562 SMITH AND WATTS: ABSORPTION SPECTRA AND

LVIII.-Absolption Spectra aizd Meltirig-point Curves of A?*onzatic Biuxoamines.

By CLARENCE SMITH and CONSTANCE HAMILTON WATTS.

THE discovery by Griess that the same diazoamine is formed by diazotising either of two primary aromatic amines and coupling the product with the other, led twenty or thirty years ago to numerous investigations which had for their object the determination of the constitution of such mixed diazoamines and the isolation of the two possible isomeric forms, ArN2*NHAr/ and ArNH*N2Ar’. The latter purpose has never been satisfactorily realised, whilst the former has resulted in a mass of such conflicting evidence that even at the present time the constitution of the aromatic diazoamines remains an open question.

In order to show that the conclusions at which we have arrived are supported by the bulk of this earlier evidence, it is necessary to recapitulate the main points made by previous investigators, the more so as an impartial survey of all the facts appears to have been omitted hitherto.

An examination of the substances obtained by decomposing with water the product of the action of carbonyl chloride on a benzene solution of a diazoamine led Sarauw to the conclusion that the imino-group is attached to the more negative aromatic nucleus, and that the initial product (not isolated) of the reaction is a diazo- carbamide, ArN2*NAr/GO*NAr/*N2Ar, in which Ar is the less negative benzenoid group. A perusal of the author’s two papers (Ber.,1881, 14, 2442; 1882, 15, 42) proves, however, that his evidence is inconclusive, for whilst phenol and dibromocarbanilide are the products arising from the decomposition by water of the diazocarbamide obtained from benzenediazoamino-p-bromobenzene, the action of water on the diazocarbamide from benzenediazoamino- p-toluene yields both phenol and pcresol and a viscous product from which only di-p-tolylcarbamide can be isolated. The viscous product may and probably does contain diphenylcarbamide, seeing that both phenol and p-cresol are formed. If such is the case, the diazoamine reacts with carbonyl chloride in accordance with both formulze, C6H5-N2-NH*C7H7 and C6H5*NH*N2*C7H,. Similar reasoning holds in the case of benzenediazo-m-aminobenzoic acid, phenol and m-hydroxybenzoic acid being the only substances isolated from the decomposition products of its diazocarbamide.

The preceding constitution of a mixed diazoamine has been far more satisfactorily established by Goldschmidt and Molinari (Ber.,

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MELTING-POINT CURVES OF AROMATIC DIAZOAMINES. 563

1888, 2 1, 25 78), by heating equimolecular quantities of the diazo- amine and phenylcarbimide in an indifferent solvent, such as benzene. The product is a diazocarbamide, which can be isolated and appears to be an individual substance; it is decomposed by water, yielding a phenol, nitrogen, and a diarylcarbamide, of wliich one aromatic group is always pheny€, and the other the more negative 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 investigations

were published which partly supported and partly opposed Goldschmidt’s conclusions. Heumann and Oeconomides (Ber., 1887, 20, 372, 904) found that diazoaminobenzene, when heated in phenol, reacted to form aniline and benzeneazophenol ; similarly, p-chlorobenzenediazoamino-p-toluene gave p-chloroaniline and p-tolueneazophenol. Benzenediazoamino-p-toluene, however, with phenol or resorcinol gave approximately equal quantities of aniline and p-toluidine and a mixture of hydroxyazc-compounds. I n these reactions, therefore, some mixed diazoamines behave as if con- stituted in accordance with Goldschmidt’s formula, others like mixtures 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-ptoluene and other mixed diazoamines to the attack of numerous reagents, and fonnd that they behaved sometimes in accordance with the formula ArN2*NHAr’, sometimes in accordance with the formula Ar*NH*N,Ar’, but generally as a mixture of both forms.

As a result of these and other investigations, two views were current regarding the constitution of mixed aromatic diazoamines, and these have not been materially modified by more recent researches. One view, which does not appear to have been urgently advanced by any single investigator, regards the mixed diazoamines as consisting of the two possible isomerides, ArN,*NHAr’ and ArNH*N2Ar’. The other theory, initiated by Goldschmidt, regards the diazoamines as being constituted so that the imino-group is attached to the more negative aromatic group. Reactions in which the diazoamine yields four products of decomposition are attributed to a migration of the iminic hydrogen atom due to the presence of water, alcohol, or an electrolyte, and are explained by an initial addition 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’ may result, and by subsequent decomposition yield each a pair of products. Goldschmidt claimed that the migration of the iminic

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564 SMITH AND WATTS: ABSORPTION SPECTRA AND

hydrogen atom does not occur in indifferent solvents, such as benzene, petroleum, or chloroform, and consequently in such solvents diazoamines behave as individual substances and not its mixtures, and to this cause attributes the success of his phenylcarbimide method 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 and Streatfeild on alkylated diazoamines. Although alkylation m a method of determining constitution has been viewed in recent years with some suspicion, yet in some instances, for example, the phthaleins and the hydroxyazo-compounds, the problem of the constitution of a substance containing a mobile hydrogen atom has been approached and to a great extent solved by replacing the migratory hydrogen atom by an immobile alkyl group. I f , there- fore, an alkylated diazoamine can be shown to have a similar constitution to that of its parent substance, Meldola and Streatfeild’s researches acquire a new and fundamental significance, and can be utilised directly to prove the untenability of the theory that in a, mixed aromatic diazoamine the imino-group is attached t o the more negative aromatic nucleus. We have been able to prove the point in question by means of the spectrograph. Meldola and Streatfeild showed that three isomeric mp’-dinitrodiazoethylaminobenzenes exist, 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 of I and 11, and absolutely identical with that of its parent diazo- amine, which is thus proved to have a constitution similar to that of isomeride 111. The constitution of this isomeride has been prac- tically proved by Meldola and Streatfeild, who find that it can be

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MELTING-POINT CURVES OF AROMATIC DIAZOAMINES. 565

synthesised by heating equimolecular quantities of isomerides I and I1 in alcohol or benzene. Meldola and Streatfeild regard I11 as a co’mpound of I and 11. For reasons given below we believe it to be an equimolecular mixture of I and 11, but for the present purpose this difference of opinion is immaterial, the main point being that I11 is composed of equal quantities of I and 11, either mixed or combined. Now, in the diazoamine under discussion, the p-nitro- benzene nucleus is probably the more negative, but whether this is really so does not affect the argument. Assuming that it is, the diazoamine will be represented by the Goldschmidt theory by the formula:

and the directly alkylated derivative, which must possess a similar constitution to that of its parent substance from the spectrometric evidence, will have the formula:

No2 that is, should be identical with isomeride I above. Since experi- ment shows that the directly alkylated derivative is I11 above, that is, a mixture of equal quantities of I and 11, it follows that the premise is incorrect, and that mp’-dinitrodiazoaminobenzene con- sists of a mixture (or compound) of equal quantities of the two individually unknown isomerides :

Of course, the acceptance of this theory at once renders intelligible the numerous reactions in which a mixed diazoamine yields four products of decomposition. It only remains t o explain why the diazoamine a t times yields only two products, decomposing as though it consisted entirely of one of the two unknown isomerides. A t present it is impossible to advance any argument satisfactorily supported by experimental evidence. The most obvious explanation is the selective attack of the reagent. If one of the two isomerides is attacked a t a much greater rate than the other, a transformation of the less susceptible into the more susceptible isomeride must occur in order to preserve the equilibrium ratio a t unity, and the diazoamine will decompose almost entirely as though it consisted of one isomeride only. The most important case to which this explanation can be applied is the phenylcarbimide reaction, since

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566 SMITH AND WATTS: ABSORPTION SPECTRA AND

this forms the main foundation of the Goldschmidt theory of the constitution of diazoamines. Goldschmidt attributes the formation of an individual diazocarbamide from phenylcarbimide and a mixed diazoamine in benzene solution to the immobility of the iminic hydrogen atom in an indifferent solvent. Dimroth, however, gives instances (Annuten, 1904, 335, 1) in which the transformation of one tautomeride into another by the migration of a mobile hydrogen atom proceeds much more rapidly in an indifferent solvent than in a hydroxylic solvent. Goldschmidt admits that this is correct in principle, 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 exist as an inseparable mixture in the mixed diazoamine. The unitary course of the phenylcarbimide reaction, therefore, may very well be due to the selective attack of the reagent, the transformation of one isomeride in the mixed diazoamine into the more susceptible form being facilitated by the indifferent solvent to such a degree that one diazocarbamide is produced only in inappreciable quantities.

It would be premature to dogmatise from the result of the examination of a single triplet of ethers originating from a diazo- amine containing aryl groups of not very different character, and we do not unhesitatingly commit ourselves at present to the views advanced above, and the less so in consequence of Pechmann’s work on amidines of the type ArN:CPh*NH*Ar’ (Ber., 1895, 28, 869), which differ from mixed diazoamines by containing the group CPh in place of a nitrogen atom. The compound C,H5*N:CPh=NH*CpH7, obtained from benzanilide iminochloride and p-toluidine, is identical with C,H,-N:CPh-NH*C,H,, prepared from benzoyl-p-toluidide iminochloride and aniline, but yields by ethylation a mixture of two ethyl derivatives corresponding with the two formuk given. When Ar and Art are different in character, however, the amidines pro- duced by the two methods are still identical, but yield only one ether, indicating an immobility of the hydrogen atom in the amidine. A similar constitution may obtain for mixed diazoamines containing aryl groups of very different character, the imino-group remaining attached to the negative nucleus as in the Goldschmidt theory. 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 mixed diazoamine has been shown by Meldola and Streatfeild to be composed of equal quantities of the two isomerides, ArN,*NR*Arf and ArNR*N2*Ar’, by boiling an alcoholic or benzene solution of

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MELTING-POINT CURVES OF. AROMATIC DIAZOAMINES. 567

these isomerides for one hour, whereby a product is obtained identical with the directly alkylated diazoamine. The authors regard this product as a compound of the two isomerides, although they found that the molecular weight in benzene by the cryoscopic method agreed with a unimolecular and not a bimolecular formula, a discrepancy which they attribute to dissociation of the compound in the benzene solution. This explanation is untenable, since the compound is produced in boiling benzene, and it is very improbable that it would dissociate in the cold solvent. Mr. T. J. Mander, to whom we proffer our thanks, has determined the molecular weights of several alkylated mixed diazoamines in boiling alcohol and benzene, and has obtained values which are always less than those corresponding with the unimolecular formula. The com- pound, therefore, does not exist in the solution, and must be produced, if formed a t all, at the instant of the deposition of the solid from the solution. To ascertain whether or not a compound is formed, we have determined the melting-point curve of mixtures of pnitrobenzenediazoethylamino-mnitrobenzene and m-nitro- benzenediazoethylamino-p-nitrobenzene. The curve is of the simple U-sbape characteristic of mixtures, and has its minimum at it point corresponding with the mixture of equal quantities of the two isomerides and a temperature identical with the melting point of the substance obtained by the direct ethylation of mp‘-dinitrodiazo- aminobenzene. This evidence, combined with that furnished by the cryoscopic and the ebullioscopic methods of determining the molecular weight, proves that the directly ethylated substance is a solid solution of equal quantities of the two isomerides mentioned above.

EXPERIMENTAL. mp’-Dinitrodiazoarninob enzene.-This compound was prepared in

the usual way from mnitroaniline and p-nitrobenzenediazonium chloride in the presence of sodium acetate. The crude substance melted at 214O; after crystallisation from a mixture of equal volumes 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 boiling alcohol and toluene, filtered while still hot (precipitate A), again when cold (precipitate B), and yet again after concentrating the mother liquor (precipitate C). The melting points of A, B, C respectively were 220°, 212.b-213°, 219-220O. After re- crystallisation from the same solvent, the melting points were 231-232O, 231-232O, 230-231°, in a bath previously heated to 180O. After a third recrystallisation, the melting points were 231-232O, 231--232O, 228O. In all cases the substance decomposes

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568 SMITH AND WATTS: ABSORPTION SPECTRA AND

at the melting point. The three samples all gave the same orange- red colour in alcoholic sodium hydroxide, and dissolved without change of colour in concentrated sulphuric acid, forming solutions which ultimately became colourless. The melting point of pp/-dinitrodiazoaminobenzene is given by Hantzsch as 2 3 3 O (decomp.), a value which we have confirmed. The melting point of a mixture of approximately equal quantities of this compound and our mp/-isomeride melted a t 208-21 lo (decomp.).

mpf-Dinitrodiazoethylanzinobenzene was prepared by heating the diazoamine with alcoholic potassium hydroxide and ethyl iodide on the water-bath for eight hours, and twice recrystallising the product from alcohol. It melted at 152--153O, dissolved in alcoholic sodium hydroxide 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 recrystallised from alcohol and toluene until the melting points were constant at 174-5-174.8O and 188*5--188*7O respectively ; the substances dissolved in alcoholic sodium hydroxide without change of colour, and gave solutions in concentrated sulphuric acid, which ultimately became colourless.

The absorption curves of mp’-dinitrodiazoaminobenzene and of the three ethylated isomerides are shown in Fig. 1. The curves of the parent diazoamine and of its directly ethylated derivative are identical throughout. The curves of the other two isomerides, although naturally very similar to, are quite distinct from, that of the directly ethylated isomeride. A comparison of the three curves, particularly in the neighbourhood of oscillation frequencies 3200 to 3600, indicates that the curve of the directly ethylated diazoamine is very much what would be expected if the substance is a mixture of 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 the different mixtures shall be as uniform and under as nearly the same conditions as possible, for it is well known that the apparent melting point of a diazoamine can be raised many degrees by rapid heating. Intimate mixtures of the two isomerides were obtained by making two solutions in benzene distilled over sodium:

m-iVitro b enzenediaao et h ylamino-p-nitro b enz ene

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MELTING-POINT CURVES OF AROMATIC DIAZOAMINES. 569

A. 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 were evaporated on the water-bath, the residue was detached from the basin, finely powdered, and transferred as completely as possible to capillary 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 46

mp'-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, having a range from looo to 200°, graduated in tenths of a degree. The bulb of the thermometer was immersed in sulphuric acid contained in the outer jacket of a Victor Meyer vapour density apparatus The temperature of the acid was raised to looQ, then the capillary tube was affixed to the thermometer, and the temperature was raised fairly rapidly to within loo of the melting point of the preceding mixture; the rate of heating was then adjusted so that the tem- perature rose lo per minute.

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570 SMITH AND WATTS : AROMATIC DIAZOAMINES.

Corrcctcd C.C. of A. C.C. of B. melting point.

20 0 178'5-1 78.8" 19 1 176.6 18 2 169.1-169*8 17 3 166.9-167.2 16 4 164.5-165.4 15 5 159.6-160.4 14 6 157.3-157'5 13 7 156.1-156.5

11 9 156 '3-156'9 10 10 155 -4-1 56 -2

12 a 156.4-156.9

Corrected C.C. of A . C.C. of B. melting point.

9 11 157'2" 8 12 160 '2 -1 61 -6 7 13 164 *5-1655 6 14 166 '8-167 '4 5 15 169.1-169'8 4 16 176.1-176.9 3 17 177.6-177'9 2 18 185.9-186 '1

0 20 193.2-1 93.4 1 19 ia6.9-ia7-2

FIG. 2.

~ O O A 90 ao 70 60 50 40 30 20 10 ox, 0 10 20 30 40 50 60 70 80 90 lOOBA

A = m- Nitrobenzencdiazoeth ylamino- p-nitrobenzene. B = p-Nitrobenzeiaediazoethylamino-m -nitrobenzeene.

It will be noticed that mixtures containing from 65 to 50 per cent. of A melt at only slightly different temperatures, but there

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ABSORPTION SPECTRA AND CHEMICAL CONSTITUTION. 571

is not the slightest doubt that the equimolecular 50 per cent. mixture has the lowest melting point, which is the same as that of the substance obtained by the ethylation of mp'-dinitrodiazoamino- benzene. In order to see whether a compound of the two isowerides is

formed under the conditions mentioned by Meldola and Streatfeild, mixtures of A and B were heated under a reflux condenser for one hour on the water-bath; the benzene was then evaporated, and the melting points of the residues determined as above. The results tabulated below show that the melting points are practically unchanged by this treatment:

Corrected C.C. of A. C.C. of B. melting point.

20 0 178.2-178 '3" 15 5 159.3-159'7 12 8 156.3-156.8 10 10 155.1-155*7

Corrected melting point. C.C. of A. C.C. of B.

8 12 160'4.-160 '9" 5 15 168'8-169'8 0 20 192'0-192.6

I n conclusion, we wish to express our thanks to the Research Fund Committee of the Chemical Society for a grant by which the expense of this investigation has been largely defrayed.

EAST LONDON COLLEGE.

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