.
2nd Annual Summer S#mpoeium
- Organic Reagents
Determination of End Unsaturation in Organic Compounds CLARK E. BRICKER
AND
KARL H. ROBERTS, Princeton University, Princeton, N . J.
A chemical method for detecting and determining terminal or end unsaturation in mixtures of unsaturated organic compounds has been developed. The unsaturated compounds are treated with a slight excess of potassium permang anate to form glycols which are split by means of periodic acid. Because formaldehyde is a product of this reaction when terminal double bonds are present, determination of formaldehyde affords a specific method for determining end unsaturation. Formaldehyde is separated from the reaction mixture by distillation and is determined spectrophotometrically by means of chromotropic acid.
A
Sulfuric acid, concentrated The distillation apparatus is illustrated in Figure 1.
VARIETY of methods have been published for the determination of olefinic unsaturation in organic compounds. The chemical methods for this determination break down into two general categories-addition reactions and hydrogenation. The physicochemical methods are rather diversified and include infrared absorption, ultraviolet absorption, mass spectrometry, Raman spectrometry, and polarography. The chemical methods for the determination of olefinic unsaturation are used chiefly for the determination of total unsaturation, whereas the physicochemical methods are used principally for the determination of the position of the unsaturation or even the exact chemical compound. At least two chemical methods have been suggested, however, for determining the position of the unsaturated group or groups in organic compounds. Dienometry, which was proposed almost simultaneously by two groups of investigators ( 2 , 6) utilizes the Diels-rllder reaction for determining the amount of conjugated unsaturation. Kolthoff and Lee ( 7 ) and later Saffer and Johnson ( 9 ) suggested methods for determining internal and external unsaturat,ion in polymers based on the difference in the rate of reaction of perbenzoic acid with there two types of bonds. Because of the lack of a simple cheniical method to determine end or teriiiinal unsaturation in organic conipcundb, it was decided to investigate the possibilities of a method which would be specific for this type of olefinic linkage. I t is wdl known that olefins are converted into glycols with potassium permanganate. Because terminal unsaturated groups are transformed to 1,2-gly~301swhich are split with periodic acid to yield formaldehyde and a higher aldehyde, the detsrmination of formaldehyde would then afford a specific indication of this type of unsaturation. The idea of using the Malaprade reaction together nith a disidlation to remove the formaldehyde for the determination of I ,2-glycols has been suggested (375, 8 , I O ) . However, in none of these investigations has a specific reagent, for formaldehyde been used. A sensitive arid specific spectrophotometric methcd for determining formaldehyde with chromotropic acid has been tlescrihed by Bricker and Johnson ( 1 ) . Therefore, a combination 'if these methods should provide a comparatively simple chemical procedure for determining end unsaturation.
PROCEDURE
Dissolve a weighed quantity of the unsaturated compound in ethanol and then dilute it to a definite volume with this solvent. Pipet a suitable sized aliquot (not more than 3 ml.) of this solution which will yield not over 2 mg. of formaldehyde into a glassstoppered test tube. Add sufficient ethanol so that a total of 3 ml. of alcohol are present and then add 1 ml. of the periodic acid solution. S o w add the potassium permanganate solution in 0.2-ml. portions until a pink color persists in the supernatant liquid for 1 minute. Alternatively, if the amount of total unsaturation is known, 0.5 ml. more than the calculated amount of the potassium permanganate solution can be added in 0.2-ml. portions without observing so closely the color of the supernatant liquid. Place the test tube in the distillation apparatus (Figure 1) and continue to distill the reaction mixture until 10 or 15 ml. of distillate are collected. Read the volume of the distillate collected and then transfer 1.00 ml. of this solution to a test tube containing 50 mg. of chromotropic acid. Add 5 ml. of concentrated sulfuric acid to the test tube and then heat the resulting colored solution in a boiling water bath for 30 minutes. Cool the test tube and finally dilute the solution to 50 ml. in a volumetric flask. Measure the optical density of this solution against a reagent blank at 570 mp.
REAGENTS AND APPARATUS
Figure 1. Distillation Apparatus
Ethanol, 95% Potassium permanganate solution, 1 yo aqueous solution Periodic acid solution, 200 mg. per ml., obtained from the G. Frederick Smith Chemical Companv Chromotropic acid (1,8-dihvdroxynaphthalen~-3,6-disulfonic acid), obtained from the Paragon Division, Matheson Chemical Company
From a calibration curve obtained from known amounts of formaldehyde and by multiplying by the number of milliliters in the distillate, the amount of formaldehyde in the entire distillate is obtained. The formaldehyde found by making a blank determination on all reagents is now subtracted to give the amount of 1331
ANALYTICAL CHEMISTRY
1332
formaldehyde actually obtained from the end unsaturation in the ample. DEVELOPMENT OF METHOD
The chemical reactions which are used in this procedure to determine end unsaturation can be shown by the following equatioILs : CHZOH
I + 2Mn& + 2Hs0 + 3CHOH I i-
I
,
+ 2Mn01
In order to investigate the triialyticitl posbibilities of these reactions, it was necessary to deternine how Tvell small amounts of formaldehyde could be distilled from solution. Although the papers dealing with the determination of glycols by periodic acid oxidation followed by distillation do not imply any difficulty in distilling formaldehyde quantitatively, it is known that this compound is strongly hydrated in solution and, therefore, is not distilled as readily as one would predict from its physical properties. About 1.3 mg. of formaldehyde in 5 ml. of water were placed in a teat tube and then distilled in the apparatus shown in Figure 1. Two 10.0-ml. distillates were collected and analyzed for formaldehyde. Inasmuch as the 6rsL distillate contained over 98% of the formaldehyde, it was decided that 10 to 15 ml. of distillate would be adequate to recover the formaldehyde quantitatively. It is important to distill the solution in the test tube almost to dryness a t least once before the distillation is stopped. This can be done easily by regulating the two burners shown in Figure 1 80 that the distillation from the test tube proceeds much faster than that in the steam-generating flask. The continuous stream of gas bubbles from the steam-generating flask prevents the reaction mixture in the test tube from bumping during the distillation and also aids in sweeping the formaldehyde into the distillate. Because the reaction mixture contained strong oxidizing agents, it was important to determine if the formaldehyde could be recovered from solutions containing potassium permanganate and periodic acid. Identical amounts of formaldehyde (about 0.5 mg.) in 2.0 nll. of water were added to 3.0 ml. of 95% ethyl alcohol and then treated with 0.00,0.50, and 1.00 ml. of potassium permanganate solution, respectively. These solutions were distilled and the distillates were analyzed according to the recommended procedure. The recoveries of formaldehyde were 99, 28, and 21%. Another series of 2.0-ml. aliquots of the formaldehyde solution was taken and 1.0 ml. of periodic acid and 3.0 ml. of water were added to each solution. No ethyl alcohol was added to any of these solutions but 0.00, 0.20, and 0.50 ml. of potassium permanganate solution were added, respectively. The recovery of formaldehyde was 99, 23, and less than 2Y0 from these solutions. To each of three additional 2.0-ml. aliquots of the formaldehyde solution, 1.0 ml. of periodic acid and 3.0 ml. of ethyl alcohol were added. Then 0.00, 0.50, and 1.0 mi. of potassium permanganate solution were added, respectively. The recovery of formaldehyde was 100, 98, and 94%. These experiments indicate the importance of the ethyl alcohol ln the recommended procedure. Formaldehyde cannot be recovered from an aqueous solution of potassium permanganate and periodic acid but it can be distilled well from an alcoholic solution, This is believed to be due to the reaction of formaldehyde with ethanol in dilute acid solution to form ethylal. This acetal is &able and undergoes oxidation rather slowly. The possibility that the formaldehyde may all be present in the distillate as ethylal causes no interference in its determination with chromo-
tropic acid. This reagent has been shown to be especially useful for determining combined formaldehyde in linear or cyclic formala In the recommended procedure, the periodic acid is added before the potassium permanganate. I t was shown in a series of experiments that when potassium permanganate was added prior to the periodic acid, about 30% less formaldehyde was obtained than when the recommended procedure wm followed. These data can be explained if it is assumed that periodic acid must be present to react immediately with the glycol as it is formed, and thus prevent the attack by excess permanganate on the glycol to yield products other than formaldehyde. This explanation implies that the rate of attack by periodic acid on a glycol is considerably faster than that of permanganate. The ethyl alcohol used in the recommended procedure alw serves to destroy the excess potassium permanganate before it can appreciably oxidize the formaldehyde or any compounds which should yield formaldehyde in the procedure. This was shown in a. swies of experiments in which constant amounts (10.4 mg.) of undecylenic acid dissolved in dilute sodium hydroxide were treated nith sulfuric acid to a pH of about 7. Then 1 ml. of periodic acid solution and varying amounts of potassium permanganate solution were added to each sample. The results of these experiments showed that the amount of formaldehyde obtained increased to a maximum value and then decreased rapidly as the amount of permanganate in excess of the theoretical amount to form the glycol was added. However, when alcohol was used in a similar series of experiments, the recovery of formaldehyde was considerably higher than the maximum value obtained when no alcohol was present. Furthermore, the recovery of formaldehyde was practically constant in these experiments as long as sufficient permanganate was present to form the glycol but not more than 6 to 8 mg. in excess. Some experiments were run to determine how long the reactiou nlixture should stand prior to the distillation. The recovery of formaldehyde was essentially constant when the reaction mixture was allowed to stand 0, 1, 2, and 5 hours between the addition of the potassium permanganate and the actual distillation. Therefore, it was concluded that the time of standing of the mixtiire prior to distillation was not a critical factor. RESULTS AND APPLICATIOAS
A qualitative study on several compounds was carried O U L 11, order to test the conclusion that only compounds that contain a terminal olefinic group would yield a positive test for formaldehyde by the recommended procedure, The following compound? gave a very strong test for formaldehyde and, therefore, indicated terminal unsaturation: allyl alcohol, allvl chloride, allyl bromide allyl isothiocyanate, allyl thiourea, styrene, isoprene, pinene citral, citronellol, itaconic acid, and a-methylenebutyrolactone The following compounds produced little or no more formaldehyde than the blank and consequently indicated that few, if any terminal olefinic linkages were present: oleic acid, cinnamic acid indene, coumarin, and indole. Citraconic and aconitic acidc gave inconclusive results because small samples showed a considerable amount of formaldehyde whereas large samples gavt very little formaldehyde. The results from these compound9 clearly indicate that the recommended procedure provides a ronvenient qualitative method for detecting end unsaturation in n variety of organic compounds. In order to determine the quantitative possibilities of the procedure, several standard solutions from a sample of commercially available 10,ll-undecylenic acid were prepared by dissolving weighed quantities of the material in ethyl alcohol and diluting to 25.0 ml. in volumetric flasks. Various sized aliquots of these solutions were analyzed by the recommended procedure. The precision of multiple analyses on the same sized aliquot was within 5 7 , or better in all cases. A plot of the total formaldehyde recovered in the distillates versus the number of milligrams of
1333
V O L U M E 2 1 , NO. 11, NOVEMBER 1 9 4 9 Table I.
Determination of Undecylenic Acid
izelaic Acid Taken
Undecylenic Acid Taken
Undecylenic Acid Found
Ml7.
Mo.
Mo.
Mo.
2.95 7.86
2.95 8.05
t0.00 fO.19
Oleic Acid Taken 18.8 19.0 19.5 85.7 160 19.0
2.02 2.02 2.02 3.00 2.02 5.05
1.90
58.0 75.0 ~05.5 87.5
8.01
1.89 1.97 2.85 1.83 4.87 4.85 4.92 4.78 8.m
-0.06 -0.13 -0.05 -0.15 -0.19 -0.18 -0.20 -0.13 -0.2i CO 09
2.17 1.98 2.17 1.96 1.84 1.86 1.86 1.82 4.92 6.04 4.87 4.76 4.79 4.84 4.87 4.79
+0.15 -0.04 f0.15 -0.06 -0.18 -0.16 -0.16 -0.20 -0.13 -0.01 -0.18 -0.29 -0.26 -0.21 -0.18 -0.26
ton
ion
6.05 5.05 5.05
Ricinoleic i c i d Taken 10.4 10.4 20.8 20.8 31.2 31.2 41.6 L1.6 11.0 11.0 21.9 21.9 32.9 32.9 43.0 43.0
2.02
2.02 2.02 2.02 2.02 2.02 2.02 2.02 5.05 5.05 6.05 5.05 5.05 5.05 5.05 5 .BS
Deviation
calibration curves indicate that the method may be upplicable and work involving these polymers is now in progress. When various sized aliquots of a standard solution of itaconic acid were analyzed, a linear calibration line was again obtained. However, this line intersected the ordinate a t a value higher than the blank. This showed that the smaller samples of this acid gave abnornially high amounts of formaldehyde and that the percentage of formaldehyde obtained decreased as the weight of sample increased. The multiplicity of functional groups in itaconic acid probably caused this discrepancy. Further work will have to be done in order to determine if the procedure can be modified to overcome this anomalous behavior. The formaldehyde obtained from compounds containing end unsaturation is not stoichiometric and, therefore, the amount of formaldehyde found cannot be used to calculate directly the percentage of end unsaturation. The percentage of formaldehyde produced by a certain end-unsaturated compound appears to be constant over a considerable concentration range but variee somewhat with each compound. Therefore, to apply this procedure quantitatively, it is necessary to prepare a calibration curve with a pure sample of the compound to be determined and then use this curve in all subsequent determinations of this sub. stance. The validity of the use of such a procedure is indicated by the agreement of multiple determinations on the same compound bv VItrinw chemists. LNTERFERENCEb
iindecylenic acid taken is shown in Figure 2. These results, which were obtained by three different chemists, indicate that the recovery of formaldehyde is linear with the smaller samples of unsaturated acid taken. With the larger samples, the recovery If formaldehyde appears to deviate slightly from linearity. After this calibration curve was obtained, a study was made to +ee whether or not undecylenic acid could be determined in the presence of azelaic acid, oleic acid, or ricinoleic acid. In each case small amounts of undecylenic acid were added to weighed portions of the other acid. The results of these experiments, listed in Table I, indicate that undecylenic acid can be determined fairly accurately in the presence of these acids. Additional quantitative studies were made with styrene and butyl acrylate. Linear calibration lines were obtained with both of these compounds, as is shown in Figure 2. These two compounds were selected in order to determine whether or not the general procedure could be used to detect terminal double bonds in styrene and acrylate polymers of low molecular weight. The
1.4
1 L
i.’ F
r
’
/
IO. I I UNDECYLENIC >
t d
5 0.2
0
Figure 2.
e
4 6 MG. OF SAMPLE
8
10
Calibration Curves for Various Compounds
Open circles, closed circles, and triangles show results of various chemists. All values for butyl acrylate were lowered so that lines would not intersect
The limitatioilv so far encountered in this procedure fall into two categories: the formation of formaldehyde from the chemical attack on compounds which do not contain terniinal olefinic groups, and the interference caused by various volatile compounds on the development of the formaldehyde color with chromotropic acid. Any organic substances other than terminal unsaturated compounds which would yield formaldehyde on treatment with either potassium permanganate or periodic acid would interfere with this method-for example, methyl alcohol or any I,2-glycols that may be present with the original sample would cause high results. A more serious limitation was observed when dealing with organic compounds in which rearrangements can take place. Citraconic and aconitic acids should not show any terminal unsaturation. However, small samples, less than 3 mg. of these compounds, gave abnormally high yields of formaldehyde but when larger samples were taken the amount of formaldehyde decreased as the size of the sample increased. Furthermore, the formaldehyde recovered from these acids w&snot too reproducible on duplicate runs. Both acids form a- and 8-hydroxy acids during the glycol formation and it is possible that these structures change so that a terminal olefinic group is actually present. As such rearrangements can cause a serious interference, it would appear that this method must be confined to the determination of end-unsaturated groups which are isolated from other functional groups. Another limitation of the method was encountered when trying to determine safrol in the presence of isosafrol. Both compounda are apparently completely oxidized by potassium permanganate and thus large amounts of this reagent must be added. This causes some difficulty in the subsequent distillation. Furthermore, the cyclic formal which is present in both of these compounds is not completely destroyed and consequently formaldehyde is found in the distillate from both of these samples. Although isosafrol gives much less formaldehyde than safrol, the difference does not appear to be sufficientlyreproducible to permit a quantitative determination or separation of these compounds. In attempting to determine end-unsaturated compounds in the presence of large amounts of benzene, phenyl ethyl alcohol,
1334
ANALYTICAL CHEMISTRY
benzaldehyde, and acetone, it has been found that these compounds are distilled with the formaldehyde and cause a serious inhibition of the development of the color with chromotropic acid. Rork is now in progress to eliminate this type of interference. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance given by W. A. Vail and C. E. Langenhop in obtaining some of the results presented in this paper. Also, they wish to thank the Decyl Pharmacal Company. Charles Pfizer and Company, and C. J. Cavallito of the Sterling-U’inthrop Research Institute for some of the chemicals used i n this investigation. Finally, they gratefully acknowledge the financial assistance received from the grant made by E. I. du Pont de Semours & Company to sponsor chemical research at Princeton Universitv.
2nd Annual Summer Sqmmeium
LITERATURE CITED
(1)
Bricker, C. E., and Johnson, H. R., IND.ENG.CHEM.,ANAL.ED., 17,400 (1945).
Ellis, B. 9., and Jones, R. A., A n a l y s t , 61, 812 (1936). (3) Elving, P. J., Warshowsky, B., Shoemaker, E., and Marnotlit. J., ASAL. CHEY.,20, 25 (1948). (4) Hoepe, G., and Treadwell, W. D., H e h . Chim. Acta, 25, 353 (2)
(1942). (5) Johnson, M. J., IND. ENQ.CHEM.,~ A L ED., . 16, 626 (1944). (6) Kaufmann, H. P., and Baltes, J., Fette u. Seifen, 43, Nos. 6-7 93 (1936). J . Polvlmer Sci., 2, 206 (1947). (7) Kolthoff, I. M., and Lee, T. S., ENG.CHEM.,ANAL.ED.,18, (8) Reinke, R. C., and Luce, E. N., IND. 244 (1946). (9) Saffer,A., and Johnson, B. L., Ind. Eng. Chem., 40, 538 (1948) ENQ.CHEY.,ANAL.ED.. (10) Warshowsky, B., and Elving, P. J., IND. 18,253 (1946).
RECEIVED July 27, 1949,
- Organic Rennente
Preparation and Colorimetric Properties of Aluminon W. H. SMITH, E. E. SAGER,
AND
I. J. SIEWERS, National Bureau of Standards, Washington, D. C .
Commercial preparations of the dye called Aluminon vary in quality; some are worthless for analytical purposes. Methylenedisalicylic acid as prepared here, which with salicylic acid is used to make Aluminon, was found to contain a low amount of monomer probably because phenolic resins are formed during its preparation. I t is not expedient to remove all the resins but rather to keep the amount of polymer low. The molecular w4ght of methylenedisalicylic acid is 288. By using a product with an average value of 310 or less, a satisfactory reagent can be made and directions are given for its preparation. I t is more soluble in water than the acid form of commercial Aluminon and therefore an
T
HE reagent Aluminon is described as the ammonium salt of aurintricarboxylic arid. Preparations of it, offered for the rolorimetric determination of small amounts of aluminum, vary in quality, frequently lack sensitivity, and have caused considerable trouble in laboratories of the steel industry and elsewhere. In the experimental work reported here, a method is described for the preparation of a reagent with properties that may be reproduced well i n successive lots. Spectrophotometric measurements have been made of dyes prepared by this method and of two commercial preparations. PREPARATION OF A REAGENT
Two procedures ( 2 , 5 ) originallv proposed for the preparation of Aluminon yicald unsatisfactory products, and modifications (6, ‘7, 1%)of them appear to produce little improvement. In the method of preparation ( 5 ) devised by K. Sandmever, sodium nitrite is added to concentrated sulfuric acid and to this mixture salicylic acid is added. Formaldehyde is then slowly introduced. In the procedure of Caro ( I ) , sodium nitrite is added to concentrated sulfuric acid and an intimate mivture of equivalent amounts of methvlenedisalicylic acid and salicvlic acid is added. The second method seems preferable, but the dye formed lacks sensitivity. Holaday ( 7 ) improved this procedure
ammonium salt is not required. Spectrophotometric measurements of different lots indicate that a reagent can be satisfactorily reproduced. Studies were made in the ultraviolet and visible portions of the spectrum of aqueous solutions of commercial Aluminons and of others made by the method described. Changes in spectral characteristics with changes in hydrogen ion concentrations were also studied. Errors which may be introduced by the presence of the colorless, carbinol “faded” form of the dye are indicated. Changes in absorbancy which accompany metal complex formation are given and typical calibration curves a t selected wave lengths that may be employed with colorimeters are shown.
by adding a second portion of nitrite and sulfuric acid and in thimanner improved the dye. The amounts of monomeric aurintricarboxylic acid must br low in products made by Sandmeyer’s and Caro’s procedures for the following reasons. In the published discussions of the preparations of methvlenedisalicylic acid it appears to be assumed that only the monomer is present. However, when a reaction mixture containing salicylic acid, formaldehyde, and a mineral acid ic heated for several hours, resinous products result, doubtless of the phenol-formaldehyde type. Clemmensen and Heitman (2) who used a 50% aqueous solution of sulfuric acid as a condensing agent, obtained a product which after washing with hot water melted at 238” C. In the course of this work, methylenedisalicylic acid was prepared by their procedure, but only a small amount oi pure monomer could be isolated from it. Kahl ( 8 ) , who used hydrochloric acid as a condensing agent, purified the crude product of the reaction by dissolving it in a 50% hot aqueous solution of acetic acid, filtering, and precipitating the Toluble material by pouring the solution into an excess of distilled water In this wav, he obtained an acid that melted at 242’ C. A rapid method for the preparation of methylenedisalicylic acid is described below. One product of this procedure was found by titration to have an average molecular weight of 300. Its melting
