J. A. Leisten University of Sheffield England
Experiments in Acid-Base ~ a t a i ~ s i s
In some cases of acid or base catalysis the rat,e of the rcaction depends upon the concentration of each acid or base in the solution. This is general catalysis. In other cases the rate depends only upon t,he hydrogen or hydroxide ion concentration (or in general the lyonium or lyate in concentration), regardless of the concentration of any other acids, or bases, which may he present,. This is specific catalysis (1). The distinction between general and specific catalysis can he crucial in studies of reaction mechanism. Suppose, for example, that studies of an acid-catalysed reaction in aqueous solution had suggested two likely mechanisms: 1) A proton is donated to a reactant in the rate-determining step; 2) The rate-determining step involves the conjugate acid of a reactant. This conjugate acid is formed to a fractional extent in a mobile acid-base equilibrium. Both are mechanisms of acid catalysis, but 1) is associated with general catalysis, because every acid in the solution is capable of donating a proton to the reactant to a greater or lesser extent. On the other hand 2) is associated with specific hydrogen ion catalysis, because the proportion of the reactant (R) converted to the conjugate acid (HR+) will depend upon t,he hydrogen ion concentration, regardless of any other acids that may be present, according to the relation .
H1P04-) are 0.05 M, 0.025 M , and 0.0125 M respectively. The Mutarotation of Glucose
This reaction can be followed polarimetrically. Weigh 5 g of D glucose into a beaker. Add 50 ml of the catalyst solution and stir until the glucose has dissolved. Rapidly fill a 20-cm polarimeter cell and place it in position. Take readings every minute a t first,, then at longer intervals until the reaction is 80-90% complete. Repeat the measurements mith the other catalyst solutions. The third solution should he left in the polarimeter for an infinity reading. Excellent first-order plot,s are ohtained eve11 without a thermostatted cell, and the whole series of measurements (other than the infinity reading) can be made in an hour. Since the reaction is reversible the observed rate constant is the sum of the rate constants for the fonvard and reverse reactions (5). However t.he catalyst does not affect the position of equilibrium, and t,he forward, reverse, and observed rate constants mill all vary in the same way with the concentration of catalyst. The Hydrolysis of Dimethyl Ketal CHI
This aspect of acid-base catalysis, general and specific catalysis as criteria of reaction mechanism, has considerable presentday importance ( 2 ) . To illustrate this theory in a laboratory class, rate measurements should he made bot,h on a specificallycatalysed and on a generally-catalysed reaction. To minimise the t,ime required, physical methods of measurement must be used and the reactions must be rather fast under the experimental conditions. The experimental conditions should he similar; in particular the same catalysts in the same concentration should he used in each reaction, for then the different patterns of results ohtained in the two reactions provide a greater challenge to the students' understanding. The experiments described here meet these requirements. Experiments Solulions. Weigh 13.61 g of KHIPO, into a beaker, add 50 ml of 1 N NaOH, and make up to 500 ml with boiled-out water. l-his A , is 0.1 M in both alld HPO,-. prepare
500 ml of 0.4A! NaCl in boiled-out water (B). All mixtures of A and B have the same ionic strength (0.4)and very nearly the same pH (7.0);the eoncentrationfi of the general aeida and bases HaPOd-and HPOa-are the only signifioant variables.
The kinet,ic;qns are carried out in three mixtures of A and B SUCKthat the concentrations of HP04- (and 132
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Journol of Chemicol Educofion
OCIHs
-
CHJ
+ H20
/ \ CH1 OCzHr
?O \
+ 2CaH,OH
/
CHs
This reaction can he followed by the change of volume in a dilatometer, which should have a capacity of 80-90 ml and a capillary of 0.5-mm bore graduated in mm. Immerse the dilatometer and a flask containing 100 ml of t,he catalyst solution for 20 minutes in a thermostat at 2 5 T . Add 0.7 ml of dimethyl ketal to the solution, shake, and fill the dilatometer mith t,he mixture. Take readings at minute intervals for approximately the time of half reaction, allow an equal time to elapse, and then resume readings, again for t,he same period and a t minute intervals. The first-order rate constant is calculated by Guggenheim's method (4). Results
Typical results, obtained by the methods described, are shown in the figure. The mutarotation was studied at 23.4-23.7'C, and the additional point on the ordinate refers to pure water as solvent. The linear -increase in rate with the concentration of the phosphate huffer shows that the mutarotation of glucose is subject to general catalysis. The relative constancy of the rate 07 hydrolysis dimethyl ketal in these solutions is consistent with specific catalysis. (In ~
of
0 0
0.025
0.05
Molar Concentration of HPOA-
Rates of reaction or o function of HPOI- concontrotion. Open circles: mutarototion of g l ~ ~ o s ed; i d circles: hydrolysis of dimethyl k e t d
fact there is a slight decrease in rate with huffer concentration. This is a salt effect. The effect, is greater if the runs are not carried out at equal ionic strength, and since higher concentrations of neutral electrolytes "salt out" dimethyl ketal it cannot be much reduced.) Extensions of the Experiments
What has so far been described is a skeleton exercise which ran be supplemented hoth experimentally and t,heoret,ically. A few possibilities will he dealt. wit,h hriefly. (1) It is desirable to confirm that the hydrolysis of dimethyl ketal is a catalysed reaction, and to decide whet,her cat,alysis is by hydrogen ions or by hydroxyl ions. One way to do this is to show by qualitative testtube experiments that hydrolysis is much faster in strongly acid solutions, and much slower in strongly alkaline solutions, than it is in pure water. The strong camphor-like odor of dimethyl ketal can be used satisfactorily t,o determine under which conditions hydrolysis has occurred. If such experiments are added to t,he exercise the additional conclusion can he drawn that the reaction is specifically catalysed by hydrogen ions. (2) The resuks do not distinguish between HPOI= and H2P04- as possible catalysts in the mutarotation of glucose. This is a good opportunity t,o introduce
the Bronsted relation,. which predicts from existsing data (5) catalytir constants of 1.41 min.-I for base catalysis by HPOa=, and 0.0015 min.-' for acid catalysis by H2P04,- hoth at 1 8 T . (The reason for ignoring base catalysis by H2P04- and acid catalysis by HPOI- provides a useful question.) Thus the Bronsted relation predictas that the effective catalyst in the phosphate huffer is the HP04=ion,functioning as a base. The slope in the figure yields an experiment,al value of 2.54 min-I at 23.6" for the catalytic constant of the buffer. (3) If the rate determinations are repeated in solutions similar to the previous ones except that the concentration of H2P04-is halved in each case, the results for t,he mutarotation of glucose are the same, within experimental error, as t,hose in the figure, showing that HP04= is the effective catalyst. The rate constants for the hydrolysis of dimethyl ketal are, again within experimental error, exactly half the previous values. The hydrogen ion concentrat,ion in the new solutions is also of course half that in the original solutions. This reartion is therefore specifically catalysed by hydrogen ions. If the student is given these additional kinetic observations he can reach the conclusions in (1) and ( 2 ) without more experiments and without applying the Bronsted relation. (4) Emphasis can be placed on the mechanistic implications of the results if this is desired. Both reartions have been discussed in recent papers (6). Litemture Cited
(1) BELL,R. P.,"Acid-Bme Catalysis," Oxford University Press, London, England, 1941, pp. 48f. (2) See, e.g., WEINSTOCK, J., PEARSON, R. G., AND BORDWELL, F. G . ,J . A m . Chem. Soc.,78,3468 (1956); BUNNETT, J. F.! AND RANDALL, J. J., ibid., 8 0 , 6020 (1958); KRESGE,A. J., AND CHIANO, Y., ibid., 81,5509(1959); BELL,R. P., RAND, M. H., A N D WYNNE-JONES, K. M. A,, T7m8.Famday Soe., 1093 (1956). (3) See, e.g. FROST,A. A,, AND PEARSON, R. G., "Kinetics and Mechanism," John Wiley & Sons, Inr., New York, 1953, n. 172. (5) BELL,R. P., o p . Cil., pp. 66 and 88. B. C., LONG,F. A,, A N D POCKER, Y.,J . Chem. Sac., (6) CHALLIS, G , A N D GRAFF,C., J . Am. 4679 (1957); R I ~ ~ E N B E RD., Chem. Soe., 8 0 , 3370 (1958); LONG,F. A,, A N D PACL,M. A., Chem. Re"., 57, 965 (1957); KDSKIKALLIO, J., A N D WEALLEY, E., T ~ a n sFwaday . Sac., 55,809 (1959).
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