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A VERSATILE EXPERIMENT IN REACTION KINETICS FOR THE PHYSICAL CHEMISTRY LABORATORY J. F. BUNNETT and H. D. CROCKFORD University of North Carolina, Chapel Hill, North Carolina
WEHAVE found that the reaction of piperidine with Stwe 1. 2.4dinitrochlorobenzene can be used as the basis of a ,NO. /~ wry satisfactory set uf nwtion kim:tics experin~e~~ts for the n~~dergrnduatephysical rhemistrv loborutory. O?S 4\-- >- 2a (thus requiring the use of equation (3)), both following the progress of the reaction by potentiometric determination of the chloride ion. A third and fourth pair can be assigned the same determinations but using the photometric method of analysis. Other pairs can be assigned the same determinations using a potentiometric acid-base titration method. Another variation is the determination of pseudo-first order rate constants from which second order constants can be derived. We find that following such a procedure produces a healthy and stimulating interchange of experiences among the various groups of students, so that each gains something from the experiences of his fellow students. At the end of the work, a session is held a t which reports from the various groups are received and the results compared. It is desirable that the values of the specific rate constant be obtained for at least two temperatures so that the energies and entropies of activation can be calculated. The two most convenient temperatures are 25" and 0". MINOR RESEARCH PROBLEMS
Time in minutes
Tppicd Plot of a Student'. ~ a t afor Detmmination of S-nd h d e r Rat- C o ~ h t with , Sprtrophotometric Andmb Temperature: OO. Initial oonoentratioas: 2,4-dioitrochlorobemene, 10-8 M; piperidine, 2.0 X 10". Rate conatant obtained: 0.186 I. mole-1 min:. (This &"dent used lower reactant oonoentrations, and therefore longer intervals between samples, than we reoommend.)
1.0 X
It is possible to determine the effectsof certain variables on the rate of the reaction without changing the basic experimental techniques. For example solvent effects can be studied. Preliminary measurements (5) indicate that solvent effects are moderate but interesting. There is an indication that the reaction series in aqueous dioxane solvents has an isokinetic temperature ( 8 ) a few degrees above O°C. Further exploration of this and other reaction series involving changes in solvent would be of interest and value. Also, salt effects could and should be studied. Such studies would constitute excellent special projects within the framework of the physical chemistry course or as separately organized undergraduate research.
PLANNING OF STUDENT ASSIGNMENTS
This experiment is rather time-consuming because it requires careful preparation by the student for a sequence of experimental operations as well as the actual performance of these operations. We find that from four to six laboratory periods of three hours each are required for a pair of students to carry out duplicate determinations by any one of the experimental techniques. We required the students t o start from "scratch" preparing all their standard solutions, etc. Part of the preliminary work could be done beforehand by the staff, but this would result in some educational loss to the student. One of the virtues of this experiment is that it provides a number of different experimental and mathematical approaches to the same physical values.
EXPERIMENTAL PROCEDURES
Materials. Eastman Kodak White Label 2,4-dinitrochlorobenzene and Practical piperidine and commercial 95 per cent ethanol can be used without further purification. For more precise work i t is desirable to recrystallize the dinitrochlorobenzene from ether, to reflux the piperidine witb sodium metal witb subsequent distillation from the sodium (9), and to redistill the ethanol. The purity of the ethanol can be checked by density measurements. The reaction rate is not highly sensitive to changes in water concentration (5) and so normal variations in the water content of "95 per cent" ethanol do not appreciably change the results. For the photometric procedure 2,4-dinitropbenyl-
VOLUME 33, NO. 11, NOVEMBER, 1956
555
Second O~derKinetics. b = fa. Photometric Analysis. The following directions are suitable for analysis by means of a Model B Beckman spectraphotometer using 1-m. cells. Some modifications of quantities may be necessary if other instruments or other sizes of cells are used. Before starting the reaction, p r e pare s. set of ten numbered 100-ml. volumetric flasks each about half full of "quenching solution" (0.1 N sulfuric acid in 1:l water-ethanol). The reaction can he set up as previously described or it can he run in a 100-ml. volumetric flask with appropriate reductions in quantities so as t o maintain the same concentrations. Withdraw aliquots from the reacting solution by means of a 5 m l . pipet and discharge them into the volumetric flasks containing the "quenching solutions." Fill each flask to the mark with the "quenching solution," shake thoroughly, and determine the optical density' of each a t 460 mp. In order to determine z, the concentration of the product at, time t, one must know the optical density of a quenched sample of the "infinity" solution (OD,). This can be obtained by mmpling the reaction after it has gone to completion, hut i t is faster and equally correct to prepare a. mock "infinity" solution. This is a. solution of 2,4dinitraphenylpiperidineof the concentration expected a t the end of the reaction. A sample bf this solution, quenched in the standard manner, has OD,. From the optical density values, calculate l l ( a - z) for each of the sample times. The value of the specific rate constant is obtained from the l/(a - 2) versus 1 plot. The edculation of (a - 2) is pomible through the fact that OD, is equal to pa, in which q is a. proportionality constant and a is the final concentration of the product. The latter is equal to the initial concentraSecond Ordw Kinetics. b = f a . Potentiomet~ieTitratia of tion of the 2,4-dinitroohlorobenmne. Calculate the value of q Chloride Ion. Prepare a standard solution, approximately 0.4 M , from this relationship. ODt is then equal to qr, and therefore of piperidine in 95 per cent ethanol. Weigh the piperidine in a (a - 2) is equal to (OD, - ODt)/p. volumetric flask, fill i t nearly to the mark with ethanol, mix A typical set of data is plotted in the figure. thoroughly, allow the flask to stand in the thermostat (or in the The above procedures assume that the optical density reading room) until the appreciable heat of ~olutionis dissipated, and is proportional to the concentration of 2,4-dinitrophenylpiperifinally fill i t t o the mark. Calculate the weight of 2,Gdinitra- dine. In the event that an instrument is used for which this is chlorobenzene containing half as many moles a8 there are males not true, a cdihration curve showing the relationship of the aptiof piperidine in a 25 ml. portion of the standard solution (about eal density reeding to the concentration should be prepared and 1.0 g. of 2,4-dinitrochlorobeneene). Weigh out this cdeulated the various concentrations determined from the graph. amount and dissolve it in about 200 ml. of 95 ner cent ethanol in a Pseud~JErstOrder Kinetics. Prepare a standard solution of 2fiU m l . volunwtriv flask. I'lnvc thii &tion, the standard piperidine. approximately 0.4 M, in 95 per cent ethanol and one pipwidirrc solutiun, and a smull flask containiug purr i o l v c ~ ~i nt of 2,4dinitroehlorobenzene in the same solvent, approximately thr thermostat and allon. thrm to romr to tlocrmnl equililrrium. 0.01 M. Prepare a. set of numbered 50-ml. volumetric flasks eaeh Before starting the run, prepare s. series of nine numbered about half full of the "quenching solution" as previously debeakers, each containing ahbout 15 ml. of 0.1 N sulfuric acid, to scribed. Pipet 10 ml. of the standard 2,4-dinitrochlorobenzene receive samples of the reacting solution. solution into a 100-ml. volumetrio flask and add ahout 70 ml. of T o s t a t the run, pipet 25 ml. of the standard piperidine solu- 95 per cent ethanol. Allow this flask, the flask with the piperidine tion into the reaction flask containing the 2,4-dinitrochloroben- solution, and a. flask containing 95 per cent ethanol to came t o zene solution. Record the time of release from the pipet. At temperature equilibrium in the thermostat. Then pipet 10 ml. once, add sufficient of the thermostatted solvent to fill the re- of the standard piperidine solution into the reaction flask, recordaction flask to the mark, mix thoroughly by vigorous shaking, and ing the time of release, add 95 per cent ethanol to the mark, shake quickly return t o the thermostat. As soon as possible, remove a vigorously, and immediately remove the first aliquot. Use s 25 ml. sample and discharge it into the first of the beakers con- 5-ml. pipet to remove the samples and record the time of taining 0.1 N sulfuric acid, recording the time of release. Re- release into the 50-ml. volumetric flasks containing "quenching move subsequent samples a t intervals of five minutes for runs a t solution." Recommended time intervals between semplerr are 25' or 15 minutes far runs a t OD. Titrate eaeh quenched sample three minutes for 25" and 15 minutes for 0% It is well to take with standard (about 0.02 M ) silver nitrate solution using s. about nine or ten samples in eaohrun. Fill the flasks with quenchpH meter. I n the titration assembly the usual calomel elec- ing solution and determine the opticd ddensities a t 390 mM. Detrode is replaced by a silver eleotrode. This electrode is pre- termine the value of OD, by allowing the reaction flask to stand pared by soldering a. short length of fine silver wire t o a brass or overnight a t room temperature, returning i t t o the thermostat, copper rod. The rod is covered with lacquer to eliminate con- and then removing two additional aliquots. These are the tact with the solution. I t is well to place a very thin film of ,z. ~nfinity"ssmples. silver chloride on the silver by momentarily making i t the anode Determine (a - 2) as previously described and plot the log of in an electrolytic circuit consisting of a dry cell, a platinum the values versus time. The specific rate constant can be decathode, and a. dilute hydrochloric acid solution. termined from the slope of the line either graphically or by the The e.m.f. (or the "pH") of the titration cell is plotted against method of least squares. An alternate and quicker procedure is the volume of the silver nitrate titrant. The midpoint of the to plot -log(OD, - OD,) versus time. The slope of this lie section of greatest slope is taken as the end point. Calculate multiplied by 2.303 gives the pseudo-first order specific rate conl l ( a - 2) for eaeh aliquot, and plot these values against the stant. The second order specific rate constant can he determined corresponding times. The resulting plot should be a straight from this by dividing by the initial concentration of the piperidine. line whose slope can he determined graphically. If the points do not all fall on a straight line the slope can be determined by the ' The term abso~bancehas been officially recommended to remethod of least squares or the method of averages. The spe- place optical h i t y (10). Both are defined s s the logarithm of cific rate constant is half the slope. the reciprocal of the trsnsmittance.
piperidine can be prepared by combining excess piperidine with 2,4-dinitrochlorobenzenein ethanol solution, refluxing for 15 minutes on the steam bath or allowing the mixture to stand overnight a t room temperature, pouring into water, and recrystallizing the precipitated material from 95 per cent ethanol. I t forms orange crystals of m.p. 91-2O. General Sampling Technique. The reaction is run in a volumetric flask, and samples are removed a t intervals by pipet and are discharged into excess acid. The time of release from the pipet is recorded for each sample. For convenience and safety, a three-foot length of rubber tubing with a glass mouthpiece is used with the pipet. Flow is regulated by squeeziug the rubber tuhe near the top of the pipet. The pipet need not be cleaned between samples; it is sufficient to rest it in a vertical position with its tip bearing on a couple of thicknesses of filter paper. For runs at 0°, pipets are kept in a "pipet chiller" except during use. The "chiller" is a glass tuhe, long and wide enough to contain the pipet, sealed a t the lower end, and immersed for most of its length in a container of crushed ice.
556 ACKNOWLEDGMENT
JOURNAL OF CHEMICAL EDUCATION (2) BLANKSMA, J. J., AND H. H. SCHREINEMACBERS, Rec. trav. ehim., 5 2 , 428 (1933). (3) BFLADY, 0.L.,AND F. R. CROPPER, J. Chem. Soc., 1950,
We are grateful to Dr. R. J. Morath for valuable advice and for the comments of the following m7 - ~hysical . . chemistry students who have worked with the general (4) BUNNETT,J. F., AND G. T.DAVIS,J. Am. Chem. Soc., 7 6 , experiment: Edgar W. Garbisch, Jr., Henry H. Dear3011 (1954). J. F., AND R. J. MORATH, J . Am. Chem. Soc., 77, (5) BUNNETT, man, George T. Davis, William &. Beard, Robert W. 5165 (1955). Meschke, and Hugh L. Medford. Our experimental (6) MELLON,M. G., "Analytical Absorption Spectroscopy," acquaintance with this reaction developed during the John Wiley Rr Sons, Inc., New Ymk, 1950, p. 98. course of some research work sponsored by the Office ( 7 ) BUNNETT, J. F., A N D MORATH, R. J.,J. Am. Chem. Soe., 77, of Ordnance Research, U. S. Army. 5051 (1955). J. E., J . Org. Chem., 20, 1202 (1955). (8) LEFFLER, LITERATURE CITED (9) BROWER, X. R., AND e. D . AMSTUTZ, J . 01q. Chem., 18, (1) BUNNETT, J. F., E. W. GARBISCA, JR.,A N D K. M. PRUITT, 1075 (1953). unpublished work. (10) HUGHES, H. K., Anal. Chem., 24, 1349 (1952).
