TUCTC : THE CONSTITUTTON OF HYDROXYAZO-COMPOUNDS. 449

XLV.--The Constitutio?z of Hydyoxyaxo-compounds. By WILLIAM BRADSHAW TUCK.

ALTHOUGH the constitution of the ethers, and acetyl and benzoyl deriv- atives of the hydroxyazo-compounds has been settled definitely by means of their reduction products, chemical methods have failed to decide whether the parent compounds are true hydroxyazo-compounds, C,H,*N:N* C6H, 0 H, or quinonehydrazones, C,H,*NH*N: C,H,:O.

I n the case of the ethers, for example, it has been found by reduc- tion with tin and hydrochloric acid that benzeneazophenetole,

C,H,.N:N*C,H,*OEt, is first converted into benzenehydrazophenetole,

C,H,*NH *NH*C,H,* OE t, which then undergoes fission into aniline and phenetidine (Jacobsen and Fischer, Ber., 1892, 25, 995). I n this way it is conclusively proved that benzeneazophenetole is a true azo-derivative, that is, its formula is C,H5-N:N*C,H,*OEt. Similarly, evidence is available in the case of the acetyl and benzoyl derivatives. It is evident, however, in the case of the free hydroxyazo-compounds that the constitiition cannot be determined by reduction, since the two alternative formulae, C,H5*N:N*C6H4*OH and C,H,*NH*N:C,H,:O, yield the same first reduction product, C,H,*NH*NH*C,H;OH. Further, no decisive con- clusions can be drawn from substitution compounds owing to the great ease with which additive compounds are produced. From the chemical evidence at our disposal, it is evident that in the case of p-hydroxyazo- compounds the ethers and benzoyl derivatives possess the azo-configura- tion ; the same is true for the ethers of the o-hydroxyazo-compounds, whilst it is probable that the benzoyl derivatives of the latter are o-quinone benzoylphenylhydrazones.

From previous spectrographic work on quinone, hydrazones, and azo- compounds (Baly and Stewart, Trans., 1906, 89, 502 ; Baly and Tuck, Trans., 1906,89,982), it seemed probable that conclusive evidence of t h e structure of the hydroxyazo-compounds could be obtained by an examina- tion of their absorption curves. In the present paper the absorption curves of several of these substances, their ethers and benzoyl deriv- atives, have been examined, and, as was hoped, it has been possible to decide definitely the constitution of free hydroxyazo-compounds.

The absorption curves were obtained by plotting the oscillation frequencies of the lirriits of the transmitted light against the logarithms of thicknesses of M/lO,OOO solution equivalent t o the concentrations used. The absorption curves are given in Figures 1 to 8, and an

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450 TUCK : THE CONSTITUTION OF HYDROXPAZO-COMPOUNDS.

examination of these show that the 0- and p-compounds produce EL

different type of absorption, and for purposes of convenience they will be discussed separately.

T h e Pcwa-compounds.

The compounds of this class which have been examined are benzene- azophenol, C6H,-N:N*C,H,*OH ; benzeneazo-m-cresol,

C,II,.N:N~/-\OK, \-/ Me

and their ethers and benzoyl derivatives. hydrochloric acid and sodium ethoxide upon them has been studied.

I n some cases the effect of

FIG. 1.

Oscillation frequencies.

. 16 182000 22 24 26 28 3000 32 34 36 38 4000 42

Full curve . . , . . . . . . . . . Benzeneazophenol in alcohol. Dotted curve ......... ? ? in concenlrated hydrochloric acid. Dot and dash curve.. Y 9 in alcoholic sodium ethoxide.

In Figs. 1 to 4 the full curves are those of benzeneazophenol, henzeneazophenetole, benzeneazo-m-cresol, benzeneazo-m-cresetole re- spectively. The curves for all these substances are very similar, and show a remarkable analogy to that of azobenzens, except that the sudden extension, which occurs between the frequencies 2000 and 2400 in the hydroxyazo-compounds, appears in azobenzene as a well-

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TUCK : THE CONSTITUTION OF HYDltOXYAZO-COMPOUNDS. 451

developed band (Baly and Tuck, Trans., 1906, 89, 982). The reason for this difference will be discussed later.

The close resemblance of the curves for the ether and hydroxyl compounds proves conclusively tha t their structure is similar, except tha t the ethyl group replaces a hydrogen atom. As it is imp ssible for the ethyl group to be tautomeric, it follows tha t the hydrogen a tom also is not tautomeiic under the conditions of the experiment, namely, when the compounds are dissolved in alcohol.

The benzoyl derivative of benzeneazophenol (dot and dash curve,

FIG. 2.

Oscillation frequencies.

16 i a 2000 22 24 26 28 3000 32 34 36 38 4000 42

Full curve ~ .. . . . . . . . . . Dotted curve Dot and dash curve.. BenxoyZben2eneaxophcnol.

Bci~..cnenxopheiietole in alcohol. ... .. . ... ? I in comentmted hgdrochloric acid.

Fig. 2) also shows a very close relation t o azobenzene, as was to be expected, since chemical evidence has shown that they have the azo- configuration.

The formula for p-hydroxyazo-compounds is therefore of the type C,H,* N: N*C,H,* OH, not C,H,*NH*N : C,H,: 0.

The same conclusion has been recently arrived a t (P. Lemoult, Compt. rend., 1906, 143, 603) by a study of the heats of combustion of these compounds, allowing 27 calories for the thermal characteristic of the azo-group -N:N-.

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452 TUCK : THE CONSTITUTION OF HYDROXYAZO-COMPOTJNDS.

FIG. 3.

Osci l la t ion frequencies.

16 18 2000 22 24 26 28 3000 32 34 36 38 4000 42 0 0

I

Full curve . . . . . . . . . . . . Benzeneazo-in-cTesol in alcohol. Dotted curve ......... > ? in concentrated hydrochloric acid. Dot and dash curve.. > > in alcoholic sodium ethoxide.

FIG. 4.

Oscillation fyequencies.

16 18 2000 22 24 26 28 3000 32 34 36 38 4000 42

24

22

20

18

16

1 4

12

10

8

6

4 Full curve . . , . , . . . . , . .

Dot and dash curve.. Dash and two dots.. .

Beir xeitcaxo-m-cresetole in alcohol.

Axobenzene in concentrated hydrochloric acid. p-Benzoquinonebenxoylphenylhydrazone.

Dotted curve ... ,. . ... > ? ,, i n concentrated hydrochloric

0 0

3

acid.

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TUCK : THE CONSTlTUTION OF HYDROXYAZO-COMPOUSDS. 453

The hydrochlorides of the p-hydroxyazo-compounds being very different in colour and reactions from the parent compounds, Hewitt and Pope ( B e y . , 1897,30, 1625) have stated that their constitution must be different, and that they are really hydrochlorides of quinone hydr- azones. I n view of this, it seems necessary to ascertain the effect that an excess of hydrochloric acid has on the absorption spectra of these substances. A n excess of hydrochloric acid is necessary, since the additive compounds hydrolyse with great ease; the ratio of the con- centration of the hydrochloric acid to the hydroxyazo-compound was not less than 10,000 : 1. The dotted curves, Figs. 1 to 4, shorn the effect produced on some of these p-hydroxyazo-compounds and their ethers. It may be seen a t once tha t the absorption spectra have altered entirely, and that the change produced is exactly the same in each case. This undoubtedly proves that the hydrochlorides of both the free p-hydroxyazo-compounds and of the ethers have similar structures, and consequently the hydrochlorides of the free hydroxyl substances have a n azo-structure. It is interesting to compare these with the absorption spectra of azobenzene in a solution cootaining a large excess of hydrochloric acid (dot and dash curve, Fig. 4).

The Ortho-compounds.

The compounds of this class which have been investigated are benzeneazo-p-cresol, p-tolueneazo-p-cresol, their ethers, and benzoyl derivatives.

The absorption curves (full curves, Figs. 6 and 8) of the ethers agree with those of the ethers of the p-compounds, and with that of azobenzene. This was to be expected, as chemical evidence has proved that they are azo-derivatives. The benzoyl derivatives (dot atid dash curves, Figs. 6 and S), on the other hand, give very different spectra. It has been suggested from chemical evidence (Goldschmidt and Brubacher, Ber., 1884, 17, 352; Odd0 and Puxeddu, Gsxxetts, 1906, 36, ii, 1) that the benzoyl derivatives of o-hydroxyazo-compounds are benzoylphenylhydrazones of o-quinone. The absorption curves for the benzoyl derivatives show a very close relation to that. of benzoquinonebenzoylphenylhydrazone, synthesised from benzoquinone and benzoylphenylhydrazine, allowing for the fact tha t the latter is a p-quinone and the former an o-quinone derivative. There can therefore be no doubt that the benzoyl derivatives are in the quinone-hydrazone form.

The fu l l curves of Figs. 5 and 7, which are those of benzeneazo- p-cresol and p-tolueneazo-p-cresol, shorn a marked agreement with those of their benzoyl derivatives, so that it may be concluded that the

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454 TUCK : THE CONSTITUTION OF HYDROXYAZO-COMPOUNDS.

parent compounds are o-quinonephenylhydrazones and are derivatives, therefore, of o-benzoquinone,

FIG. 5. Oscillation frequencies.

Uppcr curves. Ful l curve . . , . . , Be?tzeneaxo-p-cresol in alcohol. Dotted curve.... I ) ,, in conceittrated hydrochloric acid. Dot and dash ... 7 9 ,, in alcoholic sodium ethoxide.

16 18 2000 22 24 26 28 3000 32 34 36 38 4000 42

100

50

25 2 0

-@ u

N 0

10

3 5 , 2 3

250 3 4 .s s

0

er,

100 -g u

N e

50

25

10

5

FIG. 6. Oscillation freqtccncies.

Lower curvcs. Full curve .. . .. .

Dot and dash ... BenxoTllbenzeneazo-p-cresoZ.

Benxcnea~o-p-cresetolc in ako7zoZ. Dotted curve ... 7 9 ,) in concenfrnted hydrochloric acid.

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TUCK : THE CONSTITUTION OF HPDROXYAZO-COMPOUNDS. 455

FIG. 7.

Oscillufiou frepz~eneies.

Upper curves.

Fnll curve . . , ... p-Tolueneaxo-p-cresol i n alcohol. Dotted curve ... ,, , , in concentrated hydrochloric acid. Dot and dash ... 3 ,, i ? ~ alcoholic NaOEt.

18 2000 22 24 26 28 3000 32 34 36 38 4000 42

PIC,. 8.

Oscillation frequencies.

Lower curves. Full curve . . . . . . Dot and dash.. .

p- Tolueneaio-p-cresetol in alcohol.

Benxoylaxo-p-eresetole in alcohol. Dotted curve ... 9 9 ,, in coiwentrated hydrochloric acid.

The results obtained in this investigation are directly opposed to the hypothesis of ‘‘ rnesohydry ” (Oddo and Puxeddu, Zoc. cit .) , or equilibrium of the hydrogen between the oxygen and the nitrogen.

H H 2

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456 TUCK : T H E CONSTITUTION OF HYDROXYAZO-COMPOUNDS,

Such a state would, from the analogy of enol-keto-tautomerism, un- doubted1y"produce a charttcteristic ultra-violet absorption band in the case of the o-hydroxyazo-compounds, which woulcl be absent. in the benzoyl derivatives. The absence of such a band clearly proves tha t the hydrogen is not in the tantomeric condition required by the hypothesis of mesohydry.

I n the o-compounds the effect of hydrochloric acid on both the ethers and the parent compounds is exactly the same (dotted curves, Figs. 5 to 8). From this i t may be concluded that both hydrochlorides have the same structure, and that as the ethers are in the azo-form, the hydrochlorides of the parent compounds must also be in tha t form. Thus the effect of adding hydrochloric acid to o-hydroxyazo-compounds is to change them from o-quinonehydrazones to azo-derivatives :

The reason for this change probably lies in the fact tha t the hydro- chloric acid in adding on to the nitrogen atom (1) causes the latter to become more nearly saturated and therefore less able to retain the hydrogen atom against the great attraction of the oxygen.

The relation of the absorption curves of the various azo-compounds to one another is of great interest. From the curves given it may be seen that when a large amount of residual affinity is in close proximity to one of the benzene nuclei of azobenzene as in the case of the hydroxyl compounds and their ethers,

C6H,*N : N C,H; OH and C6H5 N : N C,H,* OE t,

a very great diminution in the persistence of the colour band occurs. If the residual affinity of the hydroxyl group is decreased by replacing the hydrogen atom by an acetyl or benzoyl group, the curves obtained approach more nearly to that of azobenzene.

It has been shown (Baly and Collie, Trans., 1905, 87, 1332) tha t unsaturation in the a-position to a benzene ring produces a very great alteration in the type of vibration.

These facts are of considerable interest in view of the hypothesis, recently put forward, with regard to the colour of azobenzene (Baly and Tuck, Trans., 1906,89, 982). According to this view the colour depends on the fact that the benzene nuclei vibrate mainly along the two directions indicated by the dotted lines,

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TUCK : THE CONSTITUTION OF HYDROXYAZO-COMPOUNDS. 457

the extreme phases being 5 6 6' 5'

Thus the carbon atoms 1 and 1' are to a large extent unsaturated and the action of these centres of unsnturation on the unsaturated group *N:N* sets up the isorropesis. From this i t may be seen a t once that the more the vibrations of the benzene rings are disturbed the greater will be the diminution of the persistence of the jsorropesis band observed. This agrees with the results previously obtained by substitution in benzoquinone (Stewart and Baly, Trans., 1906, 89, 618). Addition of sodium ethoxide to solutions of the ethers of hydroxyazo-compounds produces absolutely no change, but when added t o tho free hydroxyl compounds, a considerable modification of the type of absorption occurs,

Conclusions.

1 . The p-hydroxyazo-compounds and their hydrochlorides a re in the true azo-form, for example, benzeneazophenol is

2. The o-hydroxyazo-compounds and their benzoyl derivatives are derivatives of o-benzoquinonebenzoylphenylhydrazone, for example, the formule of p-tolueneazo-p-cresol and its benzoyl derivatives are

CH, CH3 CJ-&*/--\.NH.N:/-\ and CH3/-\*N---N:/-\

\-/ \-/ \-/ &.OPh\-/' 0 0

3. The hydrochlorides of the o-hydroxyazo-compounds are derivatives of true azo-compounds.

I should like t o express my hearty thanks to Prof. Collie and Mr. Baly for the interest they have taken in the progress of these experiments. I also wish to thank Mr. Baly for the facilities he has afforded me in carrying out this research. I am indebted to the Chemical Society for a grant in aid of the work.

THE SPECTROSCOPIC LABORATORY, UNIVERSITY COLLEGE,

LONDON.

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