Identification of Dicarboxylic Acids in Polymeric Esters


The complexity of the problem is apparent from Wake- man's (9) discussion of ... been stated by Bradley (2) in his survey for the American. Society fo...
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Identification of Dicarboxylic Acids in Polymeric Esters Preparation and Propprties of the Dibenzyl Amides of Eight Typical Dicarboxylic Acids

H > refluxing pdynieric esters with benLylaiiiine i n the presence c~fa c a t a l j a t , characteristic derib atives of the ronstituent dicarboxy lic acids can he obtained. The resulting dibenxjl amides can he separated into saturated and unsaturated fractions lay the use of selectibe sollents. Indi,idual dihenzyl amides can be identified in admixture bj comparison of their infrared spectra w i t h ~Jioqeof il standard *et of clihenr>l aniides prepared from ~nono~iierir esters.

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of t h e p t x w t i t ccsiitury, the 0rigiii:tl conius, thts react iori products of gl>-cerol :ind phthalic :inh>-driiic~, prrsrn ted >t Eitirl). simple :tnd)-tical prohlem. The Mituation bwanw n i o r . c 2 conip1ir:tted a a these resins bvere modified by the introduction of additional const,ituents, such as fatty acids, rosin, natural resins, and phenol and amine alrirhyde condens:ttiori producats. It was found possible, however, to adapt the Kiipprlnieier procedure ( 6 , 8) to a semiquantitativr sepaiatioii niethod (7). Originally designed :ts a saponificatioii tec~hniquefor the quantitative estimation of phthalic anhydride c:ontenl, $he Ksppelmeier procedure could also be employed as o f n qualithtive iden1ificS;itiori srhPnic (7).

pl:inri(d, tlwigued to take dvantwge of the many war-at i i n i h r i 1~ ~ developnients that have occurrtvl iii the field of physicS:il i i i s t i , t i inrntation. The present rcport is conwrned with thr :il)l)lii*:it i o i i of one such procedure. It \v:w decided to select eight ( ' i i i n n i Ily importaiit t i i t w 1)oxylic. : t d s B S a basis for investigation. The optimum 1 ) ~ liniinary slrp seemed to be the convcxrsioii of the selected :tc*iclto the diethyl esters. This proi~~clurr pc.t,mits t,he effirirrit pu1.ification of the acids by the fi,:rc*tion:ildistillation of the r'ty)cr'tive ester>. It also providtv :t w t of liquid derivatives t u wrvi. :ts tlit, sourre of additional :tn:rlytic.aI data, as well as stock ('anipound.< of known purity for. the series of planned altemitivt. :tnalytiwl procedures. Est:Httirs are particularly suitable for d i t , lat t w purpose, because they can be oonverted readily to amidr.. wlts, :mil acids. I t i v s h originally intended to complete and report on the analytiral ch:iixi-terization of the diethyl esters before proceeding further. This phase of the investigation, however, did not ai)pear to offer a quick solution. Bemuse the demand for a metmhoiI more s:ttiPf:actory than those currently available was urgcwt. attent ion \vas transferred to a procedure promising usabk wsults in :t shorter time. The investigation of the benzyl a t i i i r l t niethoil. \vhirh is the subject of this piper, was therefore srltv~tt~rl.

1

100I

I

EXPERI%ZENTAL

The eight acids chosen for this and subsequent investigat ioii.were as follows-four saturated (including phthalic in this W I I W and four unsaturated acids are included:

COOH /'

((:H2)4

>Y )OH

.Adipic 01700

1500

1300

I100

900

I

(CH,),

')'OOH I

CHI

I 700

bzvlaii,

C-CO( )fl ~1

HC-COOil

Ci t raco ti ic.

CH?

FREQUENCY IN CM;'

Figure 1. A.

Infrared Spectra of Dibenzjl 4niidrs

4dipic arid.

R.

iirlaic acid.

C.

Sebaric acid

A further :inalytical complication, however, 1iab O(.(~II int i . 1 ~ duced by the increasing variety of polycarboxylic acids and glycols. The complexity of the problem is apparent from Wakeman's (9) discussion of alkyd resins in general and contact resins in particular. The definite need for improved techniques has been stated by Bradley (8)in his survey for the American Society for Testing Materials, and has been emphasized by the experiences of Subcommittee XI of A.S.T.11. Committee D-I. In recoguition of this need, a wrirs of investigations has been

\~>OOH Phthalic

Sebacic

Approximately 1 kg. of the dieth! 1 ester of each of the above acids was prepared and purified hv frwctional vacuum distilln-

1454

1455

Y O L U M E 21, NO. 1 2 , D E C E M B E R 1 9 4 9 i ion, Iletails of thts pi~cy:ir:itioii arid characterist,ic physical 1)roIwrties determined or1 tht. wtrrs will he reported in a subseq u i ' r r t paper. Preparation of the Amides. The formation of' A\-benzyl tniclrr proceeds accordirig to the following reaction, wing dih!.l :itlipate as an example:

4.1

-. I hcay \yere choseii :ts

promising derivatives for two reasons:

Melting Points. The melting poirik of' the eight diherizyl :imides were drtrrniined hy thr capillary mrthod, using a Thirlr tube equipped with an air hubhle stirrer, arid were correcatrtl for tJnirrgeiit stern. The results, rounded to the nearest integcar, I)wmw itrid King MY listed in Table 11. Tliv dattt reported l ~ y :ire included for comparison. Infrared Examination. The applirzitioii of iiifraretl exaniiriatioii to the separtctrtl lbrnzyl amide fractions permits thca qualitative identifiration of individual wida preseiit in the niisturc,. €GSrein lies the priiiripal dvantagt. of the p r o p o s d nirthotl. Iriasniurh as the us(^ of irifrared mrthods in t h t , solutioii of chemicaal problems has breii discussed ( 1 ), 1 1 0 attempt is tiitrtic~ here to c*onsicierthr fnrrdanit~nt;ilaspects of thc nicthnci.

I ) .V-benzylamine has hrtw reported ( 3 ) to yield satisfactory iitxirtut.ts

with many of thr acids under consideration.

(2) S-

I wtiz>.luniineis a relativt~1~strong base capable of cleaving mono-

nrcst,ic. esters. A similar rr:ccition with polymeric esters \vas thereI I Y ~ considered probahlt,. Iri 1943, Dermer arid King (3j reported the results of a well l ) l : t i i i i c d and thorough investigation of the suitability of 1Yl ) c w > ~ amides l as derivatives for 90-odd carboxylic acids. The orcyaration of the present amidcs was based on their proordure, i\.tiich may be briefly summarized as follows: ,'(

Reflux 1 ml. of liquid or 1 gram of solid ester (or acid) for 1 tiour with 3 ml. of benzylamine and 0.1 gram of ammonium chlox i t l v as catalyst. Cool, ant1 \\-ash with wat'rr to reinow thta esamine. If n o solid separates, acidification with hydrochloric

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.icid sometimes precipitates t,he desired amide. If enough un:i::ictid ejter is left to keep the amide in solution, the ester may l)c esprlled by boiling the oily layer with water. Filter off t,he sidid amide, dry, wash with ligroin, and recrystallize, usually from 'iqueous acetone or ethnnnl. .\ttcsnipts to a p p l ~ .t l i c , :ii)ovta procedure to polymeric ester :(.(I to the several modificatioris ivhirh are incorporated in the ~ollon%igdirections : r i i i l ilirc~ri*scdsubsequently in some detail: I i c ~ H u s10 grams of esier fur 2 hour:, with 30 ml. of henzylaniiiie 1 gram of ammonium chloride. Pour the reaction mixture .into 300 ml. of benzene, a,nd allow to cool to room temperature. Bulky precipitates usually requirr a double precipitat,ion.) Filter off the precipitated :mitit, dissolve in a minimum volume $ i f \v:irni ethanol, and tqxeripitate by pouring into 300 ml. of e l i l u t tl hydrochloric ac+l (approximately 1 iY). Filter off thc Iwt'c*il)itxtedamide, wash w i t h cold 50% ethanol, and dry. ( 'oricrritrate the filtrate Ernin t,he first separation of amide to * I I I I S h:+lf the original volume, cool, and pour into 300 ml. of petro~ w nt i~ hvr. t Purify tiit. prt,ripit:itrd :imide as outlined ahovr. :tiid

_ _ _ _ _ _

'l'ahle I.

__

-

. .

Nitrogen Content of the Dibenzll 4mides c"

-

Prepared from Adipic ester Azelaic ester Citraconic ester Fumaric ester Itaconic ester Llaleic ester Phthalic ester qeharic ester

1)ptermined 8.53 7.47 8.98

___

Xitropen

Palrrilaied 8.64 7.69 9 09

9.48 X.Y6 9.41

9,53 9.09

7.98 7 , 2,-,

8.13 7.37

9 53

'Tlw amides prepared by the application of the niodified pro,,t,tlurvto the eight diethyl rsters were dried to constant weight in \.:Leuo. .4 portion of rwcli \v:is dissolved in ethanol, and titratrd T o a phenolphthalein t,nd point with 0.5 S alcoholic potassium Ii~.tiroxide. No titratalil(8 :icitiity was observed in any instance. (;rider the microsroprs, t hv :tmidc,s appeared as homogeneous ,,rystalline products. The determination of the various optic5:il jiroperties will be discussed in :isubsequrnt paper. Nitrogen Content. The results of Kjeldahl nitrogen deterIiiiiiations run on the c4ght dibenz>-l amides are listed in T:thle I. 'The theoretical values are inc~ludrtifor comparison.

Figure 2. Infrared Spectra of Dibenzjl hrnides A.

Phthalicacid.

H. Fumaric acid. C. Itaronic arid

The spectra shown in Figures 1 arid 2 were obtained oii Sujol oil slurries of the benzyl amides, using a Perkin-Elmer infrared spectrometer Model 12-4. The instrument was equipped with a sodium chloride prism, and had been converted to a per cent transmit,tance spectrophotometer. The spectra are limited to six curves, since, because of isomerization, the amides obtained from maleic and fumaric esters were identical, as were those from cit.raconic and itaconic esters (see also the determined melting points). To conserve space, only the region from 1700 to 700 cm.-l is shown, although furt'her characteristic differences were apparent in the rest of the spectrum. The absorption contributed by Kujol is designated by the letter S in each figure. I t is apparent that the differences illustrated are sufficient t o permit the differentiation of the amides even among fairly close members of the same homologous series-e.g., Figure 1. Qualitative Solubility Data. Inasmuch as the infrared data demonstrated the presence of only six discrete amides, the solubility measurements xere confined to these. The solvents

Table 11.

llelting Paints of 1)ihenzjl imides Reported 1zI.P..

Prepared from Adipic ester Azelaic ester Citraconic ester Fumaric ester Itaconic ester llaleic ester Phthalic ester Sebacic ester

c.

188-189 S o t reported Xot reported 903.5-905 Not reported 149-150 178-175 166-167.

;1

DrtPrryned 51.P C. 180 147 106

207 106 906

178

166

ANALYTICAL CHEMISTRY

1456 selected were benzene, petroleum ether, and 50% ethanol, all at room temperature. I n the case of the saturated amides, the solubility was negligible in all three solvents. The solubility of the unsaturated amides in petroleum ether was negligible. A low degree of solubility in benzene was observed, as well as a somewhat higher solubility in 50% ethanol. The itaconic amide was the more soluble in both solvents, with a calculated approximate maximum solubility of 750 to 800 mg. per 100 ml. of benzene, and 2000 to 2500 mg. per 100 ml. of q50% ethanol. DISCUSSION OF EXPERIMENTAL RESULTS

Preparation. I n applying the present procedure to polymeric samples, particularly to contact resins, the polyester fraction is first isolated by precipitation with a nonsolvent such as petroleum ether. The justification and advantages of the modified preparation procedure used in the present investigation may be summarized as follows: An increase in size of sample was required by the complexity of the polymeric samples, which usually include at least two acids. An increase in reaction time was required by the polymeric nature of the sample. This point will subsequently be discussed further. Substitution of benzene for water accomplishes two purposes: (1) Unreacted polymeric esters, being generally soluble in benzene, do not interfere with the filtration by appearing as an immiscible oil. (2) Benzene acts as a selective solvent for the amides of saturated and unsaturated acids, dissolving the latter in t,he presence of unreacted benzylamine. Because the unsaturated amides are not precipitated when the reaction mixture is added to benzene, the subsequent use of a nonsolvent such as petroleum ether is required to precipitate the uneaturated amides from the concentrated filtrate. Although Dernier and King ( 3 ) recommend the use of hydrochloric acid only when necessary, its invariable inclusion in the washing procedure has been found advantageous. Contaminants found with the precipitated crude amides include: benzylamine hydrochloride resulting from the reaction of the reagent with the ammonium chloride catalyst, unreacted ammonium chloride, benzylamine carbonate resulting frqm the exposure of benzylamine to air, and unreacted benzylamine. rlll these are readily removed by the use of dilute hydrochloric acid, in which the amides are substantially insoluble. The efficiency of the wash is greatly increased by the dissolution of the crude amide fraction in ethanol prior to treatment Rith dilute acid. Although no attempt was made to determine the qua~ititative conversion of the monomeric esters t,o the corresponding amides, a preliminary study of a polymeric ester of known composition indicated that only a small increase in yield of benq-1 amide was obtained on increasing the reflux time from 2 t o 4 hours. No further increase was observed when the reflux timr was again doubled; doubling the amount, of reagent was also without ohservable effect. Some attempt ~ a made a to determine the sensitivit? of thr method. It, was first evaluated qualitatively, with good results, on binary and ternary mixtures of the monomeric estem .As an instance of extreme practical dilution, it was found pospihle to detect 1 mole of unsaturated acid in the presence of 9 moles of saturated acid. The method was then evaluated on a series of known polyesters, each containing one saturated and one unsaturated acid. Good results were obtained on four such pa,irs, with compositions ranging up to 5 moles of saturated acid to 1 mole of unsaturated acid. These sensitivities do not necessarily represent limiting concentrations. Melting Points and Infrared Spectra. The identical amides obtained from maleic and fumaric esters, and from citraconic and itaconic esters, respectively, deserve further discussion. I n the case of maleic and fumaric acids, the latter is known to be the lower enerKy (more stable) mrmhrr of t,his pair of geometrical iso-

niers. Because halogens and halogen acids have been reported ( 4 as effective catalysts for the maleic -+ fumaric rearrangement, it seemed reasonable to assume that the amide isolated was that 0: fumaric acid. This assumption u 8s confirmed by reprocessing maleic and fumaric esters by the wual procedure, but omitting the hydrochloric acid wash. The faompoundobtained from fu. maric ester had a melting point and an infrared absorption spectrum identical a i t h that reiulting from the application o r the recommended procedure. In the case of maleic ester, a com pound nith a different melting point and infrared spectrum was obtained. This compound contained 10.23% nitrogen, and had 6 correctrd melting point of 150" C., compared with a melting poini of 149-150" C., reported b) Dermer and King for the dibenzyi amide of maleic acid. Because this compound was apparent!: not the maleic amide, Some further reaction was indicated The compound was converted to the normal dibenzyl amide of fumaric acid, containing 9.444, nitrogen, by treatment with dilute hydrochloric acid. The relationship between the vitraconic and the itaconic acid was not so evident, as these compounds are structural but no1 geometric isomers. Examination of the infrared spectrum of a perfluokerosene suspension of the amide obtained by the recommended procedure indicated the absence of methyl absorption These data suggest that this amide was that of itaconic acid, intc which citraconic acid had been converted during the courJe of the reaction. K'either hydrochloric acid nor the catalyst was responsible fur this rearrangement; subsequent reactions from which these agents were successively omitted produced an amide with the same melting point and spectrum as that already described. I t was assumed, therefore, that the benzylamine \vas responsible for the observed rearrangement. In partial support of this assump tion, Hickinbottom's ( 5 ) brief discussion of the shifting of the double bond in a,P- and $,?-unsaturated acids under the influence of suitable catalysts may be cited. Aliphatic amines are among the basic reagents listed as generally used to bring about this change. Solubility. The solubility of the unsaturated amides in benzene was considerably less than would be expected from their behaviol during precipitation-separation. I t is probable that the greater solubility observed in the latter inptance was caused by the pre-enrr of iinrparted henq laminr. CONCLC S I 0 3 s

The uork to date iridicateb that characteristic dibenzyl ariuattc can be prepared from polymeric as well as from monomeric esters The saturated amides can be separated from the unsaturated amides. Individual amides can be identified in admixture bx examination of the infrared spectra. L-nder the reaction conditions used, however, both maleic aiio fumaric acids yield the same derivative, as do both citraconic and itaconic acids, as a result of isomerization. hIodification O~ the procedure will be necessary to distinguish between individua members of these two pairs of acids. The scope of the present procedure is not limited to dicarbox)lic dcid esters. Experimental results not included in this paper have demonstrated the applicability of the procedure to polymeric plasticizers which also include the esters of monocarboxylic acids The method could probably be further extended to include dkvd resins in general. iCKNOW LEDGMENT

The opportunit~to plan and initiate a general analytical XIvestigation of polycarboxylic acids was made possible by R Bowling Barnes. The continuance of the program has been encouraged by R. P. Chapman, director of the .4nalytical and Testing Division. Grateful arknowledgment is also made for

145’1

V O L U M E 21, NO. 12, D E C E M B E R 1 9 4 9 the guidance and cooperation of H. Mark, director of the Institute of Polymer Research of the Polytechnic Institute of Brooklyn.

151

LITERATURE CITED

(7)

( 1 ) Barnes, R. B., Gore, R. C., Stafford, R. W., and Williams, V. Z., ANAL.CHEM., 20,402-10 (1948). 42) Bradley, J . J., Jr., Oficial Digest Federation Paint & Varnish Production Clubs, No. 266, 162-76 (1947). (3) Dermer, 0 . C., and King, Jack, J . Org. Chem., 8, 168-73 (1943). (4) Fieser, L. F., and Fieser, Mary, “Organic Chemistry,” p. 285. Boston, D. C. Heath & Co., 1944.

(8) (9)

(6)

Hickinbottom, W. J., “lieactions of Organic Compounds,” 2nd ed., pp. 43-4, London, Longmans, Green’and Co., 1948. Kappelmeier, C. P. A . , Paint,Oil Chem. Rev., 99, No. 12, 20, 22. 24 (June 10, 1937). Mattiello, J. J., “Protective and Decorative Coatings,” Vol. V Chap. 1, pp. 126-34, New York, John Wiley & Sons, 1946. Sanderson, J. M., A . S . T . M . Bull. 107, 15 (December 1940). Wakeman, R. L., “Chemistry of Comme.cia1 Plastics,” Chaps, 11 and 19, New York, Reinhold Publishing Corp., 1947.

RLCEIVEDJuly 19, 1949. Submitted by R. W. Stafford to the Polytechnic Institute of Brooklyn In partial fulfillment of the requirement8 of the Ph.D. degree.

Coulometric Titration of Iodide By Electrolytically Generated Bromine and an Amperometric End Point W 4 H R E S S. WOOSTER, PAUL S. FARRIKGTON, AND ERNEST H. SWIFT California Institute of Technology, Pasadena, Calif.

A method for the coulometric titration of iodide has been developed in which the iodide is oxidized to the unipositive state by electrolytically generated bromine. The end point is determined amperometrically by measuring the current between two platinum indicator electrodes with an impressed potential difference of approximately 140 millivolts. A study of the behavior of the indicator current during the titration has been made. Confirmatory analyses have shown an accuracy of .t294 for samples from 13 to 50 micrograms, and of =tO.S% for samples of from 65 to 2000 micrograms.

A

METHOD and apparatus for the coulometric titration of thiodiglycol have been described by Sease, Xemann, and Swift ( 7 ) , and a further study of this method has been made by Myers and Swift (6) in connection with the determination of tripositive arsenic. In these determinations, the thiodiglycol or arsenic is oxidized by means of electrolytically generated bromine, and the end point is determined by observing the current flow between two platinum indicator electrodes which have a small potential difference impressed upon them. By maintaining a constant rate of bromine generation and measuring the time of generation, the number of equivalents of bromine generated can be determined. The advantages of such secondary coulometric processes in which an intermediate reactant--in this case bromide-is provided in relatively large concentration, so that both constancy and efficiency of current generation can be maintained throughout the titration, have been discussed by Meier, Myers, and Swift (5). Inasmuch as the coulometric principle possesses certain unique advantages, especially its adaptability to automatic titration procedures, further investigations of its applicability seemed desirable. Xeither thiodiglycol and its bulfoxide nor tripositive and quinquepositive arsenic set up reversible half-cells at the generator or the indicator electrodes under the conditions of the above titrations. Therefore, no significant indicator current flows between the indicator electrodes until the titration end point has been reached. By shielding the generator cathode and by modification of the titration procedure, it should be possible to extend this method to substances involving reversible halfcell reactions. Iodide ion has been selected ab an example of such a substance. ; i study has been made of the indicator current characteristics during the oxidation of iodide to iodine monobromide, and a method for the titration of small quantities of iodide has been developed.

EXPERIMENTAL

Reagents. Six formal hydrochloric acid solut*ions were prepared from reagent grade concentrated acid. The acid available commercially was found to contain as much as 3.5 X 10-8 equivalent of reducing agent per milliliter. The amount of reducing agent present was determined by electrolytic oxidation and was removed by boiling the 6 F acid with the calculated amount of 374 hydrogen peroxide. Excess peroxide was destroyed by boiling the acid for about 15 minutes. One formal sodium bromide solutions were prepared from reagent grade salt. These solutions, when tested, were found to contain no extraneous oxidizing or reducing agents. On occasions, laboratory distilled water was found to contain as much as equivalent per liter of an oxidizing agent, presumably chlorine. This was removed by bringing the water to a boil and bubbling clean, dry air through it, Stock 0.1 F solutions of potassium iodide were made up by weight from reagent grade salt which had been dried for 1 hour at 110” C. The potassium iodide used was found to contain no iodate. The solutions were made 0.01 F in sodium carbonate to minimize air oxidat,ion. Dilutions of the stock solutions, 0.005 F i n sodium carbonate, were used directly for tit’rations. Apparatus. The apparatus used wm essentially that employed by Myers and Swift (C), except that the generator cathode was contained wit.hin a glass tube, open at the top and terminating a t the bottom in a sintered-glass disk, and filled above the level of the surrounding solution with 6 F hydrochloric acid. This modification removed the danger or reduction of oxidation products a t the generator cathode and eliminated any indicator current due to the presence of hydrogen in the titrated solution. Procedure. When not in use, the electrodes were stored in a solution 2 F in hydrochloric acid and 0.1 F in sodium bromids. Before use, the indicator electrodes, which were shorted together during storage, were made the generator anode, and then bromine was generated on their surfaccs for a period of 5 seconds, This treatment increased the st,ability of the electrode sensitivity, I n some cases, after the electrodes had been used for antimony or arsenic determinations, it was found advisablc to generate hydrogen on their surfaces for a short time, and then bromine for periods of up to 50 seconds. Sensitivity of thr indicator electrodes, in microamperes of indicator current per second of bromine generation, a t the appropri-