Micelle Catalysis of an Aromatic Substitution Reaction
Gerald Corsaro and J. K. Smith' University of Akron 0~1044325
This article describes an experiment which demonstrates micelle catalysis. The continuing research interest in the area of micelle formation, structure, and their role in reaction catalysis reflects the importance of the subject. The extensive review of Fendler and Fendler ( 1 ) and nuhlications in hook form (2-4) summarize much of thd knowledge about micelles. One of the latter, by C. Tanford, discusses biological structures and develops the subject on the basis that the structural constituents have ~ronerties & . similar to surfactants in formine organized aggregate structures. We have devised a relatively simple experiment for student work in which the iodonation of aniline reaction is shown to undereo catalvsis in solution of sodium laurvl sulfate 1NaLS) whichiorms micelles with negatively charged pseudo s&aces; The reaction in aqueous medium has been investigated by Berliner (5)who reported his findings as follows: The reaction rate is relatively slow in water, and independent of hydrogen ion concentration hut is generally base catalyzed in phosphate buffer solutions. It is inhibited bv iodide ions to the extent that
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' Student assistant,
himolecular rate constants are inversely proportional to [I-] ( 2 ) .The expected para and ortho iodoanilines are obtained as reactionpoducis and the rate determining step is likely the hreaking of the C-H bond at the suhstitution site. The Experlrnents Solutions of 0.01 M iodine in 0.1 M sodium iodide, 0.1 M sodium laurvl sulfate. and 0.1 M aniline are reauired. Sodium iodide rathe; than the potassium salt was used Since the latter precipitates the Na1.S from solution. The rate of reaction is rollowed by measurement of absorhances at 420 nm, which are assumed proportional to iodine concentrations left unreacted after different time intervals. Distilled aniline stored under nitrogen was used to nreoare . . the 0.1 M solutions as needed. 'I'he s;,dium lauryl sulfate rNaLS) was purified by mixing the powder with hot methanol, I'ilterina, the Na1.S -. rrvsrallizine from the filtrate, and drying. Two sets of rate runs are made. In one, equimolar (0.0025 M)iodine and aniline with different concentrations of NaLS are allowed to react. In another set, the aniline is used at 0.025 M with 0.0025 M iodine and NaLS. An absorbance reading with 0.0025 M iodine in 0.001 M NaLS is needed to give Ao ~~
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.001U NOLS
0150
,205M
NDLS
0.100
2.0
4.0
6.0
8.0
10.0
.
n.0 Mn.
Figure 1. Rates of iodonatian of aniline with equal molar reactants (0.0025 M).
Figure 2 Rates 01 iodonation wilh aniline-iodine ratio ten to one.
Volun?? 53,Number 9, September 1976 / 589
tions that micelle catalysis involves binding of a fraction of one reactant to a micelle oseudo surface which reacts with the other reactant in the buik phase. I t should be noted that the reaction products apuarentlv do not dissociate from the micelles. Hence, as F G r e 1 shows, increasing the NaLS concentration probably allows for increasine amounts of aniline to he bound by micelles and the subsequent increase in rates as noted. With excess aniline, and even with low concentra.tions of NaLS, most of the aniline required for reaction is adsorbed on micelles. This is reflected in the observation that only 0.0005 M NaLS induces catalysis which suggests that the critical micelle concentration, cmc, of NaLS is decreased from the approximate 0.008M in water to avalue less than 0.0005 M with excess aniline present. I t is desirable to display data including calculated rate constants. Figure 1, however, merely shows the extent of iodine reaction in terms of absorbance, A. The calculation of Figure 3. Pseudo rate constants calculated horn the data of Figure 2: slopes X 2.303 (min-'I. overall himolecular rate constants would be complex since competing rates are involved. The figure does, however, emphasize the fact that initial first rates are those involving adsorhed aniline, and that the rates converge to some maximum limit with increasing [NaLS]. This is in sharp contrast with used in the plotted data of Finure 1.The reauired volume of data of Figure 2 where useudo first-order rate constants can aniline a n d ~ a L S are delivered to a 100-ml volumetric flask he calculated and reveai in turn how increased [NaLS] effects and the iodine to another, and made UP to volume with water. greater adsorption of aniline and increased rates. Smaller volumes than 100 ml can of course he used for preThe binding of a reactant on a micelle surface serves two paring reaction mixtures. In the equimolar runs the time functions. The adsorbed reactant is favorably oriented for measurement is started on the instant of mixing the reactants attack by the other from the hulk phase and &charged Stern and absorbance readings, A , taken every minute. For the layer provides electrostatic contributions for stabilizine an second set of runs, the reacunts are mixed, rapidly transferred intermediate and/or a transition state. The iodonation reacto a colorimeter cuvet, and the time smilrted on taking the first tion may be written sumabsorbance readine. Plots such as Figures 1., 2., and 3- .---marize all the data. Figure 2 shows &at pseudo first-order kinetin is approached wherein the slopes of the linear portions of the plots are proportional to pseudo first-order rate constants, k,, and Figure 3 shows the rate constants, k,, calculated from the slopes of Figure 2 and the effect of increasing The negatively charged surface of NaLS may be expected to NaLS concentrations. stabilize (11) and enhance the C-H bond breaking probaKgure 1 shows that the initial ratesof reaction are fast and bility. that rates fall off to thoseobserved in water in theal)aenreof It is instructive to compare this mechanism with that for micelles. Figure 2, however, shows that most of the iodine the reaction between crystal violet cation, CV+, and hydroxide reacts as NaLS concentration is increased. With the lNaLSl ion catalyzed in cetyltrimethylammonium bromide solutions > 0.002 M, the reaction is complete in less than 10 s.'~tma; (6).The latter provides micelles with positively charged surappear that if lower reactant concentrations are used for exfaces and which adsorb CV+ ions. The latter is a stable carperiments summarized in Figure 2 that a slower reaction rate bonium ion hut in the positively charged environment is dewould allow for a wider range of [NaLS] and its subsequent stabilized relative to a developing neutral transition state and effect on rates. This is true only if the fraction of aniline adcatalysis is thus possible. sorbed on micelles is the same or less than when reactant The experiment with excess aniline can he used as a visual concentrations are high. This writer has observed that for demonstration. certain ionic reactions in surfactant solutions the lower the reactant concentration, the higher the rates of reaction and Literature Cited overall rate constants, which reflects that a given micellar 11) Fendler, E. L a n d Fendler, J. H., Advon. Phys Or& Chrm., 8,271 11970). concentration adsorbs a higher fraction of a reactant. 12) Cords. E H., (Editarl, "Reaction Kinetien in MirrUs:Plcnum Rss.NW York. 1973. A suggested variation in experimental procedure is to lower (3) Tanfard. D., "The Hydiophobir Effect: John Wiley & Sons.New York,1973. 14) Fendler. J. H., and Fendier. E. J., "Catalyrir in Micellar and Maeromulecular Syrtems." and reuse the concentrations of reactants duplicating either Academic Press, New Yolk, 1975. the data of Figure 1or Figure 2 with [NaLS] varied. (51 Berliner. E., J. A m m Chem Soc.. 72.4003 11950). (61 Corrro,G., J. CHEM,EDUC.,50,575 11973). These results are in conformity with the general ohserva~
590 / Journal of Chemical Education
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