Acid-Base Equilibria in Methanol, Acetonitrile, and Dimethyl Sulfoxide

From the data values of K' = [HA-]/[HA-], HA- denot- ing the intramolecularly hydrogen bonded monoanion, have been found. The monoanions of malonic an...
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M. K. Chantooni and I. M. Kolthoff

1176

Acid-Base Equilibria in Methanol, Acetonitrile, and Dimethyl Sulfoxide in Acids and Salts of Oxalic Acid and Homologs, Fumaric and o-Phthalic Acids. Transfer Activity Coefficients of Acids and Ions M. K. Chantoonl, Jr., and 1. M. Kolthoff* School of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received December 1 1, 1974)

The first and second dissociation and the homoconjugation constants of the homologous series of oxalic to azelaic acid, fumaric, and o-phthalic acids and K(HE) of some of their monoesters have been determined in dimethyl sulfoxide (DMSO) and methanol (M). From the data values of K’ = [HA-]/[HA-], HA- denoting the intramolecularly hydrogen bonded monoanion, have been found. The monoanions of malonic and o-phthalic acids in M are strongly intramolecularly hydrogen bonded (but not in water, W). In agreement with the above potentiometric data, ir spectra of biglutarate solutions yielded conclusive evidence that the biglutarate ion contains an intramolecular hydrogen bond in acetonitrile (AN) and (CD3)2SO. The ratio K’(AN)/K’(DMSO) = 101.7*0.2closely corresponds to the difference in hydrogen bond accepting capacity previously reported between these two solvents. From this it is inferred that the carboxylic acid group in HA- is not hydrogen bonded to the solvent DMSO, in contrast to that in This has been confirmed by ir spectrometry. Ir spectra of mixtures of glutaric acid with biglutarate in AN indicate the presence of an intermolecular hydrogen bond between the carboxylate group of HA- and the acid in the homoconjugate HA--.H2A. No indication of intramolecular hydrogen bonding in the bioxalate ion was found in W, M, and DMSO. From solubilities of the dicarboxylic acids and silver salts of some of the monoesters in AN and the other solvents transfer activity coefficients of the various acids and their ions between AN and W, M, and DMSO have been obtained.

m-.

Introduction In a previous paper1 we reported in acetonitrile (AN) first and second dissociation constants, K1 and K2, homoconjugation constants, Khomo,of the homologous series of oxalic to and including azelaic acid, also of fumaric and ophthalic acid, and the dissociation constants K(HE) and homoconjugation constants K(HE2-) of the corresponding monomethyl or -ethyl esters (HE). Using eq 1, first proK’ = [HA-]/[HA-] = K1/2K(HE) - 1

(1)

posed by Westheimer and Benfy? we calculated the equilibrium constants K’ between intramolecular and nonintramolecular hydrogen bonded monoanions, HA- and HA-, respectively. The corresponding values of K’ of the above acids in water are equal to or close to zero.2 In the present paper these studies are extended to the solvents methanol (M) and dimethyl sulfoxide (DMSO). Compared as solvents (not as solutes), methanol is a slightly stronger hydrogen bond acceptor than water3 and about an equally strong hydrogen bond donor. Dimethyl sulfoxide, a typical dipolar protophilic solvent, is a much stronger hydrogen bond acceptor than acetonitrile, but about an equally poor hydrogen bond donor. In contrast to water and methanol, solvation of a carboxylate ion by hydrogen bonding in AN, DMF, and DMSO is negligible.3.4 With the nonintramolecularly hydrogen bonded monoanion HA-, DMSO forms an intermolecular hydrogen bond with the carboxylic acid group. Evidence is presented in this paper that this does not occur with HA-. Consequently, the values of K‘ in DMSO should be smaller than in AN. An additional reason for the inclusion of M and DMSO in the present study is to obtain values of the transfer activity coefficients, y, of H2A, BHA- ( = HAHA-) and A2- between the amphiprotic solvents M and water (W) on one hand and AN and DMSO

+

The Journal of fhyslcal Chemistry, Val. 79, No. 12, 1975

on the other. Also, the Born effect on transfer activity coefficients of ions between the close to isodielectric solvents M, AN, and DMSO is almost negligible and small even for py(A2-). Our values of ?ion are based on the tetraphenylborate assumption which yields almost the same values for the organic solvents as the ferrocene assumption.5,6 Values of ANyS(H2A)and ANyS(HE)were determined from solubility data of the diacids and monoesters, respectively, while those of ANyS(ZHA-)and ANyS(A2-)were obtained from the relations (pK& - ( ~ K ~ ) =AsAANpK~ N = pANyS(H+) pANyS(BHA-) - pANyS(H2A) (2)

+

(pK2)s - ( ~ K P ) A=N‘AANpK2 = pANyS(H+) pANyS(A2-)- pANyS(BHA-) (3) Calculation of values of pANyS(H2A)and pANyS(HE)do not involve any extrathermodynamic assumption. From the experimental results the effect of -CH2- groups on ANyS(H2A)and ANyS(HE)was found. An ir study has been made of biglutarate and of the homoconjugate of biglutarate with glutaric acid in AN in order to obtain conclusive evidence that in the homoconjugate H2A is intermolecularly hydrogen bonded to the carboxylate group in the monoanion In order to show that DMSO is not hydrogen bonded to the carboxylic acid group in E c - , ir spectra were obtained in solutions of bisuccinate in DMSO.

+

m-.

Experimental Section Solvents. Acetonitrile,l N,N-dirnethylf~rmamide,~dimethyl sulfoxide,l and methanol1 have been purified and dispensed as described previously. Acetonitrile-ds and dimethyl-& sulfoxide were Aldrich and Baker Co. products, respectively, used without further purification.

1177

Transfer Activity Coefficients of Acids and Ions

TABLE I: Molecular Solubility of Diprotic Acids a n d Their Monoesters i n Various Solvents W

AN

M

(CH,),

Acid

[~2Alsat.

Succinic

2

Glutaric Adipic

3 4

0.70' 0.65'

[ ~ ~ I s a t . '[ ~ 2 ~ 1 s a t [. '~ ~ I s a t[~2AIsat.' .' [ 1.28

7.45d

6.26'

0.0404

~ ~ l s a

5.06'

DMSO DMF t i~2Alsat.' . ~ [~2~1sat.'[ ~ ~ I s a t . ' Solvate

3.53

4.58 0.715 1.12 5.3e 0.039 5.2e 2.86 3.69 5.12e 0.136" O.lge 0.16b 2.76 0.197 3.52 Pimelic 5 0.324' 0.85 0.028 2.29 2.78 6 0.0152' Suberic 0.34' 1.48 0.063 2.73 3.15 Azelaic 7 0.013' 0.015a Sebacic 8 0.0015' 3.4 X 0.51 3.54e 0.016 3.80' 1.86 2.59 3 ,73e Fumaric 0.051' 0.185d 0.51 1.87' 0.0055 0.4186 Solvate 3.74 5.4 5' 0.053" o-Phthalic 0.0368' 0.206d 1.15 4.91' 0.0245 2.93' 3.77 a A. Seidell, "Solubilities of Organic Compounds", 3rd ed, Vol. 2, Van Nostrand, New York, N.Y., 1941. * N. P. Komar, V. Mel'nik, K . Zimina, and A . Kozachenko, Vestn Khark. Uniu., 73,67 (1971); Chem. Abstr., 78,8478t (1973). This work. Methyl ester. e Ethyl ester.

TABLE 11: Values of pK1 a n d pK2 of Diprotic Acids i n Various Solvents and Homoconjugation Constants, Khomo,i n AN and DMSO

PKi and PK2 Wa

HZOX

H,Mal H~SUC H2Glu HZAd

Mf 6.1 10.7 7.5 12.4 9.1 11.5 9.4 11.5 9.45

11.1 H~Az H,Fum

8.03 10.44

0-H2Pth Butyric Acetic

9.97

{ :::9.72

ANf 14.50 27.7 15.3 30.5 17.60 29 .O 19.20 27.95 20.35 26.9 20.8 24.8 18.6 22.9 14.2 29.8 22.73 22.3'

Khomo

DMF

7.8b 20.8b 10.4b 19.gb

8.2' >16' 10.05' 17.2'

1 3 .5d

AN^

DMSO 6.2' 14.9' 7.2f 18.55f 9.50~ 16.7f 10.9f 15.3f 11.9f 14 .If 11.9~ 1 3 .5sf 9 .2f 11.23f 6 .2e 16.0e 12.86f 12.6i

DMSO~

4.0 x 103 0.9 x 103 0.2 x 103

5 M), values of their pANyS(HE)were also obtained from SAANpK(HE)and SAANpKsp(AgE)using SAANpK(HE)- SAANpKSp(AgE)= pANyS(H+)- pANyS(Ag+)- pANyS(HE) (4) in which y denotes transfer activity coefficients. Values of KSP of the silver salts of the monoesters were estimated from paH and paAg in their saturated solutions containing a known amount of ester.g Silver wire and glass electrodes were used to measure paAg and paH, respectively. The data are presented in Table a.1° The following values of pKBpwere obtained: AgMeSuc, 8.32 in AN, 7.4 in M, and 3.56 in W; AgMeAd, 8.9 in AN, 7.6 in M, and 4.04 in W. Under our experimental conditions both silver salts are uncomplexed and practically completely dissociated in water. However, in M and AN, complexation and incomplete dissociation are appreciable, K(AgE2-) being of the order of lo7 and Kd(AgE) = 7 X 10-j in M for both salts. Owing to the low solubility of the silver salts in AN, reliable values of these constants in this solvent could not be evaluated. [HE] were calculated Values of P(HE2-) = [H~E~-]/[HEz-] in AN from paH and paAg in mixtures of H E and AgE, as described previously for silver benzoate? and were found equal to 1.8 X lo1 and 2.7 X 101 for the monomethylsuccinate and adipate ester anions, respectively. Values of pK1, pK2, pK(HE), and Homoconjugation Constants from Potentiometric Data. In DMSO and M, pK1 and pK2 values of malonic, succinic, glutaric, and adipic acids were estimated from the paH, measured with the glass electrode, in mixtures of the diacid and monotetraethylammonium salt or mono- and ditetraethylammonium salts. Figure 1presents plots in DMSO of paH vs. log C(HzA)/(HA-)f-. In addition, in DMSO or M, paH was measured in equimolar mixtures of H2A-HA- and of HA-A2- prepared by partial neutralization of azelaic or fumaric acid with 1.1M tetramethylammonium hydroxide (in M).' Since monoesters of oxalic acid are unstable, values of K(HE) of monoethyloxalate in W, M, and DMSO were obtained from the paH of mixtures of 0.001 to 0.02 M potassium monoethyloxalate with standard hydrogen chloride. Data are presented in Table b.10 In these three solvents hydrogen chloride and the potassium salt of the monoester at

1179

Transfer Activity Coefficients of Acids and Ions Absorbonce

paH

Or

13.0-

12.0 -

11.0

10.0

9.0

3600

3200

E800

0

-1.0

+1.0

,,Y;

log CHIA/

1800

1600

1400

I200

Figure 3. Ir spectra of glutaric acid, tetraethylammonium biglutarate, solvent, (- -) and a 1: 1 mixture of the two in acetonitrile: (--) 0.261 M H~GIu,(-*-.-*-) 0.263 M E~~NHGIu, (-X-X-X-X-) 0.252 M H~GIu,0.259 M Et4NHGlu.

I

1

+2.0

2000

Wave Number cm-'

0'

I

2400

--

+3.0

CHA-f-

Flgure 1. Plots of paH vs. log C(HpA)/fC(HA-) in DMSO: (1)butyric acid (with Me4NOH in M) C(A-) = 0.00258;(2)adipic acid, C(HA-) = 0.00354;(3) glutaric acid, C(HA-) = 0.0050;(4)succinic acid, qHA-) = 0.0042.Dashed line has a slope of -1 .OO.

Absorbance O r

0.1 -

ApaH

l

"-----e1

0

1

1.0

I

CMeOH , M

3600

Figure 2. Effect of methanol on paH of mixtures of monomethylsuccinate and its tetramethylammonium salt in AN and in DMSO: (0)C, = 0.0203, C, = 0.00279 M in AN, paHo = 18.54;(0) C, = 0.00400, C, = 0.00334 M in AN, paHo = 21.06; (A)C, = C, = 0.00215 Min DMSO. paHo = 11.81.

the concentrations used can be regarded as strong electrolytes. In dilute solutions in these solvents the monoester appears to be remarkably stable. The potassium salt of the ester has too small a solubility in AN to permit making in this solvent similar measurements as above. Values of K(HE) of the other monoesters in DMSO and M and of K(HA) of butyric and mono- and dichloroacetic acids in AN were obtained from paH in equimolar mixtures of the acids and tetramethylammonium hydroxide.ll In all instances the effect of M introduced with the titrant on p a H of mixtures of H2A, HA, or HE was taken into account. It was assumed that the methanolation constants of the monovalent anions were the same as that of the monomethylsuccinate ester anion. From the effect of M on paH of nearly equimolar mixtures of monomethylsuccinate and its anion in AN and in DMSO, plotted in Figure 2, the following values were found: K(Me0H.E-) 1.0, K(2MeOH.E-) = 4.6 X lo1 in AN and 3.0 and 3.5, respectively, in DMSO. For the fumarate and azelate dianions in DMSO K(MeOH.Fum2-) = 4 (unpublished data) was used. Activity coefficients were calculated from the partially extended Debye-Huckel limiting law, -log f = Az2v'J(1 B a f i ) where A = 1.64 in AN, 1.10 in DMSO, 1.90 in M

-

+

3200

2800

2400

2000

1800

1600

I

I

1400

1200

Wave Number cm-'

Flgure 4. lr spectra in (CD&SO of succinic acid, tetraethylammonium bisuccinate, the mixture of the two, and of tetraethylammonium solvent, (- - - - -) 0.172 M succinic acid, (----) biglutarate: (--) 0.282 M tetraethylammonium bisuccinate, (-0-0-) 0.374 M tetraethylammonium biglutarate, (-X-X-X) 0.151 M succinic acid, 0.132 M tetraethylammonium bisuccinate.

and B = 0.49 in AN, 0.42 in DMSO, and 0.51 in M. In Table I1 are presented pK1 and pK2 values of diacids in W, M, AN, DMF, and DMSO and overall homoconjugation conin AN and stants, Khomo, of H2A with HA- and DMSO. Values of pK1 and pK2 in M reported by Liteanu and B1azsek-Bod6l2 were obtained from glass electrode measurements using sodium methoxide as titrant. The presence of Na+ ions may give rise to relatively large alkaline glass electrode errors. With malonic and oxalic acids our pK2 values are some 2 units greater than those reported by Liteanu et al. For these reasons their values are not included in Table 11. In Table I11 are listed values of log K' of the monoanions and pK(HE) of the monoesters in W, M, AN, and DMSO. Infrared Spectra. Ir spectra in AN of glutaric acid, tetraethylammonium biglutarate, and a 1:l mixture of the two are presented in Figure 3, while Figure 4 presents ir spectra in (CD3)zSO of succinic acid, tetraethylammonium bisuccinate, and a mixture of succinic acid and bisuccinate. A spectrum of biglutarate in (CD&SO is included in Figure 4 for comparison with that in AN.

=-

The Journal of Physical Chemistry, Vol. 79, No. 12, 1975

M. K. Chantooni and I. M. Kolthoff

1180

Discussion Transfer Activity Coefficients. The solubility in W and AN of most of the diacids is such that their saturated solutions can be considered as ideal. From the values of [H2A],,t. expressed in molarity (Table I) values of pANyW(H2A)were calculated and are listed in Table IV. It is of interest to note that pANyW(H2A)increases by 0.37 unit per - C H p group in the chain. For example, pANyW(H2A)= -1.2 for succinic and +1.0 for sebacic acids, the difference yielding 2.2/6 = 0.37 per -CH2- group. The solubility of most of the diacids, H2A, in DMSO and DMF, and of the diacids with an odd number of -CH2- groups in M, is too large to consider molarity equal to activity in the saturated solutions. For these acids values of pANyDMFvDMS0(H2A) from solubility data involve an uncertainty of f0.3 unit and they are reported in parentheses in Table IV. It is striking that for the acids with an even number of -CH2groups pANyMpDMSo(H2A) is constant and equal to -1.5 for AN M and -2.0 for AN DMSO. Hence for these acids pM,DMSoyW(H2A) also increases by +0.37 unit per -CH2group. In this connection it is of interest to note that from solubility datal3 it appears that p‘yW of normal aliphatic hydrocarbons becomes 0.48 unit more positive per -CH2group. This suggests that changes in pMyW(H2A) (H2A with an even number of -CH2- groups) with the number of -CH2- groups can be identified with the hydrocarbon part of H2A. Making the reasonable assumption that the solvation of the carboxyl groups by hydrogen bonding to the solvents is independent of A in H2A, we write -+

-

+

pANyS(H2A)= 2pANyS(H,) pANyS(n)= pANyS(Ha)+ pANyS(HE) (5) In eq 5 pANyS(H,)denotes the difference in hydrogen bond accepting capacity between AN and S and pANyS(n)the contribution of the nonhydrogen bonded nonelectric component of the transfer activity coefficient. Using values of = -1.g3 we obtain pANyM(Ha)= -1.53 and pANyDMSo(Ha) from eq 5 pANyM(n)= +1.53 and pANyDMSo(n)= +1.8 for H2A with an even number of -CH2- groups. These values of pANyM,DMSo(n) represent the effect of the two carboxyl groups, this effect being the same in H2A as in the corresponding HE. We now consider values of pANyS(HE) (reported in brackets in Table IV) calculated from eq 5 using pANyS(HzA) values in Table IV and the above values of pANyS(Ha).When S = W, a correction of +0.6 unit per -CH2group in the alcohol part of the ester has been added, as was done for the benzoate esters.14 Using the value of pANyW(Ha)= -0.8 previously reported3 and the values of pANyW(H2A) in Table IV, we calculate from eq 5 pANyW(MeHSuc) = +0.2 and pANyW(MeHAd)= +0.9. Similarly, the corresponding values for AN M are 0.0 for both acids. These values are to be compared with those obtained from eq 4. Using values of W,MAANpK(HE) (Table 111), W,MAANpKSp(AgE) (Results), pANyW(Ag+)= +3.8,6 pANyM(Ag+) = +5.1,6 pANyW(H+) = -10.2,6 and pANyM(H+) = -6.2,6 good agreement is found between the values of pANyWfM(MeHSuc)and pANyW,M(MeHAd) in Table IV obtained from eq 4 and 5. It is reasonable to expect that pANyMsDMS0(HE) 0, as the solubilities in AN, DMSO, and M of a given monoester of the homologous series of oxalic acid (with an even number of -CH2- groups) are practically the same. Intramolecular Hydrogen Bonding in Monoanions. An

-

-

The Journal of Physical Chemistry, Vol. 79, No. 12, 1975

expression describing the effect of solvent on K’ is simply derived from eq 6-8. Regarding that HA- is not intermolelog (K’)AN- log (K’)s = ANAS log K’ = pANyS(HA-)- pANyS(HA-) (6) (cf. eq 1)

+

pANyS(HA-)= pANyS(n) pANyS(HA-)el+ pANyS(Ha) (7)

+

pANyS(m-) = pANyS(n) pANyS(m-),l

(8)

cularly hydrogen bonded to the solvents (eq 8) and considering that pANyS(n)is the same in HA- as in HA-, eq 9 results ANAS log K’ = -pANyS(Ha)

+

pANyS(=-)e1 - PANTS(HA-),~ When S is an aprotic solvent, pANTS(HA-)e1 yS(HA-),l 0, and

-

-

ANAS log K’ = -pANyS(Ha)

(9) pAN(9’)

The values of K’ of the various diacids in DMSO and of succinic acid in DMF in Table I11 are all considerably smaller than those in AN, as expected. Between the solvents AN and DMSO the average value of DMSoAAN log K’ = -1.7 f 0.1, which agrees well with pANyDMSo(Ha)= -1.9: omitting azelaic acid, the K’ value of which is too small in DMSO to be considered reliable. For succinic acid in DMF DMFAAN log K’ = -1.3,15 in agreement with = -1.4.3 This agreement lends strong suppANyDMF(Ha) port to the assumptions made in deriving eq &The premise that intermolecular hydrogen bonding of HA- with the aprotic solvents is negligible is supported by the ir results (vide infra) and the fact that no appreciable effect of DMSO up to 0.7 M on the paH of mixtures of tetraethylammonium bisuccinate and succinate is observed in AN.l For biphthalate ion in DMSO Forsen16 observed a large downfield PMR chemical shift (-15 ppm from H2O as external standard) and attributed this to a strong symmetrical intramolecular hydrogen bond and to the effect of placing a negative charge in proximity of the hydrogen bonded proton.17 The fact that intramolecular hydrogen bonding is absent in bioxalate in the solvents W, M, and DMSO (K’ 0) must be attributed to steric hindrance. Considering that methanol is a slightly stronger hydrogen bond acceptor and about an equally strong donor as water, it is expected that values of K’ in M of monoanions, whose conformation would not greatly favor intramolecular hydrogen bonding in water, would be practically equal to zero. This has been found for succinic and longer straightchained acid monoanions in Table I11 and for bicyclo[2.2.2]octane-1,4-dicarboxylicacid monoanion,18 while Christol et al.19 found K’ 0 for the trans-4-cyclohexene1,2-dicarboxylic acids in methyl cellosolve (t 17) in contrast to values of log K’ equal to 1-2 for the cis isomer. It is unexpected that bimalonate and biphthalate ions are rather strongly intramolecularly hydrogen bonded in M, the values of K‘ being about 10 and 12, respectively, while K’ of these monoanions in water is zero.2,20From ANAM log K’ = 3.4 and 4.2 for bimalonate and biphthalate, respectively, and eq 9 taking pANyM(Ha) = -1.53 and assuming pANyM(HA-),l pANyM(OAc-),, = -8,3 we obtain pANyM(HA-),lof the order of -5 to -6, attesting to a large electrostatic free energy of solvation to the carboxylate group of HA- by M. It deserves further study to establish

-

-

-

1181

Transfer Activity Coefficients of Acids and Ions

TABLE IV: Transfer Activity Coefficients of Dibasic Acid Species a n d Monoesters of Diprotic Acids ~~

h

--

Acetic

w

SUCcinic

-0.4 -1.21 M -0.4 -1.50 DMF -0.2 (-1.9) + DMSO -1.2 (-1.9)“” P Y ~ H A - ) AN w -9.8 -6.5 ANM -7.0 -3.8 AN DMF +1.6 (+1.2) AN DMSO +0.5 (+1.4) py(A2‘)’ AN W -21.9 AN-M -15.1 AN-“ DMSO (+0.5) py(MeHA) AN W [+o .2]” P~(H,A) AN

AN AN AN

------t

Glutaric

suAdipic Pimelic beric Azelaic Sebacic

(-0.9)’ (-0.80)

-0.54 -1.45 (-1.9) (-l,9)a (-1.9) (-7.7) -8.3 (-4.45) - 6 . 1 5

-0.22 (-1.14) (-1.25)

+O.26 -1.48 -1.91 (-2.0)

FU-

maric o-Phthalic

+0.68 +1.02 -1.37 -1.50 (-1.6) - 2 . 0 ~ (-1.7) -2.2 -7.4

-1.0 -1.96 (-2.8) -8.5 -6.4

(-2.2) -3.4 -2.5

(+0.9) -18.7

(-0.8) -18.9 -12.7 (-1.1) +0.4]g i0.4“

(+1.2) -19.7 (-16.3) (-1 2) [+1.2Ig (+1.15)’

-0.2 -1.7

b

-+

ANpy(MeHA) AN

M

-

(+1.2) (-22.1) (-14.7) (-0.1)

(+1.0) -21.7 -15.8 (-0.4) +0 .8d [+O .9Ig (+1.44)@’f

1”’”” [+0.o5]g (0.0)ep

(+1 . l ) [+2.4Ig {(+3.05)’”

[O.O]“

{ [O.O]e*f

{ { { [-0.5Ig { [-O.2]” [-0.7Ie -0.2e

f

DMSO

Value of -1.9 taken for pANyDMSo(HzA), since pANyDMS0(HzA) is independent of chain length. Solid solvate formed. CInterpolated from plot of pANyW(H2A) vs. n in (CH2)n(COOH)~.From solubility of silver salt of monomethyl ester and eq 4 . e From solubility of monoester. f Py(EtHA), corresponding values of pANyW(MeHAd) = +0.8 and pANyW(MeHSeb) = +2.4; pANyMJ”’So(EtHA) = ANyMvDMS0(MeHA).g Calculated from py(H2A) using eq 5, see text. pANyDMF(Suc2-) = 0.0. Q

whether the stronger intramolecular hydrogen bonding in M than in W is to be attributed to the lower dielectric constant of the latter. Infrared Spectra. In 0.26-0.37 M tetraethylammonium biglutarate and biadipate solutions in CH&N, CDsCN, and bisuccinate in (CD3)zSO strong absorption starting a t 1730 cm-l to a t least 900 cm-l is observed, while little or no absorption is found at higher frequencies (Figures 3 and 4). The carbonyl band is practically absent in these spectra. This pattern indicates strong symmetrical intramolecular hydrogen bonding21 in HA- in AN and DMSO, possibly with extensive proton tunneling and absence of significant intermolecular hydrogen bonding with these solvents. As expected, glutaric and acetic acid (the spectrum of the latter not given) in CH3CN (Figure 3) and CD3CN (not in a Figure) exhibit the “free” OH stretching bandz2 from 3500 to 2900 cm-l, the absorption of glutaric acid being about twice that of an equimolar solution of acetic acid.’On the other hand, this band is absent in 0.17 M succinic acid in (CD3)zSO and is replaced by a broad absorption from 3200 to 41000 cm-l (Figure 4). A similar result is found in solutions of trifluoroacetic acid in sulfolane to which dimethyl sulfoxide has been addedz3 which has been ascribed to intermolecular hydrogen bonding of the carboxyl group with DMSO. This is supported by the fact that the carbonyl stretching band a t 1734 and 1746 cm-l for glutaric (Figure 3) and acetic acids, respectively, in CH3CN is shifted t o 1700 cm-l in succinic acid solution in (CD3)zSO (Figure 4). Of particular interest is the spectrum of the homoconjugate of the glutaric acid-biglutarate mixture in CH3CN and CD3CN. The “free” OH stretching band in the homoconjugate is reduced to about half that in an equimolar solution of the free acid (Figure 3), only one carboxylic acid group in HA-.HzA containing a “free” hydroxyl group. Strong absorption is found from 2900 cm-l to the Irtran cell cutoff a t 760 cm-l, which is much greater than that of glutaric acid

alone. Considering that the biglutarate does not absorb between 2900 and 1900 cm-l, the strong absorption in this region is attributed to intermolecular hydrogen bonding between the carboxylate ion and the acid, HzA, in the homoconjugate, while the strong absorption a t lower frequencies is attributed to both inter- and intramolecular hydrogen bonding. Furthermore, the broad carbonyl band(s) in HzAHA- occur(s) at -1700 cm-l in AN. Thus the spectrum of the glutaric acid-biglutarate mixture in AN (Figure 3) lends strong support to the conclusion drawn from equilibrium data in much more dilute solutions that in the homoconjugate the carboxylate group is intramolecularly hydrogen bonded to the carboxylic acid group in HA- and intermolecularly to H2A. The spectrum does not exclude homoconjugation between HA- and HA, the contribution of which must be very small. As expected, the spectrum of the mixture of succinic acid and bisuccinate in DMSO is roughly the sum of the components (Figure 4) as homoconjugation is negligible. Inductive Effects on pK(HE) and pK1. The values of pK(HE) of the monoesters in M, AN, and DMSO in Table I11 increase markedly a t first with increasing chain length, level off at monomethylglutarate, and become practically equal to pK(H0Ac) for monomethyladipate. This is attributed to the inductive effect of the carbalkoxyl group as in water.2 As expected, the inductive effect is particularly large in monomethyloxalate and monomethylfumarate, while in the o-phthalate half ester the steric inhibition of resonance effect operates in addition to the polar effect of the ortho carbalkoxyl group. In a given monoester the inductive effect on pK(HE) of the carbalkoxyl group is greater in the dipolar aprotic solvents than in water or methanol, which gives rise to resolution of acid strength. Thus, a plot of pK(HE) of monomethyladipate, -glutarate, -succinate, -fumarate, including pK(HA) of acetic, butyric, and monochloro- and dichloroacetic acids in AN vs. that in M is _ .

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linear with some scattering and has a slope of 1.8, which is practically equal to that observed of non-ortho-substituted benzoic acids3 of 1.71. A similar plot in DMSO vs. M (including monoethyloxalate) has a slope of 1.8. Like for the substituted benzoic acids (HA), no resolution of acid strength of the monoesters is found between AN and DMSO, ANADMSopK(HE)= ANADMSopK(HA)= 9.7 f 0.15. For fumaric acid, whose monoanion is not intramolecularly hydrogen bonded in any of the solvents used, ANADMSopK(HE)= ANADMSopK(HA)= 9.7 f 0.15. For fumaric acid, whose monoanion is not intramolecularly hydrogen bonded in any of the solvents used, ANADMsopKl= 9.4. Also, pK1 of fumaric acid lies on the plot of PK(HE)M-AN.The values of ANAMpKI(H2Fum) and ANAMpK(MeHFum)are equal to 10.6 and 10.96, respectively. From the analytical viewpoint it is of interest to mention that (pK2 - pK1) in AN as an average is 4.2 units greater than for the same diprotic acid in DMSO. Hence the break in pH at the first equivalence point with water- and alcohol-free titrant is considerably greater in AN than in DMSO. Acknowledgment. We thank the National Science Foundation for Grant No. GP-20606 in support of this work. Supplementary Material Available. Tables a and b will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be ob-

The Journal of Physical Chemistry, Vol. 79, No. 12, 1975

tained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order for $4.00 for photocopy or $2.50 for microfiche, referring to code number JPC-751176. References and Notes I. M. Kolthoff and M. K. Chantooni, Jr., J. Am. Chem. SOC., 97, 1376 (1975). F. Westheimer and 0. Benfy. J. Am. Chem. SOC., 76, 5309 (1956). M. K. Chantooni, Jr., and I. M. Kolthoff, J. fhys. Chem., 77, 527 (1973). C. D. Ritchie in “Solute-Solvent Interactions”, J. F. Coetzee and C. D. Ritchie. Ed., Marcel Dekker, New York, N.Y., 1969, pp 224-225. B. Cox and A. J. Parker, J. Am. Chem. SOC.,95, 402 (1973). I. M. Kolthoff and M. K. Chantooni, Jr., J. Phys. Chem., 76, 2024 (1972). I. M. Kolthoff, M. K. Chantooni, Jr., and H. Smagowski, Anal. Chem., 42, 1622 (1970). I. M. Kolthoff, J. J. Lingane, and W. Larson, J. Am. Chem. Soc.. 60, 2512 (1938). M. K. Chantooni, Jr., and I. M. Kolthoff, J. Phys. Chem., 77, 1 (1973). See paragraph at end of paper regarding supplementary material. Raw data will be supplied by the authors upon request. C. Liteanu and A. Blazsek-Bodo, Rev. Roum. Chim., 17, 1465 (1972). M. Abraham and J. Johnston, J. Chem. SOC.A, 1610 (1971). M. K. Chantooni, Jr. and I. M. Kolthoff, J. fhys. Chem.. 78, 839 (1974). log K’ was derived from the value of pK(H2Suc) This value of DMFAAN = 10.05 in Table It7 and assuming that ANAoMFpK(MeHSuc) = ANADMFpK(HOA~) = 8.8 (Table 11). Hence pK(MeHSuc) = 21.6 - 8.8 = 12.8 in DMF from which log K’ = 2.4 for succinic acid in DMF, yielding OMFAAN log K’= 2.4 - 3.7 = -1.3. S.Forsen, J. Chem. fhys., 30, 852 (1959). J. Pople, W. Schneider, and H. Bernsteln, “High Resolution Nuclear Magnetic Resonance”, McGraw-Hill. New York, N.Y., 1959, p.408. C. D. Ritchie and G. H. Megerle, J. Am. Chem. SOC., 80, 1452 (1967). H. Christol, D. Moers, and Y. Pietrasanta, J. Chim. fhys.. 67, 2024 (1970). D. Chapman, D. Lloyd, and H. Prince, J. Chem. SOC., 550 (1964). L. Eberson, Acta Chem. Scand., 13,224 (1959). L. J. Bellamy, “The Infrared Spectra of Complex Molecules”, Wiley, New York. N.Y., 1959, pp 162-165. J. Husar and M. M. Kreevoy, J. Am. Chem. SOC., 04,2902 (1972).