562 SMITH AND WATTS: ABSORPTION SPECTRA ANDLVIII.-Absolption Spectra aizd Meltirig-point Curves ofA?*onzatic Biuxoamines.By CLARENCE SMITH and CONSTANCE HAMILTON WATTS.THE discovery by Griess that the same diazoamine is formed bydiazotising either of two primary aromatic amines and coupling theproduct with the other, led twenty or thirty years ago to numerousinvestigations which had for their object the determination of theconstitution of such mixed diazoamines and the isolation of thetwo possible isomeric forms, ArN2*NHAr/ and ArNH*N2Ar’. Thelatter purpose has never been satisfactorily realised, whilst theformer has resulted in a mass of such conflicting evidence that evenat the present time the constitution of the aromatic diazoaminesremains an open question.In order to show that the conclusions at which we have arrivedare supported by the bulk of this earlier evidence, it is necessaryto recapitulate the main points made by previous investigators, themore so as an impartial survey of all the facts appears to havebeen omitted hitherto.An examination of the substances obtained by decomposing withwater the product of the action of carbonyl chloride on a benzenesolution of a diazoamine led Sarauw to the conclusion that theimino-group is attached to the more negative aromatic nucleus, andthat the initial product (not isolated) of the reaction is a diazo-carbamide, ArN2*NAr/GO*NAr/*N2Ar, in which Ar is the lessnegative benzenoid group.A perusal of the author’s two papers(Ber.,1881, 14, 2442; 1882, 15, 42) proves, however, that hisevidence is inconclusive, for whilst phenol and dibromocarbanilideare the products arising from the decomposition by water of thediazocarbamide obtained from benzenediazoamino-p-bromobenzene,the action of water on the diazocarbamide from benzenediazoamino-p-toluene yields both phenol and pcresol and a viscous productfrom which only di-p-tolylcarbamide can be isolated.The viscousproduct may and probably does contain diphenylcarbamide, seeingthat both phenol and p-cresol are formed. If such is the case, thediazoamine reacts with carbonyl chloride in accordance with bothformulze, C6H5-N2-NH*C7H7 and C6H5*NH*N2*C7H,. Similarreasoning holds in the case of benzenediazo-m-aminobenzoic acid,phenol and m-hydroxybenzoic acid being the only substancesisolated from the decomposition products of its diazocarbamide.The preceding constitution of a mixed diazoamine has been farmore satisfactorily established by Goldschmidt and Molinari (Ber.MELTING-POINT CURVES OF AROMATIC DIAZOAMINES.5631888, 2 1, 25 78), by heating equimolecular quantities of the diazo-amine and phenylcarbimide in an indifferent solvent, such asbenzene. The product is a diazocarbamide, which can be isolatedand appears to be an individual substance; it is decomposed bywater, yielding a phenol, nitrogen, and a diarylcarbamide, of wliichone aromatic group is always pheny€, and the other the morenegative group of the original diazoamine :Ar*N3H*Ar‘ + C,H,*NCO 4 C,H,*NH*CO*NAr’*N2Ar +C,H5*NH*CO*NHAr’ + N, + Ar-OH.In the preceding year, however, the results of two investigationswere published which partly supported and partly opposedGoldschmidt’s conclusions.Heumann and Oeconomides (Ber., 1887,20, 372, 904) found that diazoaminobenzene, when heated inphenol, reacted to form aniline and benzeneazophenol ; similarly,p-chlorobenzenediazoamino-p-toluene gave p-chloroaniline andp-tolueneazophenol. Benzenediazoamino-p-toluene, however, withphenol or resorcinol gave approximately equal quantities of anilineand p-toluidine and a mixture of hydroxyazc-compounds. I n thesereactions, therefore, some mixed diazoamines behave as if con-stituted in accordance with Goldschmidt’s formula, others likemixtures of equal quantities of ArN,*NHAr’ and ArNH*N,Ar’.Still more striking is the evidence advanced by Noelting and Binder( B e y ., 1887, 20, 3004), who submitted benzenediazoamino-ptolueneand other mixed diazoamines to the attack of numerous reagents,and fonnd that they behaved sometimes in accordance with theformula ArN2*NHAr’, sometimes in accordance with the formulaAr*NH*N,Ar’, but generally as a mixture of both forms.As a result of these and other investigations, two views werecurrent regarding the constitution of mixed aromatic diazoamines,and these have not been materially modified by more recentresearches. One view, which does not appear to have been urgentlyadvanced by any single investigator, regards the mixed diazoaminesas consisting of the two possible isomerides, ArN,*NHAr’ andArNH*N2Ar’. The other theory, initiated by Goldschmidt, regardsthe diazoamines as being constituted so that the imino-group isattached to the more negative aromatic group.Reactions in whichthe diazoamine yields four products of decomposition are attributedto a migration of the iminic hydrogen atom due to the presence ofwater, alcohol, or an electrolyte, and are explained by an initialaddition of water or the like; thus:Ar*N:N*NHAr‘+ HX -+ Ar*NH*NX*NHAr’ ;by the elimination of HX, Ar*NH*N,Ar’ and ArN,*NHAr’ mayresult, and by subsequent decomposition yield each a pair ofproducts. Goldschmidt claimed that the migration of the iminicVOL. XCVII P 564 SMITH AND WATTS: ABSORPTION SPECTRA ANDhydrogen atom does not occur in indifferent solvents, such asbenzene, petroleum, or chloroform, and consequently in such solventsdiazoamines behave as individual substances and not its mixtures,and to this cause attributes the success of his phenylcarbimidemethod of determining the constitution of diazoamines.I n arriving at this theory, Goldschmidt apparently has over-looked the exhaustive researches in 1886-1895 of Meldola andStreatfeild on alkylated diazoamines.Although alkylation m amethod of determining constitution has been viewed in recent yearswith some suspicion, yet in some instances, for example, thephthaleins and the hydroxyazo-compounds, the problem of theconstitution of a substance containing a mobile hydrogen atomhas been approached and to a great extent solved by replacing themigratory hydrogen atom by an immobile alkyl group.I f , there-fore, an alkylated diazoamine can be shown to have a similarconstitution to that of its parent substance, Meldola and Streatfeild’sresearches acquire a new and fundamental significance, and can beutilised directly to prove the untenability of the theory that in a,mixed aromatic diazoamine the imino-group is attached t o the morenegative aromatic nucleus. We have been able to prove the pointin question by means of the spectrograph. Meldola and Streatfeildshowed that three isomeric mp’-dinitrodiazoethylaminobenzenesexist, namely :I. 772-Nitrobenzenediazoethylamino-p-nitroberizene,NO,prepared from diazotised mnitroaniline and p-nitroethylaniline.11.p-Nitrobenzenediazoethylamino-m-nitrobenzene,from diazotised p-nitroaniline and mmitroethylaniline.111. mp’-Dinitrodiazoethylaminobenzene,r;r 0,obtained by the direct ethylation of mpr-dinitrodiazoaminobenzene.Isomeride I11 gives an absorption curve different from those ofI and 11, and absolutely identical with that of its parent diazo-amine, which is thus proved to have a constitution similar to thatof isomeride 111. The constitution of this isomeride has been prac-tically proved by Meldola and Streatfeild, who find that it can bMELTING-POINT CURVES OF AROMATIC DIAZOAMINES. 565synthesised by heating equimolecular quantities of isomerides I andI1 in alcohol or benzene. Meldola and Streatfeild regard I11 as aco’mpound of I and 11.For reasons given below we believe it tobe an equimolecular mixture of I and 11, but for the present purposethis difference of opinion is immaterial, the main point being thatI11 is composed of equal quantities of I and 11, either mixed orcombined. Now, in the diazoamine under discussion, the p-nitro-benzene nucleus is probably the more negative, but whether thisis really so does not affect the argument. Assuming that it is,the diazoamine will be represented by the Goldschmidt theory bythe formula:and the directly alkylated derivative, which must possess a similarconstitution to that of its parent substance from the spectrometricevidence, will have the formula:No2that is, should be identical with isomeride I above. Since experi-ment shows that the directly alkylated derivative is I11 above, thatis, a mixture of equal quantities of I and 11, it follows that thepremise is incorrect, and that mp’-dinitrodiazoaminobenzene con-sists of a mixture (or compound) of equal quantities of the twoindividually unknown isomerides :Of course, the acceptance of this theory at once renders intelligiblethe numerous reactions in which a mixed diazoamine yields fourproducts of decomposition. It only remains t o explain why thediazoamine a t times yields only two products, decomposing asthough it consisted entirely of one of the two unknown isomerides.A t present it is impossible to advance any argument satisfactorilysupported by experimental evidence.The most obvious explanationis the selective attack of the reagent.If one of the two isomeridesis attacked a t a much greater rate than the other, a transformationof the less susceptible into the more susceptible isomeride mustoccur in order to preserve the equilibrium ratio a t unity, and thediazoamine will decompose almost entirely as though it consistedof one isomeride only. The most important case to which thisexplanation can be applied is the phenylcarbimide reaction, sinceP P 566 SMITH AND WATTS: ABSORPTION SPECTRA ANDthis forms the main foundation of the Goldschmidt theory of theconstitution of diazoamines. Goldschmidt attributes the formationof an individual diazocarbamide from phenylcarbimide and a mixeddiazoamine in benzene solution to the immobility of the iminichydrogen atom in an indifferent solvent.Dimroth, however, givesinstances (Annuten, 1904, 335, 1) in which the transformation ofone tautomeride into another by the migration of a mobile hydrogenatom proceeds much more rapidly in an indifferent solvent than ina hydroxylic solvent. Goldschmidt admits that this is correct inprinciple, but is not applicable to the particular case of the diazo-amines, because one of the isomeric forms is unknown (Ber., 1905,38, 1097). Our experiments prove, however, that both forms existas an inseparable mixture in the mixed diazoamine. The unitarycourse of the phenylcarbimide reaction, therefore, may very wellbe due to the selective attack of the reagent, the transformationof one isomeride in the mixed diazoamine into the more susceptibleform being facilitated by the indifferent solvent to such a degreethat one diazocarbamide is produced only in inappreciablequantities.It would be premature to dogmatise from the result of theexamination of a single triplet of ethers originating from a diazo-amine containing aryl groups of not very different character, andwe do not unhesitatingly commit ourselves at present to the viewsadvanced above, and the less so in consequence of Pechmann’s workon amidines of the type ArN:CPh*NH*Ar’ (Ber., 1895, 28, 869),which differ from mixed diazoamines by containing the group CPhin place of a nitrogen atom.The compound C,H5*N:CPh=NH*CpH7,obtained from benzanilide iminochloride and p-toluidine, is identicalwith C,H,-N:CPh-NH*C,H,, prepared from benzoyl-p-toluidideiminochloride and aniline, but yields by ethylation a mixture of twoethyl derivatives corresponding with the two formuk given. WhenAr and Art are different in character, however, the amidines pro-duced by the two methods are still identical, but yield only oneether, indicating an immobility of the hydrogen atom in theamidine.A similar constitution may obtain for mixed diazoaminescontaining aryl groups of very different character, the imino-groupremaining attached to the negative nucleus as in the Goldschmidttheory. This view of the constitution of such diazoamines, which,of course, harmonises well with the chemical behaviour cited above,can be tested spectrometrically, and is receiving our attention.The derivative obtained by the direct alkylation of a mixeddiazoamine has been shown by Meldola and Streatfeild to becomposed of equal quantities of the two isomerides, ArN,*NR*Arfand ArNR*N2*Ar’, by boiling an alcoholic or benzene solution oMELTING-POINT CURVES OF.AROMATIC DIAZOAMINES. 567these isomerides for one hour, whereby a product is obtainedidentical with the directly alkylated diazoamine. The authorsregard this product as a compound of the two isomerides, althoughthey found that the molecular weight in benzene by the cryoscopicmethod agreed with a unimolecular and not a bimolecular formula,a discrepancy which they attribute to dissociation of the compoundin the benzene solution. This explanation is untenable, since thecompound is produced in boiling benzene, and it is very improbablethat it would dissociate in the cold solvent. Mr.T. J. Mander,to whom we proffer our thanks, has determined the molecularweights of several alkylated mixed diazoamines in boiling alcoholand benzene, and has obtained values which are always less thanthose corresponding with the unimolecular formula. The com-pound, therefore, does not exist in the solution, and must beproduced, if formed a t all, at the instant of the deposition of thesolid from the solution. To ascertain whether or not a compoundis formed, we have determined the melting-point curve of mixturesof pnitrobenzenediazoethylamino-mnitrobenzene and m-nitro-benzenediazoethylamino-p-nitrobenzene. The curve is of the simpleU-sbape characteristic of mixtures, and has its minimum at it pointcorresponding with the mixture of equal quantities of the twoisomerides and a temperature identical with the melting point ofthe substance obtained by the direct ethylation of mp‘-dinitrodiazo-aminobenzene. This evidence, combined with that furnished bythe cryoscopic and the ebullioscopic methods of determining themolecular weight, proves that the directly ethylated substance is asolid solution of equal quantities of the two isomerides mentionedabove.EXPERIMENTAL.mp’-Dinitrodiazoarninob enzene.-This compound was prepared inthe usual way from mnitroaniline and p-nitrobenzenediazoniumchloride in the presence of sodium acetate.The crude substancemelted at 214O; after crystallisation from a mixture of equalvolumes of alcohol and toluene, the melting point wils 218-219O.Since Meldola and Streatfeild give the melting point as 212-212-5O(Trans., 1859, 55, 416), the substance waa again dissolved in boilingalcohol and toluene, filtered while still hot (precipitate A), againwhen cold (precipitate B), and yet again after concentrating themother liquor (precipitate C).The melting points of A, B, Crespectively were 220°, 212.b-213°, 219-220O. After re-crystallisation from the same solvent, the melting points were231-232O, 231-232O, 230-231°, in a bath previously heated to180O. After a third recrystallisation, the melting points were231-232O, 231--232O, 228O. In all cases the substance decompose568 SMITH AND WATTS: ABSORPTION SPECTRA ANDat the melting point.The three samples all gave the same orange-red colour in alcoholic sodium hydroxide, and dissolved withoutchange of colour in concentrated sulphuric acid, forming solutionswhich ultimately became colourless. The melting point ofpp/-dinitrodiazoaminobenzene is given by Hantzsch as 2 3 3 O(decomp.), a value which we have confirmed. The melting point ofa mixture of approximately equal quantities of this compound andour mp/-isomeride melted a t 208-21 lo (decomp.).mpf-Dinitrodiazoethylanzinobenzene was prepared by heating thediazoamine with alcoholic potassium hydroxide and ethyl iodide onthe water-bath for eight hours, and twice recrystallising the productfrom alcohol. It melted at 152--153O, dissolved in alcoholic sodiumhydroxide without change of colour, and gave a solution in con-centrated sulphuric acid, which became colourless after a few hours.and p-nitro-B enzenediazoethylamino-m-nitrob enzcne were prepared from diazo-tised mnitroaniline and p-nitroethylaniline and diazotised p-nitro-aniline and m-nitroethylaniline respectively, and were recrystallisedfrom alcohol and toluene until the melting points were constantat 174-5-174.8O and 188*5--188*7O respectively ; the substancesdissolved in alcoholic sodium hydroxide without change of colour,and gave solutions in concentrated sulphuric acid, which ultimatelybecame colourless.The absorption curves of mp’-dinitrodiazoaminobenzene and ofthe three ethylated isomerides are shown in Fig.1.The curves ofthe parent diazoamine and of its directly ethylated derivative areidentical throughout. The curves of the other two isomerides,although naturally very similar to, are quite distinct from, that ofthe directly ethylated isomeride. A comparison of the three curves,particularly in the neighbourhood of oscillation frequencies 3200 to3600, indicates that the curve of the directly ethylated diazoamineis very much what would be expected if the substance is a mixtureof the other two isomerides. The most important result, however,is the proof of the similarity of the constitutions of mp’-dinitro-diazoaminobenzene and its directly ethylated derivative.To obtain the melting-point curve of m-nitrobenzenediazoethyl-amino-p-nitrobenzene and p-nitrobenzenediazoethylamino-m-nitro-benzene shown in Fig.2, it is necessary that the heating of thedifferent mixtures shall be as uniform and under as nearly thesame conditions as possible, for it is well known that the apparentmelting point of a diazoamine can be raised many degrees by rapidheating. Intimate mixtures of the two isomerides were obtainedby making two solutions in benzene distilled over sodium:m-iVitro b enzenediaao et h ylamino-p-nitro b enz enMELTING-POINT CURVES OF AROMATIC DIAZOAMINES. 569A. Containing 0-1575 gram of m-nitrobenzenediazoethylamino-p-nitrobenzene in 250 C.C.B. Containing 0.1575 gram of p-nitrobenzenediazoethylamino-m-nitrobenzene in 250 C.C.Mixtures of different volumes of these two solutions wereevaporated on the water-bath, the residue was detached from thebasin, finely powdered, and transferred as completely as possible tocapillary tubes about 2 mm.in diameter and about 15 cm. long.FIG. 1.Oscillation frequencies.22 24 26 28 3000 32 34 36 30 4000 42 44 46mp'-Di?~~t.iodinxoami~~enzene and its ethyl derivative..___..__._..__..._-._.__._ m- Nitrobenzenediaaoeth y lamino- p-nitrobenzne.____-.- p - Nitrobenmwdiuzoeth y lamino- m- nit robenzene .These tubes were attached to a thermometer 70 cm. in length, havinga range from looo to 200°, graduated in tenths of a degree. Thebulb of the thermometer was immersed in sulphuric acid containedin the outer jacket of a Victor Meyer vapour density apparatusThe temperature of the acid was raised to looQ, then the capillarytube was affixed to the thermometer, and the temperature was raisedfairly rapidly to within loo of the melting point of the precedingmixture; the rate of heating was then adjusted so that the tem-perature rose lo per minute570 SMITH AND WATTS : AROMATIC DIAZOAMINES.CorrcctcdC.C. of A.C.C. of B. melting point.20 0 178'5-1 78.8"19 1 176.618 2 169.1-169*817 3 166.9-167.216 4 164.5-165.415 5 159.6-160.414 6 157.3-157'513 7 156.1-156.511 9 156 '3-156'910 10 155 -4-1 56 -212 a 156.4-156.9CorrectedC.C. of A . C.C. of B. melting point.9 11 157'2"8 12 160 '2 -1 61 -67 13 164 *5-16556 14 166 '8-167 '45 15 169.1-169'84 16 176.1-176.93 17 177.6-177'92 18 185.9-186 '10 20 193.2-1 93.41 19 ia6.9-ia7-2FIG. 2.~ O O A 90 ao 70 60 50 40 30 20 10 ox,0 10 20 30 40 50 60 70 80 90 lOOBAA = m- Nitrobenzencdiazoeth ylamino- p-nitrobenzene.B = p-Nitrobenzeiaediazoethylamino-m -nitrobenzeene.It will be noticed that mixtures containing from 65 to 50 percent. of A melt at only slightly different temperatures, but therABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 571is not the slightest doubt that the equimolecular 50 per cent.mixture has the lowest melting point, which is the same as that ofthe substance obtained by the ethylation of mp'-dinitrodiazoamino-benzene.In order to see whether a compound of the two isowerides isformed under the conditions mentioned by Meldola and Streatfeild,mixtures of A and B were heated under a reflux condenser forone hour on the water-bath; the benzene was then evaporated, andthe melting points of the residues determined as above. Theresults tabulated below show that the melting points are practicallyunchanged by this treatment:CorrectedC.C. of A. C.C. of B. melting point.20 0 178.2-178 '3"15 5 159.3-159'712 8 156.3-156.810 10 155.1-155*7Correctedmelting point. C.C. of A. C.C. of B.8 12 160'4.-160 '9"5 15 168'8-169'80 20 192'0-192.6I n conclusion, we wish to express our thanks to the ResearchFund Committee of the Chemical Society for a grant by which theexpense of this investigation has been largely defrayed.EAST LONDON COLLEGE