In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Using Conductivity Devices in Nonaqueous Solutions II: Demonstrating the SN2 Mechanism submitted by:
Thomas A. Newton* Department of Chemistry, University of Southern Maine, Portland, ME 04104-9300; *
[email protected] Beth Ann Hill School of Applied Medical Sciences, University of Southern Maine, Portland, ME 04104-9300 John Olson Department of Chemistry, Augustana University College, Camrose, Alberta, Canada, T4V 2R3
checked by:
In the preceding article (1) we described the construction of a simple conductivity device and its use in polar protic solvents to demonstrate various aspects of the SN1 mechanism. In this article we report the use of the conductivity device to demonstrate certain features of the SN2 mechanism as embodied in the Finkelstein reaction. This transformation is described in general terms as, R1 R2
C R3
control reactions to establish which species are responsible for the flow of current in these demonstrations. This approach is well suited to the large classrooms generally required for organic chemistry courses. Chemicals Acetone
R1 X
+ NaI
R2
C
I
+ NaX
2-Heptanone
(1)
1-Bromobutane
R3
1-Chlorobutane
where X is a halide. Because of its importance in mechanistic organic chemistry, experiments involving the Finkelstein reaction are included in many organic chemistry laboratory manuals. It is used, for example, in organic qualitative analysis (2) to differentiate between primary, secondary, and tertiary alkyl chlorides and bromides. Another standard experiment involves correlating changes in the rates of substitution with changes in substrate structure (3). Both of these experiments depend on the fact that sodium iodide is soluble in acetone, while sodium chloride and sodium bromide are not. This means that the reaction in eq 1 is essentially irreversible; the appearance of a precipitate is taken as evidence that a reaction has occurred. It is also possible to use a conductivity device to demonstrate that a reaction has occurred. The accompanying article describes the construction of a conductivity device that is designed for use with small sample volumes. A solution of sodium iodide in acetone conducts a current, that is, the light bulb glows brightly. When an alkyl bromide or chloride is added to such a solution, the brightness of the bulb diminishes as the dissolved sodium iodide reacts and the insoluble sodium bromide or sodium chloride precipitates. Using two conductivity devices to make side-by-side comparisons provides dramatic demonstrations of how changes in the structure of the substrate as well as the polarity of the solvent affect the relative rates of many of the reactions implied by eq 1. Since most students are not familiar with the electrolytic properties of nonaqueous solutions, it is important to include www.JCE.DivCHED.org
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2-Bromobutane Benzyl bromide Benzyl chloride Sodium iodide
Procedures The general procedure for the substitution reactions shown in eq 1 entails adding 3 mmol of alkyl halide to 5 mL of NaI stock solution. Specific details are given for a comparison of the relative rates of substitution of 1-bromobutane and 1-chlorobutane. The appropriate volume of each alkyl halide is given in Table 1 along with the approximate time required for the glow of the light to become undetectable.
Table 1. Approximate Reaction Times for Various Alkyl Halides Volume/µL
Amount/mmol
Time/min
Benzyl bromide
360
3.01
0.1
1-Bromobutane
320
2.97
6
1-Bromobutanea
320
2.97
10
Benzyl chloride
350
3.04
7
2-Bromobutane
330
3.02
38
1-Chlorobutane
310
3.06
45
Compound
a
The solvent in this reaction was acetone. A 4:1 acetone:2-heptanone mixture was used for all other reactions.
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In the Classroom
Preparation of NaI Stock Solution Mix 120 mL of reagent grade acetone with 30 mL of 2heptanone. Save 50 mL of this mixture for use in controls. Dissolve 155 mg of sodium iodide in the remaining 100 mL of this mixture. This is enough material for 10–12 comparisons. Keep the solution in a dark bottle until ready for use. Controls Add 5 mL of a 4:1 acetone:2-heptanone mixture to a 10-mL beaker that is clamped to a ring stand. Insert the cork containing the electrodes (1) into the beaker and plug in the apparatus. Note whether the light bulb glows. Inject 320 µL of 1-bromobutane into the beaker. Stir. Again note whether the light bulb glows. In another beaker add small quantities of NaI to a second 5-mL aliquot of the solvent mixture while stirring. Note whether the light bulb glows. Repeat the third control using NaBr and NaCl. SN2 Reactions of 1-Bromobutane and 1-Chlorobutane Pour 5 mL of the NaI stock solution into each of two 10-mL beakers that are clamped securely to ring stands. Stir. Insert conductivity devices into the beakers, making sure the electrodes do not prevent the spin bars from spinning. Connect the apparatuses to a power source. Using automatic pipetters, simultaneously add 320 µL of 1-bromobutane to one beaker and 310 µL of 1-chlorobutane to the other beaker. Add the alkyl halides through the small space between the stopper and the spout of the beaker. Once the difference in rates becomes obvious, the 1-chlorobutane solution may be allowed to stir while the electrode assembly is used for another comparison, for example 1-bromobutane versus 2bromobutane. The conductivity of the 1-chlorobutane solution may be checked periodically throughout the class period. Results The results of various substitution reactions are summarized in Table 1. All reaction times are averages of at least two determinations. Discussion The first two controls demonstrate that neither the solvent system alone nor a solution of 1-bromobutane in the solvent contains a measurable concentration of ions. The third control establishes that NaI is an electrolyte, while the last control clearly shows that NaBr and NaCl are not. Since most students’ experience with electrolytes is limited to aqueous solutions, it is important to make this distinction between NaI on the one hand and NaBr and NaCl on the other in this demonstration. The utility of the Finkelstein reaction depends upon the insolubility of NaBr and NaCl in acetone. In the present demonstration we have used a mixture of acetone and 2heptanone to increase the rates of reaction relative to those in pure acetone. Also, in each demonstration the RX:NaI molar ratio is approximately 75:1. Judicious choice of the
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order in which comparisons are made allows most of the reactions go to completion within a standard 50-minute class period. Even if sufficient time is not available for every reaction to go to completion, the difference in reaction rates, as indicated by the decrease in intensity of the light bulb, is obvious within 10 minutes for any comparison. In multiple runs the variation in reaction times for 1-chlorobutane and 2bromobutane was considerable. However, this variability is not important for the comparisons described below.
Effect of Leaving Group To demonstrate that bromide ion is a better leaving group than chloride, either the comparison between 1bromobutane and 1-chlorobutane or between benzyl bromide and benzyl chloride may be used. The 1-bromobutane reacts about 7 times faster than 1-chlorobutane, while benzyl bromide reacts nearly 70 times faster than benzyl chloride. Effect of Substrate Structure The greater reactivity of primary substrates relative to their secondary isomers is easily demonstrated by comparing 1-bromobutane and 2-bromobutane where the relative rates are about 6兾1. The 60兾1 rate ratio for the comparison between benzyl bromide and 1-bromobutane provides a dramatic demonstration of the effect of an sp2 versus an sp3 hybridized center adjacent to the reaction center. Effect of Solvent Polarity Using 1-bromobutane as the substrate, it is possible to demonstrate that decreasing the polarity of the solvent leads to an increase in reaction rate. This result is consistent with, and nicely illustrates, the idea that reactions in which the charge is dispersed in the transition state proceed faster in less polar solvents than they do in more polar solvents. The comparison provides a nice contrast to the decrease in the rate of the SN1 reactions that attends decreasing solvent polarity, which we described in the preceding article (1). Literature Cited 1. Newton, T. A.; Hill, B. A. J. Chem. Educ. 2004, 81, 58–60. 2. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 4th ed.; John Wiley & Sons: NY, 2000. 3. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Organic Laboratory Techniques a Small Scale Approach; Harcourt Brace: Philadelphia, PA, 1998.
Editor’s Note See the accompanying article “Low-Voltage Conductivity Device” on page 63 for the construction of an alternate conductivity device. —Ed Vitz
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