Acidic Behavior of Sulfonic and Perchloric Acid in Acetic Anhydride Solvent Mixtures and Their Use as Titrants Donald J. Pietrzyk Uniaersity of Iowa, Department of Chemistry, Iowa City, Iowa 52240 Acidity studies with substituted sulfonic acids and HCIO, are reported for acetic anhydride (Ac20)-acetic acid (HOAc) mixtures. For a fixed set of solvent conditions, the order of decreasing acidity was found to be HCIO, > 2,4,6-trinitrobenzenesulfonic (TNBS) > 2,4-dinitrobenzenesulfonic > p-nitrobenzenesulfonic > benzenesulfonic > p-toluenesulfonic acid. It is demonstrated that the presence of AczO in a Ac,OHOAc mixture increases the potential range in both the basic and acidic direction. A major reason for the enhanced potentiometric break that is observed for a base-strong acid titration upon adding Ac20which is less basic and acidic than HOAc to the HOAc solvent is credited to the increased potential range of the solvent mixture. When the AcnO concentration is greater than 50%, the solvent mixture is a better differentiating solvent for strong acids than methyl isobutyl ketone or HOAc. The stoichiometry for the titration of several bases is examined and a specific side reaction for unsubstituted amides is discussed. Titrants composed of TNBS or HCIO, in 90% Ac20-10% HOAc are compared for the titration of a wide variety of bases in the same solvent mixture. Conductance data support the suggested order of acidity for the acids. Measurements in dioxane-Ac20 mixtures are also reported.
THEPOPULARITY of acetic anyhydride (AczO) as a solvent for nonaqueous titrations has increased in recent years and can be traced t o the fact that the solvent permits titration of a variety of weak bases which are not easily titrated in other solvents. In the early studies mixed solvents of the type acetic acid (H0Ac)-AczO (1) and nitromethane-AczO (2) were used. More recently the tendency has been toward using pure A c 2 0 o r A c 2 0 containing small amounts of HOAc. In the majority of the cases, HC10, was the acid used for the titrant and the solvent has been pure HOAc, mixtures of HOAc-Ac,O, or dioxane. Although all authors that have tried it report extensive color formation upon using HC10, in Ac,O, they differ in their opinions in regard to the titrant’s stability. Therefore, this latter titrant mixture has not been generally recommended. Many compounds which exhibit no or very weakly basic properties in water or the common solvents for nonaqueous titrations can be titrated in AczO. F o r example, amides, certain amines and ureas, and many organic and inorganic salts can be titrated (3, 4). One of the major limitations in its application is the high reactivity of AczO with some of the bases t o produce a n acetylated product of weaker basic character. But even in some of these cases the product is still basic enough t o be titrated (3, 4). The acetylation side reaction is further complicated by the fact that it is catalyzed by the strong acid in the titrant. Wimer has used AczO as the solvent in the titration of sulfoxides (5) and amine and phos(1) A. F. Gremillion, ANAL.CHEM., 27, 1837 (1955). J. S. Fritz and M. 0. Fulda, Zbid., 25, 133 (1953). C. A. Streuli, Ibid., 30, 997 (1958). D. C. Wimer, Ibid., p. 77. Zbid., p. 2060.
(2) (3) (4) (5)
phine oxides (6). Titration data for additional varieties of the latter two compounds (7, S), quarternary salts (9), nitrogen bases in crude oil ( I O ) , and various carbonyl derivatives (11) in AczO have also been reported. Indicators have been studied (12) and weak bases coulometrically titrated (13, 14) in Ac20. Although the method has been recommended for amides, Berger and Uldall (15) recently reported that a systematic error occurs in the titration of amides. They also note that the error changes with titration time and were unable to experimentally overcome the problem. Work in this laboratory involving potentiometric and indicator measurements and their catalytic effect in acetylation reactions has illustrated the highly acidic character of certain sulfonic acids in nonaqueous solvents (16-18). Consequently, it was of interest t o see if the advantages of sulfonic acids could be utilized in AczO and still retain the highly acidic nature of the HCIOI titrant. The only report to the author’s knowledge is the use of p-toluenesulfonic acid in Ac,O for the photometric and visual titration of weak bases (19). Additional data on the properties of sulfonic and perchloric acids, acidic strength of sulfonic acids, and titration stoichiometry in A c 2 0 and mixtures with HOAc or dioxane are reported. EXPERIMENTAL
Reagents. Benzenesulfonic (BSA), p-toluenesulfonic (PTS), p-nitrobenzenesulfonic (NBS), and 2,4,6-trinitrobenzenesulfonic (TNBS) acid were obtained from Eastman Organic Chemicals. 2,4-Dinitrobenzenesulfonic acid (DNBS) was purchased from Pfister Chemical and Eastman. A general procedure for the purification of the sulfonic acids has been described previously (18). Baker and Adamson was the source of 70 HClO,. Tributylarnine (TBA) from Eastman and “Analyzed Reagent” pyridine (Py) from J. T. Baker Chemical Co. were used as received. Caffeine (CAF) was recrystallized several times from ethyl alcohol-water mixtures. Other inorganic salts and organic compounds were obtained from readily available sources in good grades and used as received in most instances. Glacial acetic acid (HOAc) and acetic anhydride (>99x) (Ac20) were obtained from J. T. Baker Chemical Co. and
x
(6) Zbid., 34, 873 (1962). (7) C. W. Muth. R. S. Darlak. W. H. English. and A. T. Hamner, Zbid., p. 1163. (8) J. R. Parker and C. V. Banks, Zbid., 36,2191 (1964). (9) M. E. Puthoff and J. H. Benedict, Zbid., p. 2205. (10) I. Okuno, D. R. Latham, and W. E. Haines, Ibid., 37,54(1965). (11) D. B. Cowell and B. D. Selby, Analyst, 88, 974 (1963). (12) 0. W. Kolling and T. L. Stevens, ANAL.CHEM., 34,1653 (1962). (13) G. Durand and B. Tremillon, Bull. SOC.Chim.France, 1963, 2867. (14) W. B. Mather, Jr., and F. C. Anson, ANAL.CHEM., 33, 1634 (1961). (15) J. Berger and I. Uldall, Acta Chem. Scand., 18, 1311 (1964). (16) D. J. Pietrzyk and J. Belisle, ANAL.CHEM., 38, 969 (1966). (17) Zbid., p. 1508. (18) J. Belisle, Dissertation Absfr., 27, 1383B (1966). (19) T. Jasinski and Z. Szponar, Chem. Anal. (Warsaw), 10, 619 (1965). .
I
VOL. 39, NO. 12, OCTOBER 1967
1367
Eastman, respectively. For the measurements in 1 :1 AczO-HOAc, 97% ACZO from J. T. Baker was used. A small amount of ACBOwas added (-0.05z by volume) to the HOAc solvent and allowed to stand several days before using the HOAc solvent. (The slow rate of reaction of A c 2 0 with water can be speeded up by the addition of a trace amount of strong acid catalyst.) Dioxane was refluxed with sodium metal until a shiny surface was obtained, and then distilled. Both operations were done under Nz. All solvent mixtures are reported in per cent by volume. Instruments. The potentiometric titrations were performed manually with the Precision Scientific Dual Titrometer employing a glass electrode and a sleeve type calomel electrode. A 0.1M LiC104 solution made from the anhydrous salt in Ac2O was used as a replacement for the saturated KCI in the latter electrode ( 4 ) . The electrodes were stored in AczO when not in use and the LiC104 solution was replaced each day. End points in the Karl Fischer titration were detected amperometrically with dual platinum electrodes. Sodium tartrate dihydrate was used for standardization, and methyl alcohol of low water blank was the solvent. Water contents for TNBS, DNBS, NBS, BSA, and PTS were in the order of 10% by weight. Conductance measurements were made with the Conductivity Bridge Model R C 16B2, Industrial Instruments, Inc., using an immersion type conductivity cell with a cell constant of 0.107 cm-'. The electrodes were soaked in A c 2 0 for 1 hour prior to their use. No effect on the cell constant was observed. Procedure. The purity and waters of hydration for the sulfonic acids were established by a combination of aqueous and nonaqueous potentiometric titration with a standard basic titrant and Karl Fischer titration. The procedure for determining half-neutralization potentials, HNP, of the sulfonic acids was to weigh out the acid into a tall-form 185-ml electrolytic beaker, correct the weight for water content, and dissolve the acid in 50 ml of the respective pure or mixed solvent. The basic titrant was prepared by dissolving accurately weighed pyridine, usually O.lN, with the same pure o r mixed solvent in a volumetric flask. Electrodes were inserted and the titrant was added from a 10-ml buret under N2 with end points occurring a t 5 to 6 ml. In the cases where the Ac20 concentration was high, several minutes were allowed between additions of the titrant because of slow equilibrium. This was most noticeable in the vicinity of the end point. The data were graphed and the mid point of the titration was determined from the graph, From the titration curve, the potential break is also determined.by taking the potential difference between the values at 2~0.50ml from the end point. Care was exercised in obtaining weights of the different acids which correspond to nearly identical equivalents and, thus, the effects on H N P that can occur for different end point volumes were minimized. Procedures for the measurement of H N P and potential breaks for CAF, Py, and TBA were essentially the same as the above. The difference was that the organic bases were accurately weighed and dissolved in the appropriate pure or mixed solvent in the electrolytic beaker and titrated with a standard solution of the sulfonic acid, usually 0.1N, in the same pure or mixed solvent. Since the weights of the sulfonic acids (corrected for water) and bases were accurately measured, a double check on the stoichiometry of the titration was possible. Once a group of titrations of the series of acids or bases were started, they were carried out to completion. Undesirable electrode effects were thus minimized. The data reported represent an average of at least two independent series of titrations. N o precipitation occurred in either the titration of the sulfonic acids or of the bases. F o r the routine titration of the various basic compounds, a procedure similar to the one for H N P measurements was 1368
ANALYTICAL CHEMISTRY
used. The acidic titrants employed in these studies are listed in Table VI and were standardized by titration of potassium acid phthalate. DISCUSSION
The increased potential break that was observed upon the addition of acetic anhydride when titrating C A F with HCIOa was attributed to the removal of water and subsequent conversion of H3+0.C l o d - to the more acidic species, Hz+OAc. C104-. I n contrast mixed anhydride formation-between AczO and the acid was thought to be the reason for increased acidity (20). Although these are significant possibilities, it appears that other more important factors are involved. Acetic acid has a n autoprotolysis constant of 3.5 X 10-15 (25"C), a dielectric constant of 6.13 (20°C), a specific conmho a t 25°C (depends on puductance of about 6.0 X rity), and exists as a dimer. In comparison, A c 2 0 has a higher dielectric constant (20.5 at 25"C), a larger specific conductance mho at 25"C), and does not exist in a dimer form. (1.94 X These latter constants along with a wide variety of experimental observations, which have been discussed and reviewed in more detail (11, 21) can be rationalized by assuming the existence of reaction I. In contrast, the exchange experiments of Evans, Huston, and Norris (22)
-
0
0
//
//
e CH3C'
(CH3C-)20
0
//
+ CHIC-0-
(1)
d o not support the dissociation reaction. Although dehydration, changes in dielectric constant, or formation of solvated proton contribute to the enhanced potentiometric breaks observed in the titration curves of amides in AczO in comparison t o dry HOAc, it is suggested that additional acidic species are the principal contributors (4, I I , 2 1 ) . I n HOAc the acidic species is H2+OAc.C1O4- and under these conditions amides or other very weak bases will not be titrated, Upon the addition of A c 2 0 the observed enhancement in the titration break is attributed to the newly formed acidic species suggested in equilibrium reactions (11) and (111) (as perchlorate salts). CH3COZH2+
+ (CH3CO)?O S (CH3C0)20H+
+ CH3COOH
(11)
The higher the AczO concentration is, the greater the concentration of CH3CO+ while the greater the HAC concentration, the greater the concentration of the CH3COZHi species; the acidity decreases in the following order: CH3CO+ > (CH3C0)20H+ > CHsCO2Hi For the titration of an amide, then, the overall acid-base reaction can be viewed by the Lewis acid-base concept (Reaction IV). //O R-C-NH2
+
f CHBCO
*
ClO;
e
(20) J. Russell and A. E. Cameron, J. Am. Ckem.SOC.,60, 1345 ( 193 8). (21) H. A. E. MacKenzie and E. R. S. Winter, Trans. Faraday SOC.,44, 159 (1948). (22) E. A. Evans, J. L. Huston, and T. H. Norris, J. Am. Chem. SOC., 74, 4985 (1952).
Table I. Potential Break for Titration of Several Bases with Acidic Titrants as a Function AczO-HOAc Mixtures Titrant AczO, 7z HCIOI TNBS Potential break, mV CAF PY TBA CAF PY TBA 248 n 277 100 95 204 462 537 186 501 90 170 412 450 164 405 444 80 60 102 295 347 103 304 339 40 20 39 228 258 0 a Very slow equilibrium. * Very poorly defined titration curve break. (I
(I
0 I'
+
I1
R-C-NHZ
*
CHrCO
9
ClO;
(IV)
More recent studies by Muth et al. (7) and Paul et al. ( 2 3 , 2 4 ) tend t o support these suggestions. The former workers isolated the product
0
+? R C l O k 0-C-CHa 9
from the titration of pyridine-N-oxide with HClO, in AcpO; the results might be conveniently explained by the presence of the acetylium ion. The latter workers using a variety of instrumental techniques suggest that 1 :1 and 1 :2 protonic acid:Ac,O addition products are formed in AQO. They suggest the acidic species (V) and basic species (VI) to be T
(CH3C0)20
*
HX
$ (CH3CO)zOH
(CH3CO)zO.B S (CHaCO*B)+
+ X-
+ CH3COO-
(VI (VI)
and neutralization to occur by reaction VII. (CH3CO)2OHf
+ X- + (CHsCO.B)+ + CH3COO- e
(CH3CO),O
-t XBH.(CHsCO)?O e BH+
+ X- + (CH3CO)zO
(VII)
Although there is ample evidence, direct and indirect, to show the existence of the acetylium ion under suitable conditions, it is interesting to note that the recommended titrant, HClO, in dioxane ( 4 ) or HOAc (3-50z)-Ac20, is one in which the conditions are far from optimum for the production of this more acidic species. Therefore, it would seem that the increased potentiometric titration breaks observed in HOAc solutions containing A c 2 0 in large concentration or in AczO are due to the latter solvent having a larger potential range than HOAc. For more general details of the addition of an inert solvent (in this case AczO) to a noninert solvent, see reference (25). In earlier studies in HOAc and methyl isobutyl ketone, it was shown that several substituted aryl sulfonic acids (16(23) R. C. Paul, K. C. Malhotra, and K. C. Khanna, Ztidiurr J . Cliem., 3, 63 (1965). (24) R. C. Paul, K. C. Malhotra, and 0. C. Vaidya, Zbid., p. 1. (25) H. B. Van Der Heijde, Atiul. Cliim. Acta, 17, 512 (1957).
of AczO Concentration in ._
DNBS
-
CAF 115 111 104 90 76
PY 326 335 330 315 288
TBA 526 440 406 314 326
63 26h
250 194
279 226
18) were close in acid strength t o HC104. Therefore, it seemed of interest t o examine whether the sulfonic acids would be capable of exhibiting a similar increased degree of acidity in AczO as HCIOl does. The technique chosen to illustrate this and a t the same time yield information about the properties of AcpOas a solvent was to titrate the sulfonic acids and HC104 with a basic titrant and then d o the reverse, or, titrate several bases with the sulfonic acids and HClOl as titrants. I n both cases the same solvent composition was used for the titrant and sample. From the titration curves half-neutralization-potentials, H N P , and potential breaks were deduced. The HNP is the potential which corresponds to the mid point of the potentiometric titration curve and the potential break is arbitrarily defined as the difference between potentials recorded a t 0.50 ml before and after the end point. Acetic Anhydride-Acetic Acid. In almost all of the reported studies on titration of basic substances in Ac20, a mixed solvent occurs since the base is usually in AcnO and t h e HCIO, titrant is in Ac20-HOAc mixtures or dioxane. F r o m a practical point of view, solvent changes are minor as long as the titrant volume is small and no effect is usually observed in analysis. However, in order to see acidity or basicity changes by viewing H N P and titration potential breaks for a given set of solvent conditions, it is necessary t o maintain the solvent conditions. This was done in these series of experiments. Three bases, caffeine (CAF), pyridine (Py), and tributylamine (TBA) were chosen since they represent a wide range in basic strength, pK, of 1.22, 8.79, and 11.04, respectively. Table I lists the potential break data for the titration of the three bases with the acidic titrants, HCIO,, TNBS, and DNBS, as a function of Ac,O concentration. For each base, the observed potential break increases as the AcaO concentration increases and the acid strength of the acids for a fixed set of conditions appears t o decrease in the order HC10, > TNBS > DNBS. Although a larger potential break would tend to indicate increased acidity, one has to be careful in this case since solvent composition is also varied. I n essence, the potential break defines the difference between a basic-buffered system (prior to the end point) and an acidicbuffered system (after the end point). Therefore, changes in the solvent could affect either of the buffered conditions or both and would then indicate either a stronger degree of basicity or acidity or both. To see the effect of solvent, the half-neutralization potentials were determined and are listed VOL. 39, NO. 12, OCTOBER 1967
1369
Table 11. AHNP for Series of Bases Titrated with Several Acids as a Function of AclO Concentration
Titrant TNBS AHNP, mV5 CAF PY -44
HClOi
AC,O, 70
100 95 90
___
CAF -44 -21 O(450)
80
60 40 20
PY
TBA
b
b
-11 O( 187)
45
-49
- 15
96
54
O(146)
83
O(431)
- 12 O( 179)
83
DNBS TBA - 52 O( 140)
CAF -72 - 53 - 26 O(421) 32
TBA
- 239
- 50
- 34
O(174) 46
- 95 - 52 O( 129) 52
100 113 145
88 115
96c 144 155 a Half-neutralization potentials in parenthesis. Very slow equilibrium. Very poorly defined titration break. 0
in Table I1 in the form of AHNP. (See Figure 4 for typical titration curves.) AHNP is the difference between the H N P a t the given per cent A c 2 0 and the value a t 80% Ac20. The latter was arbitrarily chosen a s the standard since it was a common measurement to all the titrations and thus permits normalization of the data; the HNP data for the chosen standard are listed in parenthesis. As the per cent A c 2 0 in the solvent mixture increases, the AHNP becomes more negative for each base. One concludes then that AcsO is less acidic than HOAc and, therefore, as the concentration of A c 2 0 increases, the leveling power of the solvent mixture toward bases diminishes and the limiting potential on the basic side becomes more favorable. If the AHNP data are plotted
PY
- 73
137 169
against per cent AcpO, a gradual change occurs up to about 60z A c 2 0 and then increases very rapidly. Such influence of one solvent on another in determining the limiting potential range has been observed before (25). To examine the effect of the solvent mixture on the acids, the titration conditions were reversed; that is, the acids were dissolved in the solvent mixture and titrated with standard base. Pyridine was used as the base for the titrant (the same solvent mixture that was used for the acid was used for the titrant) because of its ease in drying and purification, its moderately strong basic strength, and its lack of any apparent side reactions. Since these data for each set of solvent conditions yield relative acidity, several more sulfonic acids were also included. Table I11 lists the potential break data for five aryl sulfonic acids and HC1O4. As the per cent A c 2 0 increases, the potential break increases for only the stronger acids while an apparent decrease occurs for the weaker acids NBS, BSA, and PTS. The latter occurs because of the shape of the titration curve. That is, even though the titration curve covers a wider potential range at higher per cent Ac20, the potential break appears to be less because of a decrease in slope at the end point and more curvature before and after the end point; the arbitrary definition of the potential break therefore causes it t o appear less. The comparisons between the different acids under fixed solvent conditions are more revealing and one concludes from the potential break data that the acidity decreases in the order HClO, > TNBS > DNBS > NBS > BSA > PTS. This is illustrated in Figure 1 where the normalized titration curves for these acids in 100% AcpO are shown.
Table 111. Potential Break for Titration of Several Aryl Sulfonic Acids in AczO-HOAc Mixtures with Pyridine Titrant
Ac20-HOAc ratio, 50-50 Acid
I
20
I
I
I
1
40 60 E0 100 PER C E N T N E U T R A L I Z E D
I
120
1 ;
140
Figure 1. Potentiometric titration of sulfonic acids and HCIO, with pyridine in 100 AcpO 1370
ANALYTICAL CHEMISTRY
HClOi TNBS DNBS NBS BSA PTS
80-20
100-0
Potential break, mV 343 334 293 236 159 153
397 319 329 252 159 157
519 469 345 210 106 96
’
Table IV. AHNP for Several Arjl Sulfonic Acids as a Function of Per Cent Ac?O Using a Pyridine Titrant Ac?O-HOAc ratio. “; 50-50 80 20 100-0 AHNP,-mV Acid -
l ‘ t
HClOi
171
TNBS DNBS NBS
150
BSA PTS
DNBS 0
0.05 CON CENT R AT ION, M/L
I 0.10
Figure 2. Conductivity data for stronger acids in 95 % Ac20-5 % HOAC The same conclusions regarding the acid strength of the aryl sulfonic acids and HClO, and effect of the solvent composition can be made from the AHNP data which are reported in Table IV. For these data the H N P values for BSA are designated as the standard and the others made relative to it by subtraction of the H N P for BSA from the H N P of each of the other acids, It is clearly seen from these data that a s the per cent AcpO increases, the leveling power of the solvent mixture toward acids diminishes and, consequently, the limiting potential becomes more positive. If AHNP is plotted against per cent Ac20, the largest change in the curve occurs above 6 0 z A c 2 0just as in the case of the titration of the bases. Since the potential range o n both the basic and the acidic side of the solvent mixture is extended by the addition of the inert solvent AcsO t o HOAc, one can then ascribe part or all of the increased potential break to the nonleveling solvent properties of Ac20. It is interesting t o note that TNBS is considerably stronger than DNBS. In previous work in this laboratory (16, IS) on the relative acidity of these two and other sulfonic acids in methyl isobutyl ketone and acetic acid, it was shown that TNBS and DNBS were very strong acids and nearly as strong as HCIO,. TNBS appeared to be only slightly stronger than D N B S in contrast to the data reported here for Ac20-HOAc solvent mixtures in which the former is much stronger. The reason for this is that the addition of the Ac20, which is an inert solvent and thus much less basic than HOAc, results in a mixture which is more weakly basic than the pure HOAc. Greater differentiation between the stronger acids occurs and as the per cent AcfO increases, the degree of differentiation increases. This is more obvious if one plots the AHNP for NBS, BSA, and PTS against their respective Hammett sigma value as was done for eight m- and p-substituted benzenesulfonic acids in methyl isobutyl ketone and HOAc (16). When comparing the slopes of the lines, the order of decreased slope occurs in the order 100% AcpO > 80% A c 2 0 > 50% A c 2 0 2 methyl isobutyl ketone > HOAc and, consequently, the solvents containing higher concentrations of A c 2 0 are better differentiating solvents for strong acids. Additional evidence which supports the order of acidity is reported in Figure 2 where conductivity data for HCIO,,
122
25 1 209 161
78 0
85 0
- 20
- 38
424 364 244 144 0 - 10
TNBS, and DNBS are plotted as a function of acid concentration in 95% A c 2 0 - 5 z HOAc. Since the acids contain small amounts of water as water of hydration, no attempt was made t o measure the conductivity a t 100% A c 2 0 ; the water reacts with A c 2 0t o form HOAc. Color formation of HC10, solutions in high concentration of A c 2 0 has been reported before ( 4 , 11). It was noted in the course of preparing the solutions in these studies that the rate of color formation occurs as a function of acid strength and AcpO concentration. When the AcyO concentration was 80% or greater, color formation occurred and as the A c 2 0 concentration got larger, the rate of formation was faster. For example, in 100% A c 2 0 , the solution darkens almost instantaneously upon adding Ac?O to a HClO, sample; whereas with the weaker acid, DNBS, several minutes are required before the color is discernible. Qualitatively, the rate of color formation follows the acid strength. No apparent deviations in stoichiometry were observed upon titrating the acids under these conditions with standard base. It is also interesting to note that the color formation was observed to gradually disappear as the end point was approached. Dioxane-Acetic Anhydride. Several experiments were attempted using dioxane-AcyO mixtures as the solvent since it has been suggested that HC10, in dioxane is a better titrant for amides than in HOAc-Ac,O mixtures ( 4 ) . Using large amounts of dioxane, which lowers the dielectric constant, however, has little advantage if potentiometry is used for end point detection because of greater difficulty in obtaining reproducible potentials. Initial titrations of the acids, HClO ,, TNBS, and DNBS, with standard Py titrant were carried out in a 50-50 mixture of dioxane-Ac?O. Extensive color formation occurred almost immediately with the stronger acids and comparatively slower with the weaker acids. Upon titration, electrode response was not clearly defined, color formation in part disappeared, and stoichiometry appeared to be affected. Useful H N P or potential break data were difficult t o obtain. In a very qualitative manner, however, acidity of the acids seemed to be close to that obtained in 100% A c 2 0 . Titrations were then attempted with the same acids in 1 :4.5:4.5 and 2:4:4 HOAc:AcnO:dioxane mixtures. Color formation was not observed and most of the adverse effects were not present. Also, increased potential breaks in comparison to corresponding HOAc :Ac20 mixtures was not observed and, therefore, further studies were not attempted. Titration Stoichiometry. Berger and Uldall (15) have suggested that a systematic error occurs in the titration of unsubstituted amides using the procedure suggested by Wimer (4). Since this method is suggested as a general method for the analysis (26), it was of interest to examine this further. (26) “Handbook of Analytical Chemistry,” L. Meites, Ed., McCraw-Hill, New York, 1963, pp. 12-135. VOL. 39, NO. 12, OCTOBER 1967
1371
'I I
I
4.0
5.0 VOLUME, MI
I
6.0
Figure 3. Potentiometric titration of acetamide with TNBS in 90% AczO-lO% HOAc as a function of titration time A.
10 minutes
B. 20 minutes C . 30 minutes
Berger and Uldall found that upon titrating a pure unsubstituted amide, the measured per cent purity decreased with titration time. It was suggested that the amide undergoes a side reaction presumably acetylation, in which the product is no longer basic. If this is the case N- or N,Ndisubstituted amides should be less affected a t least under these conditions. Table V lists titration data in the form of per cent purity for acetamide, N-methyl propionamide, N,Ndimethyl formamide, and pyridine titrated with TNBS or HC104 in 90% Ac20-10% HOAc as a function of titration time, The 10-minute time corresponds to reaching the end point a t about 10 minutes. For the 20- and 30-minute time periods, the titration was carried to about 75-80z of theoretical completion, allowed to set for the appropriate time, and
Table V. Per Cent Purity for a Series of Bases Titrated with HCIOz and TNBS in 90% AczO-lO% HOAc as a Function of Time Purity, Z Base Titration time, minutesHClO4 Titrant 10 20 30 78.9 73.4 88.9 AcetamideR N-Methyl propionamide 100.9 101.6 100.0 100.0 100.1 N,N-Dimethyl formamide 99.4 101.o 99.4 Pyridine TNBS Titrant 98.7 96.0 88.7 Acetamidea N-Methyl propionamide 100.8 98.2 100.0 99.7 99.8 100.4 N,N-Dimethyl formamide 99.7 100.3 Pyridine a A similar result was found for hexanamide.
1372
ANALYTICAL CHEMISTRY
J
I
I
50
100 PER C E N T NEUTRALIZED
150
Figure 4. Potentiometric titration of a variety of bases with TNBS in 90 AczO-10 % HOAc
z
A . Tributylamine B. Pyridine C . Nicotinamide D. KI E. Caffeine
F. G. H. I.
N,N-Dimethyl p-nitroaniline Dimethyl formamide Acetanilide Triphenylamine
then, the titration was continued. In the case of the unsubstituted amide, this waiting period was marked by a n abrupt rise in potential; indicates a decrease in concentration of the base. Since aliquots of a stock solution of each base were used, the comparison of per cent purity is easily made. One concludes from the data that the side reaction is specific for the unsubstituted amide under these conditions. A similar result was also found for hexanamide. I t is possible, however, that under longer periods of time, heat, or in different reagent mixtures, the substituted amide will also react. The effect of time is clearly illustrated in Figure 3 where titration curves of acetamide are shown as a function of titration time. The most likely side reaction is acetylation of the amide; for acetamide the product would be diacetimide. Even though the data in Table V for acetamide and hexanamide suggest that the side reaction is faster for the HCIOi titrant than for the TNBS titrant, a more detailed study would be required before concluding this generalization. Applications. In order to illustrate the use of TNBS a s a strong acid titrant, a wide variety of additional weak t o strong bases of different types of functional groups and salts were titrated in the solvent mixture, 90% Ac20-10% HOAc. The results in terms of per cent purity are listed in Table VI and compared to data obtained for the HCIOa titrant. Several titration curves for the TNBS titrant are shown in Figure 4. Although reasonable results could be obtained for the unsubstituted amides, the method is not recommended for these bases unless extreme care is taken in controlling the titration time. Nicotinamide, which contains a pyridinelike N and an unsubstituted amide, does not undergo the
1
TNBS
y
/
VOLUME ACID Figure 5. Titration of a fixed weight of KHP as a function of titrant storage time in 90 % Ac20-10 HOAc ( T = equivalence point) A . 2 hours B. 1 day C . 2 days
E.
5 days
7 days G . 14 days
F.
D. 3days side reaction. I n fact, the base is titrated with one equivalent of acid. Titration of several of the salts was handicapped by low solubility of the base in the solvent mixture. Acetanilide barely gave a perceptible titration curve while triphenylamine was too weak a base to be titrated by either acid. The potential breaks for the HC104 titrations are on the average about 40 mV larger than for the TNBS titrant. But, only in the case of acetanilide does the small difference in potential break hinder the selection of the end point. N o difficulty was encountered in titrating the N-, P-, or S-oxide type compounds with the TNBS titrant and other derivatives which have been titrated with HCIOa titrant (5-8, 27) should also be titratable. As can be seen in Table VI, good stoichiometric results, which are averages of a t least two titrations, although not quite as good as for the usual nonaqueous acid-base titration, are obtained. The stability of the HC104 titrant in AczO solvent has been questioned before and a gradual decomposition with time was suggested t o take place. Consequently, even though
(27) K. K. Anderson, W. H. Edmonds, J. B. Biasotti, and R. A. Strecker, J. Org. Chem., 31, 2859 (1966).
Table VI. Titration Data for Several Bases in 90 % AczO-10 HOAc with 0.1N TNBS and HCIOl as Titrants in 90 Ac20-10 HOAC - Purity, % Bases TNBS "2104 100.2 Tributylamine 98.9 Trihexy lamine 99.7 98.5 Acetanilidea 91.5 97.6 N,N-Dimethyl p-nitroaniline 102.3 100.2 Valeranideb 103.0 99.0 104.3 Hexanamideb 99.4 4-Picoline N-oxide 101.2 101.3 Dimethylsulfoxide 103.0 102.7 Trioctylphosphine oxide 101.8 103.6 96.3 Thioacetamide 99.7 100.6 Nicotinamide 100.2 Brucine 98.6 97.0 Caffeine 98.2 99.7 98.5 Strychnine 99.9 p-Phenol sulfonic acid sodium salt 101.7 99.7 KI 99.7 98.8 NaC2H302 99.2 101.8 KCP 95.5 92.8 KNO3' 98.6 96.8 a Poorly defined titration curve. Controlled titration time of 5-6 minutes. Low solubility for the base.
100% AcsO gives the largest potential break, the HCI04 titrant was made in HOAc, and the authors designed the experiment in such a way that only 10% HOAc was introduced into the sample-Ac20 mixture a t the end point ( 4 ) . The titrants used on these studies were made in 90% A c 2 0 - 1 0 x HOAc, and it was observed that the usefulness of the HCIOl titrant was not nearly the same as for the TNBS titrant. Both titrants darkened, the HC10, at a faster rate, with time and a reaction o r decomposition took place with the apparent formation of weaker acidic species. Conclusions about the normality, although it decreased, are difficult to make because of the change in the shape of the titration curve. The aged titrant causes a decrease in the steepness of the potential break and a large increase in the curvature of the titration curve just past the end point. This is illustrated in Figure 5 where KHP is titrated with TNBS and HCIO, as a function of titrant storage time (calculations based on the normality measured initially after preparation of the titrant). The TNBS titrant can be stored for up to five days with a change of about 1 in normality taking place. O n the other hand, the HCIOa titrant changes by 10-15z in one day and, consequently, must be made fresh daily and standardized often during its use.
RECEIVED for review April 10, 1967. Accepted June 26, 1967. Division of Analytical Chemistry, 154th Meeting, ACS, Chicago, Ill., September 1967. Research supported in part by the National Science Foundation.
VOL. 39, NO. 12, OCTOBER 1967
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