Determination of Functionality in Organic Compounds STIG VEIBEL Technical L'nioersity, Copenhagen,Denmark
HE symposium of six papc'rs 011 tht. determination of funcrrtionality in organic compounds (published in the Xarch 1950 issue of L 4 ~CHEMISTRY) ~ 4 was~received ~ with ~ gwat ~ interest by the department of organic chemistry. During the past 25 years this departmcnt has t d its final course in practical organic chemistry for gladuat udents on the identification of organic sulistanceu by nic'ans of funct,ional groupsthat is, t,ht%drtcaction of such groups, t,he det,ermination of thc c~quivalr~nt iwight of the su1)st:inccs with regard to the functional group (or a auliatituc~ritsuch a i a halogvn atom), and the prepara tion of dt~iivativt~a of the substance which may servv t o vc.rify tlic. awuniptioii. This papcxr has 1)ccn tvrittcsn t o supplenient the inforrnation givc.rl in thcx papvrs nic~ntionctl, especially the p,:~pc,r of Siggia (fd),a n d the tcstliook l)y the same aut,hor (16). If tht. sul)st:incv, contains :i functional group n-hich may easily I)(' tl~~tc~rniint~ti l)y titration, this group is usrd for thr d tion of thc rquivii1t:nt \wight of thr sut)stance. Such groups itrc' sulfonic, acid groups and carboxylic groups, nicrcapto groups tvhich may I)[, titrated with iodine, nitro groups which may be titrated nith titanous chloridc, arid ioriogenic halogen in hydroIinlogenidc~sof miiric:s \\.hirh may bt. drtormined by titration hy the Vol1i:ml method. Hytlrolyzablt~ compounds such as ('sters are tietc,rniintd by boiling n-ith excess standard alkali and hick-titrating the c'xcc'ss. .imides, nit,lilcs, and other compounds yielding ammonia or volatilc aliphatic amines by hydrolysis arcs hydrolyzed by refluxing with 6 S hydrochloric acid, whereupon t,he ammonia or the amine i p determined as in thc Kjtlldahl dotc~rminationof nitrogen. Sulfur in mustard oils, thioureas, and niany thiamides may he cirttlrmined by precipitation of thtl sulfur as silver sulfide in ethanolic ammoniacal solution with standard silver nitratc,, and 1):ick-titration of t,he excess silver nitrate after acidifying thc solution with nityic acid. In many instance9 thc substanctx doe!: not contain a functional group or a substituent suitablr for the c,stiniation of the equivalent \\tight,; in such iristanccs it may be convenient to prepare a dctrivativc containing a functional group or a substituent which will allow the estimation, and, after purification, to usv this derivative for the dct,erminntionof the c,quivalent weight. The following is a brief summary of some of the methods used in the author's lalioratory for thcl cietcction and estimation of diffcxrr,nt groups of substance+
ard alkali or titration of the ,,i-iiitrophenylcarbaniic est,ers \{ it h titanous chloride. Quantitative acetylation with acetyl chloride-pyridine in toluene. ~ ~ ~ CARBONYL CO3IPOUSDS
Detection. Formation of 2,4-~linitropIienyll~~drazones by waction with 2,4-dinitrophenylhydrazine in 2 S hydrochloric acid or, for substances very slightly soluble in water, addition of a solution of 2,4-dinitrophenylhydrazine in concentrated sulfuric acid to a n ethanolic solution of the substance. - Characterization. Through 2,4-dinitrophenylhydrazones, p carboxyphenylhydrazones, semicarbazones, 01' oximes. I n some instanccs formation of condenPation products n i t h dimedonc is preferable. Estimation. Titration of the ~-carboxyplien~-lliydrazones in (11 hanolic solution with standard alkali, elcctrometrically or \vit,h phenolphthalein as indicator ( 1 7 , 25), or>if this titration cannot be carried out, hydrolysis of the semicarhazones with acid, osidation of the hydrazine with iodic acid, and determination of the ammonia by current methods ( 1 8 ) . K i t h a- and P-ketoles, a- and P-diketones, P- and y-ketonic esters arid acids, and with some unsaturated carbonyl compounds, the reaction with hytiraziiit. derivatives is mor(' complicated. -4 survty is given i n (17). ACIDS
Detection. A solution of the substance has acid reaction aritl wquires a measurable amount of alkali for its neutralization. It, ip more soluble in alkali than in n-ater. Characterization. Formation of broniophenacyl ei'toIs or other phenacyl esters. Formation of benzylthiuroniuni salts by reaction with benzylthiuronium chloride (4,19, 20, 2 2 ) . Formation of amides or anilides. Estimation. Titration with standard alkali in aqueous I ~ I ' ethanolic solution. ESTERS
Detection and Estimation. By consumption of :tlkali I)y wflusing the substance with standard alkali. Characterization. Benzylthiuronium salts of the acid may 1)~; precipitated by addition of a solution of benzylthiuroniuni chloi,ide to the hydrolyzed and neutralized sample after addition of a fen. drops of 1 S hydrochloric acid. If the acid is insolulilt: in water it may be precipitated by acidifying the neutralized hydrolyzatcs. For identification of t h e alcohol, one third of the hydrolyzed solution is distilled and the distillate is oxidized n.ith chromic acid. Primary and secontlary alcohols are ositlized to carbonyl compounds, which may be characterized as dinitrophenylhydrazones or as dimedone derivatives (the latter method only for primary alcohols). Tertiary alcohols and phrnole are not found by this procedure. Y o general procedure can he given for the identification of thf, alcoholic part of wters of these substances.
HYDROXYL COMPOUKDS
Aliphatic Compounds. DETECTION. Faint yellow color reaction with ferric chloride and formation of a xanthogenate by treatment Lvith carbon disulfide and solid potassium hydroxide. The formation of a xanthogenate is established by addition of a few drops of a solution of cupric sulfate to the reaction product dissolved in water. If a xanthogenate has been formed, the cupric ions will cause a yellow to brown precipitation of a misturc of cuprous xanthogenate and dixanthogen. Formation of solid esters with p-nitroCHARACTERIZATION. tirnzoic acid or 3,klinitrobenzoic acid. Formation of m-nitrophenylcarbamic esters bj- react,ion with m-nitrobenzazide (6, 13, F 1 , . Titration of the potassium xanthogenates with I~STIMATIOS. iodine or titration of the nitrosubstituted esters with titanous cihloride. Quantitative acetylation with acetyl chloride-pyridine in toluene. Aromatic and Heterocyclic Compounds. DETECTION. Strong color reaction with ferric chloride. Decolorization of bromine water, often accompanied by precipitation of the brominated substance. The substance is more soluble in alkali than in water. CHARACTERIZATION. Formation of aryloxyacetic acids by reaction with chloroacetic acid in alkaline solut,ion. Formation of acetates or benzoates or m-nitrophenylcarbamic esters. 1':sTnfATIoN. Titration of the aryloxyacetic acids with stand-
ANISOLES
Detection. Heated with anhydrous aluminuni chloride, the anisoles are transformed into aluminum salts of phenols, which after acidification may be detected as indicated previously. Characterization. Chlorosulfonic acid transforms anisoles into sulfonyl chlorides, which by treatment with ammonia or ammonium carbonate give sulfonamides (9). Formation of disulfonamides may take place. Estimation. By treatment with hydriodic acid by the method of Zeisel ( 2 4 ) or by determination of nitrogen in thr sulfonamides by the Kjeldahl method. PRIlllARY AMINES
Detection. h-itrogen-containing substances with p-toluenesulfonyl chloride give condensation products soluble in alkali. Aromatic primary amines give diazonium ions with nitrous acid; diazonium ions, with 2-naphthol, give azo dyes. If strongly charged with electronegative groups diazotization of the amine occurs only when sodium nitrite is added to 4 solution of the substance in concentrated sulfuric acid. After dilution with water it is examined if coupling with 2-naphthol occurs.
665
ANALYTICAL CHEMISTRY Characterization. Amines often give slightly soluble salts with picric acid and 2,4- or 3,5-dinitrobenzoic acid. Acylation with acetic anhydride or with benzoyl chloride often gives solid acyl derivatives. Amines react with isocyanates or mustard oils to give substituted ureas or thioureas. Estimation. Inorganic chlorine in hydrochlorides of the amines may be titrated using the Volhard method. Sitrogen in the amine may be determined by the Kjeldahl method if no azo, hydrazino, nitro, or other disturbing groups are present. If such groups are present, a Kjeldahl determination after reduction with zinc and sulfuric acid will in most cases give the total amount of nitrogen. SECONDARY AMIRES
Detection. With nitrous acid, nitrosamines are formed, which, with phenol and concentrated sulfuric acid give Liebermann's (11, 1.2) nitroso reaction. p-Toluenesulfonyl chloride gives condensation products insoluble both in acids and alkalies. Characterization. Through the nitrosamines or the ptoluenesulfonamides. Acyl derivatives may also be used for characterization. Estimation. By titration of the hydrochlorides by the Volhard method or by titration of the nitrosamines with titanous chloride TERTIARY AMINES
Detection. Formation of insoluble salts with potassium ferrocyanide in acid solution. Characterization. Through salt formation. See primary amines. Estimation. By titration of the hydrochlorides by the Volhard method or by determination of nitrogen by the Kjeldahl method. AMINO ACIDS
Detection. The substance gives an almost neutral aqueous solution which, by addition of formalin, becomes acid. (This reaction is given by salts of primary aliphatic amines also.) Only amino acids with primary amino groups give this reaction. Characterization. Through acylation of the amino group or through its reaction with isocyanates or mustard oils (see primary amines). Also salts with 2-naphthalenesulfonic acid may be used for characterization ( 3 ) . Estimation. Through formol titration-that is, the titration of the acid after addition of neutralized formalin solution under strictly controlled conditions. When dissolved in acetone the amino group may be titrated with ethanolic hydrochloric acid; the carboxyl group, when dissolved in ethanol, may be titrated with alkali. AMIDES, UREIDES, NITRILES
Detection. Boiling with alkali for some time causes hydrolysis; the vapors will turn red litmus paper blue. Characterization. All members of this group may be characterized through the acids formed by hydrolysis. Amides and ureides are often more easily characterized as xanthydryl derivatives by boiling an ethanolic solution with xanthydrole. Estimation. Hydrolysis by boiling the substances for 1 hour with 6 .?i hydrochloric acid and estimation of the ammonia (or the amine) formed by current methods. Determination of nitrogen by the Kjeldahl method may also be used. AROiMATIC NITRO COMPOUNDS
(And other nitrogen compounds derived from a higher degree of oxidation than ammonia) Detection. The substance is reduced to a substituted phenylhydroxylamine by boiling in 80% ethanolic solution with zinc dust for some minutes. The hydroxylamine reduces Fehling's solution. It is oxidized by aqueous ferrichloride to a nitroso compound which is soluble in ether with a green or blue color. Characterization. The substance is reduced to amine with tin or zinc and hydrochloric acid. The amine is characterized as indicated previously. Estimation. Reduction with titanous chloride in sodium citrate-buffered aqueous ethanolic solution a t room temperature. Nitro and azoxy compounds requlre 6 equivalents of titanium chloride; nitroso and azo compounds require 4, but some azo compounds are rearranged into benzidines during the titration and will use only 2 equivalents of titanous chloride. HALOGEN COMPOUNDS
Detection. Through the sodium fusion test. Characterization. Aliphatic halogen compounds are conven-
iently transformed into alkylthiuronium salts by heating an ethanolic solution with thiourea. By addition of a solution of picric acid the alkylthiuronium picrates are preci itated. Bromatic halogen compounds are treated with chlorosuyfonic acid and the resulting sulfonyl chlorides are transformed into sulfonamides by treating them with ammonia or ammonium carbonate (8). Estimation. Chlorine and bromine may in most instances be estimated by the Stepanov ( I , I 6 )method by boiling the ethanolic solution with sodium. After dilution with water the solution is acidified with nitric acid and halogen and is determined by the Volhard method. Iodine may be estimated in the same manner, but by acidifying the solution with nitric acid hydriodic acid may be partially oxidized toiodine. The method of Baubigny and Chavanne (2) is more accurate; the iodine-containing substance ie oxidized with chromic acid in the presence of silver nitrate. Iodine is transformed into silver iodate, which in turn is reduced to silver iodide by sulfur dioxide, filtered, dried, and weighed. The method of Grote and Krekeler ( 5 , IO) may also be used for estimating the halogens. HYDROCARBONS
Detection. Hydrocarbons are detected by lack of reaction for all functional groups or substituents. Characterization. Aliphatic hydrocarbons are characterized by determination of boiling point, density, and molecular weight (density of vapor). Aromatic hydrocarbons are characterized through sulfonamides (7) or through nitro derivatives. Estimation. For the aliphatic compounds the methods used for characterization are used for estimation also. For the aromatic compounds titration of the nitro derivatives with titanous chloride or determination of the nitrogen in the sulfonamideq by the Kjeldahl method may be used. The course in identification is a 6-week course during which the students work in the laboratory 30 to 36 hours a week. During that time about ten identifications are completed. In principle, no limitation as to the substance given for identification exists. During the course the studenb obtain an excellent training in using Beilstein, Chemical Abstracts, and the periodicals in search of physical constants for derivatives of the substances identified. The course ends with a seventh week (36 working hours) in which the student has to identify two substances without help from the assistant. A report is written in which the student has to indicate the result of the qualitative and quantitative tests and to discuss the possibilities derived from these tests. As an example of the way in which this procedure is utilized, the following description of the identification of a substance, selected among the substances identified by the students without help, is given:
A colorless solid with a melting point of 104" C . is found, by the sodium fusion test, to contain nitrogen, but neither halogen nor sulfur. It is neutral and cannot be titrated with acid or alkali, not even by formol titration. It does not contain nitrogen in higher degree of oxidation than ammonia, gives no precipitation with 2,4dinitrophenylhydrazine (contains no carbonyl group), and cannot be acetylated (contains no hydroxyl, or rimary or secondary amino groups). Boiling with aykali does not cause evolution of ammonia. A Kjeldahl determination of nitrogen shows that 1 atom of nitrogen is contained in 255 g r a m of the substance. The substance is refluxed with standard sodium hydroxide and then back-titrated with standard acid (phenolphthalein indicator). The end point was not sharp, indicating an e uivalent weight of about 240 t o 245. After addition of formain more alkali was used. A determination of the equivalent weight, by a formol titration, gave an equivalent weight of 127. The end point was sharp. The hydrolyzed solution had a smell of phenol. The neutralized solution was extracted with ether, and from the ether layer phenol wtts isolated and identified as phenylbenzoate (melting point 70" C. When the neutralized solution was acidified and extracted with ether once more, a solid acid with a melting point of 120" C . was isolated. By titration the equivalent weight was found t o be 122, and the acid was identified a~ benzoic acid throigh formation of the benzylthiuronium salt (melting point 167" C.).
V O L U M E 23, NO. 4, A P R I L 1 9 5 1 We now know: The substance is an ester, presumably of phenol. By the hydrolysis a n ammo acid and another acid (benzoic acid) are formed. As the substance does not contain free amino or carboxylic groups, it must contain a benzoylated amino group. The Kjeldahl determination and the formol titration of the hydrolyzed substance show that no acid groups other than the benzoic acid and the amino acid are present. The structure of the substance must be C&COPXIRCOOC6-
Hj. From the equivalent weight of 255 are shbtracted C&CONH = 120 and COOCBH~= 121. R, therefore, can only be a methylene group, and the substance must be the phenylester of hippuric acid, the melting point of which (104” C.) is that found for the substance. A further proof for the correctness of the identification is obtained by hydrolyzing the substance with sodium carbonate. Hereby only the phenylester is hydrolyzed, and hippuric acid (melting point 190” C.) may be isolated. The second substance given to the student was hydratropaldehyde, the identification of which is somewhat easier. LITERATURE CITED
(1) Bacon, C.W., J . Am. Chem. SOC.,31,49 (1909). (2) Baubigny, H., and Chavanne. G., Compt. Tend., 136, 1198 (1 903).
667 (3) Bergmann, M.,and Stein, W. H., J . Bid. Chem., 129, 609 (1939). (4) Donleavy, J. J., J . Am. Chem. SOC.,58,1005 (1936). ( 5 ) Grote, W., and Krekeler, H., Angew. Chem., 46, 106 (1933). (6) Hoeke, F., Rec. trau. chim., 54,505(1935). (7) Huntress, E.H., and Autenrieth, J. S., J. Am. Chem. SOC.,63, 3446 (1941). (8) Huntress, E.H., and Carten, F. H., Zbid., 62,511 (1940). (9)Ibid., p. 603. (10) Kirsten, W., Svemk Kern. Tzd., 56, 248 (1944); 57, 69 (1945). (11) Liebermann, C., Ber., 7, 247 1098 (1874). (12) Liebermann. C.. and Kostanecki, S.v., Ibid., 17, 285 (1884). (13) Sah, P.P.T., and Woo, T.-F., Rec. trav. chim., 58, 1013 (1939). (14) Siggia, S.,ANAL.CHEM.,22,378 (1950). (15) Siggia, S.,“Quantitative Analysis via Functional Groups,” New York, John Wiley & Sons, 1949. (16) Stepanov, A,, Be?., 39, 4056 (1906). (17) Veibel, S., Acta Chem. Scand., 1, 54 (1947). (18) Veibel, S., Bull. SOC. chim. France, [4]41,1410 (1927). (19) Veibel, S.,J . Am. Chem. Soc., 67,1867 (1945). (20) Veibel, S., and Lillelund, H., Bull. SOC. chim. France, [5]5, 1153 (1938). (21)Veibel, S.,and Lillelund, H., Dansk Tids. Farm., 12,236 (1940). (22) Veibel, S., and Ottung, K., Bull. SOC. chim. FTance, 151 6, 1434 (1939). (23) Veibel, S., and Schmidt, H. W., Acta Chpm. Scand., 2, 545 (1948). (24) Zeisel, S.,and Fanto, R., 2.anal. Chem., 42, 554 (1903). RECEIVED July 10, 1950.
Quantitative Functional Group Determination In the Identijication of Organic Compounds SIDNEY SIGGIA, General Aniline & Film Gorp., Easton, Pa. DENTIFICATION of organic compounds has been largely
I dependent, in past years, on elemental analyses and on the properties of derivatives. The usefulness of quantitative functional group determinations has been realized, as shown by the introduction of micromethods for alkoxy], alkimino, acyl amino, hydroxyl, and other groups. It is the purpose of this paper to emphasize further the advantages of the latter concept, t o consider the scope of present methods, and to suggest new applications of quantitative functional group analysis. Identification problems lie in two main categories. The f i s t category concerns verification of the identity of a compoundfor example, the chemist may be rather sure of the identity of his compound inasmuch as he carried out a known reaction with known reagents, yet he needs verification of the identity before he can proceed with his work. The second category concerns the identification of unknowns. I t has been standard practice for many years for the chemist who has been working on a synthesis to send his product for the usual elemental analyses to verify the identity. I n recent years it has become iwognized as more advantageous to verify the identity of an organic compound by quantitatively determining the functional groups which should be on the molecule. It is a more meaningful analysis if it is determined that the synthesized compound contains the theoretical amount of nitro group or amino group rather than just the theoretical amount of nitrogen. iilong the same lines, the identity of a hydroxy acid is more firmly established by a hydroxyl group determination and a carboxyl group dtJttirmination than by a carbon and hydrogen determination. Functional group analysis also eliminates the uncertainty which exists when the elemental analyses are close to the theoretical. If a carbon analysis is 1% low out of a theoretical 50.070 carbon, the compound could be 98% pure, but often it is much less pure since organic interferences also contain carbon. Elemental analysis has a definite role in organic analysis, and the above should not be construed as meaning that functional group analysis should replace elemental analysis. However, the emphasis is laid on the greater specificity of the functional group determination.
I n the case of the second category, where the problem is the identification of an unknown, quantitative functional group determination is of value in not only establishing the structure of the sample by giving the ratio of groups to one another, but in obtaining the molecular weight of the material. Saponification equivalent and neutralization equivalent determinations have been used for many years to obtain the molecular weights of esters and acids or bases. However, any functional group determination can be used in the same fashion-for instance, hydroxyl, carbonyl, and amide equivalents are just as useful in obtaining the molecular weight of organic compounds containing these groups. I t is best to illustrate the above points with an example of an actual identification. It was discovered that in the reaction HCECH
+ CH,O+HC-CCH&H
several side reactions took place, and the by-products had to be identified. The product of the reaction was distilled, and two materials were isolated which did not have the physical constants of any of the known materials from the reaction mixture. All the functional groups involved in the system were known, and, no matter which functional groups were consumed in forming the by-products, there should still be some unreacted groups which would be determinable. I n the case of these by-products, the following determinations were run: hydroxyl, acetylenic hydrogen, unsaturation, and free and combined carbonyl groups (the combined carbonyl groups are acetal-like compounds). The hydroxyl and free carbonyl values were zero. However, definite values were obtained for acetylenic hydrogen, for unsaturation, and for combined carbonyl. From these values, the corresponding equivalent weight was calculated. (The term “equivalent weight” is used herein in the same sense as neutralization and saponification equivalents are used. The equivalent is actually the molecular weight of the compound when only one of the determined functional groups is on the molecule. It is one half the molecular weight if there are two such functional groups on the molecule, and it is one third the molecular weight if three groups are present, etc.)
