H. LEMAIKE .4ND H. J. LucAs
5198 [CO~TRIBUTION No. 1438 FROM
THE
Vol. 73
GATESAND CRELLINLABORATORIES OF CHEMISTRY, CALIFORNIA INSTITUTE OF TECHNOLOGY ]
Measurement of the Basic Strength of Weak Bases in Glacial Acetic Acid1 BY H.LEMAIRE~ AND H. J. LUCAS The thermodynamic constants, PK., of three indicators useful in the study of weak bases have been evaluated; they are f0.3 for 2,4-dinitroS,X-diethylaniline. The indicator method of determining the basic strength of weak bases has been extended to solutions in acetic acid, and has been found applicable for bases havingpK. values between -4.5 and 1.1. Thermodynamic constants pK,, have been calculated, as follows: acetonitrile, -4.2; dioxane, -4; p-toluenesulfonamide, -3.2; water, -2.36; acetanilide, 0.9; acetamide, +O.l; urea, +0.5. The constants of acetone, acetophenone, benzophenone, ethylene di acetate, dibutyl ether, cyclohexene and 3-hexyne do not exceed -4.5 and those of diphenylamine and o-chloroaniline exceed 1.1; all these lie outside the range of study by this method. The relationship between the thermodynamic constant, pKa, of a base and its ionization constant, K:, in acetic acid and in the prasence of perchloric acid in concentrations up to 0.2 .lf is accurately expressed by pK. - pR: = - 3 88
- 1.4 for 2-nitro-4-chloro-N-methylaniline,-0.1 for 2,6-dichloro-4-nitro-N,N-dimethylaniline, and -
Introduction thermodynamic constant of a base from measureThe high activity of hydrogen ion in glacial ace- nients made in non-aqueous media. A relation between p K a and pKZ is obtained by tic acid solutions of strong acids makes this solvent a useful medium for studying the basic strengths of adding log (H+) to Equation 3 and combining with weak bases. Hall8 investigated the strengths of a Equation 2 . large number of weak bases in acetic acid by con@Sa- pK: = Ho + log (H+) = (1 (4) ductometric titration with perchloric acid. Ham- Since PKa is a constant, and since pKA is also a conmett and Deyrup4 determined the basic strengths stant, subject perhaps to small concentration efof water and propionitrile by means of an indicator Q (Equation 4) will also be a constant, and it method, with formic acid as the solvent and ben- fects, can be evaluated from observations made on soluzene-sulfonic acid as the strong acid. Hall and tions containing a strong acid and an indicatorP Spengemad established the constants of certain in- HOcan be evaluated r e a d i l ~ . The value of (H+) dicators in acetic acid in order that the indicator can be obtained by subtracting ~ from the total conmethod might be used in this solvent. In the pres- centration of perchloric acid the concentrations of ent paper are presented the results of measurespecies containing the proton in combination. ments of a number of indicators in glacial acetic any The value of (IHf) is obtained accurately from the acid and the constants of some weak bases ob- known concentrations of total indicator and free tained by means of these indicators. indicator in the presence of acid, as determined by The constants evaluated and the symbols used the spectrophotometric readings. It is assumed are shown in Equations 1, 2 and 3, where brack- that none of the proton is with perchloreted expressions are activities (concentration-ac- ate ion.' Some of the datacombined are shown in Table 11. tivity coefficient products) , expressions in parenthe- 'The result is expressed by Equations 5 and (i. ses are the actual concentrations in glacial acetic if,, f log (H') = -3.88 (31 acid, B represents an uncharged base and BH+ its conjugate acid. When the base is colored, i.e., an P L Y , - PK; = - 3 8 8 iii1 indicator, these last two are replaced by I and IH +, respectively. The superscript s stands for solvent, The thermodynamic constant of a base may be calculated with Equation 6 from constants obin this case glacial acetic acid. tained in glacial acetic acid. Ks = [B][H+]/[BH'j S: = (B)(H')/(BHt) Ho = log (B)/(BH+)
+ #iL
(I)
j'( (31
The thermodynamic constant Ka is defined in Equation 1. It is the thermodynamic acid constant of BH+, the conjugate acid of the base. The ionization constant, IC:, in a solvent, s, is given by Equation 2. The validity of this equation, with K: constant, depends on the fact that H+/BH+ is a ratio so that the ionic activity coefficients cancel because both ions are subjected to similar ion atmospheres in the same medium, and on the fact that 713 can be taken as unity in dilute solution. The acidity function14 Hu, is given by Equation 3. With this equation it is possible to calculate the (1) Presented before the Division of Organic Chemistry, 115th Meeting of the American Chemical Society, San Francisco, California, March 29, 1949. ( 2 ) Wyandotte Chemicals Corporation, V+-ydndotte,Michigan (3) N. F. Hall, Tins JOURNAL, 63, 5115. (1930). (4) L. P. Rammett and A. J. Deyrup, ibid., 64, 2721, 4239 ( l W 2 j . (2) K. F. IIall and \I-. P. Spengenian. ihi,!., 63. 2487 ilC140)
(6) Throughout these investigations the acid used, except a s noted in footnote seven, was perchloric acid and the indicators, I, were substituted nitroanilines of the type used b y previous investigators.'s5 They are colored and their conjugate acids, IH +, are colorless. (7) The assumption that perchloric acid is completely ionized in acetic acid may not be completely justified, even though 0 has a constant value of -3.88 over a t w o thousand fold range of concentration of perchloric acid, namely, 7.6 X 10-6 M to 0.14 M. Presumably a t the lowest concentrations the solutions have become so dilute t h a t extrapolation to zero concentration would produce little further change in Q. However, solutions of sulfuric acid also show a constant value of Q in solutions in acetic acid up to 0.4 M , from observations made in the present investigation and from earlier work,S but the value of Q is different in this case, namely, about -2.58. On the basis of coiistancy of Q sulfuric acid also could be considered to be completely ionized, hut it is evident that its concentration must be twenty times that of perchloric acid to produce a solution of equal acidity. It appears, therefore, that constancy of Q cannot be considered proof of complete ionization. The referee has pointed out t h a t in a solvent of low dielectric constant there may be a n apparent compensation of neglected activity coefficient and ion association eeects and that this may explain the behavior of the two acids. For this reason there may be some uncertainty in the method of obtaining ( H f ) . To the extent t h a t the calculated d u e of (H +) is in error, the values given for pK:, will deviate from the true ionization constants, and the values given for pK. will deviate f r o m t h c true thr:rnmodynamic constants.
Nov., 1951
MEASUREMENT OF THE BASICSTRENGTH OF WEAKBASESIN GLACIAL ACETIC ACID 5199 TABLE I INDICATOR CONSTANTS
Soln. no.
Indicator name
Molar ~-~~~~ Absorp. extinct. Concn., mas., coefficient a t M X 104 mr absorp.max.
f%"
KI
x
10'
#Ka
Eq. 4
IA 4-Chloro-2-nitroaniline 5.81 410 4470 -0.91 1.22 -0.97 IB 5.53 4700 1.33 -1.00 ICa 5.52 4840 1.08 -0.91 ICb 5.52 4620 1.08 -0.91 ID 5.75 4930 0.97 -0.87 4,6-Dichloro-2-nitroaniline 5.40 408 4540 -3.67 701 -3.73 IIA IIIA 4-Chloro-2-nitro-N-methylaniline 4.93 437 5560 3.08 -1.37 IVB 2,6-Dichloro-4-nitro-N,N-dimethylaniline6 . 7 8 0.172 -0.11 2,4-Dinitro-N,N-diethylaniline 3.18 375 15,700 0.0655 +0.30 VA Absorption maximum of Indicator IV was at about 380 mp. Readings were Z . Reference five and references therein. made a t 420 mp to avoid interference from absorption by the conjugate acid of this indicator.
'
was crystallized from freshly distilled isopropyl ether, m.p. Experimental 80.0-80.8°, acetanilide from water, m.p. 113-114', and pPreparation of Materials.-Acetic acid, C.P., was frac- toluenesulfonamide, m.p. 138-138', and urea, m.p. 132.2tionally distilled through a 90-cm. column of glass helices 133.5', were crystallized from ethyl alcohol. surrounded by an electrically heated jacket and provided Measurement of Basic Strength. ( 1) Indicators.-Stock with a total-reflux partial-take off still head.* Batch lots solutions of the indicators were prepared in glacial acetic of 2.5 liters were distilled and initial cuts of 0.5 t o 0.7 liter acid in concentrations of about 0.0005 M. The optical were discarded. The melting point of the main cut was densities of diluted solutions of each stock solution were dealways 16.6', and this value remained constant throughout, termined with a Beckman spectrophotometer in l .000-cm. the weeks of use. Corex cells. Readings were taken a t a specified wave length, Solutions of perchloric acid in acetic acid were prepared usually an absorption maximum. Solutions of indicators by mixing in anhydrous acetic acid the appropriate quantities that were strong enough bases to react appreciably with the of 60% aqueous perchloric acid and acetic anhydride. Stand- solvent (those having a pK. greater than -1.0) were diardizations were made in two ways: (1) on the original aque- luted with acetic acid containing aniline or potassium aceous perchloric acid, by dilution with water, then titration tate. Concentrations were chosen for both the neutral and against standard aqueous iodide-i~date,~ and (2) on the the basic (last two) solutions and also for the test solutions h a 1 acetic acid solution by titration against a standard containing perchloric acid, so that the observed optical solution of recently distilled aniline in acetic acid with crys- densities were usually between 0.200 and 0.400. Different tal violet as the indicator. There was exact agreement be- dilutions of the same stock solution were found to obey tween the results of the two methods. Beer's law in the range in which observations were made. The indicators, if available, were purified, and if not, were The observed readings were divided by the fraction by which prepared by standard methods. 4-Chloro-2-nitroaniline the concentration of the indicator had been diminished (I) (Eastman'Kodak Co.) was crystallized from hot water, (called the dilution factor) so as t o convert all readings to m.p.10 116.0 . Chlorination of o-nitroaniline in concen- the same basis, which would be the optical density of the trated hydrochloric acid" gave 4,6-dichloro-2-nitroaniline original stock solution if Beer's law held. It is immaterial (11)which after crystallization from ligroin (90-127') had for these calculations whether or not Beer's law holds up rn.p.l2 100.7-101.0". to the concentrations of the stock solutions, because the Recrystallized I was heated with dimethyl sulfate a t 100' readings are made a t approximately the same concentration for one hour. The solid resulting from the addition of of the colored form of the indicator. aqueous sodium hydroxide was recrystallized successively The indicator constant IC: was determined by mixing from ligroin, water and ligroin; product, 4-chloro-2-nitro- known amounts of indicator solution, perchloric acid soluN-methylaniline (111), m.p.I3 106.0-106.5". Nitration of tion, and, usually, extra acetic acid. The optical density dimethylaniline1* followed by stepwise chlorination16 gave of such a test solution was determined and the value so ob2,6-dichloro-4-nitro-N,N-dmethylaniline (IV), crystals from tained was divided by the dilution factor. The quotient is ethyl alcohol, m.p.16 105-107". Diethylamine and 2,4- called the calculated optical density of the test solution. dinitrochlorobenzene in 1-butanol reacted spontaneously The value of was then calculated with Equation 7, and the solution became hot. On the addition of water where T i s the calculated optical density of the test solution, Z,Cdinitro-N,N-diethylaniline (V) separated. This was S is the calculated optical density of the stock indicator crystallized twice from alcohol, reprecipitated from its solu- solution, A is the total perchloric acid concentration in the tion in hydrochloric acid by sodium bicarbonate and re- test solution and Cis the total indicator concentration in the crystallized from alcohol; m . ~ . ' ?78.5-79.0'. test solution. The colorless bases, in addition to water, which was laboratory distilled water, were the best grade of commercially available products. They were subjected to additional purification. Acetonitrile was dried with phosphorus pentoxide and distilled, nZ5D1.3409. Dioxane was refluxed (2) Colorless Bases.-The constants of colorless bases five hours over sodium hydroxide and five hours over me- were determined from data on test solutions containing tallic sodium and then distilled, n Z 51.4195. ~ Acetamide known amounts of indicator, perchloric acid, and colorless base. The calculated optical density of a test solution was (8) F. Todd, I n d . E n g . Chem., A ~ a l Ed., . 17, 185 (1945). obtained as before. The ionization constant was then cal(9) I. M. Kolthoff. THISJ O U R N A L , 48, 1447 (1926). culated with Equation 8, where K,I is the ionization con(10) Previous value 116.2', 17. Swartz, RCC.frou. chim., S6, 131 stant of the indicator and Bt is the sum of the concentration (1916). of the colorless base and its conjugate acid. The value of (11) A. E.Holleman and F. E.van Haeften, i b i d . , 40, 67 (1921). pKi from Equation 8, when inserted in Equation 6 gives (12) Previous value,ll 101-102°. (13) Previous value, 1OSo, H. Behrens, Rcc. Ira% chim., 21, 269 (1902). (14) J. B. Tingley and F. C. Blanck, THISJOURNAL,80, 1395 (1908). (15) N. L. Drake, el al., ibid., 68, 1602 (1946). (IS) Previous value,l6 104-106". (17) Previous value, SOo, J. J. Blanksma and H. H. Schreinemachers, Rcc. trau. chim., 62, 428 (1933).
{B, - A
+ C(S - T ) / S + K:r(S - T ) / T ) K : r ( S - T ) / T A - C ( S - T ) / S - K,Bl(S - T ) / T
(8)
the thermodynamic constant of the weak base.
El. EEMAIRF, AKD H. J. L u c ~ s
5200 TYPICAL VALUES
TABLE II LOG (H') FOR VARIOUS SOLULITERATURE VALUESOF pK.
OF €10 TIONS CALCULATED FROM Jndicator ( H + ) , (I), (1%). soln. M M No. X 106 X 10, X 10' IA 128 4.25 5.4 6.27 It3 !I.i.-I 4.6 ri.64 3.1 ICa :O,t ID JO. 1 (;.Sa 3.4 !4.: 1.3 I[ $::IO 'I
Ho
PK: PKa +3.00 -0.91"
-
+2.89 +2.97
$10 -1.02 .91a - .78
-.9l5
-
+3,F2 +O.lG
.91"
-3.67"
-.68 .63 -2.80
-
+
log (I%-) -3.1,1 -3.80 -2.58 --R.9? -%8X
Reference five and references therein.
Results and Discussion Indicators.-The properties of some solutions of the indicators, and the indicator constants are shown in Table I. Four different solutions of indicator I, ;.e., IA, IB, I C and ID, were made, IA and IB from one crystallization but five weeks apart, IC and I D from two other crystallizations. Precautions were taken to exclude air from the product of the last crystallization. It is believed to be the purest. The solutions underwent a slow change but this was not significant except in the
VOl.
case of IC, for ICb represents data taken three weeks after ICa. The values of the indicator constants, PKa, varied as follows: for indicator 11, -3.74 to -3.61; 111, -1.48 to -1.37; IV, -0.17 to -0.04; and V, +02G to +0.:35. Whenever an indicator solution was being used for determining the basic strength of a colorless base, the values of the optical density and ionization constant were redetermined weekly. From the ionization constants of indicators I and I1 and literature values of the therinodynamic constants of these same indicators it has been possible to calculate the value of IJo log (H f) Equation 5 , as given in Table 11. Colorless Bases.-The data leading to the determination of the strength of seven weak bases are shown in Table 111. The values for acetonitrile were obtained from two solutions of separately purified material, the first four from one, and the last five from the other. The values of PK, were but very little d e c t e d by a three hundredfold change in the acid concentration. This result is partial justification for neglecting the effect of
+
TABLE I11
Base
Acetonitrile
Dioxaiie
T H E L)ETERMISATION OF THE STRESCTH OF \vEAK BASES 'Test solution Indicator Optical Base Indicator solution HCIOA density ( T a b l e I: dilution totdl cnncn total concn. obsd. S O M x 101 JI x 10' factor Opt dens
IB
2.60
IC11
2.55
I IA
2.452
ID
2.836
p-Toluenesulf oriamide
11)
2.836
LVater
In
2 ti0
Acrtanilidc
111.4 IB
2.742 2 60
1/4 1/ 5 5/28 1/ 8 5/28 1/4 1/5 1/6 1 15 1/ 5 1/5 1/ 5 5/23 1/ 5 1/5 5/2.3 1/ t i 1/7 1/4 5/28 1/ 5
1/4 115 111-4
2.742
1/ 8 1/ii 1/ G
Acetainide
Urea
I ca
I Ca
2.67
2 ,f i i
5/28 I/8 1/e 5/23 1/ 8 1 /6 5/23 1/ ! I
1/n
1/9 5/28 5/33
73
1.250 1 000 0,536 ,625 ,536 1.250 186.8 78.2 93.4 t
1.000
1.000 1.000 1.087 1.000 1.000 1.087 1.67 2.14 2.50 0.893 1.00 1.25 1.00 0.625 .83.3 ,833
,893
. (i25 . sa3
1.087 0.625 0.833 1 ,087 0 . 556 .556 ,556 .536 .455
951 381 680 476 680 951 7U 1 633 381 ,556 840 280 1&3 149 44.2 28.8 100
85 7 50.0 119 1.86 0.820 ,620 -410 ,940 ,470 ,302 1.802 1.202 0.941 1.470 0.982 ,769 .%I
.i91 .648
.635 ,443
0.400 ,318 ,355 ,235 ,3325 ,361 .4085 .379 ,442 .313 ,325 .3035 .315 .362 .309 .320 .326 .250 .369 .465 .3%5 .400 .3315 .2375 .395 .3i7 .394 . 3125
.sa
.435 .307 .368 .4 13.5 .2i9
.2775 .4445 .3665 .2725
Basic strength
K* '
2.15 2.96 2.66 2.50 2.42 2.14 2.17 1.47 1.69 2.74 2.66 2.30 1.73 0.200 .263 ,202 .0319 .0328 ,3000 .0266 .00121 ,00103 .00153 ,00123
P Ka -4.21 -4.35 -4.30 -4.28 -4.26 -4.21 -4.22 -4.05 -4.11 -4.32 -4.31 -4.24 -4.12 -3.18 -3.30 -3.19 -2.38 -2.40 -2.36 -2.31 -0.96 -0.89 -1.06
-0.97
-
.000885 .00126 ,00111 .000166 .mol46 .000125
+
.000169 .000174
-
.000113 .0443G ,04478 .04408 .04316 .04355
-
-
.83 .DS
.93 .IO .01 .06 .10
.I1 3- .07 .48 .44
.51 .62 .57
NO^., 1951
MEASUREEMENT OF THE BASICSTRENGTH O F WEAK BASESIN GLACIAL ACETICACID 5201
ionic strength when calculating the constants. The best value is -4.2 for P K a of acetonitrile. Hammett and his co-workers4J8found a value of -4.19 for propionitrile. For dioxane, the first value came from 0.926 M solution, the other three from 1.40 M solution in glacial acetic acid. There is a definite concentration effect. The best value is about -4. For acetamide, the first three values were obtained with one and the last three with a second stock solution. In general, the apparent basicity of the amide increases with an increase in the ratio of acid to base. This observation may be related to the tendency of one mole of strong acid to combine with two moles of acetamide.lg The best value of P K a probably is about 0.1, with the acid in excess. Results obtained in aqueous solution3 average about 0.5. Values for p-toluenesulfonamide came from one stock solution, for water from two, and for acetanilide and urea from three solutions of each. The best p K a values are -3.2 for p-toluenesulfonamide, - 2.36 for water, - 0.9 for acetanilide and 0.5 for urea. Previous values are -3.72 for water and - 1.89 and - 1.74 for acetanilide, all from measurements in formic a ~ i d , ~and tl~ an average of about -0.5 for acetamide and + O . l for urea, from measurenients in aqueous solutions.a The greater basicity observed for urea and acetamide in the present work, in comparison with earlier values, may be due in part to the high concentrations necessitated in the earlier studies in aqueous s o l ~ t i o n . ~ I t was found that constants could not be obtained for a number of other compounds because their constants lie outside the range for which the method is applicable. As a test of the method, nheptane*Owas examined with indicator solutions, IB and IIIA. In concentrations up to 0.9 M , K i = l/K&was found to be 0.00, well within experimental error. Acetone, acetophenone, dibutyl ether, ethylene diacetate, cyclohexene and 3-hexyne failed to show any basic character or else showed so little (18) L. P. Hammett and M. A. Paul, THIS JOURNAL, 86, 827 (1934). (19) T. W. J. Taylor, J . CIzcm. Soc., 2741 (19301,and references therein. (20) This was a sample of 99% n-heptane, kindly supplied by the Phillips Petroleum Company, Bartlesville, Oklahoma.
that the differences from blank readings had little significance. Most of these observations were made with indicator 111. Indicator I was unsuitable in solutions containing ketones, probably because these react with formation of Schiff bases. Diphenylamine and o-chloroaniline were too strongly basic for the method. Range of Applicability of the Indicator Method. -The various subtractions necessary in solving Equation 8 limit the range within which reasonably accurate values can be obtained. Limit for Weak Bases.-If in a test solution the values of A , the total acid concentration and of (H+), the free acid concentration, which equals Kz (S - T ) / T ,are too close together, the subtraction to give (BH+) could lead to large errors. If much base is added so as to make (H+) smaller, the nature of the solution changes and the behavior of the indicator may not be the same as in the dilute solutions, If the maximum value of (B) is taken as 0.5 M , and if it is decided that a t least 10% of the total acid present, excluding that present as I H f , should react to form BH+, then 4.5 is the Kz value of the weakest base that can be measured (Equation 9), and -4.5 is the corresponding value of pKa. K: = (B)(H+)/(BH+) = (0.5)(0.9.4)/(0.18)
= 4.5
(9)
Limit for Strong Bases.-If in a test solution the values of Bt, the concentration of all of the colorless base, and of (BH+), that of its conjugate acid, are too close together, the subtraction to give (B) could lead to large errors. If, in order to make (BH+) small, only a small amount of acid is present, the ionization of the solvent will not be suppressed, and the interaction of this with the bases becomes appreciable. If the minimum acceptable value of (H+) is taken as 0.0001 M , and if it is decided that at least 10% of the total base should not react, then 1.1 X loF6is the K! value of the strongest base that may be measured (Equation 10) and 1.1is the corresponding p K a value. K: = (B)(H+)/(BH+) = (O.lB~)(0.0001)/(0.95t) = 1.1 x 10-6 (10) PASADENA, CALIF.
RECEIVED JULY 25, 1950
