495
Feb., 1963
NOTES HYDROLYSIS OF A4CETIC AKHYDRIDE IN THE PRESENCE OF ACETATE BUFFER AND PYRIDINE BY S. L. JOHNSON
M for the acetate concentration, gives the result7 that k-1 2 37000k2. Thus acetylpyridinium ion is more selective or in the limit, has somewhat greater selectivity than 1-acetyl-3-methylimidazolinium ion under the same conditions, an unexpected result.
Mellon Institute, Pittsburgh IS, Pa.
0
It
Receiued April 88,196.9
Pyridine, in the absence of an acetate buffer, catalyzes the hydrolysis of acetic anhydride. The catalytic coefficient,for pyridine is ca. 30,000 times greater than the catalytic coefficient for acetate ion1 in 50y0 aqueous acetone a t 25'. However, in an aqueous acetate buffer (0.0153-0.0621 M ) the catalytic coefficient observed for pyridine is inversely proportional to the acetate concentration.2 Under these conditions the catalytic coefficient for pyridine experiences a solvent isotope effect2 (kEao/kDzo) of 5 i: 1. The results were explained by the mechanism shown in eq. 1. The catalytic constant for pyridine according to the preceding
C H ~ C - N , ~ bN-CH3
+
I1
Because of the above conclusions, the kinetics of pyridine catalysis of acetic anhydride hydrolysis in the presence of a sodium acetate-acetic acid buffer were followed by an independent method: the change in the nuclear magnetic absorption signal during the reaction. It was found that acetic anhydride in the presence of a buffer 1.06 M in acetate and 0.54 114 in acetic acid, experiences no cutalysis by 0.063 M free pyridine in 40% dioxane-60% water (v./v.) a t 35'. This observation supports the conclusions of Gold and Butler2as represented in eq. 1. The rate expression for the hydrolysis of acetic anhydride in an acetate buffer containing pyridine is given by
mechanism is given by
k~ -!- ka~[OAc-l
+ kp[Cd%N]
(3) where k , is the rate constant for the reaction of acetic anhydride with water, kAc is the catalytic constant for acetate ion, and k, is the catalytic constant for pyridine. The present work indicates that k, kAo. [OAc-] >> Ic,[C5H6N]. This condition is because lc, is proportional to l / k - ~[OAc-]. The catalytic coefficient for pyridine, IC,, would be expected to be ca. 30,000 times greater than the catalytic coefficient for acetate, kAc, in a solvent devoid of acetate, by analogy with Gold and Bafna's results in 50% aqueous acetone, a solvent with a dielectric constant similar to that for 60% water-40% dioxane (48.2 and 43.0, respectively, a t 25 ') In the present work no catalysis by pyridine could be detected, even upon using a ratio of [OAc-]/[C&H51\J] as low as 16.8. On the other hand, extrapolation based on Gold and Butler's2 data (obtained in 100% water) predicts a 40% increase in the observed rate constant upon addition of 0.063 M pyridine to the acetate buffer kAc[OAc-] used in the present work. Therefore, k , k, [CF,H~N]in 100% water and k , kAe[OAc-1 >> kP[CJ€6N]in 60% water-40% dioxane (for the concentrations of catalysts used in the present work). kAo[OAc-] The value of k, is reduced relative t o k , upon going to a lower dielectric constant solvent hecause the value of ICl is reduced, and particularly, because the value k-1, the ion-recombination step, is raised (eq. 2) more rapidly than k , and kAeare lowered. hbsd
=
+
If k-l[OAc-] >> 1c2[HZO]then an inverse relationship hetweeri k, and OAc- and a large solvent isotope effect3 are reasonable. The k-l [OAc-] > i%z [HzO] relationship implies a certain large selectivity of acetylpyridinium ion in its reaction with nucleophiles. This large selectivity is surprising in view of the following facts; (1) acetylpyridinium ion reacts with hydroxylamine only to the extent of 6.5% in 2 M hydroxylamine4 (an extremely effective nucleophile) ; (2) 1-acetyl-3-methylimidazolium ion (11),which should be more selective6 than acetylpyridinium ion, is actually less selective. This conclusion derives from the reported rate constants for the reaction of 1-acetyl-3-methylimidazolium ion separatelya with acetate ion and with water at 25O, which are, respectively, 16.8 1. M-I m h - l and 2.8 min.-l. Expressing the water rate constant in the same units as the acetate rate constant by division by 55.5 gives the result that k~~ = 333k~,o. Gold arid Butler's observations for acetic anhydride catalysis by pyridine require that k-1 [OAc-] >> kz[H20] for acetylpyridinium ion according to ey. 1. Placing a factor of 10 on the inequality and substituting in the values of 55.5 M for the water concentration and 0.015 (1) V. Gold a n d S. L. Bafna, J. Chem. Soc., 1406 (1953). (2) V. Gold and A. R. Butler, %bid., 2305 (1961). (3) The isotope effect wises from the conversion of H20 t o HzO-R+ in the transition state with the concomitant lowering of the zero-point energy of the water moiety. (4) D. E:. Koshland. J . A m Chem. Soc., 7 4 , 2286 (1952). (5) The greater selectivity of acetyl-N-methylin~idazolium ion is due t o the distiibution of the positive charge t o the second nitrogen. away from the reactive carbonyl center (6) J. P. Jencka and R. Wolfenden, J . Am. Chem. SOC, 83, 4391 (1961).
-
+
+
+
Experimental Materials.-Redistilled Allied Chemical reagent grade acetic acid and recrystallized, oven-dried Baker Analyzed sodium ace(7) At 0' in water, concurrent isotopic exchange and hydrolysis experiments indicate t h a t k - I cz 1390-2880bz a h e n corrected for relative concentrations (see C. A. Bunton, N. A. Fuller, S. G. Perry, and V. J. Shiner, Jr., Tetrahedon Letters, 14, 4513 (1961)). (8) G. Akerlof, J. Am. Chem. SOC.,5 4 , 4125 (1932).
NOTEB
496
tate were used as buffer components. Water de-ionized with a mixed bed exchanger and possessing a specific resistance of ca. 5 X 108 ohm/cm., and Fisher certified dioxane redistilled over sodium were used as solvent components. Allied Chemical reagent grade pyridine was fractionated over a packed column, b.p. 115". Eastman Kodak White Label acetic anhydride wm used without further purification. Measurements.-The fraction of unprotonated pyridine in the buffer solution was determined by the relationship [CsHsN]rree = .B!( - ~ H ) / ( ~ o E a=), wbere a ~a H, , and aoH are the optical densities of pyridine at 2550 A. in the buffer, in 0.1 M hydrochloric acid, and in 0.1 M sodium hydroxide solutions, respectively. The measurements were made on a Cary lModel 14 spectrophotometer in a 0.01 cm. quartz cell. The buffer solution had the same composition as that used in the kinetic determination; the concentration of total pyridine was 0.0264 M. Under these conditions pyridine was found to be 96% unprotonated. Kinetic measurements were made by following the decrease of the height of the acetic anhydride proton magnetic resonance signal which was well separated from the acetic acid-sodium acetate signal by 17 C.P.S. A Varian A-60 spectrometer was used for this purpose. The reaction was started by exliausting from a micropipet 0.10 ml. of acetic anhydride into 10.0 m!. of the desired solution, which gives 0.10 M initial concentration of acetic anhydride. Two solutions were used in the kinetic determination: (1) a blank solution consisting of 1.06 iM NaQAc and 0.53 M HOAr in 60% water-40% dioxane; (2) a solution identical with the blank solution but containing in addition 0.063 iM free pyridine. The initial concentration of pyridine under these conditions is only 2% higher than the final concentration due to the additional acid produced in the solution from the hydrolysis of acetic anhydride. This conclusion derives from the relationship
Vol. 67
was derived from the theory of Debye and Huckel.' The values of k (Table I), derived213 in 1931 from Falckenberg's measurements* of dD/dP and from molal volumes determined by Baxter and Wallace,6 could not claim high accuracy. Moreover, values of dD/dP obtained from Kyropoulos' datae led to much k. higher resu1t~'~~for
-
where Kp and K H Aare ~ the acid dissociation constants of pyridine and acetic acid, f is the fraction of unprotonated pyridine in the buffer solution, and Pt is the totaI concentration of pyridine. A plot of log ( ht - h,) us. time results in a straight line, the slope of which gives the first-order rate constants. The rate constants from solutions 1 and 2, respectively, are 5.22 X 10-8 sec.-l and 5.07 X lO-*ssec.-l.
THE MOLAL VOLUME OF ELECTROLYTES BY OTTOREDLICH Department of Chemical Engineering and InoTganic Materials Division of the Lawrence Radiation Laboratory, University of California, Berkeley, California Received June 66, 1966
Recent, obviously excellent determinations' of the dielectric constant of water between 0 and 70°, and 1 and 1000 bars, remove any doubt from a question that has been under discussion for several decades, A limiting relation, expressing the apparent molal volume of an electrolyte as a function of the valences x1 of its ions, and the dielectric constant D and compressibility p of the solvent cp =
p20+ ~
~ ~ 1 . 5 ~ 0 . ~
(3) =
E
= 4.8029 X lQ-'O
N
=
83.1469 X lo6 erg(deg. mole)-' 6.0232 X
Temp., OC.
IC
16
1.8 i 0 . 5
25
1.7
20
2.53
25 25
1.86 i 0.02 2.517
25
1.884
OR
NEAR25'
Based on
Reference
dD/dP (Falckenberg) Molal volumes (Baxter, Wallace) dD/dP ( Kyr op oulos ) Molal volumes dD/dP (Kyropoulos) dD/dP (Owen, et al.)
(2) 1931 (3) 1931
( 7 ) 1933 (9) 1940 (8) 1949 1962
TABLE I1 COEFFICIENT k Temp., OC.
IO'* d In D / d P , dyne-' em.*
10'2 @,a dyne-1 om.2
k, cm.8 (mole/l.)-O
1
0 45 14 45.42 1.539 10 45.84 44.85 1.668 20 46.65 44.52 1.809 30 47.58 44 43 1.963 50 49.78 44.85 2.318 70 52.43 46.08 2.746 a L. B. Smith and F. G. Keyes, Proc. Am. Acad. Arts. Sci., 69, 286 (1934).
By 1940, however, accurate density determinations by Geffoken and his co-workers, and by Wirth, furnished a reliable basisg for the value k = 1.86 0.02, though the difference from the value derived from Kyropoulos' data was large. The recent measurements' are in perfect agreement with the conclusions of 1940 and eliminate any reason for using arbitrary empirical values instead of the derived values given in Table 11. Moreover, no attempt need be made to make the data fit relation 1 by introducing terms of higher orderlo a t unusually low concentrations.
*
(2) 0.Redlich and P. Rosenfeld. Z . phyeik. Chem., A M s , 6 5 (1931). (3) 0.Redlich and P. Rosenfeld, 2. Elektrochem., 3 1 , 705 (1931). (4) G. Falckenberg, Ann. Physzk, [41 61, 145 (1920). (5) G. P. Baxter and C. C. Wallace, J . A m . Chem. Soc., 38, 70 (1916). (6) 9. Kyropoulos, 2. Phyaik, 40, 507 (1926). (7) F. T. Guoker, Jr., Chem. Rea., 18, 111 (1933). (8) B. B. Owen and 6. R. Brinkley, Jr., Ann. iV. Y . Acad. Sci., 61, 753 (1949). (9) 0.Redlich, J . Phys. Chem., 44,619 (1940). (10) H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions," 3rd Ed., Reinhold Publ. Corp., New York, N. Y., 1958, p. 390.
(1)
w = 0.52; v i ~ i ' (2) k = 2N2r3(2a/1000RT)o.5D-1~6(d In D/dP - p/3)
R
TABLE I COEFFICIEST k AT
e.s.u. mole-'
(1) B. B. Owen, R. C. Miller, C. E. Milner, and H. L. Cogan, J. Phys. Chem., 611. 2065 (1961). See also F. E. Harris. E. W. Haycock, and B. J. Alder, ibid., S7, 978 (1953).
CHBRGE-TRANSFER COMPLEXES OF METHYLVIOLOGEN BYAKITSUQTJNAKAHARA~ AND JIJIH. WANQ Contribution No. 1YO3 from Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut Received June 86, l g S 8
Molecular complexes with absorption spectra uncharacteristic of the components of the respective (1) On leave of absence from Institute of Chemistry, College of General Eduoation, Osaka University, Toyonaka, Osaka, Japan.
