by Reinmuth (8).These consist of the slopes of the plots of log ( ~ 1 ’ 2 t 1 ’ 2 ) / t 1 ’us. 2 E or log T ~ -’ t~1 I 2 vs. E for which we obtained 0.160 volt; of Ell4us. log io, for which 0.420 volt; of us. log C, for which 0.135; and of the ratio of cathodic to anodic transition times, for which rve obtained zero. I n the discussion below-, we shall assume the two-step reaction scheme 01 electron transfer follon-ed by radical reaction mentioned above, and shall consider the kinetic schemes suggested by Reinmuth. If the electron transfer step is irreversible. then the nature of the second step will have no effect on the chronopotentiometric behavior. This would correspond to cases 3 or 6, which involve equal slopes for each of the log plots. Aipparently Tve are not dealing with these cases. The failure to obtain reverse transition, together with the chemical evidence and the log plot slopes appear to rule out cases 1, 2, 4, 5, 7 , 11, and 12. which involve reversible electron transfer reactions or reversible electron
transfer reactions followed by a reversible chemical reaction. Case 14 is excluded by failure to obtain the correct ratio of log plots. The fact that this would involve a nonintegral number of electrons, together n i t h the fact that dimerization is not observed, eliminates this possibility. Similarly, the absence of dimerization appears to eliminate cases 13 and 15. Case 9, rapid reversible electrode reaction followed by rapid irreversible chemical reaction, is ruled out by requirement of equal log plot slopes. On examination, our results appear to eliminate all of the suggested schemes except numbers 8 and 10, rapid reversible electron transfer followed by slow reversible chemical reaction and rapid reversible electron transfer followed by slow irreversible chemical reaction. One would not expect, however, to find that the reaction of this ion-radical with the solvent would be reversible; consequently case 8 is improbable. I t therefore appears that, of the suggested
schemes number 10, reversible electron transfer followed by an irreversible chemical reaction is most probable. LITERATURE CITED
(1) Bard, A. J.,
~ A L C . H E v . 33, 11 (1961). ( 2 ) Elving, P. J., Krivis, -1.F., I b t d . , 30, 1645 (1958). ( 3 ) Kelly, >I T., . Jones, H. C., Fisher, D. J., Ibid., 31,488 (1959). (4) Lingane, J. J., 3‘Electroanalytical Chemistry,” 2nd ed., p. 456, Interscience, New York, 1958. ( 5 ) Loveland, J. K., Dimeler, G. R., ANAL. CHEM. 33, 1196 (1961). (6) Mann, C. K., Champeaux, V. C., J. Chem. Ed., 38, 519 (1961). ( 7 ) Mezoguchi, T., Adams, R. S., J . A m . Chem. SOC.84. 2058 11962). (8) Reinmuth, K. H., A X ~ L CHEW . 32, 1514 (1960). (9) Shriner, R. L., Fuson, R. C.,,Curtin, D. Y., “The Systematic,,Identification of Organic Compounds, 4th ed., p. 229, Wiley, New York, 1956. RECEIVEDfor review August 20, 1962. Accepted March 8, 1963. Work supported by the Petroleum Research Fund of the American Chemical Society.
Spectrophotometric Determination of Hydrocarbon and Carboxylic Acid for the Material Balance Data in Carbanion Oxidations HARVEY POBINER,’ THOMAS J. WALLACE, and JOHN E. HOFMANN Analytical Research Division and Process Research Division, Esso Research and Engineering Co., P.O. Box 727, linden, N. j .
b Analytical methods have been developed for obtaining material balance data in certain carbanion oxidation reactions. One method is an extraction-ion exchange-infrared technique to measure starting material and carboxylic acid product. The other method is an extraction-ultraviolet technique to measure aromatic starting material and carboxylic acid. The two methods are illustrated with mixtures of toluene and benzoic acid and 2methylnaphthalene and 2-naphthoic acid in solvent systems of base, dimethylsulfoxide, and water. Aliphatic compounds can also b e determined quantitatively.
E
ion exchange, and spectrophotometry are used in an analytical scheme for the quantitative analysis of carbanion oxidation products. The technique was developed for determining both conjugated and nonconjugated carboxylic acid products and unreacted hydrocarbon in basic solutions. It was extensively used for 680
XTRACTION,
ANALYTICAL CHEMISTRY
obtaining material balance data in a general study of the base-initiated oxidation of carbanions in selective solvents (2, 15). Carbanion oxidations are thought to involve proton abstraction by base in selective solvents. Molecular oxygen then reacts with the carbanion to form Oxidation products by an anion-radical mechanism. This mechanism and certain experimental data have been described (2, Q-li?.15). Tmo procedures are discussed. One is an extraction-ion exchange-infrared procedure for starting hydrocarbon and a carboxylic acid derivative in a system of solvent, base, and water. This method is indicated for either conjugated or nonconjugated hydrocarbon and acid. The other is an extractionultraviolet procedure for a conjugated starting material and its carboxylic acid derivative in a similar system. Both of the procedures rely on the initial removal of unreacted starting material by an extraction step with cyclohexane. This effectively removes any spectral interference with the sub-
sequent determination of carboxylci acid. The carboxylic acid is present as the potassium salt in the solvent, dimethylsulfoxide (DMSO). Hydrochloric acid is added to liberate the carboxylic acid in solution. The HCl also prevents the hydrogen bonded adducts that are reported to form betneen weak acids and polar solvent systems (S, 4, 6-8). If the carboxylic acid that remains in the aqueous raffinate is aromatic, it iq directly determined by ultraviolet spectrometry. If the carboxylic acid that remains in aqueous raffinate is aliphatic. or only presents weak ultraviolet absorption, it is determined by an ion exchangeinfrared method. This involves treating the aqueous raffinate Kith Amberlite LA-2, an anionic exchange resin. The Amberlite-RCOOH adduct forms and is extracted with carbon tetrachloride. The extracted carboxylic acid is then
1 Present address, General Precision Aerospace Group, 1225 McBride Avenue, Little Falls, N. J.
quantitatively determined by infrared spectrometry. Earlier work by rltlanis and S w a n ( I , 14) has shown the value of absorption spectrometry for determining aromatic acids in alkyd resins. The use of Amberlite LA-2 was recently demonstrated by Doiinsky and Stein ( 5 ) in a technique fo:. solubilizing sulfonic acids for infrared studies. The spectrophotonietric det'ernlination of carboxylic acids in these solvent systems was chosen in preference to alternate determinations, such as potentiometric titration and gas chromatography. It was found that inconsistent blank titers, and, occasionally poorly defined inflection points, gave unsatisfactory analyti1:al recovery from blends. The spectrophotonietric approach appeared more sensitive and direct for both aroms.tic and aliphatic carboxylic acids when compared with gas chromatographic techniques. The latter generally require conversion of the carboxylic acids 1)o derivatives for suitable chromatographic resolut'ion. EXPERIMENTAL
Two analytical schemes are presented. T h e extraction-ion exchange-infrared procedure is illustrated with a mixture of toluene and its oxidation product, benzoic acid, in a system of potassium tert-butoxide-DMSO-lvater. It was developed initially fo:. aliphatic compounds. The extraction-ultraviolet procedure is illustrated with 2-methylnaphthalene, and its cxidation product, 2-naphthoic acid, in the same solvent system. The latter technique is primarily indicated for aromatic compounds. Reagents. Amberlite LA-2 (liquid anion exchange resin, available in pound quantities from Rohm & Haas Co., Philadelphia 5 , Pa.), cyclohexane (Fisher, Spectranaly2:ed, KO.C - 5 5 5 ) , methanol (Baker Analyzed Reagent No. 9070). Carbon tetrachloride (Baker Analyzed Reagent S o . 1512). Calibration standards for starting hydrocarbon and carboxylic acid product. For the examplcs cited below, 2-methylnaphthalene (LIatheson Coleman 8: Bell, No. MX 1185, m.p. 3435' C.), 2-naphthoic acid (NCB, S o . NX 105, m.p. 183-5' C.)) toluene (Baker's Analyzed Reagent, No. 9460), and benzoic acid (hlCB, primary st'andard, Xo. CB 1OOil). An Amberlite LA-2 solution, 5% volume in carbon td,rachloride. is prepared. Extraction-Ion Exchange-Infrared Procedure. Calibration. PreDare t h e following solutions of toluehe a n d benzoic acid. Weigf 0.1 gram, correct t o 0.1 mg., of toluene into a 100ml. volumetric flask a n d dilute t o t h e m a r k with cyclohexsne. Dilute as necessary in cyclohexane t o obtain t h e ultraviolet spectrum and calculate absorptivity at 262 mp. Use a doublebeam recording spectrophotometer, such as the Beckman 1)K-2 and 1.0-em. absorption cells.
Weigh 2 grams of benzoic acid, correct to the milligram, into a 100-ml. volumetric flask. Simulate the normal composition of oxidation samples b y adding about 2 grams +O.l gram of the base (K-tert-butoxide) and 25 ml. of the solvent (DMSO). Dilute to the mark with distilled water. Pipet 25 ml. into a 150-ml. beaker. Add dilute HCl until indicator paper shows a p H of 7.0. Pipet in 20 ml. of 1N HC1 and heat to a boil. Keep at the boiling temperature for about 2 minutes. Cool. Extract with five IO-ml. volumes of the 5% Amberlite in CCll reagent. Collect the extracts together in a 150ml. beaker. On the hot plate, boil down to a volume of 10 ml. and quantitatively transfer to a 25-ml. volumetric flask containing a few crystals of anhydrous SanSOa. Dilute to the 25ml. mark with carbon tetrachloride. Obtain the infrared spectrum in a 0.25-mm. cell us. a reference solution of CCL. Dilute the solution further in CC1, to resolve the carbonyl band a t 1700 cm.-l a t absorbances from 0.2-0.6, to serve as the calibration. Draw a calibration curve of -i21i00cm-I us. grams of benzoic acid. Use a base line drawn from 1800-1625 cm.-l A double beam infrared spectrophotometer, such as the Perkin-Elmer Model 421 or 21, can be used. Prepare a blank solution of base, solvent, and mater and carry through the entire ion exchange-infrared procedure. The blank is useful for determining optimum base line points and for correcting absorbances at any analytical bands other than the carbonyl. There are usually no interferences a t the acid carbonyl. Analysis of Samples. Transfer the entire anionic ovidation sample of 100 to 200 ml. t o a 500-ml. separatory funnel. If water has not been added t o t h e sample a t t h e reactor (to stop t h e oxidation), then a d d about 50 ml. of distilled water. If necessary, add more water to dissolve any solid base which is present. The solution should show a pII level with indicator paper of 10 or greater. If necessary, add a 10% aqueous NaOH solution to achieve this p H level. Extract with seven 20-ml. volumes of cyclohexane. Collect the extracts together in a 250-ml. volumetric flask and dilute to the mark with cyclohexane. Save the water plus solvent raffinate. Run an ultraviolet spectrum of the cyclohexane phase us. a cyclohexane reference to resolve the analytical wavelength of the extracted hydrocarbon starting material (toluene). Measure the volume of the water plus solvent raffinate phase to the nearest milliliter. This raffinate contains the carboxylic acid salt. Pipet a n aliquot equal to that of the calibration procedure (25 ml.) into a 150-ml. beaker. Proceed with the ion exchange procedure, beginning with the adiustment to p H 7.0. In the final spectral solution for infrared dilute as necessary to resolve the carbonyl band of benzoic acid at 1700 cm.-l in the absorbance calibration range of 0.2-0.6. Calculate the grams of unreacted
toluene in the cyclohexane phase and of benzoic acid in the aqueous phase. Determine the material balance. Extraction-Ultraviolet Procedure. Calibration a n d analysis. Obtain t h e ultraviolet absorption curves, from 210-360 mp, of the aromatic starting material @-methylnaphthalene) and of the acid product (2-naphthoic acid). As in the first procedure, extract with cyclohexane. Determine the 2-methylnaphthalene in the ultraviolet a t 276 mp. Measure the volume of the aqueous raffinate which contains the salt of 2-naphthoic acid. Weigh 10 ml. of this aqueous phase in a 100-ml. volumetric flask. Pipet in 10 ml. of concentrated HC1 and dilute to the mark with methanol. Determine the 2naphthoic acid in the ultraviolet at 280 mp. Determine the material balance from the grams of unoxidized 2-methylnaphthalene and from the grams of 2-methylnaphthalene equivalent to the 2-naphthoic acid found. RESULTS A N D DISCUSSION
Selection of Method. T h e extraction-ultraviolet procedure is indicated for aromatic starting hydrocarbon a n d its carboxylic acid product. Solvents, such a.; DMSO, do not interfere with t h e ultraviolet rebolution of compounds t h a t maxiniiw above 240 mp in the anionic3 oxidation qamples analyzed. If it is neceqsary to resolve maxima lower than 240 mp, or if other oxidation solvent- interfere n ith the ultraviolet method then the extractionion exchange-infrared method is indicated. Of course, for aliphatic systems or for those component3 having n eak ultraviolet absorption, the extraction-ion exchange-infrared method is always indicated. I n that event, infrared is used in analyzing both the cyclohexane extract containing starting material and the Amberlite-CCl4 extract containing acid. Calibrations, as described, are required for both methods. Recovery Data. T h e validity of t h e analytical techniques is demonstrated b y t h e recovery d a t a from synthetic blends in Table I. l s shown, t h e described methods are suitable for determining unreacted starting material, aromatic and aliphatic carboxylic acids, and sulfonic acid
