PROPERTIES O,F BASESIN ACETONITRILE AS SOLVENT
Jan., 1962
89
(as.)) and other known thermochemical data,40 too negative and the above estimated heat from which it was derived must be in error. Acknowledgments.-We wish to thank Mr. D. E. ReClsi=-Re04- couple. This potential is much LaValle for the preparation of the samples of KzReCle, and Mr. Q. V. Larson and Dr. R. A. (40) From Latimers8 AFfO(€IiO(l.)) = -66.69 koaL mole-’ and Gilbert for assisting with the heat capacity measnFfo(Ci-(aq.)) -31.35 kcal. mole-,. prom J. W. Cobble et urements. aZ.,a AFfO(RoOlo) have been studied, the conductivities in ((dry” acetonitrile were so low that it generally was unavoidable to work with solvent corrections which ranged from 6% for the most concentrated to 50% for the most dilute solutions studied. However, in experiments in which the effect of small concentrations of water was studied, the solvent correction was smaller. The main impurities which are likely to be present in acetonitrile are water and the various hydrolysis products of :icetonitrile, namely acetamide, ammonium acetate, ammonia and acetic acid. It is evident that if any of these impurities (particularly acetic acid) is present in the solvent, the proper solvent correction for solutions of bases will not be the same as the conductivity of the solvent alone. The effect of water was &died in some detail, and is described in subsequent sections of this paper. Ammonia can be detected and determined polarographically a t concentrations higher than approximately 5 X 10-5 M in acetonitrile.’ The solvent used in this study always contained less than this concentration of ammonia, which is sufficiently low to be ignored, since only much more concentrated solutions of compounds which are all stronger bases than ammonia were studied. The presence of acletic acid in the solvent would be much more objectionable, because acetic acid is a much stronger acid than acetonitrile (or water), and reactions such as the following B HOAc I? BH+OAcBH+ + OAc- (1) and possibly also
z
+
B
+ BH+OAc--
91
PROPERTIES OF BASESIN ACETONITRILEAS SOLVENT
(BH:B)+
+ OAc-
(2)
would increase the conductivitv of solutions of bases. Special attention therefore was devoted to the elimination of acetic acid in the purification of the solvent, by treatment with activated alumina. No evidence was obtained for the presence of acetic acid in the purified solvent. Any traces of acetic acid which might nevertheless have been present would have resulted in an essentially constant “background” conductivity, because in all cases the base would have been present in large excess. We have verified that even when a solvent correction which is 3 times as large as the solvent conductivity is applied, the important features of the relationships obtained do not change eufficientlf
P C’
-2 0
_. Q
-15
qx
?
-10
2
W, 8, M U ~ AND P J. F. COETZEE
92
\
-e 0
I3
I
A , %pe=-053
Y91. 66
extrapolate properly to small positive values of 1/A,,. The situation becomes even more obscure when the effect of sm:dl concentrations of water is considered, because addition of 0.1 or 1 M water to solutions of n-butylamine or pyrrolidine gives results similar to those described for diethylamine in the "dry" solvent. In pure water, the conductance behavior of pyrrolidine is represented quite well by equation 4b, and the dissociation constant which we calculated from a Shedlovsky plot (pK8 = 11.1) agrees fairly well with the literature value (11.3) obtained by potentiometric titration.17 Information about the possible over-all ionization reactions of nitrogen bases in acetonitrile can be obtained by considering the log A, us. log C plots given in Figs. 2 to 5. Additional information about these plots is given in Table I. The salient features are the following. (1) In the majority of cases, the log A, vs. log C plots are linear over wide concentration ranges. (2) The slope of the Iinear section of the plot usually is equal to one of 3 values: -3,/4, or 0. The significance of these different values of the slope will now be discussed. Slopes of log A, vs. log C Plots.-For an electrolyte which undergoes simple 1 4(1 1)ionization in a given sdvent, for example B .t CHsCN B H + CHiCN(5)
_ I !
20
33 -!os
50
40
c
Fig. 2.-Pyrrolidine in acetonitrile: A, no water added; B, C and D, 0.11, 1.1 and 16 M water added, respectively.
+
I A a'
%-1
10 .
15 'log
c.
30
Fig. 3.-1,3-Diphenylguanidine in acetonitrile: A, no water added; B, 0.10 M water added.
+
the log A, ZIS. log C plot is linear, and its slope is equal to as long as the degree of ionization of the electrolyte is negligible as compared to unity over the concentration range studied. A slope of zero is obtained in cases such as 2B $. CHsCN j- (BH:B)+
-2.03 0
35
!
4 0 -log
c.
+ CH2CN'
(6)
where hydrogen bonding occurs between the free base and its cation. Analogous hydrogen bonding of acid anions by the acid itself has been observed with a number of acids in several solvents, including acetonitrile (for example, see ref. 6). It is more difficultto account for a slope which ismore negative A slope of - a/4 requires that the ionizathan tion reaction produces 4 ions per mole of solute. Hence the possibility exists that those nitrogen bases which give 8 slope of -3/4 are dimerized. Thus, if the over-all ionization reaction is Bz+ 2CHaCN 1 72BH++ 2CHzCN- (7a) or, jf sufEcient water is present BS + 2H10 2BH+ + 20H(7b) and if a is the fraction of the dimer ionized at a molar concentration Cs of the dimer, it follows that the over-all ionization constant, Kb', of the base will be given by
45
Fig. 4.-lll-Dirnethylguanidine in acetonitrile.
If a is given by the conductivity ratio, A,/Ao, and if it is negligible as compared to unity over the concentration range studied, it follows from equation 8 that Q6
I
05
1.5
10 -!ag
20
I
(9)
8.5
c.
Fig. B.--n-Butylamine in acetonitrile: A, no water added (0 and A: 2 independent runs); B, 1 M water added.
(17) 8. Elearlell, M. Tamres, F. Blook and L. A. Quarterman, J . Am. Cham. Soo., ' I & 4817 (1018).
93
PROPERTIES OF BASESIN ACETONITRILE AS SOLVENT
Jan., 1962
TABLE I CONDUCTIVITY OF NITROGEN BASESIN ACETONITRILE Water preaent, M
p K , in
water
Base
Slope of log log C plot
AM UR.
Concn, range,
M X 10sa
at
.io x 108
c=
0.1 M
C
- 0,1a t s
Mb
1.7 X low6 9 -700 2.8 1.0 x 10-4 60 +1,000 22 9 x 10-5 -77 7 -900 1.8 10.98 l . 6 X 10-8 Diethylamine 2.8 X 11.27 1.OX 10“ .53 3 +400 4.7 Pyrrolidine 8 . 1 x 10-6 0.05+400 17 0.11 .75 32 1 . 5 x 10-4 -76 .02+2d 1.1 2.4 X lo-* .04 -+ 100 500 16 .57 c 2 X 10-8 .76 .02 4 0 . 3 ‘ 10 ,000’ 4 . 8 x lo-*’ 1,l-Dimethylguanidine 4 x 10-6 0 30 4 3 0 0 6 10 * 00 1.0 x 10-8 1,3-Diphenylguanidine 4 x 10-5 30 4 2 0 0 7 0.10 0 7 x 10-6 20 +1,000 1.5 .75 10.78 1 . 6 X lo-’ Triethylamine Degree of 0 Concentration range over which the slope remains constant a t the value given in the previous column. ionization = a = &/b;assumed & values: 160 for diphenylguanidine, 165 for n-butylamine and pyrrolidine in “dry” Virtually a strong base in water. Irregular behavior a t highcr acetonitrile, and 210 for all other cases (see also text). M. concentrations. e Limited solubility. f At C = 10.62
n-Butylamine
1.5X 10-8
1.1
-0.50 .78
-
-
-
-
-
*
Possible over-all ionization reactions correspond- 0.08 M water, the conductivity of 0.1 M n-butyling to the 3 different slopes of the log A@vs. log C amine has double the value for the dry solvent. It follows from equation 8 that for reaction 1 in plots observed in this study are summarized in Table 11. It should be noted that a slope of - l / z Table 11,a plot of l/& OS. should be linear, with an intercept of l/A, and a slope of 16/&’Ac4. can be obtained with 2 different reactions. Such a plot is shown for the case of diethylamine in TABLE I1 “dry” acetonitrile (plot B in Fig. 1). It is to be SLOPESOF LOG A. vs. LOG C PLOTSFOR DIFFERENT IONIZAnoted that in all figures shown, for purposes of simTION REACTIONS plicity and intercomparison, the quantity C refers Over-all ionization to the molar concentration of (monomeric) basc Possible over-all ionization reaction“ constant b Slope added; if dimerization occurs, the concentration 3 2a4C“ - -4 Czof the dimer will be given by C2= C/2. 2s(1) B1 + 2SH *2BH+ The main features of the results listed in Table I ffzcc 1 and in Figs. 2 to 5 are the following. S(2) €32 + SH F? (BH:B)+ 2 2 1. The conductance behavior of l,&diphenyldiffers markedly from that of the other - -1 guanidine (3) B + SH @ B H + S(UZC bases studied. The log Ac us. log C plot given in 2 Fig. 3 for this base in “dry” acetonitrile, and also CY* O (4) 2B + SH e (BH:B)+ + Swith 0.10 M water added, has a slope of zero at 4 concentrations above approximately 5 X 10-2 M . SH = proton donor, either CH,CN or H20, at a constant equilibrium concentration. * For values of a that are For such solutions the predominant ionization reneg!igible as compared to unity. For purposes of com- action therefore must be number 4 in Table 11. , in all cases C refers to the molar concentration of In more dilute solutions the simple ionization rease added, calculated for B, not Bz. action (no. 3) becomes predominant. Formation of It is not possible to decide from only the value the species (BII:B)+ has been observed before’s of the slope whether the base is protonated by in the spectrophotometric titration of 1,3-diphenylacetonitrile or by water. The lowest water con- guanidine with the indicator acid bromophthalein centrations attained in the purification of the sol- magenta E in benzene as solvent. 2. Perhaps the most striking feature of thc vent amounted to approximately 1 X 10-3 M (2 X low37 0 by volume), which is not negligible results listed for the remaining 5 bases is thc freas compared to the very low ion concentrations quencg with which a slope of approximately produced by even the strongest nitrogen bases - is obtained over wide concentration ranges. studied, which are all only very slightly ionized in It already has been shown that an over-all ioniz:jacetonitrile. Howcver, conductometric titration tion reaction involving dimers of these bases would of 0.1 M n-butylamine in acetonitrile (which al- produce a slope of -3/4. The following mechnready contained 1.5 X M water) with water nisms whereby dimerization of nitrogen bases iii over a concentration range of 0.01 to 1 M water acetonitrile could occur wcre considered. (a) gave a virtually linear increase of conductivity with Association of these bases by N-H-N hydrogen increasing water concentration over the whole bonding range, and extrapolation to zero water concentration gave a conductivity value only 2% lower than )N-H + +>x-H.. .N-, I or that measured with 1.5 X M water present, IH I H Hence we conclude that in acetonitrile with a water H concentration no greater than a few millimolar, the acetonitrile itself acts as the major proton donor. As the water concentration is increased, the water / rapidly takes over the role of proton donor. At H
+ +
+
-
(1
rson
:A-
>d“”< ‘k
W. 6. MUNEYAND
94
Primary and secondary amines are known to be coiisiderably associated in the pure, liquid state, even at temperatures near the boiling point. However, tertiary amines cannot associate in this way, and yet triethylamine also gives a slope of Furthermore, it is not to be expected that the association of primary and secondary amines will result in the formation of actual dimers, and particularly not that such dimerization will persist over a wide concentration range. (b) Interaction between the base and acetonitrile. One possible reaction is H
hypothesis simply as one which is consistent with the conductance data obtained. It seems desirable to determine the molecular. weights of selected nitrogen bases in acetonitrile, and this is the subject of further study in this Laboratory. It is of course also possible that the mechanism of ionization of ionophoric bases in acetonitrile may be entirely different from, and perhaps much more complex than that applying in common protogenic solvents, such as water and the alcohols. In this connection it is interesting to note that Usanovich and DulovaiG postulated that picolines ionize in acetonitrile by the reactions B:
B I n this connection it is noteworthy that Zhukova,19 who found from infrared studies that pure acetonitrile is extensively associated, postulated the formation of an analogous dimer in the pure nitrile
+ -
CH3--C=N I
(
NG-CH~
-+
(c) Various possibilities involving what may be termed “effective dimerization” of nitrogen bases in acetonitrile also were considered. For example, the ion pairs produced by reaction of these bases with water in particular would have a highly unsymmetrical charge distribution, and it is conceivable that these ion pairs will “dimerize” to quadrupoles 2B f 2H20-2(BH+OW-)
BH +OH e(OH-BH+)
However, it can be shown that in this case the log --l/Z, not -3/4. Similarly, formation of a 2 : l adduct betmeeii base and added water 2B: + H-0-H B:H-0-H:B cannot account for a slope of -3/4, unless the reaction goes to completion and sufficient vater is present to combine with virtually all the base. We obtained no evidence for the formation of such a 2 : 1 adduct (videinfra),and in any event a slope of was obtained in several experiments in which insufficient water was present to combine with all of the base. Of the various possible reactions whereby nitrogen bases could conceivably dimerize in acetonitrile, those involviug reaction between the base and acetonitrile appear most logical. We carried out a preliminary investigation of the infrared absorption spectra of several of the bases in acetonitrile, but obtained no conclusive evidence for the occurrence of significant reactions between these bases and acetonitrile, although the spectral evidence does not rule out the possibility of such reactions occurring. We therefore present the dimerization A, vs. log C plot should have a slope of
(18) If. M. Davis and H B Hetaer, J . Research A’atl. Bur. Standards, 46, 496 (1951). (19) E. L. Zhu1;ove. Optzka z. Spektroskopzya, 4, 750 (1958)
+ CH&N
B:CH3C?U’
BCH3+.
+ CY-
However, such a mechanism seems energetically unfavorable, and in any event would not account for our slopes of -3/4. We also have considered the possibility that the ionization of nitrogen bases in acetonitrile may occur by a simple 1 -+ (1 1) over-all reaction, and that the increase in the slope of the log .I, vs. log C plots to values more negative than - l / 2 is due to one or more of the following factors. (a) The uncertainty in the magnitude of the solvent correction (discussed before). If the proper solvent correction is larger than that which actually is applied (for example, if the solvent contains significant concentrations of acetic acid), the observed slope will indeed be too negative a t low concentrations of base. However, at higher concentrations of base (above approximately 0.01 M ) , the slope is virtually unaffected by even a 3 or 4-fold increase in the solvent correction. Extensive calculations showed that it is impossible to reduce the observed slope of - ”4 to - ‘/z over more than a very restricted concentration range by simply assuming larger solvent corrections. Furthermore, different batches of solvent and bases (which are not likely to contain exactly the same concentrations of impurities) (b) Eongave a reproducible slope of -3/4. equilibrium conditions. It actually was observed that the conductance of all bases studied exhibited in the “dry” solvent a rapid increase of up to 10% within the first fern minutes, followed by a much slower further increase, which continued without reaching an equilibrium value for 2 weeks, at which time the conductance was about double its initial value. A somewhat similar “aging” phenomenon occurs with solutions of many weak acids in acetonitrile (see for example, reference 6). It is likely that the slow drift which is still occurring after a few minutes with solutions of nitrogen bases in acetonitrile is due to subsequent reactions involving decomposition of the solvent (base-catalyzed polymerization, amidine formation. etc.) . We therefore have made all conductance measurements after 30 min. The reason for the initial drift may be that removal of a proton Crom acetonitrile by an amine-type base is not a rapid process. The further “aging” process was not studied further. In those experiments in which water was added, drifts mere smaller, and virtually stable conductance values were obtained within a few minutes. It was verified that reproducible and equilibrium values were measured by repeating the measure-
+
Jan., 1962
PROPERTIES OF BABES IN ACETONITRILE AS SOLVEN'P
ments with a new sample of freshly purified pyrrolidine, in a new batch of solvent to which 0.11 ilil water had been added, and ma,king measurement8s both after 1 hr. and after 24 hra. In both cases the original slope of - 3 / 4 was reproduced. We therefore believe that the slopes of - 3 / 4 observed in this study are real, and have definite significance. (e) Changes in dielectric const'ant, and particularly in viscosit,y, of the medium at high concentrations of base. The viscosity values of diethylamine (3.67 cp. at 25') and triethylamine (3.63) are quite close to that of acetonitrile itself (3.45). Hence it is unlikely that the bulk viscosity of the medium will change greatly as the concentration of these bases is varied, particularly in t,he more dilute solutions studied. Very large variations in viscosity are required to change the slope from --"4 to --I/z. The same is true of variations in the dielectric constant of the medium. (d) Changes in the nature of the ion species with changing concentration of the bases. Such changes will cause curvature in the log A,, vs. log C plots over relat'ively narrow concentration intervals (see Fig. 3 as an example), and therefore cannot account for the fact that in several cases the slopes remain constant a t -3//4 over wide concentration ranges.20 Primary Hydrogen Bonding Reaction of Nitrogen Bases with Water. Spectrophotometric Titration with Water.-The primary hydrogen bonding reaction Gf n-butylamine and pyrrolidine with small concentrations OS water in acetonitrile was studied by using the wat,er band in the infrared first overtone region. The first overtone band was used, rather than the fundamental, because it was much better resolved. Solutions of n-butylamine and of pyrrolidine in acetonitrile, containing varying concentrations of water, were scanned over the range from 1290 to 1640 mp. Several bands were obtained, a~ndthe following assignments made: 1410 mp (0-H stretch of free water), 1465 (0-H stretch due to hydrogen bonding giving H20-H20 association), 1530 (very broad, possibly also H20-H20 polymers), 1528 (N-H stretch of free amine), and 1540 (N-Hstretch of amine-water adduct). An unassigned band a t 1497 mp also was observed. The free water band a t 1410 mp .\vas the only completely resolved band useful for cpantitative calculations. The absorbance at the wave length of maximum absorption, which remained constant a t 1410 mp with varying water concentrat'ion, followed Beer's law. The results are presented in (20) I n this connection i t is interesting t o note t h a t French a n d Roe,a who obtained log Ac us. log C plots with slopes more negative t h a n -I/% for picric acid i n a.cetonitrile, attributed the deviation of t,he slope from - I / % t o more negative values to the fact t h a t the nature of t h e anion changes from Pi- t o (Pi:HPi) - a t higher concentrations. However, i t can be shown b y calculation, using a variety of numerical values (within reasonable limits) for the formation constant of t h e (Pi: HPi) complex a n d for the ratio Xo(pi:~pi)-/Aoppi- t h a t while i t indeed is possible t o account on this basis for a slope which is slightly more negative t h a n -I/z (perhaps as negative as -0.55) this is possible only if i t is assumed t h a t the value of the ratio X'pi.~pi)-/X@pi- is considerably smaller t h a n I/z (actually, '/z would be a reasonable value), and even then it is possible only for t h e very restricted concentration range over which both types of anions are present i n eignificant concentrations. At lower Concentrations the slope must have a. limiting value of - '/z (as was pointed o u t by French a n d Roe), b u t a,t higher concentrations i t must approachsero (see Table 11). It seems quite possible t h a t in the inore concentrated solutions of French a n d Roe picric acid was associated t o a certain extent.
-
95
Table 111. The slight decrease in molar absorptivity at higher water Concentrations may be the result of increasing assooiatioii of water with itself. TABLE111 APPLICABILITYO F BEER'SLAW TO F R E E WATER BAND 1410 mp IN ACETONITRILE AS SOLVEKT Water concn , M
.zoo
a
Molar absorptivity b
Absorbancen
0 100 .400 .600 ,800 1.60 2.20 Celllength = 1cm.
AT
0.038 0 380 .385 077 ,388 .155 .229 ,382 .384 307 ,583 .364 .785 ,357 ea". = 0.384(for first 5 solutions).
In Table IV the results are presented for the spectrophotometric titration of n-butylamine with water. The concentrations of free water were calculated from the absorbance values a t 1410 mp, using a value of E = 0.384 for the first 5 solutions, and E = 0.364 for the sixth. It is evident that it is TABLE IV SPECTROPHOTOMETRIC TITRATION OF +BUTYLAMINE WITH
Total
[HzO]
WATER I N Absorbanoea ( a t 1410 nip)
ACETONITRILEAS SOLVENT Free [HzOl
[HzO] Reacted
Formation constant of adduct MechMechanism AB anism B c
0.13 0.060 0.27 0.092 0.240 .10 .088 .24 .158 .412 .600 ,181 .472 .128 .31 .17 ,900 .38 .22 ,267 ,695 ,205 1.000 ,295 ,770 ,230 .39 .23 2.000 ,574 1.610 ,390 .40 .20 a Experimental conditions: 0.99 i W n-butylamine, in 1cm. cell, lis. blank containing same concentration of base, but without any water added. b B: HzO @ B :H-0-H. Bg 2H20 F P 2( B :H-0-H) . 0.300 ,500
0
+
+
not possible to decide from the equilibrium constant values obtained which of the 2 mechanisms given in Table IV actually applies. In both cases the equilibrium constant values exhibit a trend to decrease a t lower water concentrations. The same trend is evident in the case of pyrrolidine, which gives practically the same equilibrium constant values as those obtained with n-butylamine under the same conditions. It is simple to show that this trend cannot be the result of simultaneous formation of a 2 : l adduct (B:H-0-H:B), which on theoretical grounds is possible, but not likely. It seems reasonable to assume that the observed trend is caused by additional interaction between mater and the 1: 1 adduct, such as hydration of the ion pairs, BH+OII:-, produced. Degree of Ionization of Nitrogen Bases in Acetonitrile.-Nitrogen bases are too weakly ionized in acetonitrile for the evaluation of AO values from the conductivity data for the bases. However, approximate A, values can be estimated from values for ion conductivities available in the literature. Waldeu and Birr21reported Xo values for a number of substituted ammonium ions in acetonitrile at 25 O , includinc: the following: isobutylammonium 89.8, diethylammonium 94.7, triethylammonium 89.2, piperidin(21) P. Walden a n d E. J. Birr, Z. physik. Chem., 144, 269 (1929),
W.
96
E?.MUNEYAND J, F. COETZEB
Vol. 66
ium 93.3, and diisoamylammonium ion 76. With noted that a degree of ionization of approximately the exception of the last example, these values are 1 X lo-’ at a concentration of U.1 M , which applies all in the vicinity of 90, which will be used as a to n-butylamine, diethylamine and pyrrolidine in reasonable average value for the species BH+ of 5 the “dry” solvent, corresponds to a “conventional” of the bases in the present study; for the species p&, value of 11, calculated (for purposes of com(BH:B)+, half that value, or 45, wil be used. parison with PKb values in protogenic solvents) on For diphenylguanidine, a value of 75 will be as- the basis of simple 1-c (1 1) ionization. This is sumed for the species BH+, and approximately 40 8 powers of ten smaller than in water. It is clear for the species (BH: B) +. Of the anions studied by that, of the bases studied, 1,l-dimethylguanidine Walden and Birr, thiocyanate ion, with a Xo value is by far the most extensively ionized in acetoniof 123.2, approaches the CH2CN- ion closest in trile, as it is in water. Its degree of ionization of size and structure. A XO value of 120 will be used for approximately 5 X at a concentration of 1 X the CHzCN- ion. Since the conductometric titraM corresponds to a “conventional” PKb value tion of n-butylamine with water in acetonitrile as of 7 in acetonitrile (also calculated for simple 1 -+ solvent indicated (2r.s.) that the mobility of the OH- (1 1) ionization, for purposes of comparison), ion in acetonitrile does not differ greatly from that whereas in water it is virtually a strong base. The of the CH2CN- ion, a Xo value of 120 will be adopted differences in the degree of ionization values of nfor the OH- ion as well. The possibility that the butylamine, diethylamine, pyrrolidine and trimobility of the lyate ion, CHzCN-, may be en- ethylamine in acetonitrile are not very large, but nhanced by a “proton-jump” mechanism (as with butylamine definitely occurs higher in the order of the OH- ion in water) seems unlikely, because it is strengths: pyrrolidine > n-butylamine > diethylnot to be expected that the mobility of the OH- amine triethylamine, whereas in water the ion will be similarly enhanced in acetonitrile as order is: pyrrolidine > diethylamine > triethylamine > n-butylamine. solvent . Hence, the approximate degree of ionization It is interesting to note that, although extensive values listed in Table I were calculated by assuming interaction occurs between the bases studied and the following A. values: for diphenylguanidine: added water, as described in the previous section, (40 120) = 160, for n-butylamine and pyrrol- only a moderate increase in conductivity occurs. idine in “dry” acetonitrile (assuming the ionization If the ionization of a nitrogen base in the presence of added water occurs by a series of reactions such is represented by reaction 2 in Table 11): (45 120) = 165, and in all other cases: (90 120) = as 210, Naturally, these calculations are only very Ki Ka K1 approximate, but the uncertainty in the a-values B: HsO B:H-0-H _C BH+OHlisted in Table I should not exceed *250/0. BH’mlvatsd OH-nolvstsd Diphenylguanidine appears to be the only base then the product K2Ksmust be very small. Walden of those studied for which the actual over-all ionization reaction can be given unambiguously (zJ.~.),and Birr21 found that the halides, especially the and for which a definite ionization constant can chlorides, of incompletely substituted ammonium therefore be evaluated. At concentrations above salts (for example, ethylammonium chloride) are approximately 5 X 10-2 M , the over-all ionization quite weak electrolytes in acetonitrile. It is to be constant, Kb‘, corresponding to reaction 4 in Table expected that the corresponding hydroxides will possibly be even weaker, since the hydroxyl ion is XI, is given by very small. Hence KPis undoubtedly quite small, CY2 and a major cause of the extreme weakness of = 4 x 10-10 Kb’ = (10) nitrogen bases as electrolytes in acetonitrile conM , where h, = taining even moderate concentrations of water At a concentration of 3 X 1.5 X the simple ionization reaction ( number must reside in the stability of the BH+OH- ion 3 in Table 11) predominates, and the over-all ioniz* pairs. Finally, it is interesting to note that the over-all tion constant, Kb, then is given by ionization constants of n-butylamine and triethylamine in methanol (dielectric constant = 33; compare acetonitrile = 38) are only 2 or 3 powers of = 2 x 10-11 (11) ten smaller than in water.22 The much weaker It follows from equations 10 and 11 that the forma- ionization of these bases in acetonitrile therefore is tion constant, Kf, of the species (BH:B)+ for 1,3- mainly the result of the fact that the acid properties (proton donating power, as well as the capacity diphenylguanidine is given by to solvate anions) of this solvent are very weak. Acknowledgment.-Financial support by the Research Corporation and by the National Science It is evident from the a-values listed in Table I Foundation (under grant number NSF-G14502) is that the bases studied are all extremely poorly ion- gratefully acknowledged. ized in acetonitrile, even when up to 16 M (29% (22) J. R. Sohaefgen, M. 8. Nemman and F. R. Verhoek, J . Am. by volume) of water has been added. It should be (?kern. Soc., 66, 1847 (1844).
+
+
-
+
+
+
+
+
