M.GARRETT,T. TAO,AND W. L. JOLLY
824
The Protonation and Deprotonation of Sulfamide and Sulfamate in Aqueous Solutions
by Michael Garrett,’ Terence Tao,’ and William L. Jolly Department of Chemistry, University of California, Berkeley 4, California
(Received October 17, 1963)
The width of the n.m.r. signal of water in aqueous solutions of sulfamide varies with the pH; maxima are observed a t pH values 4.1 and 5.7. The data have been interpreted in terms of reactions of sulfamide with hydrogen ion and hydroxide ion. Similar data for aqueous solutions of sodium sulfamate have been analogously interpreted.
Introduction Sulfamide acts as a very weak acid in aqueous solutions. Thus, although various salts of sulfamide have been isolated from alkaline aqueous solutions, solutions of pure sulfamide are practically nonconducting. On the other hand, apparently no basic character has ever been detected in sulfamide in its aqueous solutions. In the present study, we have measured the broadening of the proton magnetic resonance signal of water in sulfamide solutions as a function of pH, and have interpreted the data in terms of an acid-catalyzed exchange reaction and a base-catalyzed exchange reaction. Each of these reactions has been studied in pH regions of moderately slow exchange (separate signals for sulfamide and water) and of fast exchange (one merged signal). The sulfamate ion, XH2S03-, is known to be weakly basic4 and t o be very weakly acidices Although we never observed a separate n.m.r. signal due to the sulfamate ion, we found that the n.m.r. line broadening data of sulfamate solutions, like those for sulfamide solutions, could be interpreted in terms of acid- and base-catalyzed exchange reactions. Experimental Sulfamide from the General Chemical Division of Allied Chemical Corporation was purified by repeated recrystallization from 95y0 ethanol, followed by recrystallization from an acetone-ethyl acetate mixture. The purified product had a melting point of 90-93’ (lit.2m.p. 93”) In each experiment, purified sulfamide was dissolved in the appropriate amount of buffer solution to make a solution 1.5 Ill in sulfamide. The T h e Journal of Phvsical Chemistry
pH of the solution was determined with a pH meter, and the solution was transferred to an n.m.r. tube. In the pH range 3.2-5.2, 0.1 iM acetic acid-acetate buffers were used; in the pH range 5.2-7.3, 0.1 M dihydrogen phosphate-monohydrogen phosphate buffers were used. Sulfamic acid from the G. F. Smith Chemical Company was dissolved in the appropriate amount of sodium hydroxide solution to make solutions 2 M in sodium sulfamate. The pH was adjusted with potassium dihydrogen phosphate and borax a t concentrations near 0.03 M. A Varian Model A-60 n.m.r. spectrometer was used.
Results Suljamide. The measured line widths a t half height of the water signal AvIl2 are plotted us. pH in Fig. 1. The curve shows two maxima (at pH values -4.1 and -5.7) and a minimum (at pH 5.1). In the pH range between the two maxima, a broad signal due to sulfamide was observed approximately 104 C.P.S. to low field of the water signal. At pH values outside of this range, only one signal (essentially that of the water) was observed. We shall interpret the data in terms of the following two reactions, each of which (1) Work performed by M. Garrett and T. Tao in partial fulfillment of the requirements for the B.S. degree. (2) L. F. Audrieth, M. Sveda, H. H. Sisler, and M.J. Butler, Chem. Rev., 26, 49 (1940). (3) E. C. Franklin, “The Nitrogen System of Compounds,” Reinhold Publishing Corp., New York, N. Y . , 1935, pp. 169-170. (4) E. G. Taylor, It. P. Desch, and A. J. Catotti, J. Am. Chem. Soc., 7 3 , 74 (1951); E. 5. King and G. W. King, ibid., 74, 1212 (1952). (5) Ref. 3, pp. 167-168.
PROTONATIQN AND
DEPROTONATION OF SULFaVIDE
I
I
I
I
825
ANlD SULFAMATE
I
I
our purposes, eq. 3 is a fairly good approximation. Because R = 4(XHzSOzXH~)/rs, and because, a t the minimum, the acid- and base-catalyzed reaction rates are equal, we may write 2 / r 8 == Icl(H+) = k,(OH-)
Using the H + and OH- concentrations corresponding to pH 5.1, we calculate k , = 3.2 X lo7M-l sec.-l and ICz = 2.0 X loi1M - I sec.-l. At pH 4.1 the two signals coalesce to one broad signal. and we may write 2nr(vw0 - v,o)
Figure 1. N.m.r. line width as a function of pH for 1.5 M sulfamide. The circles and squares represent separately determined data. It is believed that the squares represent for more accurate dat,a than the circles. AvI/,O, the value of the buffer solutions in the absence of sulfamide, has the values 1.27 and 1.0 C.P.S. for the circles and squares, respectively.
leads to an exchange of a proton between sulfamide and water. YH2S02NHz -$- H + + NH$OZNHS+ NHzSOzNHz
+ OH-
NHZSOzNH-
-3
+ H20
The sum of the rates of these reactions is represented by the equation
R
=
ki(H +) (NHzSOzNHJ +kz(OH-) (SH2SOzNHz)
At the minimum in the A v , / ~us. p H curve, we use the relation 1/r
==
l/Tz’ - l/Tz
(3)
where r is the mean lifetime of a proton on a parLicular site, and Tz’ and Tz are the transverse relaxation times in the presence of exchange and in the absence of exchange, respectively.6 By using eq. 3 and the data Av1,%= 3.2 c.p.c;. and Avl/,O = 1.0 c.P.s., we obtain an approximate value for 7 , ( T for protons on water). From the relation TS/TW
=
ps/pw
(4)
(where r8 is r for the protons on sulfamide, and p, and p , are the fractions of the protons on sulfamide and water, respectively) we calculate T, = 7.95 X see. Equation 3 is valid only when 2nr(vw0 - vso) >.> 1, where v W o - vso is the chemical shift (c.P.s.) between water and sulfamide in the absence of exchange.6 From our data we calculate that, at the minimum, 2 n ~ ~ ( v , vao) ~ = 5 ; thus, for
(5)
where T is a “reduced” lifetime for the s y ~ t e m . We ~~~ take T, as a good approximation for r. The term ka. (OH-) is negligible compared to kl(H+); thus 4 / r s E: kl(H+). The data yield IC1 = 3.1 X lo7 M-I sec.-’. At p H 5.7, the two signals coalesce to one broad signal, and again we use eq. 5 . At this pH, we may neglect the term kl(H+); thus 4 / r s = kz(OH-). The data yield kz = 5 X 1011M-l sec.-l, When 2nr(vw0 - vSo)
