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Wilford J. Teerlink. Jeanette Asay,' and James M. Sugihara ~
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University of Utah Salt lake City
A Second Order Kinetics Experiment
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The nucleophilic displacement reaction of an appropriate substrate by iodide ion in acetone has received extensive mechanistic study and clearly proceeds by the SN2 mechanism.= By the proper selection of variables, this reaction may be used in a student experiment t o demonstrate second order kinetics. The application of ethyl p-toluenesulfonate as the substrate is advantageous for several reasons. The
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The Experiment
The following solutions are prepared: 0.100 A! NaI in acetone (reagent); 0.100 M ethyl p-toluenesulf ~ n a t e ,mp ~ 334", in acetone (reagent); 0.030 M AgNOa in water; and an eosin indicator solution.4 The sodium iodide and the ethyl p-toluenesulfonate solutions are thermostated a t 25-35% Data obtained a t 26.3, 32.0, and 36.CJoC, with temuerature variance of * 0 . 0 5 ~ ,are presented.. ~eaction-mixturesare prepared by pipetting 5 ml of the ethyl p-toluenesulfonate solution into each of six t o eight 18- X 150- nun test tubes, conveniently immersed in the water bath by use of a wire basket. Five rnl of the sodium iodide solution is added to each tube, recording zero time when one half of the reactant had been introduced. The test tubes are stoppered loosely. Test tubes are withdrawn a t periods of 20 min, 30 min, or 1 hr depending upon the temperature applied. The reaction is killed by pouring the contents of the tube into 100 ml of cold water. Ten drops of the eosin indicator and 2 ml of 6 M acetic acid are added to the solution before titrating with silver nitrate solution until the pink color has changed to yellow and then to a red. The endpoint is readily determined with a high degree of precision.
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TIME (HOURS1
Figure 1. Rate of displacement of ethyl p-tolvenesulfonoto by sodium iodide in acetone.
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compound has about the desired order of reactivity. (Methyl p-toluenesulfonate is displaced by iodide ion t o the extent of about 60% in 20 min a t 25O.) It is relatively nonvolatile. The departing group does not interfere in the titration of iodide ion.
A participant on an Undergraduate Science Education Program sponsored by the National Science Foundation. ' Gouw, E. S., "Mechanism and Structure in Organic Chemistry," Hemy Holt and Co., New York, N. Y., 1959, pp. 275-7. a FERNS,J., ANDLAPWORTH, A,, J . C h a . Soc., 101,274 (1912). Ethyl ptoluenesulfonate may be purchased from Distillation Products Industries, Rochester, N. Y. KOLTHOFF, I. M., AND SANDELL, E. B., "Textbook of Quantitative Inorganic Analysis," 3rd ed., The Macmillan Co., New York, 1952, p. 544.
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Figure 2. log k h / k T v e r w 1 /T for the dirplosement reaction of ethyl p-toluenerulfonate by sodium Iodide in ocetone.
Results
Figure 1 shows the second order rate plot for kinetic measurements made a t three temperatures. Rake constants were determined by using the method of least squares and found to be 11.1 X 6.25 X 10W4,and 3.24 X l/mole sec a t 36.9, 32.0, and 26.3', respectively. The extent of reaction after two hr was found to be 29.5, 17.2, and 11.0% a t the three temperatures. Activation parameters were determined by the method of least squares and are shown in Figure 2. Enthalpy of activation was calculated to be 20.7 kcal/ mole; entropy of activation was found to be -5.3 cal/ mole deg.
Volume 41, Number 3, March 1964
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