relatively slight reactions (which may,
at least partly, be attributable to
Table I. Relative Response of Various Compounds on Molar Basis
Compound Formaldehyde Acetaldehyde Propionaldehyde Butyraldehyde n-Valeraldehyde Glyoxylate Chloral hydrate Glucose Acetone Pyruvate a-Ketoglutarate Benzaldehyde
aliphatic-aldehyde impurities). Spectra of Products. T h e absorption spectra of t h e reaction products derived from glyoxylate and the first five straight-chain aldehydes, Figure 4, exhibit a single peak in t h e visible region, with a maximum between 430 and 435 mp. The similarity of these spectra suggests that the various products are structurally similar; additionally, the spectra closely resemble the one given by the product of the reaction of o-aminobenzaldehyde with glutamic 7-semialdehyde ( 5 ) . The wavelength (440 mp) recommended for the present procedure gives adequately low reagent blanks, and yet is not too far removed from the absorption masima of the products.
Absorbance 0.96 1.00 0.92 0.99 0.81 0.88 0.02 0.01
0.01 0.09 0.05
0.09
ployed because it is readily water soluble, available a t reagent grade purity, and relatively inexpensive. The effect of methylamine hydrochloride concentration on the course of the reaction is illustrated in Figure 2. I n this experiment, 0.10M sodium pyrophosphate was included in the reaction mixtures. Since the concentration of methylamine hydrochloride has a n effect on the pH, adjustments to p H 8.4 were made. On the basis of the data obtained, a concentration of 1.0-11 methylamine hydrochloride in the final reaction mixture was selected as adequately effective. The effect of o-aminobenzaldehyde concentration on the reaction can be seen in Figure 3. These results led to the choice of a Concentration of 0.004M o-aminobenzaldehyde in the final reaction mixture. Although higher concentrations of this compound can yield increased reaction rates and extents of reaction, the concentration chosen gives satisfactorily low blank readings, and moreover is not wasteful of the material. Response of Illustrative Compounds. The relative response of
LITERATURE CITED 400
450
WAVE
500
550
L E N G T H , rnp
Figure 4. Absorption spectra of products derived from various aldehydes used at appropriately staggered concentrations 1. 2. 3.
Sodium glyoxylate Formaldehyde Acetaldehyde
4. 5. 6.
(1) Schopf, C., Komzak, $., Braun, F., Jacobi, E., Ann. 559, l ( 1 9 4 8 ) . (2) Schopf, C., Oechler, F., Ibid., 523, 1 i1936).
i..
558.
Propianaldehyde Butyraldehyde n-Valeraldehyde
several aliphatic aldehydes having free functional groups and t h a t of certain other carbonyl compounds or their derivatives are listed in Table I. The straight-chain aldehydes from CI to Ca and also glyoxylate respond comparably. Chloral hydrate or glucose, in which the aldehyde function is not free, or acetone, the lowest aliphatic ketone, is virtually inert. Benzaldehyde, as a representative of the aromatic series, and the keto acids, pyruvate and a-ketoglutarate, give
RECEIVED for review November 28, 1961. Accepted January 10, 1962. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. Based on material from a dissertation submitted by Alberta M. Albrecht in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Rutgers, The State University, June 1961. Alberta Albrecht acknowledges a predoctoral traineeship under U. S. Public Health Service Training Grant 20-507. Work was aided by a contract between the Office of Naval Research, Department of the Tavy, and Rutgers, The State University, and by research grants from the Damon Runyon Memorial Fund and the U. S. Public Health Service.
Evaluation of Amides and Other Very Weak Bases in Acetic Acid TAKERU HIGUCHI, CHARLES H. BARNSTEIN, HOSSEIN GHASSEMI,
and WALDO E. PEREZ
School of Pharmacy, University o f Wisconsin, Madison, Wis.
A modified version of the Type II plot proposed earlier has been developed for bases so weak that they exist to a significant extent in their free forms in the presence of excess perchloric acid in acetic acid. The relative basicities of compounds studied ranged over five orders of magnitude with dimethyl acetamide as the strongest and diethyl ether as the weakest measurable. Extremely precise titrations are possible for the more basic
400
ANALYTICAL CHEMISTRY
amides. Less basic compounds such as acetanilide are, however, determinable with less precision. Basicity data are presented for 40 different compounds.
D
that titration of various weak bases in acetic acid has been studied extensively for a number of years, its application to systems essentially nonbasic in water has been relatively limited. This communicaESPITE THE FACT
tion is concerned with results of an investigation designed to show some of the practical and theoretical limitations of this technique. Although the applicability of the method to both qualitative and quantitative determination of amides has been particularly stressed, a number of other functional groups have also been investigated as a part of this study. The basic relationships governing interactions between acids and bases
I c LINE x- Cn
A
LINE
Cl0 ,
B
--+
X
Figure 2 . Plots for photometric titrations of chloroacetamide with 0.4926N perchloric acid Indicator i s Sudan 111 Line A, modified Type II plot Line B, simple Type II plot
Figure 1 . Comparison of modified and conventional Type I I plots
in acetic acid and the use of photometric titration plots have brlen pointed out (4-6). Conventional Type I1 photometric titration plots are based on the following relationship ( I )
where S is the amount of acid added at a n y point, S is the amount of acid equivalent to the total base in solution,
K,, the indicator exchange constant ( I ) , 1
and f the ratio of indicator base to a indicator acid. Upon consideration of the normality of standard perchloric acid titrant and the total solution volume, bhe amounts X and S are readily determined from the volumes of standard acid added. L-nmodified Type I1 plots do not yield satisfactory straight lines for very n-eak bases because of the high degree of solvolysis present even in the presencc of escess perchloric acid. This problem was solved in one manner by Connors and Higuchi ( I ) by plotting photometric differences between results olitsined for high initial base concentration and those for low base concentration. This approach also obviated to some extent difficulties presented by the presence of variable quantitim of n-ater. For the present study i t was fclt that a simpler technique based on complete elimination of water by addition of a known sniall excess of acetic anhydride in the presence of free perchloric acid permitted more straightforward interpretation. Thus in sy5tenis ivhrre a significant extent of solvol~ added concentration of perchloric acid solution, would be X'
=
+
C B H C ~ OC~H C I O ~
Anti S',the total initial base concentration ivould. on addition of the mineral acid. be
+
8' = CBHCIO~CB
I t iy apparent then that Equation 1 must be modified for these systenis to
tions performed in this study was a solution of 0.25.11 redistilled acetic anhydride in acetic acid which had been purified by the method of Eichelberger and LaNer ( 8 ) . Since many substances known to possess basic properties in acetic acid react with the anhydride, only compounds having
Whereas K,, may be obtained directly from the slope of the linear plot of Equation 1, it is necessary t o know C H C ~ O , to plot the data according to Equation 2 . The concentration of the free acid, Table 1. Basicities of Amides and CHCIOa, may be determined from the Other Organic Compounds in Acetic indicator color and indicator constant Acid Relative to Sudan 111 obtained from titration of a blank. Since the indicator is the only base IC,, Compound titrated in a blank solution, the C H C ~ O ~S,.l--Dimethylacctaniide 0.011 value corresponding to a given indicator 0.014 l--\cetylpiperidine 0.019 ratio can be determined from the inAV,S-Dimethylcaprylaniide 0.019 S,S-Dimethylcapraniide dicator perchlorate formation equilib0.020 .V,A17-Dimethyllauramide rium 0.020 AV,.l~-Dimethylmyristaniide 0.023 iY-Methplacetamide I HClOa eIHClO4 (3) 0.024 S,S-Dimethylpalmit amide 0.10 Acetamide 0.11 Lauramide 0.12 S,A7--Dimethyl-2-naphthamide 0.13 S,S-Dimethylbenzamide 0.13 Pelargonamide 0.13 A7-hfethylformaniide 0.11 S.S-Dimethvlformamide 0.14 2-Pvrrolidone When (Ia/Ib) n-as plotted us. G"CIO, in 0.20 .Y,A--Dimethyl-l-naphthamide the blank solution, a straight line passing 0 . 31 2,6-Diniethylacetanilide 0.37 through the origin (8) as predicted by S-Methylhenzamide 0.53 Formamide Equation 5 was obtained. It is evident, 0.61 Acetanilide therefore, that the amount of perchloric 0.64 .I\cetvl-~-naphth?-lallline acid consumed in indicator perchlorate 0.64 -\cetyl-p-naphthylnniine formation is negligible; thus values of 0.84 Benzamide 0.8t1 p-Saphthamide Caclo, corresponding to various in1. G a-Saphthamide dicator ratio values are obtainable from 1.8 p-Chloroacetanilide blank titration plots for insertion into 2.6 m--4cetamidobenzoic acid Equation 2 . I n the absence of water 2.7 ita-Chloroacetamide 'L!I p-hcetamidobenzoic acid linear plots of Equation 2 yield repro16 2,4,6-Tribronioacetanilide ducible exchange constants for very 18 Benzanilide weak bases with Sudan 111 indicator 18 Chloroacetamide in acetic acid. 1400 Diethyl ether >500 Kicotinamide (amide S : Because water behaves as a base in >500 Phenobarbital acetic acid, its presence in the solvent >2000 ;\cetonitrile system is undesirable since i t interferes >2000 Dichloroacetamide with the interaction between a weak >2000 Trichloroacetamide Chloroamphenicol (acetylated) >2000 base and the reference acid. The solvent nhich was employed in all titra-
+
VOL. 34,
NO. 3, MARCH 1962
401
Table II.
Comparison of Effect of N-Alkylation on Basicities of Several Amides and Ammonia
K,, Values R
0.10
R-NH2 R-NHCHs R-N( CHs)i
0.023
0.011
0.53 0.13 0.14
0.11
0.84 0.37
...
0.13
0.02
nonacylable basic functions are reported here. EXPERIMENTAL
The apparatus, reagents, and procedure employed in this study were essentially the same as those used previously (1, 7). RESULTS
I n Figure 1 the ratios of the base form of Sudan 111 to its acid form during titration of N,N-dimethylcapramide with approximately 0 . W perchloric acid are shown plotted according to Equations 1 and 2. I n the figure the X values are expressed in terms of actual volume of the titrant added ranging from 2.630 to 3.090 nil. The final volume of the titrated solution was approximately 30 ml. ; the volume change during titration being essentially negligible. It is evident from the curvature exhibited by the upper curve, that even for this relatively strong base a significant and detectable degree of solvolysis exists. The extrapolated end point shown for the simpler relationship is slightly less than 0.04 ml. too large, corresponding to a little more than a per cent error (high) for the unmodified case. The residual end curvature for the corrected plot is probably due to a very small tendency of these amides to take up a second proton. This departure from linearity. hoivever, does not prevent extremely precise determination of the stoichiometric end point. The relative basicities of these stronger bases are reflected in the slopes of the straight line portions of these plots and are nearly the same by either plot. This is not the case, however, for extremely weakly basic compounds. I n Figure 2 the photometric data obtained for chloracetamide are shown again plotted in both ways. I n this instance the conventional plot is in considerable error and correct values can be obtained only from the modified plots. Diethyl ether was the weakest of the compounds which gave evidence of possessing basic properties by this method. The modified Type I1 plot of data for the photometric titration of
402
ANALYTICAL CHEMISTRY
x-
CHCIO,
Figure 3. Modified Type II photometric plot for titration of diethyl ether with 0.4703N perchloric acid Indicator i s Sudan Ill
diethyl ether is shown in Figure 3. While the plotted points elhibit a considerable degree of scatter, their distribution describes a sufficiently straight line to allow for calculation of the exchange constant, K,, = 1400 =t 200. The relative basicities with respect to Sudan I11 of various amides and other organic compounds as determined by these plots are shown in Table I. Since the perchlorate formation constant for Sudan I11 is of the order of TOO ( 6 ) , the perchlorate constants for all the compounds listed can be estimated readily from the table. DISCUSSION
Nonbasic Compounds. Six of t h e compounds i n Table I are evidently nonbasic, with exchange constants with respect to Sudan I11 estimated t o be in excess of 500 or of 2000, depending upon the concentrations of t h e solutions analyzed. Loss of basicity of the amide function of nicotinamide probably follows as a consequence of protonation of the relatively strongly basic amine nitrogen adjacent to it. The apparent nonbasicity of phenobarbital in acetic acid is in agreement with previous findings of Higuchi and Connors ( 4 ) . Dichloroacetamide and trichloroacetamide show
1.6
0.86
0.2
0.12
...
...
1.8 x 10-5
4 . 4 x 10-4 5.0 x 10-4
no evidence of poswssing basic properties, due presumably to the inductive effects of the adjacent chloro substituents. This same polar effect is probably responsible in part for the lack of basicity of acetylated chloramphenicol, which is a mono N-substituted dichloroacetamide. Acetanilide Series. Substitution of the negative groups, -C1 and -COOH, in the meta and para positions of acetanilide produced a marked decrease in t h e basicity of the parent compound as expected. Hammett substituent constants, u, obtained from the literature ( 3 ) , were observed to vary linearly with the logarithms of the exchange constants, yielding a specific reaction constant, p , of 1.4 =t0.1. 2,6-Dimethylacetanilide ( K e x= 0.31) is a stronger base than acetanilide (ITex = 0.61), while 2,4,6-tribromoacetanilide ( K e x = 18) is, relatively, a very weak base. These differences suggest that substituents on the ring influence the basicity of the anilides significantly but relatiwly weakly. The increased ability of the orthodimethylated compound t o attract perchloric acid may be partially ascribable to steric interferrnce ton-ard coplanarity of the ring with the resonating amide function. Effect of .Y-Alkylation. The influence on amide basicity of methyl substitution on the amide nitrogen is also evident from Table 11. I n the series of formamides and acetamides it is evident t h a t methyl substitution of one hydrogen of the amide resulted in markedly enhanced basicity whereas similar substitution of the second hydrogen produced little further change in basicity. This effect more or less parallels the changes in the basicity of the parent amine as +own in the last column of Table 11. Effect of Aromatic Substitution. Substitution of a n aromatic group reduced the basicity of t h e amide function by yirtue of its tendency t o a t t r a c t the unshared electrons of t h e amide nitrogen and oxygen. Acetan ilide, acetyl 0-naphthylamine and acetyl 0-naphthylamine appear to possess unusual base strength relative to acetamide, however, if comparison is made
with basicities of the amines which are obtained on hydrolysis. Ammonia is a t least 50,000 times more basic than aniline or the naphthylamines, but acetamide is only six times more basic than acetanilide or the N-acetyl naphthylamines. This may be due a t least in part to the lack of coplanarity between the ring system and the resonating amide plane.
LITERATURE CITED
(1) Connors, K. A., Higuchi, T., ANAL. CHEM.32. 93 (1960).
”,
1940. (4) Higuchi, T., Connors, K. A., J. Phys. Chem. 64, 179 (1960). ( 5 ) Higuchi, T., Feldman, J. A., Rehm, C. R., ANAL. CHEM.28, 1120 (1956).
( G ) Kolthoff, I. M., Bruckenstein, S. J. Am. Chem. SOC.78, 1 (1956). (7) . . Rehm, C., Bodin, J. I., Connors, K. A,, Hieuchi’. T.. ANAL. CHEM. 31. 483
(1g59). ’ ’ (8) Rehm, C. R., Higuchi, T., Zbid., 29, 367 (1967). RECEIVED for review September 5, 1961. Accepted December 18, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., September 1961.
Microdetermination of Carbon and Hydrogen Using Nondispersive Infrared and Thermal Conductivity Analysis J. A. KUCK, J. W. BERRY, A. J. ANDREATCH, and P. A. LENTZ Microanalytical and Instrument Groups, American Cyanamid Co., Stamford, Conn.
b An instrumental method has been developed for the rapid determination of carbon and hydrogen in 2-mg. samples. The conventional combustion tube i s retained but instead of weighing the gaseous combustion products, the combustion gas mixture i s collected to a constant volume and adjusted to a standard reference pressure for analysis. Carbon i s determined as carbon dioxide with a nondispersive, infrared gas analyzer, and hydrogen i s determined as the gas with a thermistor-type thermal conductivity cell. The hydrogen which i s equivalent to the Chemically bound -H in the sample i s obtained b y passing the water vapor from the combustion over calcium hydride. This new C and H method i s applicable to sample weights in the range of 4.0 to 0.5 mg. and i s operable with as little as 200 pg. of material. Trace analysis for carbon and hydrogen i s also possible. Satisfactory analyses have been obtained with compounds containing hetero atoms like B, N, P, and S, as well as with samples containing only C, H, and 0. Preliminary experiments have shown that the method here reported i s as good os or better than the classical gravimetric method of Pregl. On the basis o f its scientific principles, it is a strong competitor with other methods for use in possible automation.
A
for micro amounts of COZ and H20 has long been needed t o improve the classical Pregl procedure. Recent thinking along this line has centered on gas chromatography because of its sensitivity and good resolution (2, 18, 20). Attempts to improve the micro carbonRAPID INSTRUXEKTAL METHOD
hydrogen determination by this technique have been inadequate for two reasons-gas chromatography does not currently have the accuracy or precision required; and the thermal conductivities of 02 and COX lie close together. The latter fact precludes the use of an 02 sweep in the combustion tube if thermal conductivity is to serve for the COz measurement. The alternative-ie., burning the sample in helium by oxidation with inorganic oxidants in order to measure COP in a more favorable carrier gas-is not attractive from a chemical point of view. Electroconductimetry is another new approach. Malissa (13) determined carbon and sulfur by measuring the change of conductivity produced by COz and SO2 in suitable absorbing solutions. Gel’man and coworkers a t the University of Moscow (4, 5 ) have likewise used electroconductimetry to determine COz and H20 in the micro C and H . In this technique, the HzO in the oxygen stream is separated from the CO2 by a dry ice-acetone trap before the latter is adsorbed for conductimetric measurement. The water is subsequently vaporized and converted to an equivalent amount of C 0 2 which is determined in the same manner. This conversion is accomplished by the water gas reaction in which water is converted to CO by passage over carbon a t 1140’ C. The CO is then oxidized to COP with hot CuO. Haber and Gardiner ( 7 ) in their instrumental method employ an electrolytic determination of hydrogen as water. Carbon dioxide from combustion of the sample is chemically converted to water which is similarly electrolyzed, and calculation for carbon is based on the equivalent amount of hydrogen produced in the second electrolysis. Fur-
ther refinement of the method is still in progress in an effort to bring the precision of the carbon analysis within the required limits. This paper describes still another instrumental approach to the carbonhydrogen determination. A rapid micro method involving both infrared absorptiometry and thermal conductivity is utilized. The carrier gas mixture is collected to a constant volume and adjusted to a standard reference pressure for analysis. Instead of being collected in a cold trap, water is converted t o hydrogen by reaction with calcium hydride. The combustion gases are collected in three 100-ml. glass hypodermic syringes. Determination of C 0 2 is made with a nondispersive infrared gas analyzer (1,12, 16, 1 7 ) . This instrument responds only to CO2 molecules. I t is insensitive to NO1 and other combustion gases, which therefore cannot interfere with the carbon dioxide determination. Hydrogen, equivalent t o the water produced in the combustion, is determined by its unusual thermal conductivity. Since thermal conductivity is not a method that is specific for hydrogen, interfering gases such as SO8, SO2, or any of the halogens must be removed. This is accomplished by fixation upon hot silver gauze within the combustion tube. I n burning the sample, a rapid combustion technique (3, 16) is used which combines the “empty tube” arrangement of Korshun (8) or Kuck (10) with the mixed cobaltous-cobaltic oxide catalyst as reported by T‘eceFa (19). In the search for a reliable, rapid technique, examination of the combustion products with a CO nondispersive infrared analyzer indicated the presence of CO when either of the above methods was used separately, VOL. 34, NO. 3, MARCH 1962
403
