now ready for making measurements of the meniscus height. The cathetometer is a device for measuring the relative height of the liquid column in the capillary. It consists of a vertical steel rod with a scale marked along its length and a telescope that runs up and down the rod. In operation one measures the height of the liquid column by moving the telescope so that the cross-hair is focussed on the meniscus of the liquid column. The position of the telescope (and thus the meniscus) is then read off the scale on the bar with the aid of a vernier. Ensure that you can read the vernier scale (refer to Appendix B on “Reading a Vernier”) and can operate the telescope (focus, movement up and down, and leveling) before proceeding with measurements.

Reagents Diacetone alcohol, ~ 0.40 M NaOH.

Waste Disposal A 4-litre bottle for the collection of wastes is supplied with the experimental set up. All excess stock reagents and reaction solutions should be disposed of in this bottle. The glassware can then be given a single small rinse into the waste container before being cleaned further in the sink. In preparing reaction solutions only remove as much reagent from the stock container as is necessary to make the reaction mixtures. Page 2 - 3

Chemistry 2740 Laboratory Experiment 2 Procedure Notes: 1) In order to finish this lab in the time allotted, students must be well organized and prepared to start this experiment at the beginning of the period. 2) The ~ 0.40 M NaOH solution will need to be standardized by each group. This can be done before or after the experiment is completed, but must be done before the calculations for the report are started. Students can arrange a suitable time for this with their instructor. (Note: A similar task was performed in Chemistry 1000 lab; it may be helpful for you to review that procedure.) Three kinetic runs should be performed at hydroxide ion concentrations of approximately 0.100, 0.200 and 0.400 M. Prepare 100 mL each of 0.100 M and 0.200 M sodium hydroxide solutions from the 0.400 M solution provided. Allow a dilatometer to thermostat in the 25° C water bath. Pipette exactly 50 mL of 0.100 M NaOH solution into a 200 mL Erlenmeyer flask, stopper the flask, and allow it to thermostat in the bath as well. When the dilatometer and sodium hydroxide solution have been thermostated for at least 10 minutes, start the reaction by adding with a pipette 2 mL of diacetone alcohol into the flask containing the 50 mL of 0.100 M NaOH solution. Stopper the flask, shake it vigorously to ensure mixing and then let it stand in the water bath for a short period to allow the bubbles to settle. Pour the settled solution into the filling tube of the dilatometer and proceed to fill the dilatometer as outlined above. When the solution enters the capillary close the stopcock on the filling tube ensuring that no bubbles remain in the bulb. Clamp the dilatometer firmly in place in the bath so that the expansion bulb is covered with water. Commence reading the height of the meniscus in the capillary column with the cathetometer and continue to do so at exactly 3-minute intervals for at least 15 readings (45 minutes). The first reading can be obtained by clamping the telescope so that the cross-hair is just above the meniscus; start the clock as the meniscus climbs to the crosshair. (Because the telescope inverts its image, the meniscus will appear to be below the cross-hair when it is actually above and the meniscus will appear to be travelling down when it is actually travelling up the capillary.) Subsequent readings will require close cooperation between lab partners. One person should follow the meniscus with the telescope while the other partner gives out the time so that the first partner can clamp the telescope in position at exactly 3-minute intervals.

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Chemistry 2740 Laboratory Experiment 2 When the readings have been completed put the dilatometer aside and proceed to the second experiment. While the first experiment is being performed, the dilatometer and the 50 mL of sodium hydroxide solution for the second experiment should be clamped in the bath to thermostat. Repeat the procedure using 0.200 M NaOH and 0.400 M NaOH in place of 0.100 M NaOH and with time intervals of 1.5 and 0.75 minutes respectively. In the case of the run using 0.400 M NaOH, allow the reaction to go to completion and then read the height of the meniscus.

Before leaving the laboratory, please enter names, date, and experimental data into the computer. DO NOT FORGET TO ENTER YOUR STANDARDIZATION DATA INTO THE COMPUTER ONCE YOU HAVE OBTAINED IT.
Calculations and Report Use the Guggenheim method to calculate the apparent first-order rate constants (k’) for each run. For the last run, also calculate k’ using equation (3). Compare the rate constants calculated by the two methods and discuss the validity of using the Guggenheim method to calculate rate constants (i.e. discuss if the value calculated using the Guggenheim method compares favourably to the value calculated using the standard method). Calculate the second-order rate constants (k) in each case and discuss this confirmation of the first-order dependence on hydroxide ion concentration.

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