In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Using Conductivity Devices in Nonaqueous Solutions I: Demonstrating the SN1 Mechanism Thomas A. Newton* Department of Chemistry, University of Southern Maine, Portland, ME 04104-9300; *
[email protected] submitted by:
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:
Numerous articles describing the use of conductivity devices have appeared in this Journal (1–7). While conductivity devices are normally used with aqueous solutions, they may also be put to good advantage in nonaqueous solvents. This article describes the use of conductivity devices in polar protic solvents to demonstrate certain features of the SN1 mechanism. The accompanying article presents a related group of demonstrations involving the SN2 mechanism in polar aprotic solvents (8). The SN1 mechanism is outlined in general terms as: R1 R2
R1
ROH
C X
R2
Cⴙ
ⴚ
X
R1 R2
R3
R3
Tygon tubing, 3.2-mm i.d. Silicone caulk 18-2 lamp cord
Equipment Beakers, 10-mL (2) Clamps, 3-fingered (2) Ring stands (2) Magnetic spin bars (2) Magnetic stirrers (2)
C OR + HX
(1)
R3
Automatic pipetters (2) Pipetter tips (2) Graduated cylinders, 10-mL (4)
This transformation constitutes the basis for several classic undergraduate laboratory experiments including kinetic measurements (9), structure–reactivity correlations (9), and organic qualitative analysis (10). The latter two experiments are performed in ethanol in the presence of silver nitrate. In both cases the formation of a precipitate of silver halide is taken as evidence that a reaction has occurred. In the demonstrations described in this article, the glow of a light bulb supplies that evidence. The neutralization of NaOH solutions by mixtures that conduct a current provides supporting evidence that a reaction occurred. While it is possible to perform these demonstrations using commercially available conductivity devices,1 we chose to make our own device to minimize the volume of solution required.
Latex gloves
Chemicals Methanol Benzyl bromide Benzyl chloride 2-Bromobutane 2-Methyl-2-bromopropane 2-Methyl-2-chloropropane (1-Bromoethyl)benzene 0.1% Phenolphthalein solution NaOH
Materials
Procedures
Conductivity Detector
Construction of the Conductivity Device A schematic diagram of the constructed conductivity device is presented in Figure 1. Using a drill press, drill two holes into the #12 cork on 10-mm centers. Cut approximately 13–14 mm from the top of the cork, leaving a slice that is approximately 18-mm thick. Separate two strands of 18-2 lamp cord to a length of 2 m using a utility knife. Strip ap-
Electrical plug Receptacle 1-1/2 in. 6-32 stainless steel machine bolts (2) 7.5-W Frosted light bulb Cork, size 12
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In the Classroom
proximately 10 mm of insulation from the end of one strand. Insert the bare wire into a 30-mm length of Tygon tubing. Insert the stem of a machine bolt, which has the head cut off, into the tubing from the same end that you inserted the bare wire until the lower end of the bolt projects about 8 mm beyond the lower end of the Tygon tubing. Press this combination into one of the holes in the cork so that the top end of the Tygon tubing is recessed below the top edge of the cork. Seal the space above the Tygon tubing with a dab of a silicone compound. Press a 3-mm length of Tygon tubing onto the lower end of the machine bolt. Repeat this procedure for the second electrode. Connect one electrode directly to one of the terminals in the base of the light bulb receptacle. Connect the other electrode to one of the terminals of the plug. Connect the second terminal of the plug to the second electrode using a single strand of 18-2 lamp cord. All of the connections should be insulated.
General Procedure The general procedure for the solvolysis reactions entails adding 1 mmol of alkyl halide to 5 mL of solvent. The appropriate volume of each alkyl halide is given in Table 1 along with the approximate time required for the glow of
7.5-W frosted light bulb
receptacle
the light to become visible. Running two reactions simultaneously makes a dramatic effect. Demonstrations involving the less reactive halides should be performed as early as possible to maximize the time available for their reactions. However, each of the three demonstrations required to illustrate the effects of the reaction variables considered in the discussion section may be completed in less than 4 minutes. Specific details are given for a comparison of the solvolysis rates of 2-bromo-2-methylpropane and 2-bromobutane.
Solvolysis of 2-Bromo-2-methylpropane and 2Bromobutane Pour 5 mL of 4:1 (v:v) CH3OH:H2O into each of two 10-mL beakers that are clamped securely to ring stands. Stir. Insert the conductivity devices into the beakers, making sure the electrodes do not touch the spin bars. Using automatic pipetters, simultaneously add 115 µL of 2-bromo-2methylpropane to one beaker and 109 µL of 2-bromobutane to the other beaker. Add each alkyl halide through the small space between the stopper and the spout of the beaker. Testing for the Formation of HBr Prepare a stock solution of 0.1 M NaOH in CH3OH:H2O by dissolving 400 mg of NaOH in 100 mL of 4:1 (v:v) CH3OH:H2O. Store this mixture in a plastic bottle. Add 4 drops of 0.1% phenolphthalein solution to 1 mL of the stock NaOH solution in each of three 10-mL beakers. Place the beakers on an overhead projector and add 1 mL of the 2-bromo-2-methylpropane mixture to one beaker, 1 mL of the 2-bromobutane mixture to another beaker, and 1 mL of 4:1 CH3OH:H2O to the third beaker. In the case of 2-bromobutane, this test could also be performed on the solution from the preceding section before and after the glow of the light bulb becomes visible. Safety Considerations
18-2 lamp cord
plug
silicone caulk
All of the alkyl halides may cause central nervous system depression. They all irritate eyes, skin, and the respiratory and digestive tracts. Demonstrators should wear latex gloves. Since the apparatus used to collect the data in Table 1 uses 120 V ac, there is a potential for electrical shock associated with these demonstrations. We designed the conductivity device shown in Figure 1 so as to minimize the possibility of contact with conductive surfaces. Furthermore, the latex
cork 18 mm bare Cu wire
Table 1. Approximate Solvolysis Times for Various Alkyl Halides
Tygon tubing 8 mm
10 mL beaker machine bolt Tygon tubing
Time/min
2-Bromo-2-methylpropane
115
(1-Bromoethyl)benzene
136
5 mm
(1-Bromoethyl)benzenea
136
12
3 mm
Benzyl bromide
120
3
2-Chloro-2-methylpropane
116
2
Benzyl chloride
117
29
2-Bromobutane
109
48
spin bar 10 mm Figure 1. A schematic diagram showing the details of the construction of a conductivity device for use with small samples.
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Volume/µL
Compound
•
0.1 0.2
a
Solvent was absolute methanol. In all other reactions a 4:1 methanol:water mixture was used.
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In the Classroom
gloves we recommend to prevent skin irritation virtually eliminate any possibility of a shock. In fact, a voltmeter does not record any voltage across the electrodes when they are covered with a latex glove.
chloride. In the former comparison, the bromide compound reacts 25 times faster than the chloro analogue, while in the latter comparison the bromide compound reacts 10 times faster than the chloro analogue.
Results
Effect of Solvent Polarity In our experience, the relationship between solvent effects, charge types, and reaction rates in substitution reactions is conceptually one of the most difficult challenges for students in introductory organic chemistry. If the rate determining step leads to a transition state in which there is a concentration of charge relative to that in the reactants, the reaction will proceed faster when run in a more polar solvent. This is the case for all of the reactions described in this article as shown in eq 1. Students easily understand that a 4:1 mixture of CH3OH and H2O is more polar than pure CH 3 OH, so they are not surprised that (1-bromoethyl)benzene reacts about 60 times faster in the mixed solvent than it does in absolute CH3OH.
The results of various solvolysis reactions are summarized in Table 1. All reaction times are averages of at least two determinations. They were taken as the time when the glow of the light bulb first became visible. The larger of the two values for (1-bromoethyl)benzene was measured using absolute methanol as the solvent. Discussion The most important variables in the SN1 mechanism are the substrate structure, the leaving group, and the solvent. Using the compounds listed in Table 1, it is possible to demonstrate how changing each of these variables affects the rates of solvolysis. The pedagogical value of these demonstrations is greatly enhanced by engaging students in predicting the outcomes of each comparison or by having them rationalize the results of a specific comparison.
Effect of Substrate Structure A comparison of 2-bromobutane and 2-bromo-2methylpropane vividly demonstrates the fact that tertiary substrates undergo solvolysis much faster than their secondary analogs: the tertiary substrate reacts in about 0.1 minutes, while the glow of the bulb does not become apparent with the secondary isomer for nearly 48 minutes. Demonstration of the even lower reactivity of 1-bromobutane is not necessary. The fact that all secondary carbocations are not equally reactive is made obvious by comparing the rates of reaction of (1-bromoethyl)benzene and 2-bromobutane: the resonance stabilized secondary carbocation is formed in about 0.2 minutes, while the simple secondary carbocation requires nearly 48 minutes. Note that it is possible to make this comparison using the same solution of 2-bromobutane that was used in the first comparison. Using an overhead projector makes it easy to demonstrate the formation, or lack of formation, of HBr in these three reactions. Once the light bulb is glowing brightly, addition of a 1-mL aliquot of the solvolysis mixture decolorizes the indicator in a 1-mL sample of 0.1M solution of NaOH. If an aliquot of the 2-bromobutane mixture is added to the base before the glow of the bulb becomes visible, the solution remains pink. Effect of the Leaving Group The effect of the leaving group can be seen by examining the time results for 2-bromo-2-methylpropane versus 2chloro-2-methylpropane and benzyl bromide versus benzyl
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Note 1. Fisher Scientific Education catalog sells a conductivity device (number CQS43478) for $30.50. This item may be purchased online at http://www.fishersci.com (accessed Sep 2003).
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
Gilbert, E. C.; Pease, C. S. J. Chem. Educ. 1927, 4, 1297. Suter, H. A.; Kaelber, L. J. Chem. Educ. 1955, 32, 640. Thomas, W. B. J. Chem. Educ. 1962, 39, 531. Zawacky, S. K. S. J. Chem. Educ. 1995, 72, 728. Battino, R. J. Chem. Educ. 1991, 68, 79. Harvilla, J. W. J. Chem. Educ. 1991, 68, 80. Mercer, G. D. J. Chem. Educ. 1991, 68, 619. Newton, T. A.; Hill, B. A. J. Chem. Educ. 2004, 81, 61. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Organic Laboratory Techniques a Small Scale Approach; Harcourt Brace: Philadelphia, PA, 1998. 10. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 4th ed.; John Wiley & Sons: New York, 2000.
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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