NORMAN BAUERAND Jos6 A. REINOSA
1430
TABLE V MOLARPOLARIZATIONS ( I N CC.) AND D I P O L E MOMENTS (IN D ) OF ALKANETHIOLS IN BENZENE SOLUT~ON Substance
PI
Ethanethiol l-Propanethiol ZPropanethiol l-Butanethiol 2-Methyl-l-propanethiol 1-Methyl-l-propanethiol 2-Methyl-Zpropanethiol
64.06 70.58 72.70 76.47
PE
PA
PO
p
18.63 0.50 44.93 1.48 23.10 .62 46.86 1.51 23.20 .63 48.87 1.55 27.63 .75 48.09 1.53
75.97 27.66
.75 47.56
1.53
78.69 27.69
.75 50.25 1.57
80.05 27.90
.75 51.40 1.59
P a r t i n g t ~ none ~ ~ calculates the value 1.48 D for ethanethiol, 1.44 D for l-propanethiol, and 1.39 D for l-butanethiol. From the data of Walls and S m ~ t hthe , ~ value ~ 1.51D is obtained for l-butanethiol. It is seen that the value determined from Hunter and Partington’s data for ethanethiol agrees with the one listed in Table V, while the (24) E. C. E. Hunter and J. R. Partington, J . Chem. Soc., 2062 (1931); 2812 (1932). (25) W. 9.Walls and C. P. Smyth, J . Chsm. Phys., 1, 337 (1933).
Vol. 62
values for l-propanethiol and l-butanethiol are somewhat lower. For this last compound the value obtained from the data of Walls and Smyth agrees with the one listed in Table V. In comparing the dipole moments of the alkanethiols determined in the liquid state (Table 111) with the results obtained in benzene solution (Table V), it is seen that, from the three equations used in the liquid state, it is Onsager’s equation which agrees the best with the values in benzene solution. Furthermore, it is the value 1.57 D determined by means of this equation for ethanethiol in the liquid state which agrees, better than in benzene solution, with the only value available in the gaseous state, 1.56 D determined by Kubo.22 In the case of the isomers of l-propanethiol and l-butanethiol, the dipole moments in benzene solution show deviations in the same order, though much less pronounced, as in the liquid state. Acknowledgment.-The authors gratefully acknowledge the financial assistance given to this Laboratory by the Rockefeller Foundation (New York) and the Conselho Nacional de Pesquisas (Rio de Janeiro).
THE VAN SLYICE REACTION BETWEEN NITROUS ACID AND PHENYLALANINE BY NORMAN BAUERAND Jos6 A. REINOSA Chemistry Department, Utah State University, Logan, Utah Received M a y 18, 1968
Rates of nitrogen evolution from the reaction of phenylalanine with nitrous acid in the pH range 2.4-2.8 were measured, using both gas volumetric and mass spectrometric techniques and making corrections for nitrous acid decomposition. None of the possible first-, second- or third-order rate expressions involving the three amino acid species and other principal ions or molecules can account for the observed dependence of rate on reactant concentrations. An analysis of the secondary salt effect for systems containing zwitterions shows that the activity coefficients for amino acid dissociation must be known before the kinetics of this Van Slyke reaction can be worked out in detail. However, the corrections for the secondary salt effect are not great enough to ex lain the variation in rate constants observed for the aim le rate law expressions. The pH dependence of secondary salt e g c t s is different, under certain conditions, depending on wfether the amino acjd catipn of the zwitterion is the reactant; this should prove useful in distinguishing alternative reaction mechanisms involvmg Zwitterions.
Although there is some information on the special behavior of amino acids in the Van Slyke reaction,2,athe rate laws are not established and no data are available for the case of phenylalanine. We find that this amino acid is particularly suitable for study because (1) the nitrogen it produces is not seriously diluted with other gases from .side reactions, in contrast to glycine2; and (2) because its reaction rate is so much greater than for simple amines that the crucial low pH range (2 to 3) may be investigated without incurring excessive corrections for nitrous acid decomp~sition.~We report here a preliminary study of the reaction, including a discussion of the particular role which the secondary salt effect plays in this case where zwitterions as well as cations and anions may be involved in the rate law. ( 1 ) Supported by Western Regional Project W-31, Utah State Agricultural Experiment Station. (2) A. T. Austin, J . C h m . Soc., 149 (1950). (3) J. C. Earl, Reaearch (London),8, 120 (1950). (4) G. J. Ewing and N. Bauer, THIBJOURNAL, 62, 1449 (1958).
Experimental The two techniques used here for measuring the gas evolution rates and correcting these for nitric oxide production have already been described.4~6 The stock reactant solutions were mixed so as to maintain a constant ionic strength (1.0) and a desired pH by adding the necessary amounts of sodium chloride and phosphate. The reaction temperature was maintained a t 30.0’.
Results and Calculations Table I presents the results of five gas volumetric runs in the pH range 2.40-2.80. The Values Tblank and ‘?‘net, where T = Tnet -k Tblank, represent the initial rates of gas evolution from the nitrous acid blank and from the Van Slyke reaction itself, respectively. The gas volume us. time curves,b whose tangents at time zero gave the Tvalues, showed a considerably greater curvature than in the corresponding reaction between methylamine and nitrous acid.4 The slope decreased by a factor of about two over a period of three hours for (5) J. A. Reinosa, Thesis, Utah State Agricultural College, 1957.
Nov., 1958
THEVANSLYKEREACTION BETWEEN NITROUS ACIDAND PHENYLALANINE1431
the conditions specified in Table I ; whereas for methylamine under virtually the same conditions the slopes are linear within a few per cent. This decrease is suggestive of side reactions competing with the Van Slyke reaction for amino acid; or it may merely reflect a relatively rapid consumption of reactants. Analysis of the evolved gases given below shows that the products of possible side reactions are largely confined to the aqueous phase or that in the gas phase they are indistinguishable from the normal products.
reasonable lower limit for Y H + when extrapolated to 1.0 M. The Guggenheim constant PH+, ci- = 3-0.27 is known experimentally; and a considerably lower value of &+, H,PO,- = +0.15 may be inferred from the regularities of for the halides of H+, Li+, Na+ and K+ and for the acid phosphates of Na+ and K+. For the buffers used in the present study (0.62 M HaP04-, 0.48 M Cl-) we calculate that YH c = 0.906 in rational (molal S molar) units. The corresponding value is YH+ = 0.80 when the ionic strength is 0.80 with the same buffer type. TABLE I The importance of the above activity coefficients RATESOF GASEVOLUTION AND MOLAR CONCENTRATIONS OF is reflect.ed in the fact that appreciable differences MOLECULAR SPECIESIN VAN SLYKE REACTIONWITH in calculated rate constants aro found when Y H + PHENYLALANINE AT 30.00”,IONIC STRENQTH 1.0 alone is changed by easily attainable amounts. 9’bhk Thus values of the “constant” k:” (cf. Discus(HNOd (NOS-) (R+) (R?) )-:( m (mole/l.) x x x X X see.-‘ sion) decrease by 7y0 at pH 2.4 when YH+ goes PH 104 101 101 101 io0 10s x 108 from 0.90s ( p = 1.0) to 0.80 ( p = 0.80); and the 2.40 81.0 19.0 0.476 0.774 1.80 38.9 33.9 corresponding decrease is 12% a t pH 3.5. This 2.60 72.9 27.1 1.40 3.61 13.2 59.8 32.2 example of the secondary salt effect on zwitterion 2.70 6 8 . 1 31.9 0.294 0.955 4.46 38.5 31.5 concentration shows that the kinetics of this kind 2.75 65.7 34.3 1.08 3.92 20.4 . 7 7 . 7 30.8 of reaction cannot be worked out in detail until the 2.80 63.0 37.0 0.49 2.01 11.7 31.4 30.4 above estimates are replaced by more direct deterThe molar concentrations of possible reacting minations of the activity coefficients involved. Results from Mass Spectrometric Analysis.species,. given in Table I, were calculated from expressions for the equilibrium between ions and An independent verification for the assumption corresponding undissociated molecules, using the that rnet in Table I represents essentially the initial following reasonable estimate of activity co- nitrogen evolution rate is given by a direct mass efficients t o correct for the effect of the 1.0 molar spectrometric analysis of the entire gaseous product. sodium chloride-phosphate environment (second- Using the technique4 of carrying out the reaction ary salt effect). The equilibrium constants used in an argon atmosphere, the following composition of the evolved gases was obtained in an experiment were: for nitrous acid4 where the initial concentrations were 0.0375 M phenylalanine, 0.0150 M total nitrite, pH 3.2, ionic strength 0.80 (NaC1-NaHzPO4): 78.7 mole % Nz, phenylalanine (Rei3, R-+ + H+) 18.0% NO, 2.3% NzO (and/or COJ, 0.8% CO(C2H4?) and 0.14qib Hz. The mass spectrum was (H+)(R-+) rl = 7.16 x 10-a KI recordedgup to m/e = 88, but no fragments heavier (R+ 1 than m/e = 44 (and isotopes) were in evidence. A corrected from the data of Nevenzel, et a1.,6 to 30” knowledge of the total volume of Na gas produced by assuming its temperature coefficient is similar to (29.L cc. S.T.P.) at 30.0” from the 0.180 1. of soluthat of serine?; for its second acid dissociation tion during the 255 min. required to collect the above gas sample allows a comparison of rate “constants” k;tf in the two types of experiments, to be discussed below. For this the initial concentrations The activity coefficient factors Fa = YH+YNO,-/ could be corrected to their mean values during the Y H N O ~ , rl = YH+YR;/YR+ and rz = YH+YR-/ YR; were evaluated from the following estimates: run by assuming the Van Slyke stoichiometry and Y H N O ~G 1.00, YNO,- Z 0.67, according to Ewing by measuring the pH a t the end of the run. [(E and Bauer6; rl = 1.00 from the fact6 that the N O z ) =7.18 X lo-’ M ; ~ 4 . 2X 2 lo-’ pK’l value for phenylalanine remained constant M ; (R+) = 2.66 X M ; (h) = 7.3 X when the sodium chloride concentration was lo-’ M ; (H+)p~ = 1.58 X MI. changed over the range 0.05 t o 0.2 M ; YR- E Discussion of Rate Data.-Inspection of Table I 0.61, assuming it to lie midway between the mean activity coefficients for sodium formate and sodium shows that none of the many simple first, secondhydrogen malonate.7 In calculating (R-) from or third-order rate expressions possible for the species R+, (RT), (R-), (NOz-), (HN02) a?d r2the estimate of YR; = 1.0 was adequate. For hydrogen ion in the presence of HzP04- and (H+) can account for the observed differences in C1- we used Guggenheim’s relations for mixed the net NZ gas evolution rate a t different concenelectrolytes with the assumption that it gives a trations. This conclusion assumes the absence of accidental experimental errors greatly in excess of (6) J. C. Nevenzel, W.E. Shelberg and C. Nieman, J . Am. Chem. the maximum to be expected from all sources (Soc.. 71,3024 (1949). (7) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 16% in k,”’). Although these data are not extenAcademic Presa. New York. N. Y., 1955,p. 496. sive enough to establish a rate law in this complex
(m-)
(8) E. A. Guggenheim, “Thermodynamics: An Advanced Treatment for Chemists and Physiciats,” 3rd Ed., Interscience Publ., New York, N. Y., 1957,pp. 357-360.
(9) “Mais Speotrometer Analytical Service,” Consolidated Electrodynamins Corp., Pwadena, Cali.
1432
NORMAN BAUERAND Josfi A. REINOSA
Vol. 62
system, they do point to the exclusion of the anion
(R-) is negligible (acid side), and they take into ac-
that R- may be an inhibitor. Among the above mentioned, the two rate expressions giving the closest representation of the facts are k,”’ = r,,ef/[(R;)(HN02)(H+)], where k~”’ = 1.6 70.5 (mole/l.)-2sec.-1; and kz”’ = met/ [(R+)(NOz-)(HN02)], where k i f ’ = 4.0 f 1.3 (mole/l.)-2 see.-’. However, in each case there is a 3-fold variation between extreme k-values. The secondary salt effect, according to the calculations mentioned in the preceding section, is not great enough to account for these variations in k over the small pH range used here. Probably the reaction is complex. It is of interest to compare the above values of kl”’ (1.6 0.5) determined from initial gas evolution rates with the corresponding mean value &“’ (1.25) obtained from the single experiment based on mass spectrometric analysis. The good agreement may be fortuitous because some unknown correction fsctor in the range of 1.5 to 2.5 must be appled to kl”’ as a means of converting it t o the value based on initial conditions of concentration, k:”, This follows from the curvature of the gas volume vs. time curves, mentioned above. Use of the Secondary Salt Effect.-Even if the relatively simple expressions k: or k21” should prove capable of representing the predominant rate law, there are alternatives typical of systems having zwitterions in equilibrium with protons. Thus, at low pH, k i f ’ = (rI/Kl)k;’, where K1 and rl for phenylalanine are defined above and k,‘’ = m e t / [(R+)(HN02)]; also, at low pH, k i f ’ = (Ka/rB)]k,”’, where K, and rafor nitrous acid are defined above and k i f ’ = T~,~/(R;)(HNOZ)~. It is suggested here that the secondary salt effect may be utilized to distinguish between the kinetic effects of zwitterions and of amino acid cations. This distinction should be a real one in spite of existing equilibria because of differences in electrostatic charge distribution on the particles forming the activated complex. Thus the activation free energies should be distinctly different for processes involving R+ or R; in otherwise equivalent rate laws. The expressions (1) and (2) for the concentrations of R+ and of R7 show that these are influenced in similar, but in two senses opposite, ways by changes in the activity coefficients connected with the amino acid dissociation.
on concentration (H+) and the apparent value (H+),H = Y’H+ (H+) determined with a pH meter. Ewing and Bauer4state the conditions under which Y’H+ equals the true activity coefficient for hydrogen ion in the given solution, YH+. When such an equality is maintained, it is evident from eq. 1 and 2 that the concentrations (R;) and (R+)-and their pH dependence-are influenced mainly by l / ( y ~ ; / y ~ += ) YR+/YR; = g. As g increases (R+) decreases appreciably (as much as proportionately) unless ( H + ) p >> ~ Kl. On the other hand (R,) increases appreciably with increasing g unless ( H + ) p
