e
FUNCTIONAL GROUPS BY CHEMICAL METHODS M. 6. YAKUBIK E. I. du Pout d e Nemours & Co., Inc., Polychemicals Department, Wilmington, Delaware
THE analytical laboratory requires a wide variety of chemical and instrument,al methods for determining funct.iona1 groups. The choice of technique for a particular analysis depends on a number of factors including the physical characteristics of the sample, the concentration of the desired constituent, the type and quantity of other substances present, the precision and accuracy required. Presented as part of the Symposium on An An&& Group in an Industrial Research Organization before the 1)iviaions of Chemical Edtlcation and .4nalytical Chemistry st the 130th Meeting of the American Chemical Society, Atlantic City, September, 1956.
VOLUME 35, NO. 1, JANUARY, 1958
One of the essential requirements of any analysis is that the sample submitted be truly representative. In sampling, the purpose must be considered and proper judgment used. Too often samples are submitted for analyses which have become contaminated by exposure to the atmosphere or by reaction with parts of the container. Volatile constituents may hare been lost. In many cases these difficulties can he overcome through the use of small sealed vessels for sampling and analysis. These containers are pharmaceutical-type serum bottles with pressure seal stoppers. The pressure seal feature permits transfer of the sample
KARL FISCHER REAGENT-4
many cases due to the presence of interfering materials in the sample to be analyzed or to the nature of the organic compound in question, some research often is necessary to adapt a procedure to a particular situation.
I1
DETERMINATION OF ACID FUNCTION
The acid function is an example. If total free acidity is required, direct neutralization with standard base often can he used. Where applicable, the simplest and most reliable method for mixtures containing carboxylic and stronger acids is titration with standard caustic in an aqueous system. However, easily hydrolyzed esters, aldehydes, weaker acids (K. anhydrides and acyl halides will interfere (see Tahle 2). The use of methanol as solvent and sodium methylate
-
MAGNETIC STIRRING BAR Fig".*
Smdl 6.d.d
1.
Vra.1 for Moiatu.. D.t.rmin.ti.3".
(P.P.~.)
through hypodermic needles and syringes without exposure to the atmosphere (1). This equipment allows collecting, storing, and analyzing of samples from which atmospheric moisture and carbon dioxide must be excluded. Modified vessels are readily made for specific applications as illustrated in Figure 1. The small sealed vessel shown in the figure is used for the determination of moisture a t concentrations of parts per million in an alcohol stream. A sample is taken directly via hypodermic needles into a specially conditioned vessel. Conditioning involves stabilization of the thin film of moisture on the inside of the container prior to use by careful drying a t about 140°C. followed by overnight storage at about 5'C. After inserting the electrodes, the moisture is determined by direct titration with Karl Fischer reagent to the dead-stop end point. Typical organic functional group and compound determinations to which chemical methods are applicable are presented in Table 1. Water has been included in the tabulation because of the importance of this determination in organic compound analysis. Actually a variety of procedures for the determination of a specific function must he available since no one method is generally applicable to all situations. In TABLE 1 Typical Functional Group Determinations
GTOUP 07
P&cipd
9mthod8 analusis
subslanee
Of
Ref.
AcidAnhydride Acyl halides Ester Hydroxyl*
Neutralisation, esterification Hydrolysis-estedication Hydrolysi~esterification Saponification Acetylation, esterification
(8) (3) (5,4) (5) (4,3 7, 8,
Carbonyl
Oximration
Alkoxyl~ Peroxide UnsaturateAmine Amide
Zeisel Reduction Halogen addition, hydrogenation Neutralization Hydrolysis
(7,-'8, 10, 11, 38) (7,19,301 (18)
(14)
(7,161 (7,16, 16, ,I\
Nitrile Thioether Sulfide Salt Waterm
Hydrolysis Oxidation Oxidation Acid titration Karl Fiseher reagent titration
(4;'' (13) (18) (8) (4) Further details on determination of these substances are given in this paper.
TABLE 2 Acidimetric Methods for OrganSc acids
Sample
Reagent-System
RCOOH, HX RCOOH. HX in m-esence of easily hydrolyzed esters, aldehydes RCOOH, HX in presence of u-eaker acids or RCOOH, HX, and phenols RCOOH in presence of anhydride
NaOH or KOH in water NaOCH. in methanol KOH or KOCHI in dimethylformrtmide; NaOCH&& NHs in ethylenedismine (CnH.).N in acetic anhydridebenzene; (CJH,)*N in a c e tone
as titrant eliminates the interference of easily hydrolyzed esters and aldehydes (19). A basic solvent such as dimethylformamide (2, $0, dl) or ethylene diamine (2, 2 2 , 2 ) will differentiate between carhoxylic and weaker acids. A tertiary amine in a nonaqueous environment can be used as titrant for determining acids in the presence of anhydrides (2, 25, 26). Diferentiation among acids of varying strengths is best made in nonaqueous systems by potentiometric methods. An alcoholic environment will give separate inflections for mineral and carboxylic acids. A basic environment will permit separation of mineral acids, carboxylic acids, and phenols. For some systems acidimetric methods are not applicable. Consequently a variety of nonacidimetric methods are necessary as illustrated in Tahle 3. Carboxylic acids often can be determined directly in the presence of large concentrations of other acids or reactive esters by an aquametric method based on esterification with methanol in the presence of boron TABLE 3 Nonacidimetric Methods for Oreranic Acids
Type of acid Carboxylic Lower fatty acids Aromatic a-Aminortcids
Reaction BFz RCOOCHl RCOOH CHaOH---a GRCOOH KIO. 5KI 3L 3H20 6RCOOK Cu CO1 RCOOH LRH RCH(NH,)COOH
++ +
+
+ H20
++
a:>cHoH + RCHO + CO? + NHa
JOURNAL OF CHEMICAL EDUCATION
trifluoride as catalyst (9, 4). An iodometric method is useful for small amounts of the lower fatty acids, particularly where thiosulfate is present t o react with the iodine as it is formed (9, 27). Aromatic acids often may be determined selectively by decarboxylation in the presence of a metal catalyst such as copper (2, 98). Aminoacids may he determined by acidimetric titration in a basic or acidic system. A specific reaction for a-aminoacids, however, is based on reaction with triketohydrindene hydrate (ninhydrin) in which the evolved carbon dioxide is measured (2,29). ANALYSIS FOR ALKOXYL GROUPS
The analysis for the methoxyl and ethoxyl function was originally proposed by Zeisel (SO), and was based on cleavage by hydriodic acid to form the alkyl iodide followed by gravimetric measurement of the halide as silver iodide. The more common methods a t the present time, however, are volumetric and based upon oxidation of iodide to iodate which is measured iodometrically. The apparatus used in our laboratories for the determination of alkoxyl groups is shown schematically in Figure 2. A number of factors regarding the de-
2.
Fig-
Modified % i d A p p u a t w
termination were found to be critical for obtaining high precision a t very low alkoxyl levels. The necessity of using scrupulously clean glassware cannot be over emphasized. Furthermore, oxygen present even in trace amounts interferes with the analysis by causing nonreproducible and large blanks. Iodic acid probably is formed which is not removed by the red phosphorus scrubber. For a reproducible and controlled reaction the reagents and sample often must be mixed a t dry ice temperature. The reaction flask is warmed slowly to room temperature and if the reaction becomes violent, quickly cooled with dry ice; a positive flow of oxygen-free nitrogen is maintained a t all times. Once a t room temperature and with the reaction under control the flask is then immersed in an oil bath a t 145" 5°C. and refluxed for one hour. After reaction is complete, the scrubber is drained into a flask containing potassium acetate solution which has been flushed with and maintained under a
*
VOLUME 35, NO. 1, JANUARY, 1957
blanket of oxygen-free nitrogen. Before the titration, the solution is thoroughly sparged with nitrogen. Failure to maintain the system completely oxygen free a t this point can cause an increase in the blank titer by a factor of ten. Contrary to the observations of some others concerning the quality of hydriodic acid and its effect on the results, it was found in this work that reagent grade hydriodic acid was suitable. Perhaps the best criterion for this determination is the blank. Using Merck reagent grade hydriodic acid without further purification, reproducible blank titers as low as 0.007 ml. of 0.1 N sodium thiosulfate can be obtained. Reliable results for as low as 0.004% methoxyl have been obtained using a 0.3-gram sample. DETERMINATION OF THE HYDROXYL FUNCTION
Alcoholic hydroxyl groups usoally are determined acidiietrically by acetylation procedures with acetic anhydride or acetyl chloride (Sf). Investigation in our laboratories of the use of acetic anhydride-pyridine reagent a t concentrations of about 1.5-2.0 M anhydride indicates that many hmdered alcohols can be acetylated in less than three hours. The reaction is carried out either in pressure bottles a t 98"-100°C. or a t reflux temperatures of about 118°C. The latter procedure is recommended for sterically hindered alcohols which are not completely e s t e d e d in a reasonable time using the pressure bottle technique. Careful control and strict adherence to procedural details provides a high precision method for the determination of hydroxyl groups. Nearly all primary and secondary hydroxyl groups can he analyzed satisfactorily by the esterification procedures using acetyl chloride or acetic anhydride. The methods, however, lack sensitivity where the materials contain considerable amounts of water or acids. Water reacts with the acetylating reagent in competition with alcohols and necessitates the use of smaller analytical samples in order to maintain the necessary excess of reagent. Furthermore, tertiary hydroxyl groups usually are partially dehydrated by these reagents and, therefore, cannot be determined. Amines and amides are acetylated so that hydroxyl groups cannot be determined in the presence of these materials. A hydroxyl determination which remains precise in the presence of large amounts of water or acids involves esterification of the hydroxyl group with acetic acid using boron triflnoride as a catalyst (4,39) according to the reaction: ROH
+ CH,COOH
BFa
CH,COOR
+ H20
A measure of the water of reaction which is equivalent mole for mole to the hydroxyl content is obtained by direct titration with Karl Fischer reagent. The latter procedure represents an important method for hydroxyl groups since it can be applied to the determination of tertiary hydroxyl groups as well as the primary and secondary ones in the presence or absence of amide groups. Furthermore, differences in the relative esterification rates of the aliphatic and strictly aromatic alcohols have made possible a means for differentiating between these two classes of compounds by this procedure.
DETERMINATION OF UNSATURATES
A number of difficultiesare associated with many of the halometric methods for the determination of unsaturation. Many factors such as time of reaction. temperature, excess of reagent and solvent enter int,o the halogenation process, and substitution as well as addition is frequently encountered. The over-all result is that most of the halometric methods are generally applicable only to certain compounds under special conditions. A more reliable method for determining unsaturates employs quantitative hydrogenation. The unit illustrated in Figure 3 was designed for use a t approximately room temperature and atmospheric pressure (53). The measuring equipment and reaction vessel are housed in an air thermostat which maintains the temperature a t about 30' O.l°C. Pressure is held constant a t several millimeters above atmospheric by the automatic leveling device consisting of a reversible motor activated by contacts in the mercury manometer. When hydrogen is absorbed, the upper contact of the manometer is broken and the relay causes the motor to raise the leveling bulb until contact is again made. A second relay then starts the reversing water which lowers the bulb until the contact is broken preventing overdrive. The response of the system is so rapid that even a t high hydrogen uptake the pressure remains constant within +0.1 mm. The amount of solvent in the gas buret is limited to that diffusing through the capillary against the mass transfer of hydrogen into the reaction vessel. Analysis for solvent vapor in the gas buret after a determination showed no more than 100 p.p.m., indicating that no correction is necessary for the vapor pressure of solvent.
*
OTOR
2
RELAY
-
1r-
HERMOREGUL
Fi.ure 3.
Hydrogenation Amarat".
The gas buret used is composed of two sections, the lower of approximately 40 ml. capacity (accurately calibrated) and the upper of 10 ml. with 0.05 ml. graduations allowing either macro or semimicro scale determinations. The operation involves charging the double reaction vessel with solvent and catalyst on the one side and sample on the other and flushing both sides with hydrogen. The flasks are then stoppered and the catalyst stirred until no more hydrogen is ahsorbed. (This step eliminates a blank correction.) After readjustment of the volume of hydrogen and transfer of catalyst, the system is allowed to operate until the volume is constant. Results are calculated as millimoles of hydrogen per gram of sample (hydrogenation number), the reciprocal of which is the weight in grams equivalent to one double bond. Palladium and platinum were found to be the best catalyst systems, the former, in neutral solution being nearly specific for aliphatic unsaturation, and the latter suitable generally for all types including benzenoid structures. Glacial acetic acid is the most useful solvent for use with platinum while ethanol or dimethyl formarnide is used with palladium. A variety of i u ~ saturated materials have been hydrogenated with a precision of about +0.3%. DETERMINATION OF WATER
The determination of water is of great importance in analytical chemistry and should be included in nearly every complete analysis. In many organic systems, water represents a common interference which may affect or inhibit the course of a reaction. The presence of excessive amounts of moisture in polymers may adversely affect their physical properties. The estimation of water, therefore, has been the subject of a great deal of analytical activity. Karl Fischer reagent has become highly useful for the determination of moisture associated with a wide variety of compounds and reactions. A complete summary of the analytical details and applications of Karl Fischer reagent has been presented (4). Several specific applications have been made for using the reagent for the determination of moisture in polymeric materials. A compact field kit based on the use of small sealed vessels has been devised for the determination of moisture in acrylic molding powder (54). The principle of the kit is the same as the standard laboratory method, namely, the titration of a chloroform solution of the polymer with dilute standardized Karl Fischer reagent which has been protected from moisture in the atmosphere with pressure seal stoppers. Fischer Scientific Company-stabilized Karl Fischer reagent (Cat. No. SO-K-3) diluted with methyl "Cellosolve" to a water equivalence of about 2 mg. H,O/ml. was found to have excellent stability. In those cases where a polymer is insoluble in a suitable solvent or soluble only a t high temperatures, an alternate technique is available for the determination of moisture. This method is based upon a vacuum distillation of low boilers including water from the polymer sample into a trap maintained a t about -70°C. The water present in this condensate is determined by titration with standardized Karl Fischer reagent. The apparatus used for this determination is shown in IOURNAL OF CHEMICAL EDUCATION
rigura 4.
Appauilt".
for Determination of Moisture in Polymers
Figure 4. The sample is accurately weighed into t.he sample tube, which is connected as shown to the cold trap and to the vacuum, and heated for a specified period of time in the aluminum block. For nylon a temperature of 260" 2°C. is used and the distillation is carried on for 30 minutes. The exact time and temperature used should he determined for each polymeric system investigated. After completion of the d i d lation the cold trap and sample tube are isolated from the vacuum source and slowly vented through a drying tube. The trap is then disconnected from the unit, washed thoroughly to a specified volume with dry methanol and titrated with Karl Fischer reagent. A blank is run with each set of determinations using an empty test tube and fdlowing the same procedure for the sample. The sensitivity of the visual Karl Fischer titration is about 0.1 mg. water.
*
ANALYSIS OF MIXTURES
cation to the determinat,ion of hydroxy and carbonyl compounds has been published (55). The absorbance at 625 mp is a specific measure of the cyclohexanol content in the mixture. Both constituents absorb a t 535 mh and the determiuation of cyrlohexanone is based upon correction of this absorption for the presence of the alcohol. The formic acid and methyl formate in t,he mixture may also he determined simultaneously. The analysis is hased on the titration of the acid with standard caustic to the bromthymol blue end point followed by saponification of the ester in the same solution. The method for the determination of methanol in the mixture is based on the reaction of low nlolecular wight aliphatic alcohols with nitrous acid to form the alkyl nitrite (56). The apparatus used for t,he determination has been modified by R. H. Kinsey of our laboratories and is shoxm schematioally ill Figure 5. Since methyl nitrite is a gas, (b.p., 12'C.), it is readily removed by sparging with carbon dioxide. The methyl nitrite is hydrolyzed in acid solution to form nitrous acid and methanol. The presence of excess potassium iodide reduces the nitrous acid releasing an equivalent amount of iodine. The quantity of iodine released is proportional to the methanol present and is measured iodometrically with sodium thiosulfate under a blanket of carbon dioxide to exclude oxygen. The interference due to oxides of nitrogen and other reducible decomposition products of nitrous acid carried out of the reactor are removed by the permanganate and bicarhonate scrubbers. A recovery of 80.0% 6% relative is obtained in the range of 0-20 mg. of methanol. The method is sensitive to as little as 0.3 mg. of methanol x~itha precision of *8% relative, and, since sample size is not limited, the sensitivity can he extended to the 0-20 p.p.m. range. Methylal, a coudensation product of one mole of formaldehyde with 2 moles of methanol, can be determined in the mixture spectrophotometrically. The method is based on acid hydrolysis of the acetal to produce formaldehyde which forms a purple dye with chromotropic acid (57). The resulting color is measured a t 570 m.p. The other substances present in the above mixture do not interfere with the determination which is sensitive to about 0.05 mg. of methylal. The accurate analysis for water in the mixture is
The organic analytical chemist often requires procedures for the direct analysis of mixtures of similar or dissimilar compounds. I n many cases a combination of chemical or physical techniques can best be used to solve such problems. Consider the analysis of a hypothetical mixture containing the following components in the ranges specified: Cyclohexanol (20-700 p.p.m.), cyclohexanone (20-700 pp.m.) formic acid (0-0.5%), methanol (0-0.5%), methylal ( 0 . 3 % ) , methyl formate (0-50 p.p.m.) and water. In such cases, methods which are based on selective reactions with the constituents of the mixture as well as functional group methods become necessary. Cyclohexanol and cyclohexanone in the above mixture can be determined simultaneously. The method, a colorimetric procedure, is based on the action of para- hydroxyhenzaldehyde SlNTERED GL MEDIUM PORO in the presence of sulfuric acid to yield a blue dye. The reaction is often referred to as the Komarowski reaction and the appli~ig~7.e S. A P P ~ Z ~ ~for US ~ VOLUME 35, NO. 1,
JANUARY, 1958
*
(BASIC) e t h Nitrite y ~ ~~~~~d~~~for M ~ O H~ . t . ~ ~ i ~ ~ t i . ~
9
primarily dependent upon the amount of water and the sampling technique used. The analytical reagents, interferences, and limitation of the Karl Fischer method offer very little difficulty over the entire range from 1 p.p.m. to essentially aqueous samples. The titration using Karl Fischer reagent is preferably made on a sample containing from 20-200 mg. of water. The chances for error in this determination are greater as the water content is increased. Samples containing above 10% water are best handled by weighing a suitable amount of the sample into a known volume of dry methanol. An appropriate aliquot of this solution, corrected for the methanol blank, will provide the opportunity for greater accuracy than the direct titration of smaller portions of the original sample. Quantitative chemical methods are essential in the modern analytical laboratory. I n many cases the accuracy and precision of the chemical methods are better than is possihle with instrumental techniques. Frequently such methods are used to calibrate the more relative instrumental procedures. Chemical analytical methods in conju~lction with instrumental procedures are necessary to handle best the variety of problems encountered. LITERATURE CITED (1) SM~TH, D. M., J . MITCHELL, JR., A N D A. M. BILLMEYER, Anal. Chem., 24, 1847 (1952). (2) MITCHELL, J., JR., B. A. ~ ~ O N T A G U E ,AND R. H. KINSEY, in "Organic Analysis," Vol. 111, Interscience Publishers, Inc., New York, 1956, pp. 1-95. (3) HAXMOND, C. W., ibid., p. 97-128. ( 4 ) MITCHELL, J., JR., A N D D. M. SMITH,'(Aquametry," Interscience Publishers, Inc., New York, 1948. (5) HALL,R. T.,AND W. E. SHAEFER, in "Organic Analysis," Vol. 11, Intencience Publishers, Inc., 1954, pp. 2l370. (6) MEHLENBACHER, V. C., ibid., V d . 1, 1953, pp. 1-65. (7) SIGGIA,S., "Quantitative Organic Analysis via Functional Groups," 2nd ed., .John Wiley & Sons, Inc., New York, 1954. (8) STONE,K. G., "Determination of Organic Compounds," McGraw-Hill Book Co., h e . , New York, 1956. (9) DALNOGILRE, S., A N D A. N. OEMLER, Anal. Chem., 24, 902 (1952).
(10) MITCHELL, J., JR., in "Organic Analysis," Val. I, Interscience Publishers, Inc., New York, 1953, pp. 243-328 JR., '4nal. Chem., 22, 750 (11) SMITH,D. M., A N D J. MITCHELL, (1950). \----,.
(12) SAMEL,E. P., AND J . A. MCHARD, Ind. Eny. Chem., Anal. Ed., 14, 750 (1942). (13) WAGNER,C. D., R. H. SMITH,A N D E. D. PETERS,Anal. Chem., 19, 976 (1947). (14) POLGAR, A., AND J. L. JUNGNICKEL, in "Organic Analysis," Vol. 111. Interscience Publishers. Inc.. New York. 1956. p. 203. (15) HILLENBRAND, E. F., AND C. A. PENTZ,ibid., pp. 129-201. (16) MITCHELL, J., JR., A N D C. E. AGHBY, J . Am. Chem. Soc., 67, 161 (1945). (17) MORGAN, W. A., A N D T. 0.NORRIS,J . Dental Research, 30, 288 (1951). (18) DAr. NOGARE, S., in "Organic Analysis," Vol. I, Intencience Publishers, Inc., New York, 1953, p. 329-402. (19) MITCHELL, J., JR., AND D. M. SMITH,4nol. Chem., 22, 746
(1948). (23) BROCKMANN, H., AND E. METER,Be?., 86, 1514 (1953). (24) GRAN,G., AND B. ALTHIN, Aeta Chem. Srand., 4,967 (1950). (25) MCCLURE, J . H., T. hl. RODER,AND R. H. KINSEY,Anal. Chenr., 27, 1599 (1955). (26) SIGGIA,S., A N D N. A. FLORAMO, ibid., 25, 797 (1953). (27) KOLTHOFF, I. M., AND E. B. SANDELL, "Texbbook of Qusntitative Inorganic Analysis," 3rd ed., The Maernillan Co., New York, 1952. (28) HUBACHER, M. H., Anal. Chem., 21, 945 (1949). (29) VANSLYKE, D. D., D. A., MACFADYEN, A N D P. HAMILTON. J . Rid. Chem., 141, 671 (1941). (30) ZEISEL,S., Monatsh., 6, 989 (1885). (31) SMITH,D. M., AND W. hf. D. BRYANT, J . Am. Chem. Sac., 57, 61 (1935). (32) BRYANT, W. M. D., J. MITCHELL, JR., A N D D. M. SMITH, ibid., 62, l(1940). (33) O ~ M L EA. R ,N., K. H. OBOLD, W. HAWKINS, AND J. IMITCH-
JR.,paper presented at 7th annusl Delswere .4.C.S. Symposium, Univ. of Delaware, Feb. 19, 1955. (341 . JR... Anal. Chem.. 28, 1502 ~~ , ROTE. C. F.. A N D J . MITCHELL. ELL,
(1956). ' (35) DAL NaoAnE, S.,
- - --
(IQ5RI , ,.
AND
J . MITCHELL, JR., ibid., 25, 1376
(36) ENDER,W., 4ngew. Chem., 47, 227 (1934). (37) BRmKER, C. E., A N D H. R. JOHNSON, Ind. Eng. Chem. .4nal. Ed., 17, 400 (1945). (38) MITCHELL, J., JR., AND 15. R. ROE,ibid., 23, 1758 (1951).
IOURNAL OF CHEMICAL EDUCATION
