MERCAPTO PROTON EXCHANGE FOR 2-MERCAPTOETHANOL IN AQUEOUS SOLUTION
2287
Rates of Mercapto Proton Exchange for 2-Mercaptoethanol
in Aqueous Solution’*’b
by Maurice M. Kreevoy,lcDale S. Sappenfield, and William Schwabacher School of Chemistry, University of Minnesota, Minneapolis, Minnesota
66466
(Received January 1 1 , 1966)
The rate of exchange of the mercapto proton of 2-mercaptoethanol with aqueous solutions has been studied by an n.m.r. line-broadening technique in acetic acid-sodium acetate buffers and also in perchloric acid. It was concluded that the exchange is catalyzed by bases but not by acids, even up to 11 M HC10,. Presumably, the mechanism of catalysis is abstraction of a proton from the mercaptan by the base. At room temperature the rate constant for catalysis by acetate ion is -2 X lo31. mole-1 set.-' in the limit of low mercaptan concentration and somewhat lower at substantial mercaptan concentration. The rate constant for the uncatalyxed reaction (and/or the water-catalyzed reaction) is -32
set.-'.
Recently developed techniques based on n.m.r. line broadening permit the measurement of rates with half-times in the range lo-’ to sec.2 They are particularly effective in measuring proton-exchange rates. Such a technique has now been applied to the exchange rate of the mercapto proton of 2-mercaptoethanol in aqueous solution at room temperature. The reaction is so sensitive to catalysis by bases that rates had to be measured in somewhat acidic solutions (pH 4 to 5 . 5 ) .
Experimental 2-Mercaptoethanol (Eastman grade, Distillation Products Industries) was redistilled and had b.p. 153-154’, n 2 5 . 1.4987 5~ (lit.3b.p. 157-158”, n Z 01.4996). ~ Acids and bases, and sodium chloride solutions, were made up from analytical reagent grade materials and were standardized in the usual way. The pH of several solutions was measured with a pH meter, and values in reasonable agreement with those calculated from dissociation constants were obtained. The I1.m.r. spectra were obtained with a Varian Model 4300B high resolution spectrometer (modified so as to be equivalent to Model HR-60) operating at 56.442 Mc. Typical spectra were produced with a radiofrequency attenuation of 80 db. below 0.5 W. The sweep rate was typically 1 cycle set.-', SO that the width a t half-height being measured was several
millimeters. These widths were measured with a ruler; sometimes with the aid of a bow divider. It was shown that reducing the sweep rate by a factor of 5 did not materially alter the results. Relative frequencies were first established by the side-band technique, using a Hewlett-Packard counter to determine the side-band frequencies. Once the frequency separation of the two aqueous solution triplets was known, most subsequent spectra were calibrated by assuming that this frequency separation remained constant. Values of intensity ratios and unbroadened peak widths were obtained by averaging measurements made on from 6 to 16 spectra. Between 12 and 16 spectra were initially obtained. Widths at halfheight were measured on the broadened as well as the unbroadened peak, and those spectra were rejected ~
~
(1) (a) This research was supported, in part, by the U. S. Army
Medical Research and Development Command through Contract DA-49-193-MD-2027. Reproduction in whole or in part is permitted for any purpose of the United States Government; (b) taken, in part, from the Ph.D. thesis of D. S. Sappenfield, University of Minnesota. 1962; (c) Sloan Foundation Fellow, 1960-1964; National Science Foundation Senior Postdoctoral Fellow, 1962-1963. (2) (a) A. Lowenstein and T. M. Connor, Ber. Bunsenges. phyaik. Chem., 67, 280 (1963); (b) H. Strehlow, “Investigation of Rates and Mechanisms of Reactions,” Part 11,2nd Ed., S. L. Friess. E. S. Lewis, and A. Weissberger, Ed., Interscience Publishing Division, John Wiley and Sons, Inc., New York, N. Y., 1963, p. 865. (3) G. M. Bennett, J. Chem. SOC.,2139 (1922).
Volume 69, Number 7 J u l y I966
2288
M. M. KREEVOY, D. S. SAPPENFIELD, AND W. SCHWABACHER
in which the product of the height and the width a t half-height for the broadened and the unbroadened peak differed by more than 10%. Certain other spectra were rejected because their general appearance was unsatisfactory.
Results In fairly concentrated carbon tetrachloride solution, the n.ni.r. spectrum of 2-mercaptoethanol consists of a triplet of weight 1, r = 8.49 p.p.m4 (SH); a quadruplet of weight 2, r = 7.36 p.p.ni. (SCH2); a triplet of weight 2, T = 6.34 p.p.m. (OCHz); and a singlet of weight 1, r = 5.85 p.p.m. (OH). Both the low intensity triplet and the quartet show second-order splitting. In neutral or basic aqueous solution both the low intensity triplet and the singlet disappear, presumably by exchange with the water. The rest of the spectrum takes on the appearance of a slightly distorted AzX2system. As the aqueous solution is made more acidic, however, beginning a t about p H 5.5, the high-field triplet broadens, until, a t about pH 4, it is hardly clistinguishable as a triplet. This broadening is most notable in the central line of the triplet because it is substantially sharper than the others to start with. The appearance of the spectrum then remains roughly the same up to 11 M HClO?. The high-field band does not sharpen, and the SH signal does not become distinguishable. The low-field triplet does not change systematically with pH. Representative spectra are shown in Figures 1 and 2. These changes can be qualitatively understood if there is an uncatalyzed or water-catalyzed exchange of the mercapto proton and also a base-catalyzed exchange but no acid catalyzed exchange. The hydroxyl proton exchanges very rapidly in all of these solution^.^^^ Coupling with the mercapto proton would double each line of the triplet if its exchange were slow by comparison with the inverse of the coupling constant. If they are comparable, the lines of the triplet are broadened.2 Following this interpretation eq. 1 was used to determine the half-time for niercapto proton exchange. Equation 1 is a rearranged version of eq. 7 in the work of Takeda arid StejskaL6
Figure 1. N.m.r. spectrum of a 207; d u t i o n of 2-mercaptoethanol in carbon tetrachloride.
-8
src.y
362 S E C . - ’ d
Figure 2. N.m.r. spectra of 2.4 , l4aqueous solutions of 2-mercaptoethanol: A, in neutral water; B, in 5 X loe4 M perchloric acid; C, in 11 M perchloric acid. The multiplet separation and the width at half-height of a typical unbroadened peak are shown. The spectrum in relatively concentrated perchloric acid has a poorer signal to noise ratio than the others owing to the absorption of energy by the solvent.
absence of exchange. In the limiting case, as 1/10 approaches unity, eq. 1 simplifies to eq. 2. Although
_ -- (Il10)Tz (6w) 1 7 8(1 - 1 / 1 0 ) where T = the mean lifetime of a mercapto proton, the height of the broadened peak, 10 the height of I the unbroadened peak, 1/Ts = t h e width a t half-height of the unbroadened peak, and 6w the separation, in set.-', of the two peaks that would be observed in the T h e Journal gf Physical Chemietry
(2)
eq. 1 was actually used throughcut in the present work, values derived from eq. 2 would not be very (4) G.
V. D. Tiers, J . P h v s . Chem..
6 2 , 1151 (1958).
( 5 ) J. D. Roberts, “Nuclear Magnetic Resonance,” McGraw-Hill
Book Co., Inc., New York. N. Y.. 1959, pp. 64-66. (6) M. Takeda and E. 0. Stejskel, J . Am. Chem. Soc.. 82,25 (1960).
MERCAPTO PROTON EXCHANGE FOR 2-MERCAPTOETHANOL IN AQUEOUS SOLUTION
different. Table I shows the values of 1,'. obtained in a variety of acetic acid-sodium acetate buffers. I n all of these deterininations the ionic strength was maintained at 0.22 M by addition of appropriate quantities of sodium chloride. The temperature in the probe could not be controlled, but it was measured on a number of occasions and consistently found to be between 25 and 30". The reproducibility of l / ~ , as judged froin repetitive determinations, shown in Table I, would seem to be about &lo%,and a good part of this scatter is probably due to temperature variations. With an activation energy of 10 kcal. mole-' a temperature change of 3" would change 1 / ~ by 18%. Since 1 / has ~ the significance of a pseudo-first-order rate constant, eq. 3 expresses quantitatively the idea that there is an uncatalyzed (and/or water-catalyzed) exchange and also the possibility of a base-catalyzed exchange for any base present in solution. The bases 1,'.
=
ki
+ E ~B(B)
(3)
B
actually present in the solutions studied are the alcohol end of the substrate, acetate ion, thiolate ion, hydroxide ion, and alcoholate ion. The last three are derived from the mercaptan end of the substrate, the water, and alcohol end of the substrate, respectively, on equilibrating with the acetate ion. The acetate ion catalytic coefficient, ~ o A ~ -was , ob~ a function of acetained as the slope of plots of 1 / as ~
Table I : Exchange Rates in Acetic Acid-Sodium Acetate Buffers a t Room Temperature (RSH), M
10a(OAc-), M
10a(HOAc), M
l / r , sec.-1
0.5 0.5 0.9 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 2.4 2.4 2.4 2.4 2.4 2.4 2.4
2.7 10.0 2.7 87 43 17 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.0 2.0 2.0 2.0 216 83 22
3.2 1.0 3.2 123 61 24 3.7 3.7 3.7 11.5 5.8 0.12 0.12 5.8 2.9 1.9 0.58 204 79 21
53 353
71 223 149 125 78 67 63 45 65 153 154 38 55 75 171 340 162 127
2289
tate concentration at constant buffer ratio and constant ionic strength for substrate concentrations, 2.4 and 1.3 M . The buffer ratio, (OAc-)/(HOAe), was 1.06 a t the higher mercaptan concentration and 0.71 a t the lower. At 2.4 M mercaptan, four values of 1 / ~ were obtained for acetate concentrations ranging from 2.0 X to 2.2 X lo-' M . They gave a slope, ~ o A ~of - 1.2 , x lo31. mole-' see.-' with 50yoconfidence limits of *O.l X lo3 and 90% confidence limits of *0.3 X lo3 1. mole-' set.-'. The intercept, which is the quantity, kl - k o ~ ~ - ( O A c - ) ke(B), has the
+
B
value, 81 sec.-', with 50% confidence limits of *4 set.-'. At 1.3 M mercaptan, six values of 1 / ~gave a slope, k O A c - , of 1.9 X lo3 1. mole-' set.-' and an ~ B ( B of ) , 70 see.-', intercept, kl - k o ~ ~ - ( O A c - )
+
B
with similar confidence limits. These values, and their uncertainties, were all obtained by the method of least squares.' The best estimate of k O A c - in infinitely dilute solutions would seem to be about 2 X l o 3 1. mole-' set.-'.
For each substrate concentration the uncatalyzed rate, Icl, uas obtained by plotting ( 1 / ~- k o ~ ~ - ( O A c]- ) as a function of the buffer ratio. These plots are satisfactorily linear. The intercept, kl. is 34 + 4 set.-' at 1.3 M substrate and 31 f 6 see.-' at 2.4 M substrate. The values and the uncertainties (which are 50% confidence limits) were obtained by the method of least squares' from 10 values of 1/7 a t 1.3 ill mercaptan and 7 values of 1,'. at 2.4 ilf mercaptan. In the same way, but much less reliably, kl, 25 see.-', can be obtained from the two pieces of data available for 0.5 mercaptan. These values do not suffer badly from whatever uncertainty exists in ~ o A hecause ~ most of the solutions having low buffer ratios, which are most important in deterniining these intercepts, also have low acetate concentrations, -3 X X . Thus, the k o ~ ~ - ( O A c -term ) subtracted from 1'7 is -5 see.-', fairly small by coniparison with kl. If it is assumed that the spectra in strong perchloric acid are near the coalescence point, which s e e m reasonable from their appearance (Figure 2, B and C ) , a value of 30-40 see.-' is obtained for This must be identified with kl, as no basic species are present. Thus, kl seems to have a value of -32 set.-', fairly independent of the medium. After kl and the acetate term have been accounted (7) e. A. Bennett and N. L. Franklin, "Statistical Analysis in Chemistry and the Chemical Industry," John Wiley and Sons, Inc.. New York, N. Y . , 1954, pp. 36-40, 231. (8) J. A. Pople. W. G. Schneider, and H. J. Bernstein, "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., Inc., New York, N. Y . , 1959, p. 223.
Volume €9, 'Vumber 7 J u l y 1966
2290
M. M. KREEVOY, D. S. SAPPENFIELD, AND W. SCHWABACHER
for, the remaining terms under the summation in eq. agreement with those obtained by the other method. 3 are the hydroxide term, which can be written koH-KwThey confirm the conclusion that a t least one and (OAc-) /KrroAC(HOAc) , the intramolecular alkoxide perhaps several of the terms in 2 must contribute term, which can be written ~ ' R O-KRoH(OAC-)/KHOA~- substantially to the observed rates. (HOAc) , the iriteriiiolecular alkoxide term, which Can be written ~ R -KRoH(OAC-) O (S)/ KHOA~(HOAC) , and Discussion The general base catalysis and the absence of detectthe mercaptide term, which can be written ~ R -KRsHS able acid catalysis strongly suggests that proton ab(OAc-) (S)/KHOA,(HOAC).(In these terms Kw is straction by base is the predominant exchange mechthe autoprotolysis constant of water, K B H is the acid anism. If this interpretation can be accepted, ~ o A ~ dissociation constant of BH, and S is the substrate.) 2 X lo3, is the rate constant for the abstraction of a proAll of these terms are directly proportional to the buffer ton from the niercaptan by acetate ion. Froni the acid ratio, giving rise to the linear plots of { l / ~ - ~oA~-dissociation constants of 2-niercaptoethanol, 3.7 X (OAc-) us. buffer ratio a t constant substrate con10-10,9 and acetic acid, 1.75 X 10-5,10the equilibriuni centration noted in the previous paragraph. If the 2.1 X for the reaction of the mercaptan constant, constants involved were all invariant under changes with acetate ion can readily be calculated. Froni this in substrate concentration, the slopes of these plots lo8 1. mole-' set.-' is obtained for the rate the value, would either be constant (if the intermolecular alconstant of the reverse reaction, that of acetic acid with coholate term and the mercaptide term were negligible) mercaptide ion. This is probably about two powers of or would increase monotonically with the substrate. ten below the diffusion rate for these two species." The actual slopes are 25 sec.-' a t 0.5 M substrate, The simplest interpretation of Icl is that this is the 42 see.-' at 0.9 -41 substrate, 51 f 3 see.-' a t 1.3 M rate constant for the abstraction of a proton by water substrate, arid 41 f 4 see.-' a t 2.4 M substrate. The from the mercaptan. The rate constant of the relatter two were obtained from the least-squares analyverse reaction, that of the proton with the mercaptide sis mentioned in the previous paragraph, and the ion, then becomes 9 X 1Olo 1. mole-' set.-'. This seems uncertainties are 50% confidence limits. They are rather large, even for a diffusion-controlled considerably more reliable than the others. For 1.3 An alternative is to suppose that a significant part of and 2.4 *I/ substrate ( the measured values of k 0 k o kl is due to internal proton transfer to give the zwitterwere used. For both 0.5 and 0.9 M substrate a value ion, HzO+CH&H&. of ICoAc- had to be assunied and 2.0 X lo3 1. mole-' The lack of an acid-catalyzed exchange bespeaks see.-' was used. For 0.9 substrate a value of kl the very feeble basicity of the niercapto group.13 also had to be assunled, and 32 set.-' was used. From Eigen and Maass seem to have measured the rates these results, it is plainly impossible to sort out the of proton abstraction from 2-niercaptoethanol by a contributioIis of the various bases other than acetate ion. variety of and obtained results analogous to The results probably show the influence of the uncerthose presented here. Unfortunately, the numerical tainty in kOAc-,arid it is likely that the various k g are values of their rate constants do not seem to have been not invariant under changes in substrate concentrapublished. Sheinblatt and Luz14 have observed both tion, as k 0 A c - s e e m not to be. acidand base-catalyzed proton exchange in liquid 2I t is possible to examine the results a t 1.3 M mermercaptoethanol, but this niediuni is so different from captan and '2.4 -If mercaptan for internal consistency. dilute aqueous solution that their results are not diThe intercepts, I , of the plots, mentioned above, of rectly coniparable with ours. 1/7 us. the acetate concentration a t constant buffer ratio are given by eq. 4. Froni the known values of Acknowledgment. Part of this work was done while I , the buffer ratios, and kl, 2 is readily obtained. It AI. SI. Iireevoy was a guest in the Physical Chemistry Laboratory, Oxford. He wishes to thank Professor I = IC1 Z(OAc-)/(HOAc) (4)
1
+
z
+
+
+
{ ~ O H - K W~ ' R O - K R O H~RO-KROH(S) ~RS-KRSH(S) ]/KHOAC
should be identical with the slopes cited in the previous paragraph. At 1.3 M substrate, with buffer ratio 0.71, I was 70 sec.-i, from which is 54 sec.-i. At 2.4 M mercaptan, with buffer ratio 1.06 and I 81 set.-', z is 46 see.-'. These Values are in excellent The Journal of Physical Chemistry
(9) M. hl. Kreevoy, E. T. Harper, R. E. Duvall, H. S. \Vilgus, 111, and L. T. Ditsch, J . A m . Chem. SOC..82.4899 (1960). (10) G. Kortum, W.S'ogel, and K. Andrussow, "Dissociation Constants of Organic Acids in Aqueous Solution," Butterworth and Co. Ltd., London, 1961, p. 241. (11) (12) (13) J. A (14)
Pure AZJZJ'.
Chem.,
6p
97 (1963).
M.Elgens AWew. Chem., 75,489 (1963). E. M.Amett, C. Y. Wu, J. N. Anderson, and R. D. Bushlck. ~ them. . sot., 84, 1674 (1962). Unpublished results.
- ,
MERCURY-PHOTOSEXSITIZED DECOMPOSITION OF ISOPENTANE
Sir Cyril Hiiishelwood and Mr. R . P. Bell of that laboratory for their hospitality. He also wishes to thank
229 1
Dr. A. Loewenstein, of the Technion, Haifa, for a very useful consultation on this work.
The Mercury-PhotosensitizedDecomposition of Isopentane
by Robert R. Kuntz Department of Chemistry, University of Missouri, Columbia, M i s s o u r i
(Received J a n u a r y
14,1966)
The Hg(3P1)-sensitized decomposition of isopentane has been studied at 25" and 10 em. pressure, and the hydrogen atom reactions have been studied by means of added trimethylethylene. The average value for the disproportionation/conibination ratio of the various isopentyl radicals is 2 . 5 . Rate constant ratios for H atom reactions with isoperitane and trimethylethylene compare favorably with liquid phase results. The product distribution is similar to that observed in the liquid phase.
Introduction The mercur~r(~P1)-sensitizeddecomposition of a number of alkanes has been studied in detail. These studies have indicated the primary interaction between Hg(3P1) and alkane molecules results in hydrogen atoms and free radicals: Hg* RH I>, t H Hg. A mechanism has been proposed for this process in the vapor phase.' The liquid phase decomposition gives evidence of more than one primary process,2 but the behavior of the final products of this interaction is the same. Recently, the niercury-photosensitized decompositions of some alkanes have been studied in the liquid phase, arid values for the disproportionation/combination ratio of the alkyl radicals measured.* This series of studies was initiated in order to compare rate constant data for free radicals in the vapor phase with those observed in the liquid phase photolysis and radiolysis studies.
+
-
+
Experimental Pure grade isopentane from the Phillips Petroleum Co. was used in all cases. All olefin and polar impurities were removed by the peak isolation technique on 0.64-cm. X 2.75-111. P-P-oxydipropionitrile column. Triniethylethyletie of stated 99% purity was obtained from Xitheson Coleman and Bell and used
without further purification. Hydrocarbon samples were degassed by the freeze-thaw technique and stored over P206. The reaction vessel was constructed of Vycor tubing 30 X 250 mm. connected through a graded seal to a mercury cutoff which doubled as a manometer. The light source was a Hanovia low pressure, mercury resonance lamp. The reaction vessel was mounted 50 em. from the resonance lamp in order to reduce the intensity such that low conversion experiments might be carefully timed. L4manual shutter allowed the lamp t o be warmed up prior to exposure. Products not condensable at liquid air temperature were pumped directly into the chromatograph inlet by a Toepler pump. This gas consisted, within analytical error, entirely of hydrogen. The hydrogen was analyzed by the change in thermal conductivity of a nitrogen stream passing through a silica gel column after sample injection. Results obtainable in this manner were reproducible to + 3yG. All experiments were done a t 10 cni. isopentane pressure and 25'. The hydrogen was removed at intervals such that never more than 0.3V0of t'he excited (1) T.Rousseau, 0. 1'. Strausz, and H. E. Gunning, J . Chem. Phys., 39, 962 (1963). (2) R. R. Kuntz and G. J. Mains, J . Am. C'hem. Soc., 8 5 , 2219 (1963).
V o l u m e 69, .Vumber 7
July 1966
