In the Laboratory
The Reduction in the Rate of Hydrolysis of Diphenylbromomethane by the Common-Ion Effect Richard Cameron-Holford, Tarini Ratneswaren, and D. E. Peter Hughes* Westminster School, Dean's Yard, London SW1P 3RW, United Kingdom *
[email protected] The hydrolysis of a secondary bromoalkane, such as diphenylbromomethane, may take place by one of two mechanisms. The first mechanism is a one-step SN2 reaction, k1
RBr þ H2 O a ROH2þ þ Br k-1
where R is an alkyl group, in this experiment, Ph2CH-. The rate of reaction is k1[RBr][H2O], but as [H2O] is usually in large excess, the overall kinetics is first order with respect to [RBr]. The second is a two-step SN1 reaction: k1
RBr a R þ þ Br k-1
k2
R þ þ H2 O sf ROH2þ As the carbocation Rþ is highly reactive, k-1 and k2 will usually be much larger than k1 and the overall kinetics are again first order with respect to [RBr]. Thus, determination of the overall order does not enable one to distinguish between the two mechanisms, an important point to stress. The classical solution to the problem is to add excess Brions (1). If the reaction were taking place by a SN1 mechanism, the equilibrium of the first step would be displaced to the left (the common-ion effect) (2), which would reduce [Rþ] and hence slow down the reaction. Unfortunately, the addition of ions increases the speed of the reaction by increasing the polarity of the medium (3). Thus, on increasing [Br-], the reaction initially speeds up and then slows down (4). To compensate for the change in polarity of the medium as [Br-] increases, it was necessary to keep its ionic strength constant by adding an inert salt. The salt chosen was NaClO4; the reason for this was because the ClO4- ion is a very poor nucleophile, being derived from the very strong acid HClO4 and thus took no part in the reaction. The temperature was maintained at 25.0 °C in a thermostatic water bath, the solvent was 80% propanone/20% water by volume, and the total ionic strength was maintained at 0.20 mol L-1. These precautions ensured that all factors were kept constant except [Br-], which was the variable under investigation. Other authors have followed the rate of the reaction by titration (5), but this necessitates using a reaction with a fairly long half-life to obtain reasonable results. By measuring the increase in conductivity, which is proportional to the increase in concentration, it is possible to use a much faster reaction and thus carry out the four determinations required by this experiment in a 2-h period. The increase in conductivity is an exponential curve, superimposed on the background conductivity caused by the salts initially added to the reaction mixture. The disadvantage of using conductivity measurements is that as there 848
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Table 1. Composition of the Solutions Used in the Experiment Tube
Volume of 1.0 M NaBr/mL
Volume of 1.0 M NaClO4/mL
Volume of Propanone/mL
1
1.0
3.0
16.0
2
2.0
2.0
16.0
3
3.0
1.0
16.0
4
4.0
0
16.0
is only a small change on top of the background conductivity due to the added salt, the accuracy of the measured rate constants was not very good (20%). The results were most conveniently analyzed using a Guggenheim plot, which does not rely on finding the infinity conductivity reading. It is sensible to avoid having to find the infinity reading because these are difficult to measure accurately and necessitate leaving the experiment to run for a long time. Materials and Methods The experiments were conducted in 150 mm boiling tubes. A dipping electrode assembly (cell constant 1.08 cm-1 supplied with the WPA CM35 conductivity meter) was fitted with a rubber stopper and the height adjusted so that it went nearly to the bottom of the boiling tube. The electrodes were attached to the conductivity meter and the output attached to an AD converter (PicoScope ADC 3200) and hence to a Dell SX520 computer. A program was designed so that the output from the conductivity meter was rapidly sampled and averaged, and the output was displayed every 10 s. With the AD converter and the computer being used, this corresponded to an average of over 106 readings and ensured that the output was steady to within 0.1%. If the PicoScope program for use with ADC3200 has been loaded, the conductivity program designed for this experiment is available on the Web (6). Hazards Propanone is volatile, flammable, and dangerous if inhaled. In this experiment, a solution in water was used, which was much safer. Diphenylbromomethane is a lachrymator, causes skin irritation and burns, and may cause severe and permanent damage to the digestive tract. Results and Discussion The following solutions (Table 1) were made up in boiling tubes that were placed in a water bath at 25.0 °C. Each tube contained 16.0 mL of propanone and 4.0 mL of water; this was sufficient water to promote the hydrolysis. The electrode assembly
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Vol. 87 No. 8 August 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100264b Published on Web 06/10/2010
In the Laboratory
Summary
Table 2. A Typical Set of Student Results Tube
[Br]/(mol L
-1
)
Rate Constant/s
1
0.05
0.0027
2
0.10
0.0022
3
0.15
0.0016
4
0.20
0.0013
a
-1 a
A student familiar with SN1 and SN2 mechanisms can carry out the experimental work in less than 2 h. The following are some of the features brought out by the experiment.
• That it is often impossible to distinguish between SN1 and SN2 mechanisms by simple kinetic measurements. • The importance of keeping the ionic strength (and other factors) constant. • The reduction in rate by the common-ion effect supports (but does not prove) that the reaction is taking place by a SN1 mechanism. • A Guggenheim plot is a convenient method to evaluate the rate constant for a first-order reaction. • The steady-state approximation can be used to analyze the results in a more detailed fashion.
Six other students obtained results within 20% of these typical values.
was placed in tube 1, left for 10 min to take on the temperature of the bath, and 500 μL of 1.0 M diphenylbromomethane in methylbenzene was injected from a syringe. After 100 s, the computer was started and allowed to run for a further 200 s, taking readings every second. The results were analyzed using a Guggenheim plot program to find the value of the rate constant. The experiment was repeated with the other tubes. Twelve sets of data have been obtained by seven students, and a typical set of student data is given in Table 2. The reduction in the rate constant as [Br-] increased suggested that the mechanism of the reaction was SN1. The following more sophisticated treatment strengthens the evidence. For a SN1 reaction k1
RBr a R þ þ Br k-1
Acknowledgment My thanks to Anthony Sheehy who wrote the conductivity program. Literature Cited
k2
R þ þ H2 O sf ROH2þ rate ¼ k2 ½R þ ½H2 O As [Rþ] is small and almost constant, we can use the steadystate approximation to give d½RBr k1 k2 ½RBr½H2 O ¼ rate ¼ dt k - 1 ½Br - þ k2 ½H2 O and the measured rate constant k = k1k2[H2O]/(k-1[Br-] þ k2[H2O]) rather than k1. The simplest way to evaluate k1 is to plot 1/k against [Br-], which as an approximation is assumed to remain constant. This gives a straight-line graph whose intercept is 1/k1 and slope equals k-1/(k1k2[H2O]). The linearity of this plot supports the SN1 mechanism but does not yield values for the constants k-1 and k2.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
The results show that all these aims can be achieved in the allocated time.
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1. Bateman, L. C.; Hughes, E. D.; Ingold, C. K. J. Chem. Soc. 1940, 966–971. 2. Isaacs, N. S. Physical Organic Chemistry; Longmans: Harlow, U.K., 1987; p 399. 3. Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Sausalito, CA, 2006; pp 646-647. 4. Danen, W. C.; Blecha, M. T.; Burkett, A. R. J. Chem. Educ. 1982, 59, 659–660. 5. Herbrandson, H. F. J. Chem. Educ. 1971, 48, 706–707. 6. Westminster School. http://www.westminster.org.uk/chemistry/ conductivity.zip (accessed May 2010).
Supporting Information Available Student handout; instructor notes. This material is available via the Internet at http://pubs.acs.org.
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Vol. 87 No. 8 August 2010
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