stability of the anion results, reflected by a decrease in acidity of the substituted malonic ester. Ethyl cyanoacetate could not be titrated in the presence of its monosubstituted derivatives by Procedure A. If resonance is involved a t all, the anions of cyanoacetic esters would be stabilized by more linear forms,
Table II. Titration of Diethyl (1Methylbutyl)malonate (DMBM) Alone and in Mixture with Diethyl Ethyl( 1 -rnethylbutyl)malonate Wt. of
S a
R
\c/
C
//
Sample, Gram
DMBM, % Present Found
1.00 1.00 1.00 1.00 1.00 0,1004
2.27 5.13 8.27 10.15 12.25 100.
2.27 4.90 8.02 9.84 11.73 98.1
I
0 ’
‘OEt
which would not be subject to as much hindrance by a large R group. If the acidity of cyanoacetic esters is caused largely by inductive effects, then the coplanar requirements of resonance become irrelevant. I n either case, the acid weakening effect of substitution would be less apparent in the
cyanoacetic ester series than in the malonic esters. Table I1 summarizes the results obtained by titrating according to Procedure B a series of weighed mixtures of diethyl (1-methylbuty1)malonate with diethyl ethyl( 1-methylbuty1)malonate. Procedure B has been used successfully in analyzing the following mixtures of esters: diethyl (1-methylbutyl) malonate in diethyl allyl( 1-methylbutyl)
malonate, diethyl isopropylmalonate in diethyl ethylisopropylmalonate, diethyl sec-butylmalonate in diethyl sec-butylethylmalonate, diethyl n-butylmalonate in diethyl n-butylethylmalonate, diethyl isopentylmalonate in diethyl isopentylethylmalonate, and diethyl phenylmalonate in diethyl ethylphenylmalonate. LITERATURE CITED
W. G.. Eberlv. K.. J . Am. Chem. &c. 62,113 (1940j.‘ ’ (2) Fritz, J. S., “Acid-Base Titrations in Nonaqueous Solvents,” p. 31, G. F. (1) Brown. \-I
~~
Smith Chemical Co.. Columbus, Ohlo,
1952. (3) Fritz, J. S.,ANAL, CHEII. 24, 674 (1952). (4) Hammond, G. S.,in “Steric Effects
in Organic Chemistry,” M. S.Newman, ed., pp. 442-54, Wiley, New York,
1956. (5) Pearson, R. G., J. Am. Chem. SOC. 71, 2212 (1949).
RECEIVEDfor review January 6, 1958. Accepted March 31,1958.
Nonaqueous Titration of 2,4-Dinitrophenylhydrazones A. J. SENSABAUGH, R. H. CUNDIFF, and
R. J.
P. C. C.
MARKUNAS
Reynolds Tobacco Co., Winston-Salem, N.
b 2,4-Dinitrophenylhydrazones of aldehydes and ketones can be titrated as weak acids with tetrabutylammonium hydroxide. This provides another simple and rapid method for the identification of this important group of carbonyl derivatives.
R
Fritz, RiIoye, and Richard (6) demonstrated that nitroaromatic amines can be titrated quantitatively as acids in pyridine with tri-ethyl-n-butylammonium hydroxide. Their study included the potentiometric titfation of nitroaromatic compounds that are acidic yet do not have functional groups that are generally considered acidic. 2,4-Dinitrophenylhydrazine, which contains a basic functional group and can be titrated as a base in glacial acetic acid (IO), was also titrated as an acid with tetrabutglammonium hydroxide in the authors’ laboratory. This suggested that 2,4-dinitrophenylhydrazones should titrate as weak acids and thus afford another simple method for the characterization of this important group of derivatives. The 2,4dinitrophenylhydrazones of aldehydes and ketones have been identified by elemental analysis, melting point, paper chromatography, absorption spectroscopy, and infrared spectroscopy ECENTLY,
(2, 9, I S ) . The proposed method should be a useful adjunct to these other means of isolation and identification. By the proposed procedure, samples as small as 1 to 2 mg. can be analyzed with an accuracy of *2%, This was possible by the use of 0.01 to 0.02.47 titrants. Although quaternary ammonium hydroxide titrants have been employed by several investigators ( I , 3-6, 7 , 8 ) , none suggested the use of a titrant weaker than 0.1N. Pifer, Rollish, and Schmall demonstrated the usefulness of titrants as weak as 0.001N in other nonaqueous solvent systems (11, 12). Useful titrants as low as 0.002N can be obtained by diluting 0.01N tetrabutylammonium hydroxide in 10 t o 1 benzene-methanol with additional benzene. The preparation and use of these more dilute titrants will be discussed more completely in a subsequent article; in the present investigation, the dilute titrants are homogeneous and appear stable on standing. As these solutions contain considerably less methanol than the 0.1N titrant, they have proved exceedingly versatile. REAGENTS A N D APPARATUS
Tetrabutylammonium hydroxide, 0.01 and 0.02N. Prepare 0.1N tetrabutyl-
ammonium hydroxide as previously described (4). Dilute 100 and 200 ml. of this 0.1N titrant to 1 liter with benzene. The dilutions contain benzene-methanol ratios of 100 to 1 and 50 to 1, respectively. Standardize each by titration against benzoic acid in pyridine solution. Restandardize as may be required. Pyridine. Allow technical grade pyridine to stand overnight over sodium hydroxide pellets, then flash distill. Precision-Shell Dual AC Titrometer (Precision Scientific Co., Chicago, Ill.) , using the electrodes described previously (3). PROCEDURE
The procedure consisted of dissolving 2 to 20 mg. of the 2,4dinitrophenylhydrazone in 50 ml. of pyridine and titrating potentiometrically in an inert atmosphere, with either 0.01 or 0.02N tetrabutylammonium hydroxide. After correcting for the solvent blank, the end point was determined from a plot of the millivolt readings us. volume of titrant used. EXPERIMENTAL
All of the 2,4-dinitrophenylhydrazones tested titrated as weak acids. The use of indicators was not warranted because of the intense coloration imparted to the solution by the 2,4dinitrophenylhydrazones. VOL. 30, NO. 9,
SEPTEMBER 1958
1445
/---
tlOOC
y)t800
+ J 0 ? J 2
-
I
,600
f40C
/
Figure 1. Titration of 2,4-dinitrophenylhydrazones in pyridine with 0.02N tetrabutylammonium hydroxide
t 2001
Curves shifted for clarity 1. Formaldehyde 2. Acetone
Figure 1 shows the potentiometric curves for the 2.4-dinitrophenylhydrazones of formaldehyde and acetone when titrated m-ith 0.02.Y tetrabutj-lammonium hydroxide. The curves for the titration of vanillin and glyoxal 2.4dinitrophenylhydrazones are given in Figure 2. Both compounds titrate as dibasic acids; the second acid equivalent in vanillin 2,4dinitrophenylh;drazone came from the phenolic grouping on the vanillin molecule, whereas the glyoxal derivative is a dinitrophenylosazone. Representative 2,4-dinitrophmylliy-
Table I.
Figure 2. Titration of 2,4-dinitrophenylhydrazones in pyridine with 0.02N tetrabutylammonium hydroxide Curves shifted for clarity 1. Vanillin 2. G l y o x a l
drazones of both aldehydes and ketones were determined. Table I records the average results from three determinations of each compound. The neutralization equivalents listed were calculated from the final end point in each titration. This value is synonymous with the molecular weight in the monobasic compounds; with the dibasic 2,4-dinitrophenylhydrazones of glyoxal, vanillin, 2,4-hexanedione, butanedione, 2,3-pentanedione, and a-
Determination of 2,4-Dinitrophenylhydrazones of Aldehydes and Ketones by Titration with Tetrabutylammonium Hydroxide
2,4-Dinitrophenylhydrazone of
Formaldehyde Crotonaldehyde Isovaleraldehyde Glyoxal Furfural p-Isopropylbenzaldehyde Benzaldehyde r'anillin .-icetone Methyl n-propyl ketone (2-pentanone) 2,5-Hexanedione Aketophenone 3-Pentanone Butanedione Cyclohexanone 2-Butanone-1-hydroxyacetate
"3-Pentanedione Ethyl acetoacetate a-Ketoglutaric acid
1446
I/'
ANALYTICAL CHEMISTRY
Melting Point, C., Seutralization Equivalent Uricorrec ted Theoretical Experimental 210 04 211 04 169-70 250.21 252.18 198-9 266 2 5 267.63 122-3 209 13 219 82 ... 2i7.76 231-1 276 31 333. i 2 328 34 247-8 294.36 286 26 243-4 165 56 272-3 166 14 238 84 238.20 125-7 139-41 263-4 253 l5i-8 ... 162-3
94 5-95 264-70 91-2 174-9
266.23 237. 19 300 28
265 98 241.5i 304.18
266 26
274 09
~ .~. .
223.16 278.26
310.28 230.13
310.28
163.17
224.49 279.85 308.73 223 41 309 76 165 50
c 'F
Purity 99 57
99 21 99 49 95 13 99 48 98 40 97 20 100 35
99 73
100 13 98 19 98 67 97 14 99 41 99 44
100 50
103 06 100 16 98 56
ketoglutaric acid, the neutralization equivalent is one half the molecular weight. The standard deviation for this series, based on purity determinations, v a s 0.57. The final end point must be used in calculation of the neutralization equivalent for accurate results, because of the possible presence of isomers. I n titration of the 2,4-dinitrophenylhydrazone of a-ketoglutaric acid, three inflections !!-ere noted in the potentiometric curve; only the third end point, which included all isomers, yielded logical results. Pyridine was the most desirable soli-ent, from the standpoint of both effective titrations with tetrabutylammonium hydroxide and solubility of the 2,4-dinitrophenylhydrazonesin this solvent, Several 2,4-dinitrophenylhydrazones n-ere titrated in solvents other than pyridine, including acetone, acetonitrile, and dimethylformamide. The hydrazones vere not sufficiently soluble in isopropyl alcohol to evaluate this solvent. The samples which were soluble and titratable in each of the other three solvents yielded potentiometric curves with much more gradual inflections than when titrated in pyridine. DISCUSSION
Although 0.01 and 0.02X tetrabutylammonium hydroxide solutions were used to titrate the 2,4-dinitrophenylhydrazones, the 0.1N titrant was also
applicable. The lower normality titrants were advantageous when only a small amount of the deriva t I\' .e 14.2,s available. As isomerism does not affect the neutralization equivalent, this procedure appears more reliable than iiirlting point data as a means of identification. The phenj-lhydrazone, p-nitrophenylhydrazone. and phenylosazone derivstives as \yell as 2,4dinitrophenylhydrazine rrere titratable by this procedure. I n addition, 5,5 - dimethyl - 1.3cyclohexanedione (Dimedon), a specific reagent for aldehydes, as well as derivatives of dimedon TTere titratable. No differentiation between dimedon and aldehyde derivatives was possible.
ACKNOWLEDGMENT
The authors express their appreciation to James D. Frederickson for providing the 2,4-dinitrophenylhydrazones and the melting point data, as well as valuable advice in the course of this investigation. LITERATURE CITED
(1) Bruse, D. B., Kvld, G. E. A , , X 4 . 4 ~ . CHEM.29, 232 (1957). ( 2 ) Buyske, D. -2 Owen, L. H., Wilder, P., Hobbs, M. E., Ibid., 28, 910 (1956). (3) Cundiff, R. H., Markunas, P. C., Ibid., 28, 792 (1956). (4;Ibid., 30, 1450 (1958). i:5) Deal, V. Z., \T7yld, G. E.il., I b i d . , 27, 47 (1955). ~
(61 Fritz, J. S., Moye, A . J., Richard, 11.J., Ibid., 29, 1685 (1957). ( i )Fritz, J. S., Yamamura, S. S., Ibid., 29, 1079 (1957). (8) Harlow, G. A,, Xoble, C. 11..TTgId, G. E. A,, Ibid., 28, 787 (1956). ('3) Lynn, W. S., Steele, L. A., Staple, E., Ibzd., 28, 132 (1956). (10) Nakayama, S . , Japan Analyst 5, 4.59 (1956). (11) Pifer, C. W.,\.T'ollish,E. G., Schmall, 1L3A \ . ~ L . CHEK 25, 310 (1953). (1'2) Pifer, C. W., Kollish. E. G., Schmall, 11..J . Am. Pharm. Assoc., Sci. Ed 42, 509 (1953). (13) Pippen, E. L., Eyring, E. J., Xonaka, 11.,AXAL.CHEM.29, 1305 (1957). RECEIT-ED for review January 3, 1958. .lrcepted May 9, 1958.
Titration of Acids in Nonaqueous Solutions with Tetrabutylammonium Hydroxide Reaction of Solvents with Strong Acids ROBERT H. CUNDIFF and PETER C. MARKUNAS R. J. Reynolds Tobacco Co., Winston-Solem, N. C.
b A number of solvents commonly employed for acid determinations in nonaqueous media react in varying degrees with strong acids. Acetone, isopropyl alcohol, methyl isobutyl ketone, dimethylformamide, acetonitrile, and pyridine were investigated; only pyridine was completely satisfactory for the analysis of mixtures Containing strong acids.
D
further work with the tetr:iliutylammonium hydroxide titrant (Z), errors similar t,o those 011tained with the nonexchanged b a w \yere obtnined occasionally with t,he anion exchanged titrant. Fritz and Yanianiura (3) substantiated that tlie new source of error result'ed frmn the reaction of strong acids with the solvent used. They found that tlir major limitation of acetone as a solvent for differentiating titrations of acid inistures was its react'ion with strong acids. A study was made of the behavior of strong acids with a number of d v e n t s comnionly employed in the nonaqueous titrations. The solvents used were acetone. isopropyl alcohol, methyl isobutyl ket,one (4-methyl 2-pentanone dimethylformamide, acetonitrile. and pyridine. Of these solvents, only pyridine was completely satisfactory for use n.itli strong acids. All the other solvents reacted in varying amounts. The error caused by reaction of a11 ia3 ~ dividiial acid with a solvent 1 ~ iiclr CRISG
sufficiently large in most instances to preclude the use of a n y one solvent. Hoxever, in differentiating titrations ~ l f acid niivtures containing a strong acid, a marked effect on the accuracy of analysis was observed. Observations similar t o those of Fritz and Yamamura, but unreported, may have dissuaded others from using thi; procedure for differentiating titrationc of strong acids. The purpose of thiq ktudy is to report the magnitude of the error caused by this reaction and to shon t h a t exact quantitative data \.an be obtained if proper experiniental conditions are employed. EXPERIMENTAL
N o a t of the reagents are described in
t h e following article ( 2 ) . Acetone, ace-
tonitrile, isopropyl alcohol, and dimethylformamide (DMF) were all of commercial grade; methyl isobutyl ketone ,MIBK) was purified as described bv Rruss and Wyld (1). The same titrimetric procedure was used as in the following work ( 2 ) . If the acid sample contained appreciable I\ ater, the methyl isobutyl ketone solutions n ere not homogeneous. Homogeneity was maintained in these samples b y addition of acetone. The solvent-solute interaction may tle illustrated by comparing results c h a i n e d in the respective solvents I\ lien determining sulfuric acid, hydrochloric acid, and a mixture of these 1\30 acids (Tables I , 11, and 111).
Figure 1 shows the potentiometric curves obtained in the titration of sulfuric acid in acetonitrile, dimethylfornianiide, pyridine, isopropyl alcohol, acetone, and methyl isobutyl ketone, respectively. Table I lists the results obtained on titration of 10% sulfuric acid in each of the above soh-ent's. Resuks in pyridine solution Tvere excellent. Results in the other solvents, if calculated for total acidity: \\-ere not too far from theoretical. However, there was considerable variance on the basis of the volume of titrant for the individual equivalents. This was particularly noticeable in the titrations in dimeth?-lforinaniide. Three inflections \\-ere present in the potentioimtric curve, as indicated in Figure 1; tlie additional inflection resulted from the hydrolysis of diniethylforiiiamide to formic acid. Although formic acid may be determined in this titration, only two equivalence points, the first and third, were used for t,he calculations in Table I. The longer the sulfuric acid remained in contact with diinetli~-lformaiiiide, tlie greater the amount of formic acid formed and t,he greater the difference in volume for the tn.o equivalents of sulfuric acid. The c u r w in Figure 1 for the titrat,ioii of sulfuric acid ivas obtained from a sample which \!-as held in diniethylfornianiide for 15 minutes prior to titration to emphasize blie solvent-solute reaction. If sulfuric acid VOL. 30, NO. 9, SEPTEMBER 1958
1447
