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Estimation of pH and autoprotolysis constants in mixtures of aliphatic amides with water: Medium effect on the 4-aminoazobenzene system
Ta&nta, Vol. 40, No. 4, pp. 479-484, 1993 Printed in Great Britain.All rightsresewed
003%9140/!?3 s6.00 + 0.00
copyright @ 1993Per&amen PressLtd
ESTIMATION OF pH AND AUTOPROTOLYSIS CONSTANTS IN MIXTURES OF ALIPHATIC AMIDES WITH WATER: MEDIUM EFFECT ON THE 4-AMINOAZOBENZENE SYSTEM AGUSTING.
A~UERO,* MARIA A. HERRADO R and A. GUSTAVOGONZALEZ
Department of Analytical Chemistry, Faculty of Pharmacy, The University of Seville, 41012-Seville, Spain (Received 24 March 1992. Revised 14 Augmi 1992. Accepted 26 Asqust 1992) Summary-Correction factors to the glass electrode and autoprotolysis constants of miztures of aliphatic amides with water: N,iV-dimethylformamide (DMF), NJV-dimethylacetamide @MA), N-methylformamide (NMF), N-methylacetamide (NMA), formamide (P) and acetamide (A), have been determined. The acidity constants of 4-aminoazobenzene referred to the standard state of the mixtures of these aliphatic amides with water as well as the medium effect on the 4-aminoazobenzene system have been evaluated from spectrophotometric measurements.
Amidewater mixtures are useful polar reaction media and solvents for a variety of purposes. In spite of this, there is relatively little dataje4 available on the analytical use of mixtures of aliphatic amides with water. Correction factor to be made to the glass electrode, autoprotolysis constants and acidity constants of 4-aminoazobenzene (HB) in mixtures of aliphatic amides with water: N,N-dimethylformamide (DMF), N,N-dimethyl-acetamide (DMA), N-methylformamide (NMF), N-methylacetamide (NMA), formamide (F) and acetamide (A), have been evaluated in this paper. An attempt has been made to interpret the medium effects on the basis of hydrophobic interactions. EXPERIMENTAL
Apparatus All titrations were made with a Crison digilab 517 pH-meter and an Ingold electrode (order No. 04023 11) assembly (glass-AgCl/Ag) (CH-8902 Urdorf/Switzerland). In order to avoid the glass membrane dehydration, the electrode was soaked in water when not in use. Prior to the m~u~ment of pH, the electrode was soaked in a solution of the desired solvent composition. Titration vessels used consisted of tall beakers sealed into a water jacket through which water circulated from a Techne C-100 water bath. Temperature *Author for correspondence.
was controlled at 25 f 0.1” by means of a Selecta 320-R water bath, Titrations were carried out by using 2.0-ml micro burette with magnetic stirring. In the determination of densities, a Paar DMA 60 densimeter fitted with a Paar DT 100-20 densitothermometer, a Tecam 1000 heat interchanger and a Techne C-400 circulator were employed. A Spectronic 2000 Bausch & Lomb spectrophotometer equipped with silica cells of l-cm path length was used for the absorbance measurements. 4-Aminoazobenzene was synthesized according to the method indicated by Vogel.* The crude product was recrystallized from methanol and its melting point was 127” (reported value 128”). The purity of the dye was checked by thin layer chromatography as described by Vytras et aL6 Solutions of 4-aminoazobenzene at 0.1 O/O v/v in the various pure liquid amides were prepared. In the case of acetamide, a solution containing 50 mg of 4-aminoazobenzene in 50 ml of 1OM acetamide in water was employed. Reagents ~,~-~ime~ylfo~~ide, ~,~-dimethyla~tamide, ~-methylfo~~ide and N-methyla&amide (E, Merck Darmstadt), acetamide (Merck Schuchardt) and formamide (Normasolv, Ferosa, Barcelona) were stored in 4 A molecular sieves for at least one week. Analytical reagent potassium monohydrogenphthalate (Riedel-H5en) was used in the 479
AGUSTIN G. AWERO et al.
480
Table I. Relative~~itti~ti~~
%.
v/v
10 20 30 40 50 60 70 80
of diierent wa~r~p~tic
amide mixturesat 25”
DMF
NMF
F
DMA
NMA
77.32 74.86 72.40 69.52 65.95 61.74 56.73 51.04
80.73 83.62 87.11 91.47 96.93 103.89 113.04 126.77
82.82 87.43 93.20 97.23 101.79 105.12 110.00 111.28
76.62 74.62 72.30 69.47 66.39 62.70 57.99 52.46
79.27 79.96 81.36 83.42 86.18 89.92 95.93 105.88
standardization of alkaline solutions. Potassium chloride (Merck, Darmstadt) was used for the adjus~ent of ionic strength.
Procedures Correction factors. In 50-ml standard flasks prepare three solutions containing a suitable volume (or weight in the case of a&amide) of amide, 1, 2.5 and 5 ml of a 0.01064M solution of hydrochloric acid, respectively, and distilled water to the mark. Measure the pH at 25 + 0.1”. Au?o~ro~oZy~i~constants. In a IO@ml standard flask add a suitable volume (or weight in the case of a&amide) of amide, 10 ml of 1M potassium chloride, 1.3 ml of 1M hydrochloric acid and distilled water to the m&k, at 25 f 0.1”. Transfer 75 ml of this solution to the titration vessel and titrate with 1M potassium hydroxide. Measure the pH at 25 f 0.1”. Procedure for the evaluation of acidity constants. Solutions for absorbance and pH measurements were prepared in 25-ml standard flasks by mixing 1 ml of 7.61 x 10m4M stock solution of 4-a~no~o~~ene in the appropriate amide, 2.5 ml of 1M potassium chloride, the necessary volume of pure amide to give the required solvent composition in the final mixture, a few drops of hydrochloric acid or sodium hydroxide at different concentrations and distilled water to the mark. Absorbance was measured at 489, 496 and 505 nm, against a solvent blank and the pH checked after the absorbance measurements. The temperature was kept at 25 + 0.1”.
%, mol.
A
4.96 9.83 14.60 19.30 23.90 28.45 32.90
81.67 84.49 86.79 88.33 88.49 90.25 90.64
RESULTS AND DISCUS!3ION
Plots of potential cell against pH-meter reading (in mV) for each solvent mixture at the various acid concentrations and ionic strength tested give straight lines whose slope values (from 60.15 to 58.32) are very close to the theoretical nernstian slope (k) value of 59.16. By using the t-test’ on the slopes one may assume that the behavior of the electrode is reversible. Correction factors The relative ~~ttivity of the mixtures of amides with water have been previously reported* (Table 1). The amide molar fraction X,, was determined from
where dAMand d, are the densities of the pure amide9 and the mixture containing VAMpercentage v/v of amide, respectively, and MW is the amide relative molar mass. The dAM values obtained for the different mixtures studied are compiled in Table 2. The correction factors to the glass electrode, 6, for transforming the pH-meter readout, R, (when the calibration is done with aqueous buffers) in the logarithm of the proton activity in the studied medium, pH*, obtained for the different 6 =R-pH*,
Table 2. Density of differentwater-aliphaticamide mixturesat 25” (d:‘) %, v
DMFf
NMF
F
DMA
NMA
%, mol.
10 20 30 40 50 60 70 80
0.9964 0.996I 0.9959 0.9956 0.9950 0.9928 0.9878 0.9789
0.9984 0.9998 1.0012 1.0026 1.0040 1.0054 1.0068 1.0082
1.0128 1.0282 I .0427 1.0561 1.0693 1.0823 1.0950 1.1060
0.9951 0.9956 0.9963 0.9972 0.9968 0.9953 0.9886 0.9790
0.9957 0.9967 0.9984 W999 1.0002 0.9980 0.9931 0.9849
4.96 9.83 14.60 19.30 23.90 28.45 32.90
tTaken from Ref, 2.
A 1.0032 l.OOQ6 1.0155 1.0215 1.0273
1.0329 1.0384
Estimation of pH and autoprotolysis constants
481
Table 3. Correction factors to the glass electrode for several water-aliphatic
%,
v/v
10 20 30 40 50 60 70 80
DMF+ 0.088 f 0.093 f 0.182 f 0.309 f 0.386 f 0.476 f 0.536 f 0.491 f
0.099 0.006 0.018 0.005 0.010 0.01 I 0.007 0.009
NMF
F
0.820 f 0.025 1.169f0.040 1.513 f 0.070 1.803 f 0.099 2.167 f 0.134 2.539 f 0.156 2.978 f 0.155 3.460 f 0.175
1.815 f 0.580 2.656 f 0.618 3.244 f 0.522 3.645 f 0.437 4.063 f 0.333 6.504 f 0.144 4.766f0.111 4.917 f 0.223
NMA
%, mol.
A
0.021 * 0.010 0.176+0.040 0.358 f 0.004 0.525 f 0.003 0.735 f 0.010 0.958 f 0.010 1.182 + 0.030 1.425 + 0.040
4.96 9.83 14.60 19.30 23.90 28.45 32.90
0.178f0.011 0.299 f 0.029 0.603 f 0.061 0.813 f 0.035 1.091*0.147 1.062 f 0.114 1.152 f 0.103
DMA 0.030 f 0.150 f 0.342 f 0.509 f 0.683 f 0.854 f 1.019 f 1.098 f
0.004 0.014 0.007 0.008 0.005 0.010 0.008 0.012
amide mixtures at 25”
*Taken from Ref. 2.
solvent mixtures assayed are compiled in Table 3. The relatively small standard deviation observed in most cases indicates that the liquid junction potential is practically constant’O for each solvent composition tested. However, the reproducibility of measurements in formamide-water mixtures is somewhat poor, and measurements in water-NMF and water-A at high v/v amide to water ratio (or w/v in the case of A) also suffer some dispersion. This behavior may be qualitatively explained because all mixtures studied are more basic than pure water; the hydrogen ion is expected to be more stabilized in these media, owing to specific interactions between the hydrogen ions and the basic amides. The free standard energy of transfer from water to the mixed solvent, AGf(H+), will be negative and hence, the medium effect, log fn, also [AGF(i) = 2.303 RT logy(i)]. The presence of alkyl substituents on a nitrogen atom enhances its basicity. Therefore, the theoretical order of basicity of amides studiedwhich is based in the facility to donate the electron pair of the nitrogen atom-should be DMF>NMF>F
DMA>NMA>A.
However, an opposite arrangement is found, surprisingly, if we take into account “the virtual basicity” indicated by the values of 6 DMF
DMAzNMA
This fact may be attributed to some other interactions that take place between the hydrogen ions and the various solvent species such as ion-multipole interactions, structural effects or other solvatation effects. Nevertheless, a positive value of 6 does not necessarily imply a negative medium effect on the hydrogen ions since the liquid junction potentials are unknown. Autoprotolysis constants The p& values obtained for the different amide-water mixtures are compiled in Table 4. In the case of water-F mixtures, the cologarithm of the autoprotolysis constant passes through a minimum as the percentage of liquid amide in the mixed solvent increases, unlike the behaviour shown by the other water-amide mixed-solvent mixtures. The standard deviations of the p& values were smaller than 0.04. It can be shown (as expected) that a minimum in the p& value is found between pure water and 10% v/v of amide composition (O.O-4.96% molar in the case of a&amide) with the exception of the water-F system. The influence of the relative permittivity of the medium on the autoionization of mixed aliphatic solvents has been envisaged. Several different types of correlation”~‘* between the autoprotolysis constants and acidity constants of weak acids, respectively, and solvent composition have been suggested. The Born equationI predicts a
Table 4. Cologarithm of the aufoprotolysis constants of water-aliphatic
amide solvents
%, v/v
DMF
NMF
F
DMA
NMA
%, mol.
A
10 20 30 40 50 60 70 80
13.77 13.90 14.13 14.45 14.85 15.35 16.14 17.28
13.98 14.25 14.32 14.46 14.79 15.15 15.68 15.92
13.50 13.27 11.68 11.72 12.04 12.20 12.42 12.71
13.60 13.65 13.91 14.09 14.42 14.82 15.50 16.36
13.75 13.94 14.07 14.19 14.45 14.77 15.22 15.72
4.96 9.83 14.60 19.30 23.90 28.40 32.90
13.72 13.80 13.92 14.02 14.17 14.32 14.44
AGUTIN G. A.WEROet al.
482 18 -
12
k
t
10’ 0.5
I
I
I
I
1.0
1.5
2.0
2.5
100/e* Fig. 1. Dependence of ply, with the reciprocal value of the permittivity for several water-fonnamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (0) water-F mixtures, (V) water-DMA mixtures, (0) water-NMA mixtures and (V) water-A mixtures.
linear relationship when the pK, is plotted against the reciprocal of the permittivity. Plots of this kind are depicted in Fig. 1, showing a linear behavior with the exception of the A and F-water mixtures. Thus, in a first approach this fact may suggest a predominance of electrostatic effects on the ionization of aqueous mixtures of DMF, NMF, DMA and NMA. Evaluation of acidity constants CAminoazobenzene undergo a may prototropic tautomeric solvent dependent azonium-ammonium I4 equilibria
single ionization process. According to the basic properties of the mixed solvents studied and the greater facility of protonation of the amine-nitrogen with respect to the azonitrogen form I seems to be stabilized. Previous UV-visible and Hi NMR spectral5 of Caminoazobenzene and similar azodyes in polar solvents such as water-amide mixtures also indicates the potentially tautometric dye exists as true aminoazo species. 4-Aminoazobenzene in its ionization is quite strong and it is not entirely cationic even at pH 1. The pK,* (K,* being the acidity constant at the standard state of the mixed solvent) values in the media studied are collected in Table 5 and were spectrophotometrically evaluated’ taking into account that the limiting absorbance of HB+ is unavailable. The standard deviations were smaller than 0.06 except for the case of acetamide- and formamidewater mixtures whose values were somewhat superior. Estimation of the medium eflect The solvent effect in the dissociation process of HB+ can be expressed as followsJ6 AG, I HB +/B I sys= AG:(H+) + AG:(B) - AG:(HB+) = 2.303RT log f lHB+/Bl,,.
If we assume that the electrostatic effects may be estimated by using the Born model,” we have logf IHB+/Bls,s = bf,,
H3N+ - C6H5-N=N-C6H5 I
IHB+/Bl,,,
+F($-&+)
#H2N-C6H,--N=N+H-C,HS. II
x
Table 5. Acidity constants
($_A > L
The S-shaped form of the absorbance versus pH* graphs obtained in all cases suggests a (pK:)
%, vlv
F
NMF
10 20 30 40 50 60 70 80
1.02 0.19 -0.34 -0.87 -1.11 - 1.62 - 1.97 -2.15
1.83 1.41 1.40 0.81 0.31 0.00 -0.50 -1.14
tTaken from Ref. 1.
(2)
= l%: - PK (3)
where log f,, I HB+/B (sysis the non electrostatic contribution to the medium effect.
of the Caminoazobenxene amide mixtures
system in water-aliphatic
DMFt
NMA
DMA
%, mol
A
2.45 2.21 1.99 1.79 1.56 1.20 1.07
2.65 2.46 2.18 2.01
2.61 2.30 1.95 1.65 1.33 0.98 0.53 0.14-0.05
2.90 5.80 8.70 11.60 14.50 17.40 20.30 23.20
2.64 2.50 2.45 2.31 2.14 2.10 1.92 1.73
1.86 1.37 1.21 0.95
483
Estimation of pH and autoprotolysis constants Table 6. Medium effects (-ApKJ
on the AAB system? in the water-aliphatic amide mixtures studied
%, v/v
F
NMF
DMF§
NMA
DMA
%, mol.
A
10 20 30 40 50 60 70 80
1.93 2.76 3.29 3.82 4.06 4.57 4.92 5.10
1.12 1.48 1.55 2.14 2.64 2.95 3.45 4.09
0.50 0.74 0.96 1.16 1.39 1.75 1.88
0.30 0.49 0.71 0.94 1.09 1.52 1.74 2.00
0.34 0.65 1.00 1.30 1.62 1.96 2.42 2.81-2.90
2.90 5.80 8.70 11.60 14.50 17.40 20.30 23.20
0.31 0.45 0.50 0.64 0.81 0.85 1.03 1.22
tThe p& value of aminoaxobenxene in water was found to be 2.95. #Taken from Ref. 1.
The medium effects in the various mixtures studied are reported in Table 6. The permittivity contribution depends on the difference in the ion sizes of H+ and HB+ ions. That the permittivity plays such a minor role in the dissociation process is confirmed by plotting ApK, against the difference of reciprocal permittivity values (l/c* - l/c) (Fig. 2). In the case of a&amide derivatives and DMF, non linear graphs are obtained. Nevertheless, though in the case of F and NMF solutions a linear plot is observed, the estimation of r& (given that rH+= 1.7 A) yields meaningless results. This behaviour indicates that the non-electrostatic interactions will play a primary role. Since all ApK, values in the different media are negative (pK,* < pK,), taking into account the predominance of the non electrostatic terms, we may assume that
logbonIHB+/BIsys < 0
d ? 3 0
2-
and then AG:(H+) + AG:(B) - AG:(HB+) < 0.
(5)
The AGf(H+) values are negative because of the major basicity of partially aqueous mixtures with respect to pure water. Assuming” that the liquid junction potential is < 6 we may calculate the AC!(B) - AGf(HB+) values. In all cases AGf(B) < AGf(HB+). Thus, the neutral species B is more stable than the charged one HB+ in the solvent mixtures studied. The 4-aminoazobenzene is sparingly soluble in water, but soluble in the water-aliphatic amide mixtures studied. Though a theoretical discussion concerning the specific solvation by part of the mixed solvents exceeds the frame of this paper, a brief discussion on the basis of the basic nature of cosolvent and hydrophobic interactions would explain the results obtained in a qualitative way. According to the virtual basicity of cosolvents indicated above, and considering that this feature plays a capital role in the solvation process, the more basic the solvent mixture the more the 4-aminoazobenzene would display its acidic features (less pK, value). Thus, at a first insight the acidity of 4-aminoazobenzene is expected to be in the same arrangement as the virtual basicity of cosolvents. This is found for the formamide family F > NMF > DMF.
l-
However, in the case of acetamide derivatives the sequence is
16, 0 -60
-40
-20
0
20
40
60
80
104(l/ef- l/e)
-
Fig. 2. Dependence of ApK, (medium effect) against 10’ (l/c* - l/r) for several water-formamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (V) water-F mixtures, (v) water-DMA mixtures, (0) water-NMA mixtures and (W) water-A mixtures.
DMA > NMA > A. In consequence, some other effects in addition to solvent basicity seem to play an important role in this context. The virtual basicity of the formamide family is hierarchically greater than that of the a&amide one. Accordingly so, the basicity may be considered as the most
484
AGU~TING. A~UEXOet al. 6
r
e: T ,k, 5
4
$3
the presence of aliphatic amides as cosolvents enhances the acidity of the Caminoazobenzene system. The plots of pK,* versus mole fraction of amide were non linear in all instances as shown in Fig. 3. Acknowledgements-The financial assistance of the “Dire&on General de Investigation Cientifica y T&mica de Espatia” through Project PB-89-0630 is gratefully acknowledged.
V
0
l-
2 o
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.1
Molefractionofamtde
Fig. 3. Dependence of -ApK, (medium effect) with the mole fraction of amide for several water-formamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (V) water-F mixtures, (v) water-DMA mixtures, (0) water-DMA mixtures and (m) water-A mixtures.
important factor for explaining the acidity of 4-aminoazobenzene in mixtures of F, NMF and DMF with water. Conversely, in the a&amide family, hydrophobic interactions would be competitive against basicity and the outcome agrees with the experimental arrangement. Hydrophobic interactions contribute to a more negative value of medium effects.’ These hydrophobic interactions occur between the “aniline moiety” of the uncharged rdaminoazobenzene (B) and the methyl groups of the a&amide derivative (hydrophobic interactions cancel with the left hand ring in HB+ and B species). Accordingly, the acidity of 4-aminoazobenzene will increase as the number of methyl groups in the amide molecule increases and therefore agrees Well with the experimental sequence. Consequently, by hook or by crook
REFERENCES 1. A. G. Gonzalez, M. A. Herrador and A. G. Asuero, Anal. Chim. Acta, 1991, 246,429. 2. A. G. Gonzalez, D. Rosales, J. L. Gomez-Ariza and A. G. Perez, Talanta, 1986, 33, 105. 3. A. G. Gonzalez, D. Rosales, J. L. Gomez-Ariza and J. Femandez Sanz, Anal. Chim. Acta, 1990, 228, 301. 4. A. G. Gonzalez., M. C. Mochon, J. L. Gomez-Ariza and A. G. Perez, ibid., 1991, 224, 109. 5. A. I. Vogel, A Textbook of Practical Organic Chemistry, 4th Ed., p. 720. Longman, 1978. 6. K. Vytras, J. Kalous and J. Cema-Frybortova, Talanta, 1990, 37, 1025. 7. J. B. Kennedy and A. M. Neville, &sic Statistics Methodr for Engineers and Scientists, 2nd Ed., Harper & Row, New York, 1976. 8. P. Rohdewald and M. Miildner, J. Phys. Chem., 1973, 77, 373. 9. The Merck Index, 8th Ed, Merck and Co., Rahway, New Jersey 1968. 10. K. Das, A. K. Das and K. K. Kundu, Electrochim. Acta, 1981, 22, 54. 11. L. Sucha and St. Kotrly, Solution Equilibria in Analytical Chemistry, Van Nostrand Reinhold, London, 1972. 12. E. J. King, Acid-Base Equilibria, The International Encyclopedia of Physical Chemistry and Chemical Physics, p. 250. Pergamon Press, New York, 1965. 13. J. G’M Bockris and A. K. N. Reddy, Modern Electrochemistry, Vol. 1, Plenum Press, New York, 1973. 14. R. L. Reeves, J. Am. Chem. Sot., 1966, 88, 2240. 15. L. Skulski, W. Waclawek and A. Szurowska, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 28, 463. 16. K. p. Ana, J. sol. chm., 1975, 4, 949. 17. A. L. &Blectrochim. Acta, 1984, 29, 471.
003%9140/!?3 s6.00 + 0.00
copyright @ 1993Per&amen PressLtd
ESTIMATION OF pH AND AUTOPROTOLYSIS CONSTANTS IN MIXTURES OF ALIPHATIC AMIDES WITH WATER: MEDIUM EFFECT ON THE 4-AMINOAZOBENZENE SYSTEM AGUSTING.
A~UERO,* MARIA A. HERRADO R and A. GUSTAVOGONZALEZ
Department of Analytical Chemistry, Faculty of Pharmacy, The University of Seville, 41012-Seville, Spain (Received 24 March 1992. Revised 14 Augmi 1992. Accepted 26 Asqust 1992) Summary-Correction factors to the glass electrode and autoprotolysis constants of miztures of aliphatic amides with water: N,iV-dimethylformamide (DMF), NJV-dimethylacetamide @MA), N-methylformamide (NMF), N-methylacetamide (NMA), formamide (P) and acetamide (A), have been determined. The acidity constants of 4-aminoazobenzene referred to the standard state of the mixtures of these aliphatic amides with water as well as the medium effect on the 4-aminoazobenzene system have been evaluated from spectrophotometric measurements.
Amidewater mixtures are useful polar reaction media and solvents for a variety of purposes. In spite of this, there is relatively little dataje4 available on the analytical use of mixtures of aliphatic amides with water. Correction factor to be made to the glass electrode, autoprotolysis constants and acidity constants of 4-aminoazobenzene (HB) in mixtures of aliphatic amides with water: N,N-dimethylformamide (DMF), N,N-dimethyl-acetamide (DMA), N-methylformamide (NMF), N-methylacetamide (NMA), formamide (F) and acetamide (A), have been evaluated in this paper. An attempt has been made to interpret the medium effects on the basis of hydrophobic interactions. EXPERIMENTAL
Apparatus All titrations were made with a Crison digilab 517 pH-meter and an Ingold electrode (order No. 04023 11) assembly (glass-AgCl/Ag) (CH-8902 Urdorf/Switzerland). In order to avoid the glass membrane dehydration, the electrode was soaked in water when not in use. Prior to the m~u~ment of pH, the electrode was soaked in a solution of the desired solvent composition. Titration vessels used consisted of tall beakers sealed into a water jacket through which water circulated from a Techne C-100 water bath. Temperature *Author for correspondence.
was controlled at 25 f 0.1” by means of a Selecta 320-R water bath, Titrations were carried out by using 2.0-ml micro burette with magnetic stirring. In the determination of densities, a Paar DMA 60 densimeter fitted with a Paar DT 100-20 densitothermometer, a Tecam 1000 heat interchanger and a Techne C-400 circulator were employed. A Spectronic 2000 Bausch & Lomb spectrophotometer equipped with silica cells of l-cm path length was used for the absorbance measurements. 4-Aminoazobenzene was synthesized according to the method indicated by Vogel.* The crude product was recrystallized from methanol and its melting point was 127” (reported value 128”). The purity of the dye was checked by thin layer chromatography as described by Vytras et aL6 Solutions of 4-aminoazobenzene at 0.1 O/O v/v in the various pure liquid amides were prepared. In the case of acetamide, a solution containing 50 mg of 4-aminoazobenzene in 50 ml of 1OM acetamide in water was employed. Reagents ~,~-~ime~ylfo~~ide, ~,~-dimethyla~tamide, ~-methylfo~~ide and N-methyla&amide (E, Merck Darmstadt), acetamide (Merck Schuchardt) and formamide (Normasolv, Ferosa, Barcelona) were stored in 4 A molecular sieves for at least one week. Analytical reagent potassium monohydrogenphthalate (Riedel-H5en) was used in the 479
AGUSTIN G. AWERO et al.
480
Table I. Relative~~itti~ti~~
%.
v/v
10 20 30 40 50 60 70 80
of diierent wa~r~p~tic
amide mixturesat 25”
DMF
NMF
F
DMA
NMA
77.32 74.86 72.40 69.52 65.95 61.74 56.73 51.04
80.73 83.62 87.11 91.47 96.93 103.89 113.04 126.77
82.82 87.43 93.20 97.23 101.79 105.12 110.00 111.28
76.62 74.62 72.30 69.47 66.39 62.70 57.99 52.46
79.27 79.96 81.36 83.42 86.18 89.92 95.93 105.88
standardization of alkaline solutions. Potassium chloride (Merck, Darmstadt) was used for the adjus~ent of ionic strength.
Procedures Correction factors. In 50-ml standard flasks prepare three solutions containing a suitable volume (or weight in the case of a&amide) of amide, 1, 2.5 and 5 ml of a 0.01064M solution of hydrochloric acid, respectively, and distilled water to the mark. Measure the pH at 25 + 0.1”. Au?o~ro~oZy~i~constants. In a IO@ml standard flask add a suitable volume (or weight in the case of a&amide) of amide, 10 ml of 1M potassium chloride, 1.3 ml of 1M hydrochloric acid and distilled water to the m&k, at 25 f 0.1”. Transfer 75 ml of this solution to the titration vessel and titrate with 1M potassium hydroxide. Measure the pH at 25 f 0.1”. Procedure for the evaluation of acidity constants. Solutions for absorbance and pH measurements were prepared in 25-ml standard flasks by mixing 1 ml of 7.61 x 10m4M stock solution of 4-a~no~o~~ene in the appropriate amide, 2.5 ml of 1M potassium chloride, the necessary volume of pure amide to give the required solvent composition in the final mixture, a few drops of hydrochloric acid or sodium hydroxide at different concentrations and distilled water to the mark. Absorbance was measured at 489, 496 and 505 nm, against a solvent blank and the pH checked after the absorbance measurements. The temperature was kept at 25 + 0.1”.
%, mol.
A
4.96 9.83 14.60 19.30 23.90 28.45 32.90
81.67 84.49 86.79 88.33 88.49 90.25 90.64
RESULTS AND DISCUS!3ION
Plots of potential cell against pH-meter reading (in mV) for each solvent mixture at the various acid concentrations and ionic strength tested give straight lines whose slope values (from 60.15 to 58.32) are very close to the theoretical nernstian slope (k) value of 59.16. By using the t-test’ on the slopes one may assume that the behavior of the electrode is reversible. Correction factors The relative ~~ttivity of the mixtures of amides with water have been previously reported* (Table 1). The amide molar fraction X,, was determined from
where dAMand d, are the densities of the pure amide9 and the mixture containing VAMpercentage v/v of amide, respectively, and MW is the amide relative molar mass. The dAM values obtained for the different mixtures studied are compiled in Table 2. The correction factors to the glass electrode, 6, for transforming the pH-meter readout, R, (when the calibration is done with aqueous buffers) in the logarithm of the proton activity in the studied medium, pH*, obtained for the different 6 =R-pH*,
Table 2. Density of differentwater-aliphaticamide mixturesat 25” (d:‘) %, v
DMFf
NMF
F
DMA
NMA
%, mol.
10 20 30 40 50 60 70 80
0.9964 0.996I 0.9959 0.9956 0.9950 0.9928 0.9878 0.9789
0.9984 0.9998 1.0012 1.0026 1.0040 1.0054 1.0068 1.0082
1.0128 1.0282 I .0427 1.0561 1.0693 1.0823 1.0950 1.1060
0.9951 0.9956 0.9963 0.9972 0.9968 0.9953 0.9886 0.9790
0.9957 0.9967 0.9984 W999 1.0002 0.9980 0.9931 0.9849
4.96 9.83 14.60 19.30 23.90 28.45 32.90
tTaken from Ref, 2.
A 1.0032 l.OOQ6 1.0155 1.0215 1.0273
1.0329 1.0384
Estimation of pH and autoprotolysis constants
481
Table 3. Correction factors to the glass electrode for several water-aliphatic
%,
v/v
10 20 30 40 50 60 70 80
DMF+ 0.088 f 0.093 f 0.182 f 0.309 f 0.386 f 0.476 f 0.536 f 0.491 f
0.099 0.006 0.018 0.005 0.010 0.01 I 0.007 0.009
NMF
F
0.820 f 0.025 1.169f0.040 1.513 f 0.070 1.803 f 0.099 2.167 f 0.134 2.539 f 0.156 2.978 f 0.155 3.460 f 0.175
1.815 f 0.580 2.656 f 0.618 3.244 f 0.522 3.645 f 0.437 4.063 f 0.333 6.504 f 0.144 4.766f0.111 4.917 f 0.223
NMA
%, mol.
A
0.021 * 0.010 0.176+0.040 0.358 f 0.004 0.525 f 0.003 0.735 f 0.010 0.958 f 0.010 1.182 + 0.030 1.425 + 0.040
4.96 9.83 14.60 19.30 23.90 28.45 32.90
0.178f0.011 0.299 f 0.029 0.603 f 0.061 0.813 f 0.035 1.091*0.147 1.062 f 0.114 1.152 f 0.103
DMA 0.030 f 0.150 f 0.342 f 0.509 f 0.683 f 0.854 f 1.019 f 1.098 f
0.004 0.014 0.007 0.008 0.005 0.010 0.008 0.012
amide mixtures at 25”
*Taken from Ref. 2.
solvent mixtures assayed are compiled in Table 3. The relatively small standard deviation observed in most cases indicates that the liquid junction potential is practically constant’O for each solvent composition tested. However, the reproducibility of measurements in formamide-water mixtures is somewhat poor, and measurements in water-NMF and water-A at high v/v amide to water ratio (or w/v in the case of A) also suffer some dispersion. This behavior may be qualitatively explained because all mixtures studied are more basic than pure water; the hydrogen ion is expected to be more stabilized in these media, owing to specific interactions between the hydrogen ions and the basic amides. The free standard energy of transfer from water to the mixed solvent, AGf(H+), will be negative and hence, the medium effect, log fn, also [AGF(i) = 2.303 RT logy(i)]. The presence of alkyl substituents on a nitrogen atom enhances its basicity. Therefore, the theoretical order of basicity of amides studiedwhich is based in the facility to donate the electron pair of the nitrogen atom-should be DMF>NMF>F
DMA>NMA>A.
However, an opposite arrangement is found, surprisingly, if we take into account “the virtual basicity” indicated by the values of 6 DMF
DMAzNMA
This fact may be attributed to some other interactions that take place between the hydrogen ions and the various solvent species such as ion-multipole interactions, structural effects or other solvatation effects. Nevertheless, a positive value of 6 does not necessarily imply a negative medium effect on the hydrogen ions since the liquid junction potentials are unknown. Autoprotolysis constants The p& values obtained for the different amide-water mixtures are compiled in Table 4. In the case of water-F mixtures, the cologarithm of the autoprotolysis constant passes through a minimum as the percentage of liquid amide in the mixed solvent increases, unlike the behaviour shown by the other water-amide mixed-solvent mixtures. The standard deviations of the p& values were smaller than 0.04. It can be shown (as expected) that a minimum in the p& value is found between pure water and 10% v/v of amide composition (O.O-4.96% molar in the case of a&amide) with the exception of the water-F system. The influence of the relative permittivity of the medium on the autoionization of mixed aliphatic solvents has been envisaged. Several different types of correlation”~‘* between the autoprotolysis constants and acidity constants of weak acids, respectively, and solvent composition have been suggested. The Born equationI predicts a
Table 4. Cologarithm of the aufoprotolysis constants of water-aliphatic
amide solvents
%, v/v
DMF
NMF
F
DMA
NMA
%, mol.
A
10 20 30 40 50 60 70 80
13.77 13.90 14.13 14.45 14.85 15.35 16.14 17.28
13.98 14.25 14.32 14.46 14.79 15.15 15.68 15.92
13.50 13.27 11.68 11.72 12.04 12.20 12.42 12.71
13.60 13.65 13.91 14.09 14.42 14.82 15.50 16.36
13.75 13.94 14.07 14.19 14.45 14.77 15.22 15.72
4.96 9.83 14.60 19.30 23.90 28.40 32.90
13.72 13.80 13.92 14.02 14.17 14.32 14.44
AGUTIN G. A.WEROet al.
482 18 -
12
k
t
10’ 0.5
I
I
I
I
1.0
1.5
2.0
2.5
100/e* Fig. 1. Dependence of ply, with the reciprocal value of the permittivity for several water-fonnamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (0) water-F mixtures, (V) water-DMA mixtures, (0) water-NMA mixtures and (V) water-A mixtures.
linear relationship when the pK, is plotted against the reciprocal of the permittivity. Plots of this kind are depicted in Fig. 1, showing a linear behavior with the exception of the A and F-water mixtures. Thus, in a first approach this fact may suggest a predominance of electrostatic effects on the ionization of aqueous mixtures of DMF, NMF, DMA and NMA. Evaluation of acidity constants CAminoazobenzene undergo a may prototropic tautomeric solvent dependent azonium-ammonium I4 equilibria
single ionization process. According to the basic properties of the mixed solvents studied and the greater facility of protonation of the amine-nitrogen with respect to the azonitrogen form I seems to be stabilized. Previous UV-visible and Hi NMR spectral5 of Caminoazobenzene and similar azodyes in polar solvents such as water-amide mixtures also indicates the potentially tautometric dye exists as true aminoazo species. 4-Aminoazobenzene in its ionization is quite strong and it is not entirely cationic even at pH 1. The pK,* (K,* being the acidity constant at the standard state of the mixed solvent) values in the media studied are collected in Table 5 and were spectrophotometrically evaluated’ taking into account that the limiting absorbance of HB+ is unavailable. The standard deviations were smaller than 0.06 except for the case of acetamide- and formamidewater mixtures whose values were somewhat superior. Estimation of the medium eflect The solvent effect in the dissociation process of HB+ can be expressed as followsJ6 AG, I HB +/B I sys= AG:(H+) + AG:(B) - AG:(HB+) = 2.303RT log f lHB+/Bl,,.
If we assume that the electrostatic effects may be estimated by using the Born model,” we have logf IHB+/Bls,s = bf,,
H3N+ - C6H5-N=N-C6H5 I
IHB+/Bl,,,
+F($-&+)
#H2N-C6H,--N=N+H-C,HS. II
x
Table 5. Acidity constants
($_A > L
The S-shaped form of the absorbance versus pH* graphs obtained in all cases suggests a (pK:)
%, vlv
F
NMF
10 20 30 40 50 60 70 80
1.02 0.19 -0.34 -0.87 -1.11 - 1.62 - 1.97 -2.15
1.83 1.41 1.40 0.81 0.31 0.00 -0.50 -1.14
tTaken from Ref. 1.
(2)
= l%: - PK (3)
where log f,, I HB+/B (sysis the non electrostatic contribution to the medium effect.
of the Caminoazobenxene amide mixtures
system in water-aliphatic
DMFt
NMA
DMA
%, mol
A
2.45 2.21 1.99 1.79 1.56 1.20 1.07
2.65 2.46 2.18 2.01
2.61 2.30 1.95 1.65 1.33 0.98 0.53 0.14-0.05
2.90 5.80 8.70 11.60 14.50 17.40 20.30 23.20
2.64 2.50 2.45 2.31 2.14 2.10 1.92 1.73
1.86 1.37 1.21 0.95
483
Estimation of pH and autoprotolysis constants Table 6. Medium effects (-ApKJ
on the AAB system? in the water-aliphatic amide mixtures studied
%, v/v
F
NMF
DMF§
NMA
DMA
%, mol.
A
10 20 30 40 50 60 70 80
1.93 2.76 3.29 3.82 4.06 4.57 4.92 5.10
1.12 1.48 1.55 2.14 2.64 2.95 3.45 4.09
0.50 0.74 0.96 1.16 1.39 1.75 1.88
0.30 0.49 0.71 0.94 1.09 1.52 1.74 2.00
0.34 0.65 1.00 1.30 1.62 1.96 2.42 2.81-2.90
2.90 5.80 8.70 11.60 14.50 17.40 20.30 23.20
0.31 0.45 0.50 0.64 0.81 0.85 1.03 1.22
tThe p& value of aminoaxobenxene in water was found to be 2.95. #Taken from Ref. 1.
The medium effects in the various mixtures studied are reported in Table 6. The permittivity contribution depends on the difference in the ion sizes of H+ and HB+ ions. That the permittivity plays such a minor role in the dissociation process is confirmed by plotting ApK, against the difference of reciprocal permittivity values (l/c* - l/c) (Fig. 2). In the case of a&amide derivatives and DMF, non linear graphs are obtained. Nevertheless, though in the case of F and NMF solutions a linear plot is observed, the estimation of r& (given that rH+= 1.7 A) yields meaningless results. This behaviour indicates that the non-electrostatic interactions will play a primary role. Since all ApK, values in the different media are negative (pK,* < pK,), taking into account the predominance of the non electrostatic terms, we may assume that
logbonIHB+/BIsys < 0
d ? 3 0
2-
and then AG:(H+) + AG:(B) - AG:(HB+) < 0.
(5)
The AGf(H+) values are negative because of the major basicity of partially aqueous mixtures with respect to pure water. Assuming” that the liquid junction potential is < 6 we may calculate the AC!(B) - AGf(HB+) values. In all cases AGf(B) < AGf(HB+). Thus, the neutral species B is more stable than the charged one HB+ in the solvent mixtures studied. The 4-aminoazobenzene is sparingly soluble in water, but soluble in the water-aliphatic amide mixtures studied. Though a theoretical discussion concerning the specific solvation by part of the mixed solvents exceeds the frame of this paper, a brief discussion on the basis of the basic nature of cosolvent and hydrophobic interactions would explain the results obtained in a qualitative way. According to the virtual basicity of cosolvents indicated above, and considering that this feature plays a capital role in the solvation process, the more basic the solvent mixture the more the 4-aminoazobenzene would display its acidic features (less pK, value). Thus, at a first insight the acidity of 4-aminoazobenzene is expected to be in the same arrangement as the virtual basicity of cosolvents. This is found for the formamide family F > NMF > DMF.
l-
However, in the case of acetamide derivatives the sequence is
16, 0 -60
-40
-20
0
20
40
60
80
104(l/ef- l/e)
-
Fig. 2. Dependence of ApK, (medium effect) against 10’ (l/c* - l/r) for several water-formamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (V) water-F mixtures, (v) water-DMA mixtures, (0) water-NMA mixtures and (W) water-A mixtures.
DMA > NMA > A. In consequence, some other effects in addition to solvent basicity seem to play an important role in this context. The virtual basicity of the formamide family is hierarchically greater than that of the a&amide one. Accordingly so, the basicity may be considered as the most
484
AGU~TING. A~UEXOet al. 6
r
e: T ,k, 5
4
$3
the presence of aliphatic amides as cosolvents enhances the acidity of the Caminoazobenzene system. The plots of pK,* versus mole fraction of amide were non linear in all instances as shown in Fig. 3. Acknowledgements-The financial assistance of the “Dire&on General de Investigation Cientifica y T&mica de Espatia” through Project PB-89-0630 is gratefully acknowledged.
V
0
l-
2 o
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.1
Molefractionofamtde
Fig. 3. Dependence of -ApK, (medium effect) with the mole fraction of amide for several water-formamide derivative mixtures: (0) water-DMF mixtures, (0) water-NMF mixtures, (V) water-F mixtures, (v) water-DMA mixtures, (0) water-DMA mixtures and (m) water-A mixtures.
important factor for explaining the acidity of 4-aminoazobenzene in mixtures of F, NMF and DMF with water. Conversely, in the a&amide family, hydrophobic interactions would be competitive against basicity and the outcome agrees with the experimental arrangement. Hydrophobic interactions contribute to a more negative value of medium effects.’ These hydrophobic interactions occur between the “aniline moiety” of the uncharged rdaminoazobenzene (B) and the methyl groups of the a&amide derivative (hydrophobic interactions cancel with the left hand ring in HB+ and B species). Accordingly, the acidity of 4-aminoazobenzene will increase as the number of methyl groups in the amide molecule increases and therefore agrees Well with the experimental sequence. Consequently, by hook or by crook
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