A Symposium for Students N. C. Chokotho a n d J. A. Leisten University of Malaki, Box 280, Zomba, Mala&ti,
Central Africa
Research projects and talks by students are both accepted components of chemistry courses a t different levels, but in departments with limited staff and material resources research projects for large classes present difficulties, and student talks have a way of swallowing up teaching time. The present article describes an economical method of providing for both of these activities. It has been worked out with university students, but with adjustments here and there it might well he tried in high schools. The hasic idea is to have students working individually on different aspects of a single theme. Our theme has heen the halogenation of ketones. The common basis of the projects makes it mssihle for the supervisor to look after more students than i f thestudmts were working upon diverse st~bjrcts,and the lahorator\. administratim can be simpliiied In :1 sharinr of equipment and reagents. In their talks a t theend of the oroiects students can explain their work in a relatively short iime, lwcnuse they have an audience which is fam~linrwith the backrnntnd, and this makes it ~r,ssihltt o hold all the talks and disc~ssionson the same day. Hence the title: "a symposium for students.'' T h e possibilities of this approach can he illustrated by our experience with the halogenation of ketones, hut the choice of themes seems unlimited, and the one we have chosen could doubtless he handled in many different ways. We prefer students to start with ouite a eood understandine of the theme. for paradoxically t h k leads& more questions i f the kind that can he turned into projects. For this reason our students begin, in the year following a course of CHEM Study standard, by carrying out together some kinetic runs on the iodination of acetone CHBCOCH,
+ I2
-
CH3COCHd
+ HI
and bv studvine its well-established mechanism. (1 ). In the fir& r;n (referred to later as the standardrun) the initial reaction mixt,ure is 0.5 M in acetone. 0.1 M in hvdrochloric acid, and 0.005 M in iodine. I t is kept a t 35'C, and 10 cm3 samples are withdrawn a t intervals for titration with 0.0025 M thiosulfate after quenching with sodium acetate solution. We obtain the result which has surprised a good many generations of students, a zero-order plot, the concentration of iodine decreasing linearly with time, and we consider the consequence of this result: That since the iodine concentration is the only one which changes appreciably during the run, the rate of iodination must be independent of the iodine concentration. The followinz week our students derive the full rate equation by carrying out two similar runs in which the acetone and acid concentrations are doubled in turn: rate = k[aeetone] [acid] Studying the mechanistic interpretation of this rate equation, students easily see that in the first, reversible, acid-base step the equilibrium must lie to the left, for they already think of acetone as being a t best a very weak hase:
+
+
CH,COCH~ ~ : , = b C H ~ C ( = I ~)CHZ H HZO
We explain that this equilibrium causes the small concentration of protonated acetone to he proportional to the concentrations of acetone and hydrochloric acid. Hence the rate of the next step, which determines the rate of the overall re490
Journal of Chemical Education
action, must also be proportional to the concentrations of acetone and hydrochloric acid:
-
C H ~ ( = ~ H ) C+HHz0 ~ CHsC(OH)=CH2 + ~ $ 6 T h e enol which is formed in this slow step is snapped up by the iodine as fast as it is formed, and the iodination, therefore, oroceeds a t the same rate when the iodine concentration is low i s when it is high. The mechanism thus accounts for the rate equation which the students have observed, including its interesting zero-order-in-iodine feature. If the mechanism is presented in the way we favor, with the emphasis on physical insight rather than on rate-equation algehra, a student isalmost certain to ask: What is the function of the first, pre-equilibrium, step? Why does the enolization need to be acid catalvzed? This is a fundamental auestion. best answered with the help of curly arrows:
Water a d s as a hase in the enolization step, hut it is only a very weak hase, and the breaking of the C-H bond needs the electrostatic pull of the proton on the carbonyl group to help it along. Although the preparation is now complete we like to give the projects in the following year, because students appear more motivated to study the reaction when it has become old and familiar to them. After a brief reminder of the reaction and its mechanism our students are asked to suggest aspects that mieht he investieated further. and the choice of oroiects actuallidepends on tkese suggesti&s, as well as upon'the size of the class, and the strengths and weaknesses of individuals, including the different extents to which they are specializing in chemistry. The projects suggested below are intended to keep a class of twenty busy for five weekly three-hour periods, but manv of the . oroiects could be snlit into two or more if the " class were larger. For example in Project I, the variation of rate with temperature, one student might he asked to work above 35OC and to give his talk a t the symposium on the experimental problem of measuring rates over a wide range of magnitude, while a second student might be asked to work below 35O and present and discuss the combined results. 1) Study the uariation in the rate of iodination of acetone with temperature. ' I l k worker will wrely uhwin an Anlwlius plot, and he will prohnhly explain in hla ralk that the variation i n r:ate must hsveadependcnce on the pre-equilibrium step as well as upon the rate-determining enolization step. The discussion of AHt and AS* for a complex mechanism, however, is too academically specialized for our aims, and we usually divert this student from the transition state theory and ask him instead to derive an arithmetic formula 6f use to some ofhis colleagues for converting rates in the region 33'-31° to our standard temperature of 35'. 2) Compare the catalytic efficiency of hydrochloric acid with that of other mineral acids ' 1 ' 1 ~qtudent fmdithst w i t h ~ nhiseqwrimental errur equal m8larities 8.1 nitric. pcrvhlridspnluwr equal ritrc uf reaction. Perhaps these results are needed to prompt his realization that the acid in all the solutions is the same, H30+. He finds that sulfuricacid is less than twice as effective as the monohasic mineral
acids and explains in his talk that the second ionization of sulfuric acid is weak and perhaps tries to correlate the rate with the calculated hydronium ion concentration.
3) T h e reaction becomes slower when t h e concentration of acid is reduced. Find whether i t becomes slower still when t h e acid is replaced bv a base. This researcher discovers hase catalysis, and he explains a t the symposium that if the hase water in the enolization step of the mechanism is replaced by a stronger base, the first, pre-equilibrium step becomes unnecessary; and that acid catalysis then gives way to base catalysis. The project offers further problems depending on which bases are tried. The stroneer bases. for examole. cause further reactions which produce iodoform.
4) Deuise a quick qualitative methodof comparing the effi-
ciencv o f catalysts and apulv .. . i t to a series o f acids o f diminishing strength. With a few trials the student arrives a t a test tube method depending upon visual observation of the iodine, and from a table ofK, values he chooses perhaps the following series: HC1, HSO;, CHJCOOH, HCO,, HPOk-. His tests show that the rate along this series a t first falls, hut he is surprised to find that HCOF is agood catalyst and can hardly believe that HPOZ- is actually better than HCI! This worker also has stumbled on base catalysis: he now realizes that severalof his acids are amphoteric and that while their K , values fall along the series their Ka values are actually increasing. Further tests with unamhieuous. " . non-amohoterie acids and bases such as boric aeid and carbonate ion support his interpretation. 5) A Nuffield chemistry course uses sodium hydrogen car-
appropriate experiment gives no evidence of reversibility in the case of bromination.
9) Make the reaction as fast as possible without raising the temoerature.
and decides perhaps to lower his reaction temperature rathe; than to look for other wavs of followine the reaction. In the end. after d e ~ the concentration of acid eventually leads to diminishing returns, for although this has a favorable effect on the pre-equilibrium it retards the enolization step, which requires a base. A more advanced student might bring the Hammett acidity function into his discussion.
10) Prepare bromoacetone by modifying thestandard kinetic run. This is a favorite topic, for the application of principles rather than recipes to organic preparations seems worth encouraging. Every stage of the preparation and work-up is open to reason. An especially good student might be asked to prepare ioduacetone, in which reversihility provides an extra problem. (He might think of progressively removing iodide with lead or iodate to drive the reaction forward.) I t should he remembered t h a t these substances a r e laehrymatory. 11) Modify the standard kinetic r u n s o as to determine the
heat of bromination of acetone. Given a simple calorimeter and a thermometer graduated in O . l 0 , the main oroblem is to n r d u c e enough chemical reaction in a sufficientlv
bonate instead of sodium acetate for quenching the samples ( 2 ) .Find which is better. This project gives unexpected scope and mental exercise. The quencher is a hase whieh neutralizes the hydrochloric acid in the samples, hut unless the base is weak enough the quencher itself catalyzes the reaction whieh is why hydrogen esrhonate is less effective than acetate. The basemust not he too weak on the other hand or its conjugate acid, produced when the sample is added, will he dangerously strong. Our student finds that acetate ion and its conjugate acid have catalytic effects of the same order, and he concludes from this that sodium acetate is a well-chosen quencher: but he continues t o look for an even better one. (For example, a weaker base which yields an insoluble aeid?). Perhaos also he eomoares chemical auenchinewith dilution or cooling.
6) Calculate K, for chloracetic acid by assuming that its catalytic e f f e c t o n the reaction is due to the hydronium
This worker obtains good zero-order plots with chloracetic acid and has no trouble in comparing the rate with that produced by a fully ionized acid (hvdrochloric acid). hut his calculated K , value is far too hie. He wouldhave found similar results had he worked on HSO;.
and the reaction rate are reduced progressively by successiveadditions of solid sodium chloracetate reveals a residual rate which can be attributed only to catalvsis bv the chloracetic aeid molecule itself. This resourceful student then estimates the rate due to catalysis by H30' and so arrives at a reasonable value of K, after all.
compared with a calculated value. 12) Modify the standard kinetic run in order to demonstrate
that t h e halogenation of acetone is autocatalytic. A suitable reaction mixture is arrived a t usually with much trial and error. For a clear autocatalytic effect, assuming an initial halogen concentration of 0.005 M, the initial concentration of hydrochloric acid should he 0.002 M or less. To comoensate fur the resultine reduction of the initial rate the concentration of acetone must be greatly increased. Patience eventually leads to the dramatic curve of autocatalysis. 13) Follow t h e halogenation of acetone by change of elec-
trical conductance. Our researcher soon realizes that the increase in eonduetsnee during the standard kinetic run isconcealed by the high conductance of the hydrochloric acid already present, and in devising a run which can be followed conductimetrically she is driven toward the reaction mixture of project 12, withlow aeid and high acetone concentrations. She suspects that the curved plot of conductance against time is due to inter-ionic mobility effects, but a calibration with known additions of electrolyte eventually convinces her that her results show autocatalysis.
14) Follow the iodination o f acetone by change i n light absorption.
is as much a consequence of the proposed mechanism as the zero-order feature. (Chlorination is slightly more difficult to follow ( 3 ) and we have not yet tried it as a project.)
Since the temperature of reaction mixtures in the photoelectric colorimeter cannot be controlled, this worker takes higher than usual concentrations of acetone and hydrochloric acid so that her runs last only a minute or two. She uses a metronome far timing, and with a blue filter and the reaction tube in the light path, and the instrument reading zero, she starts the reactions by adding and stirring in a drop of concentrated iodine with a rubber policeman. She finds that the optical densityltime readings are non-linear but gets good zero-order plots by preparing a calibration graph and using it to convert optical densities to iodine concentrations. She carries out enough runs to verify the rate equation and also looks a t the reaction's reversibility. 15) Obtain a normal decay curue for the bromination of ac-
8) Find whether the halogenation of acetone is reuersible.
etone.
The reversibility of idination is soon demonstrated by a run with ten times the usual concentration of iodine, because much of the iodine now remains unreacted. The effect on the position of equilibrium of changing the concentration of the various species is studied, and since the equilibrium seems particularly sensitive to added iodide the results are discussed in terms of the equation
Since the standard run is zero-order because the acetone is in large excess, the obvious experiment design is t o reduce the acetone eoncentration t o that of the halogen and to use more hydrochloric aeid to maintain a convenient rate. The result is yet another surprise, for althoueh there is a stead" lowerine of rate. the reaction still ends abrup& with the disappe&ance of t i e bromihe. It is now realized that consecutive reactions can replace more of the hydrogenatoms in the acetone molecule, and that in the run with equal initial concentrations, the acetone is still present in excess. This realization leads to further
7) Compare the rates of iodination and bromination. It is shown in preliminary experiments thatthe bromine can be estimated hy running samples into sodium acetate quencher containing excess potassium iodide and then titrating with sodium thiosulfate. Iodination runs are then comoared with similar runs in which 0.005
which takes account of the relative stahility of the +iodide
ion. An
Volume 58
Number 6
June 1981
491
runs with reduced amountsof acetoneand tospeculationsabout the relative rates of the consecutive reactions. 16) S t u d y some effects of structure o n t h e r a t e of halogenation of ketones. This offers a wide choice. One tempting study, in view of current interest in alicyclicreactivity,is the effect of cyelization; but our student decides instead to compare acetone with monobromoacetone. She predicts that the bromine atom should have an accelerativeinductive influence on the enolization step, and she is pleased to find the hromination of bramoacetone proceeds much faster than that of acetone when the catalvst is a base (acetate ion). The difference is surnrisinely smnll. . ~ ~howeve;. . ~ ~.in~the . acid-cstalvzed reaction. and she exviains this 31 thesymposium i l l tcrmsofthemechanisrnl. although the bnmmt atom accelerates the endization step, it puches the pre-equlhhrium to the left makpi I ~ ketme P less basic, ;and the rs'o effects a l m ~ s l balance out. 17) Analyze the products of the bromination of acetone. It was written in the days before gas-liquid chromatography: "The f m t ~solahleproduct in the base-catalyzed hslogenat~ond aretww 1s rheunsymmetr~laltrihalogenderrvat~vr,t..g.,CH~COCBr.~. It is that . ..the mono- and dibrom- compounds read relatively rapidly, as swn as they are formbd, and never attain a sufficient concentration to permit isolation" (4). An obvious challenge! A GLC study of the mid-catalvzed is worthwhile also. oarticularlv since the ... . , - reaction ~ dliierences betawn the acid and base ralnlwa produ~tpatterns c.m he dtscussed reasunaldy and at a jimple 1rwI. 18) Try to brruk the zero-order r a t s luw by making the r n d r e t u r n to t h e keto form faster t h a n i t reacts with t h e halogen. Pondering over the mechanism our student decides to search for the required breakdown of zero-order kinetics at low iodine concentrations and high acidities. By using a speetrophotometer, she finds that she can start with iodine concentrations as low as 5 X 1 0 - 5 M . Eventually she obtains zero-order plots which tsil off toward the end and shows that the tsil is significant by careful calibrations with known iodine and iodide concentrations. She explains that this is theeffect to be expected as the reaction tends to become first order. (Bromination can be made fully first order by reducing the initial bromine concpntrntion to 106M and followine ...... " the reaction ootentiometridv ( 5 ) . hut we have not used this as a project.) 19) Determine the rate ronslonl /or l h r enolizotion s t e p of t h e acid catalyzed halogenation of acetone This is a relatively dvanced toplc hecaui rl ,n\olves measuremrnts with concentrated iulfuric acid oolutiuns, iar beyond the r n n v ill which pH has significance, and the student must, therefore, cope with the Hammett acidity function. She scan shows that k2 = klKt, where k z is the required rate constant, k is the overall rate constant, which she calculates from a orevious run. and KI is the eauilibrium constant of the first, pre-equilibrium, step. K I must he estimated by finding the acidity of the solvent which brings about the half-protonationof acetone. We have made rough estimates of this from differences in the smell of acetone over aqueous sulfuric acids containing equal acetone concentrations (certainly a method to be frowned upon). NMR provides the best methods (6).Our student uses UVspectrmcoov. . . . but she admits in her talk that her estimate of Kt is only apawtrme ts not a good Harnn~rtlbase. proximate 20) Devise demr,nslralions lor o i~ctureron t h e iodinalion of acetone. A project for rhc pmspective teacher! T ~ Same P student could h asked t u determine the east of the iodinntwn of acetone as a &.lass exercise and suEat.;t wonomies. In her i1r.1 drmonsrration, perhaps. she csrriesout thereaction in two large beakersunder identical canditions, except that in one the iodine is added all at once, whereas in the other, it is added in portions, each addition being made as the ~
~~
~
~
~~~
~
~
~
~
~
iodine color fades. The two reactions finish a t almost the same moment, illustrating the independence of the rate upon the iodine concentration. A similar demonstration shows the effect of the acid or the acetone concentration. The effect of temoerature is revealed bv the increasmg frrqu~ncyof iod~nur~rld~t~ona when lhe reuctim is carried out ~ I C I 4 stirrer-hot plate. There are many possihili~irs. T h e syinpo,ium is noa o w r . lr has been idealilrd for brevity. Although the reality may have more mistakes and obscurities, it is always lively, and also enjoyable, even though we are unable t o orovide t h e hanauets and excursions which make real symposia more popular! I t is stage managed t o facilitate exposition by arranging t h e topics t o follow on from one another: for example, project 13, on conductance experiments, is much easier for t h e student t o explain and for t h e class t o understand because it immediately follows a talk on autocatalvsis. Time limits are riaorously. applied so t h a t t h e .. symposi& can be completed without sacrificing question periods. Whde the projects are in p n w c s s the students should be receivine iusr enough help t o avoid discouragement, nnd no more; and t o keep-; large class working co&uctively t h e supervisor needs t o have a strong interest in physical organic chkmistry. On t h e other hand, &npler and shorter projects could be tried in high schools which make no special demands on t h e supervisor. For example, six students could effectively study six acid catalysts in just a period or two, with results that would give them another view of K , values a n d illustrate t h e levelling effect of solvents. Other projects could be modified i n a similar way. Perhaps t h e best application of our approach is in develooineu countries. where new " eraduates mav sometimes find themselves without more experienced colleagues t o consult, and where there is a oarticular need for universities t o enrourage scient~licentrepreneurs. We feel committed m a n active kind of tcachine whirh we h o w will accustom students to meeting problemsand being independent, a n d t h e drive behind t h e halogenation of ketones approach has been t h e desire to provideproject work in all fivgiears of our chemistry courses without over-stretching- t h e inventiveness and work capacity of t h e staff. Perhaps a theme i n applied chemistry would be better for our a i m i t h a n t h e one &have chosen. Nevertheless, t h e halogenation of ketones brings in a wide range of those hasic principles which all chemistry students must absorb, however much their teachers may wish to emphasize applications, and from t h e intent expressions d u r i n g t h e symposium, i t might he thought t h a t t h e students learn more about kinetics and equilibrium by talking t o each other than they d o from our lectures. ~~~~~~~~
~
.
Lnerature Cited I11 Seea.g. la) Bell, R. P."Acid-BaseCatalysi8,"Oxford University Plus.1911:ibl Hammdt, L. P. "Physical Organic Chemistry: McGraw-Hill. New Yurk. 1940;kl Ingold. C. K. 2nd Ed.,"SUuclumand Me~hsnirminOrganicChemirVs,"Bell,1.undon. 196% Id) Could, g S. "Mechanism and Structure in Organic Chemi~try."HoiLHineharh and Winston. New Yurk. 195% Him, .I. 2nd Ed.. "i'hvsicai Organic ChemiLry: ~ c ~ r a w ~ ~1962. i l l : 12) "Nilffield Advanced Seienee: Chemistry: Teachers' Guide 11, Penguin. 1970.58 18) Ball. R.P..andYates. K.,J Chem Soe., I927 il9621. I41 (11 Ib),p.Z40. IS) YaUr.K..and Wright. W. V..Con. J. Che7.,11.2882 11963). 16) Butler. A. R.. J C. S. Perkin 11.959 11976).
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