918 PHILIP: THE REFRACTION AND

LXXXV.-The Refruction and Dispersion of Triuxo- compounds.

By JAMES CHARLES PHILIP.

THE spectrochemistry of nitrogen has been made the subject of exhaustive investigation by Briihl (Zeitsch. physikal. Chem., 1891, 16, 193, 226, 497, 512; 1897, 22, 3 7 3 ; 1898, 25, 5 7 7 ; 26, 47) , and from his researches it appears that the contribution which the nitrogen atom makes to the molecular refraction or dispersion OF a compound depends very markedly on the character and arrangement of the other atoms or groups with which the nitrogen is linked. This result obviously suggests that the determination of the optical constants of n compound containing nitrogen may be useful in throwing light on its constitution, and Briihl himself has made frequent use of refraction and dispersion for this purpose.

It has been found possible to add a little to our knowledge of the spectrochemistry of nitrogen by a study of the interesting triazo- compounds recently prepared and described by Forster and Fierz (this vol., 72, 669; also Proc., 1908, 24, 102). Unlike the majority of the azoimides hitherto known, many of these triazo-compounds are liquids, and are therefore particularly suitable for a study of refractive and dispersive power.

Acknowledgment must here be made of the author's indebtedness to Dr. Forster and Dr. Fierz, who very kindly put at his disposal the materials required for this and the following investigation. Without their co-operation the work could not have been undertaken.

Two of the compounds examined, namely, phenylazoimide and benzylazoimide, were very kindly prepared by Dr. Fierz for the special purpose of this investigation. Two specimens of the former were made, one (a;) by Emil Fischer's method, and the other ( b ) by Noelting and Michel's method. The specimen of benzylazoimide, prepared by heating benzyl chloride with sodium azide, boiled a t 72' under a pressure of 8 mm. ; on redistillation, the middle fraction, boiling at 64' under n pressure of 3 mm., was collected, but i t was found that the molecular refraction was not altered by the redistillation. The description of the preparation and properties of triazoethyl alcohol, N,*CH,*CH,*OH, another of the substances examined, will appear in a future com- munication by Dr. Forster and Dr. Fierz.

The refractive indices at 24.9' for the Ha, D, and H, lines weie determined with a Pulfrich refractometer. A steady temperature was attained by circulating water from a thermostat by a small

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DISPERSIOS OF TRIAZO-COMPOUNDS. 919

rotary pump in the way described by Lorvry (Trans. Furaday Soc., 1907, 3, 119). This method of providing a current of water at constant temperature is very satisfactory, and may be highly recom- mended.

The densities of the various liquids were determined with a pyknometer, and the values recorded below are those obtained at 2 4 * 9 O , the temperature a t which the refractive indices were determined. The weighings were in all cases corrected for displaced air.

The observed values of density and refractive index are recorded in the following table :

TABLE I. Substacce.

Ethyl triazoaceta te .............. Ethyl a-triazopropionate ......... Ethyl 8-triazopropionate ......... E thy1 bis triazoace ts te ............ Triazoethyl alcohol ............... Ristriazoethane .................... Renz y lazoimide ..................... Ethyl triazoformate .............. Phenylazoiniide ( a ) ............... a-Naphthylazoi mide ...............

> ) ( p , ...............

d. 1.1191 1.0583 1.0798 1.2204 1.1436 1'1699 1.0656 1'1082 1.087 1 1.0896 1.1713

nu. 1.43231 1.42610 1.43581 1 *46090 1'46488 1'47629 1 *52923 1 '4 1353

1.55227 1.64481

-

n,. 1.43487 1'42857 1'43833 1 '46400 1'45778 1'47976 1'53414 1'41623 1'5588 1.55886 1 *65501

ny. 1.44657 1.43994 1'45010 1'47839 1'47095 1'49583 1 5 5 7 4 8 1-42861

The colour of phen ylazoimide and a-naphthylazoimide prevented the

I n the following table I1 are given in the second column the values

of the molecular refraction [M.R.], = - - * in the third column,

the values of the molecular dispersion CM.R.1,- [M.R.],; in the fourth column, the contribution to the molecular refraction which is to be assigned to the N,-group, and in the fifth column the amount contributed to t h e dispersion by the ' N,-group. Except where specially noted, the values in the fourth and fifth columns are obtained by subtracting from the experimental values of the molecular refraction o r dispersion the total contributions of the other atoms in the molecule. For this purpose, the following values of the atomic refraction and dispersion have been employed : refraction, C, 2.501 ; H, 1.051 0 (hydroxyl), 1.521 ; 0 (ether), 1.683 ; 0 (carbonyl), 2.287 (see Ostwald-Luther, Physiko-chemische Messungen, p. 234) : dispersion, C, 0.039 ; H, 0.036; 0 (hydroxyl), 0.019 ; 0 (ether), 0,012; 0 (carbonyl), 0,086 (see Bruhl, Zeiisch. physikal. Chem., 189 I ,

determination of ny for these substances,

n2,-1 M n;+?J'd '

7, 191).

VOL. xcm. 3 P

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920 PHILIP : THE REFRACTION A N D

TAkLE 11. Contribution of N,-group to

Stibstance. [M .R.],. Ethyl triazoacetate ........... 30 -07 Ethyl a-triazopropionate.. .... 34 '80 Ethyl 8-triazopropionate ... 34 *79 Ethyl bistriazoacetate ...... 38 -42 Triazoethyl alcohol ............ 20 -75 Bistriazoethane.. ................ 27 -18 Benzylnzoimide.. ................ 38 '82 Ethyl triazoformate ......... 26.05 Phenylazoimide ( (1.) ............ 35 -33

7 9 ( b ) ............ 35.25 a-Naphthylazoimide ......... 52.95

[hl.R.]y - [M.R.],. 0.856 0.977 0.984 1'242 0'626 0-942 1'701 0'826 -

Refraction. 8.74 8.87 8 '86 9 -07 8.97 8 -98 8.85" 9 '32

10'27" 10'19" 10 '56f

Dispersion. 0-35 0.36 0.37 0 -38 0 *35 0.36 0.37" 0 43 -

* I n these cases, the contributions of the phenyl and benzyl radicles have becn obtained by subtracting the atomic refraction (or dispersion) of hydrogen from the molecular refraction (or dispersion) of benzene and toluene (for latter data, see Perkin, Trans., 1900, 77, 273).

t The contribution of the a-naphthyl radicle was obtained by subtracting the atomic refraction of bromine from the molecular refraction of a-bromonaphthalene (see Briihl, Zeitsch. physikal. Chem., 1897, 22, 408).

The only one of these substances the refraction of which has already been determined is phenylazoimide. Bruhl (Zeitsch. physikd. Chem., 1894, 16, 222) gives 35.93 as the value of [M.R.], for this substance, whilst on the basis of Perkin's data (Trans., 1896, 69, 1232) the value is 35.14. The values recorded in the foregoing table are in good agreement with Perkin's figure, and it is worthy of note that the specimen of phenylazoimide examined by him was prepared by a different method, namely, by the action of nitrosyl chloride on phenylhydrazine (Tilden and Millar, Trans., 1893, 63, 256). Briihl's value is undoubtedly too high. On reference to table 11, it will be seen that, apart from ethyl

triazoformate, phenylazoimide, and a-naph thylazoimide, the exceptional behaviour of which will be discussed later, the increment of the refraction or dispersion due to the N,-group is remarkably uniform, Ethyl triazoacetate gives lower values than most of the substances, whilst ethyl bistriazoacetate, in which two N,-groups are attached to the same carbon atom, gives figures which are rather above the average. The mean value of the increment in the refraction for the first seven compounds in the table is 8.91 ; the mean value of the increment in the dispersion is 0.36. From Briihl's researches, it appears that the atomic refraction of nitrogen in -NH,, as deduced from a study of tahe hydrazines, hydroxg-lamine, ammonia, and aliphatic amines, is 3.48, and the atomic dispersion 0.08. The optical effect therefore of the N3-group, whether considered in reference to refraction or dispersion, is more than three times that of the single

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DISPERSION OF TRIAZO-COMPOUNDS. 921

nitrogen atom in its simplest linkings. On the other hand, the value just deduced for the refraction of the N,-group is not much greater than that found for the two nitrogen atoms in diazoacetic ester; according to Briihl, the value of [M.R.], for this substance is 28.71, whence the increment due to the N,-group is 8.43. The dispersion due to the two nitrogen atoms in diazoacetic ester is found by Briihl t o be 0.90, or more than twice as great as the increment due to the three nitrogen atoms in the triazo-group.

Attention has frequently been directed (see, for example, Curtius, Ber., 1896, 29, 772) t o the fact that a compound containing the N3-group exhibits a marked similarity to the corresponding compound in which a halogen atom takes the place of this group. This parallelism is further emphasised by the values of the optical constants. The refraction increment due to the N,-group is, as already stated, 8.9 1 ; the atomic refraction of bromine is 8.93. The dispersion increment due to the N,-group is 0.36 ; the atomic dispersion of bromine is 0.35. This almost complete coincidence between the optical influence of the triazo- group and the bromine atom finds its first expression in the fact tha t the molecular refraction and dispersion of the triazo-compounds are practically the same as those of the corresponding bromine compounds. Thus, for example, the value of [M.R.], for bistriazoethane is 27-18, p d for ethylene dtbromide, 26.99, whilst the molecular dispersions of the two substances are respectively 0.94 and 0.95. Again, the molecular refraction of ethyl a-triazopropionate is 34-80, whilst that of ethyl a-broimpropionate is 34.70.

I n discussing the conclusions to be deduced from the data in table 11, the abnormally high figures obtained for the refraction of ethyl triazoformate, phenylazoimide, and a-naphthylazoimide have not yet been considered. There appears to exist i n these cases some special influence which raises the refraction increment for the N,-group considerably above the normal value. This is best seen by comparing 8.85, the contribution due to the triazo-group in benzylazoimide, with 10.23, the mean value of the increment .due to the same group in phenylazoimide. Such a different influence of the benzyl and phenyl groups on the refraction O C an associated group has frequently been observed by earlier workers. Thus, for example, the refraction increment due to the *NH,-group is 4.41 in benzylamine, and 5-50 in aniline; the refraction increment due to the *CN-group is 5-22? in phenylacetonitrile, and 6.53 in benzonitrile.

Not only is the refraction increment due to the N,-group in ethyl triazoformate and phenyl- and a-naphthyl-azoimides abnormally high, but so also is the dispersion increment, So far as ethyl triazoformate is concerned, this will be obvious from table 11, but, since nny could not be determined in the case of phenyl- and a-naphthyl-azoimides, it mas

3 ~ 2

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922 PHILIP: THE REFRACTION AND

not possible to test the dispersive power of these substances in the same way. The refractive indices, however, for the Ha and D lines were ascertained, and the values of [ M.R. ID and [M.R.], - [M.R.], have been worked out for these azoimides and the corresponding bromine compounds.

TABLE 111.

Phen ylazoimido .................... 35.29 0 ‘34 Bromohenzene ..................... 33.83 0.25

Substance. [M.R.]D. [M.R-]D- [M.R.],.

a-Naphth ylazoimide ............... 53.95 0.66 a- Bromonaphthalene ............ 51-32 0.54

These figures show very clearly that the refractive and dispersive powers of the triazo-compound, instead of being, as one might expect, equal to those of the corresponding bromine compound, are very much higher.

The experimental data which have been submitted make it evident tha t the optical influence of the N,-group is intensified in ethyl triazoformate, phenylazoimide, and a-naphthylazoimide, as compared with the other substances containing this group and enumerated in tables I and 11. The question then arises whether any intelligible explanation can be given of such a difference in behaviour.

I n this connexion, it is noteworthy that the phenomenon of a given group possessing an intensified optical influence in certain positions has been successfully reEerred to the contiguity of unsaturated groups. Bruhl especially has shown (Trans., 1907, 91, 115; Bey., 1907, 40, 878, 1153) that the presence in a molecule of the grouping C:C*C:C or C:C*C:O invariably produces a marked increase of both refractive and dispersive power (compare also Perkin and Kay, Trans., 1906, 89, 839 ; Sir W. €3. Perkin, Trans., 1907, 91, 806 ; Smedley, this vol., 372). It is interesting therefore to find that in each of the triazo-compounds which exhibit exceptional optical behaviour, the N,-group is adjacent to a doubly-linked carbon atom. I n the case of ethyl triazoformate, this adjacent carbon atom is doubly linked to oxygen ; in the cases of phenyl- and a-nnphthyl-azoimides, the carbon atom which is linked to the N,-group is also doubly linked to one of the adjacent carbon atoms in the ring. From this point of view,-ethyl

triazoformate, 8 may be compared with ethyl crotonate, N,*C*OEt ’

8 the molecular refraction and dispersion of which are C,H,: CH* C-OEt’ exceptionally high, wbilst the effect of the contiguous ethylene linking on the optical influence of the N,-group in phenyl- and a-naphthyl- azoimides is similar in kind to the effect of such a linking on t h e optical influence of the NH,-group in o-toluidine,

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DISPERSION OF TRIAZO-COMPOUNDS. 923

Opticdly, therefore, the N,-group behaves as an unsaturated complex. On analogy with the ethenoid group, one might be tempted to conclude that the particular nitrogen atom in the triazo-group which is linked to carbon must be doubly linked to one or both of the other nitrogen atoms. Such a conclusion, however, is hardly warranted, for Bruhl has shown that the optical influence of the NH2 and OH groups, in which there are no double linkings, is intensified in conjuga- tion with the ethenoid group. The amino- and hydroxyl groups each contain an element the valencies of which are not all satisfied, and they may therefore be regarded as complexes with residual affinity. Since the N,-group is built up out of one of these elements, i t is not surprising that, so far as the optical evidence goes, it also should behave as a complex with residual affinity.

On the interesting question how the linkings of the thrte nitrogen atoms in the triazo-group should be represented, one fact at least brought out by this investigation throws some light. The complex N, may be represented in various ways, namely :

I. I I. 111. I v. The first of these representations is the one which is generally

employed by those who have occasion to deal with the N,-complex, although it does not appear that there is any definite evidence in favour of formulation I as against the others. Some years ago (Proc., 1893, 9, 57), Armstrong argued against the custom of representing phenyl-

N N azoimide as C,H,*N<I I, suggesting that it might be regarded as a

dinitrile, <N-r-N>, on the assumption of the existence of latent

affinities. The possibility of the formulation IV has recently been entertained by Forster and Pierz (this vol., 75), and compounds have been prepared by Dimroth (Bey . , 1907, 40, 2376) which seem to show that the ring, if i t exists, may be opened without difficulty. Thus the reduction of phenylazoimide leads readily to the formation of phenyltriazen, which, according to Dimroth’s investigations, exists in two chemically isomoric forms ; for these isomerides, the following

C,f%

N H NH‘ formulae are suggested : C,H,*N:N*NH, and C6H5*N< I

The one fact brought out by this investigation which seems to have a bearing on the question is the relative optical infiuence of the diazo- aIid triazo-groups. It has already been pointed out t h a t , whilst the refraction value for the N,-group is normally 8.91, the refraction value of the N2-group in ethyl diazoacetate is very little less, namely,

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924 THE REFRACTION AND DISPERSION OF TRIAZO-COMPOUNDS.

8.43, and, further, tha t the dispersion increment due to the two nitrogen atoms in ethyl diazoacetate is more than twice as great as the dispersion increment due to the three nitrogen atoms in ethyl triazoacetate and similar compounds. This is very striking, and lends support to the view that formulation I is to be preferred for the triazo-group, for it is the common experience of workers in this field that the accumulation of multiple linkings markedly raises the refraction and dispersion values, and the evidence just quoted shows that the optical influence of the triazo-group is relatively low. Consideration of the evidence, in fact, shows tha t formulations 11, 111, and I V for the triazo-group are unlikely, and that the arrangement

*N<R is the most probable. N If this view is adopted, it is difficult to account for the relatively

much higher refractive and dispersive power of the diazo-group in ethyl diazoacetate on the basis of the ordinarily accepted formula :

R>CH-CO,Et. The optical evidence taken alone would seem to N suggest tha t one or both of the nitrogen atoms in ethyl diazoacetate are quinquevalent, as in the formula NiN:CH*CO,Et or

E.>CH* C0,Et. N

It is, indeed, customary to introduce quinquevalent nitrogen only in the formulation of compounds of the ammonium chloride type, but, in view of the optical evidence, the two foregoing formula cannot

be ignored. The expression Hy>C*CO,Et for ethyl diazoacetate,

although perhaps accounting for the optical exaltation by a Thiele conjugation, would involve a different type of structure for the higher members of the series, and, in absence of information relating to the optical properties of these, such an assumption would not be justified. Whatever be the explanation, the fact remains that the optical influence of the triazo-group is relatively lower than tha t of the diazo-group i n similar compounds.

Briihl’s discussion of the relative influence of the diazo- and triazo- groups (Zeitsch. physikal. Chern., 1898, 25, 597) is not convincing, since he compared the diazo-group in ethyl diazoacetate with the triazo-group in phenylazoimide; as has been shown in this paper, the latter compound exhibits an optical exaltation by reason of which such a comparison cannot be considered legitimate. In fact, Briihl himself admits tha t the optical constants of tha t nitrogen atom in phenyl- azoimide which is next the benzene ring are possibly higher than the normal values.

N

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DISSOCIATION CONSTANTS OF TRIAZO-COMPOUNDS. 925

It has not yet been possible to examine triazen derivatives in which the nitrogen atoms, are undoubtedly arranged in an open chain. When this can be done, further interesting information as to the spectrochemistry of nitrogen, and possibly as to the relation of the nitrogen atoms in the triazo-group, will be available.

ROYAL COLLEGE OF SCIENCE, LONDON, S.W.

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