462 CHATTAWAY: THE ACTION OF LIGHT ON

XL1X.-The Action of Light on BenzaZdehydephenyZ- hydrazone.

By FREDERICK DANIEL CHATTAWAY.

ALTHOUGH light has been observed in many cases to cause profound alteration of the properties of both inorganic and organic substances, in comparatively few have the changes been at all fully investigated or any adequate explanations of the phenomena been put forward. Roloff (Zeit. physikal. Chem., 1898, 26, 335) distinguishes between chemical and physical actions of light, including among the former only those in which different molecules interact ; known instances of isomeric change, however, and others probably due to this which he includes in the latter group are more properly considered as strictly chemical.

Marckwald (Zeit. physikal. Chem., 1899, 30, 140) uses the term “phototropy” to designate a type of change which he regards as purely of a physical nature, where a colour which appears on exposing a compound to the light disappears on heating or on placing the substance again in the dark.

Among organic sc bstances which behave thus, benzaldehydephenyl- hydrazone is perhaps the one most easily obtained. Fischer, who first

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BENZALDEHY DEPHENYLHYDKAZONE. 463

prepared the ordinary stable a-modification of this compound, noted (Annalen, 1878, 190, 135) that when exposed to the air it reddened. H e appears to have regarded this reddening as due to slight oxidation similar to that which so many amino-compounds undergo in contact with oxygen and dismisses the phenomenon without further comment.

Biltz (Annulen, 1899, 305, 171, and Zeit. physikul. ClLenz., 1899, 30, 527) recalls this observation and states that the foregoing hydrazone and a number of osazones are sensitive to light in the same way as two compounds mentioned by Marckwald (Zoc. cit.). H e states that in all these cases the substances which are greyish-white or yellow become red on exposure to light, and that in the dark or on heating the original colour returns, and also that the red colour shows signs of fading if the exposure to light lasts a week or thereabouts. Reutt and Pawlewski (Bull. Acad. Sci. Cracow, 1903, 503) record the same facts with regard to this hydrazone; they appear to have regarded the reddened product as a third modification of the substance, but beyond noting the changes of colour made no further observations.

Benzaldehydephenylhydrazone does not redden a t all, even on pro- longed exposure to the air, if light is excluded; a specimen freely exposed on a watch-glass for a year in a wooden box, and thus screened from light, showed no trace of red colour or of decomposition and its melting point remained unaltered, On the other hand, if exposed to the action of light, its colour alters in a remarkable way; in diffused light, reddening takes place slowly and is only perceptible after some hours, but in sunlight the change becomes very noticeable in a few minutes and the colour quickly deepens until after a few hours a maximum intensity is reached, when the crystals have a brilliant scarlet colour resembling that of azobenzene.

The presence of air is not essential, as the change takes place equally well in an atmosphere of dry hydrogen. Only those parts directly exposed to light are affected, whilst subjacent layers o r any portions screened by black paper, sheet copper, or yellow glass remain un- coloured. Seen under the microscope the crystals appear not to have undergone any change in shape or brilliancy ; the surfaces reflect light exahtly as before and the colour only is altered.

The light from an electric arc playing between iron poles, which is extremely rich in ultra-violet rays, also brings about the change, the velocity of which falls off in the ordinary way as the distance from the arc is increased ; exposed in a quartz tube a t a distance of five centi- metres, the change was about as rapid as i n direct winter sunlight. The transformation, however, appears not t o be due to the ultra-violet rays, as its rate is not affected by interpqsing a sheet of ordinary crown glass two millimetres, or a sheet of !ead glass two centimetres, in thick- ness, or by both together.

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464 CHATTAWBY: THE ACTION OF LIGHT ON

The light from an incandescent mantle, giving a continuous spectrum but deficient in violet rays, also brings about the coloration very slowly. Placed at a distance of about five centimetres from the glass of the vacuum tube and exposed for over an hour to a power- f ul stream of = rays, benzsldehydephenyl hydrazone undergoes no change whatever. Exposed to light under various coloured glasses which absorb different parts of the spectrum, it is found that only the violet, and to a less extent the blue, portions are active, the other portions of the spectrum causing no appreciable colour change.

If a solution of the compound in alcohol or acetic acid is exposed to light, no change of colour occurs ; if, however, crystals separate, these are slightly reddened on the side exposed to the light. If, when the colour change has reached its maximum, the scarlet crystals are dis- solved in warm alcohol, a pale yellow solution results from which ordinary benzaldehydephenyl hydrazone separates on cooling, no appreciable amount of any other substance being produced.

On allowing the scarlet crystal3 to remain for LZ long period screened from light at the ordinary temperature, the colour slowly fades ; this fading is more rapid if the temperature is increased and takes place in a few minutes if the coloured product is heated a t 100". The pale yellow product thus regenerated is ordinary benzaldehyde- phenylhydrazone and is indistinguishable from the original pure sub- stance. On rapidly heating the coloured product, fading takes place as the temperature rises, and when about 150" is reached the red colour completely disappoars ; the product is then indistinguishable from the original material, both substances melting at exactly th6 same tempera- ture (158--16OO). On exposing the faded product again to light or on powdering and exposing the melted product, reddening again occurs as before. On powdering benxaldehydephenylhydrazone which has been melted, a smell resembling that of benzaldehyde is noticed ; this is also observed on powdering the coloured product after transformation and melting have taken place; it is not noticed on powdering the coloured product which has lost its colour by exposure to a temperature of loo", and consequently appears not to be connected with the trans- formation of coloured to colourless material, but to be due to some decomposition of ordinary benzaldehydephenylh ydrazone itself at this temperature.

The colour also disappears in a curious way on long exposure to sunlight. Quantities of benzaldehydephenylhydrazone contained in sealed tubes of thin glass filled with air or hydrogen were exposed in tho open to direct sunlight ; when the colour had deepened to a maximum, it began slowly to fade. After several days of bright sunshine, the loss of colour was unmistakable, and after several weeks all red t int had completely disappeared. Continued exposure during the whole of a

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BENZALDEHYDEPHENYLHYDRAZONE. 465

summer caused apparently no further change. The crystals exposed in hydrogen, after this treatment, stuck to the sides of the tube somewhat more than at first, but did not seem otherwise altered or decomposed, and melted within 2 degrees of the original melting point. The orystals exposed in air, on the other hand, had evidently undergone slight decomposition, as those sticking to the inner sides of the tube had lost their sharp edges and had a faint brown tint. On opening the tubes, a slight smell of benzaldehyde was noticed in both. On dissolving the contents in warm alcohol, a pale yellow solution was obtained from the product exposed in hydrogen and a slightly brown solution from that exposed in air, but from each solution pure benzsldehydephenylhydrazone crystallised on cooling, only a very slight amount of brown residue being left, apparently not more than is obtained on recrystallising the pure hydrazone from alcohol and evaporating the mother liquor to dryness.

Several of the tubes exposed during the whole summer out-of-doors became cracked, so that air had entered freely. I n these, considerable decomposition had taken place under the combined influence of light, air, and moisture, and the inner surface of the tubes had become coated with a soft, adhering, sticky, brown film, but even from this decom- posed residue benzaldehydephenylhydrazone was obtained pure in considerable quantity by a few crystallisations from alcohol.

The colour only appears to give any clue as t o what takes place under the influence of light, and supplies a possible explanation of the nature of the change, even if we do not accept in i ts entirety Armstrong and Robertson's recent contention (Trans., 1905,87, 1285) that arguments based on colour are already among the most absolute at our command.

It seems probable that in these transformations of benzaldehyde- phenylhydrazone we have a definite reversible intramolecular re- arrangement brought about by the action of light, and if we accept colour as our guide in interpreting it, we are led to the conclusion tha t it is a change from the hydrazino- to the azo-configuration, thus :

C,H,*CH:N*NH*C,H, C,H5*CH,*N:N*C6H,.

Benzaldehydephenyl hydrazone is not colourless as generally de- scribed, but has a distinct although very pale yellow colour which is quite marked in alcoholic solution, the grouping -CH:N- being only slightly colour-producing ; the grouping -N:N--, on the other hand, gives rise to strongly colonred compounds, and i t is worth again noting that the colour developed by l ight in benzaldehydephenyl- hydrazone is almost identical with that of azobenzene itself. The hydrazone configuration of the compoun d u d ~ ~ cD22S~d8T2&t,,m js und0~' ordinary conditions the more stable, and the coloured product having

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466 ACTION OF LIGHT ON RENZALDEHYDEPHENYLHYDRAZONE.

the azo-configuration, or consisting of an equilibrium mixture of both forms, passes readily and completely again into the former on stand- ing or heating or dissolving in alcohol.

This explanation of the colour change as being due to a reversible intramolecular rearrangement is slipported by the behaviour of benzaldehydediphenylhydrazone, C,H,*CH:N*N( C,H,),, of benzalde- hydephenylbenzylhydrazone, C,H,* CH:N-N( C6H5) * C H2-C,H,, and of benzaldehydeacetylphenylh ydrazone, C,H,*CH:N *N( CO*CH3)*C6H5, which resemble benzaldehydephenylhydrazone in structure, but in place of the labile hydrogen atom have the groups C,H,, C,H,-CH,, and CO*CH,, which are not labile and consequently would not be likely to pass from the nitrogen to the carbon under the disturbing influence of light. As might be expected, none of these compounds under- goes a colour change on exposure to sunlight.

Further investigation is needed to decide what causes the dis- appearance of the colour on prolonged exposure to light, but as some slight decomposition undoubtedly takes place, the reverse transforma- tion may be brought about under the influence of one of the decomposition products, or may be due to slight superficial decom- position leading to the formation of a slightly orange-coloured film which absorbs the blue and violet rays and so allows the normal reverse change leading to fading to gc. on. The latter seems the more probable reason, as on powdering the faded crystals the powder was readily coloured by light, showing that only the surface is involvod.

As showing tha t a group linked to nitrogen in the hydrazone con- figuration exhibits a marked tendency to pass to the carbon, the easy transformation of diphenyldibenzylidenehydrotetrazone into F-benzil- osazone, observed by Ingle and Mann (Trans., 1895, 67, 606), may be cited.

It is worth noting tha t benzophenonephenylhydrazone, contrary to what might be expected, is not at all affected by light ; the presence of the two phenyl groups attached to the carbon atom may, however, offer steric hindrance to the transference of the hydrogen from the nitrogen to the carbon atom and so prevent change occurring.

The consideration of this intramolecular rearrangement caused by light and accompanied by marked colour-change leads one to put forward the suggestion that the fading of organic colouring matters i n sunlight may, in some cases at least, be due to a similar reversible isomeric change. Light, 3s is well known, can cause definite intra- molecular rearrangement o r can cause such vibration or oscillation within the molecular structure as makes such rearrangement possible. I n the case described in this pgper, the change under the influence of light is from the colourless,to t h e strongly coloured configuration ; in

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STUDIES ON OPTICALLY ACTIVE CARBIMIDES. 111. 467

others it may b8 from coloured to colourless. In this connection, it would be of interest to ascertain whether any faded dyed material has ever been observed to recover after being kept screened from light for a long period.

CEIEMICAL LABORATORY,

ST. BARTHOLOYEW’S HOSPITAL, E.C.

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