THE COLORED SALTS O F SCHIFF'S BASES. A Contribution to Our Knowledge of Color as Related to Chemical Constitution. r3v I:. J . \rooitis
AS])
K . I). G A L E .
KeceivmI J a n u a r y 6 . 19oh.
I. The Hydrochlorides of Bases Formed by Condensing p-Amino Dimethylaniline with Aromatic Aldehydes. l'hc starting point of the prcwnt invcstigatiori was a cliaiic(~ol)ser\-ation iiiade upon the conipouiicl pr-oducd by condensing ~-aiiiiiiodinietli~! aniline wit11 piperonal. This product is 0,. a light orange color- :itid lias the formula
When this substance, eithcr in the dry state or iii clthercal or I)ciizcrie solution, is treztetl with dr!. hydrochloric acid gas, o m niolccule of thr latter is first added t o form a salt of a dcel) blood-rcd color. This salt can furtlicr add another moleculc o i thcl acid to foriti a dih!drochloridr, 'fhr color of tlie latter salt. iii sharp contrast to that of t h e f'orn1r.r. i> ;L bright lemon-yellow. We found this phenoincnon so striking that we determined to prepare a liumber of compounds of analoqou.; constitution :inti study the color of their salts. 'l'he first sulistancvs selected werc tliosc. most strict 1y analogous t o the one already nientioned, namely, t h e condensation pro(Iucts of ~-aminodiiiiethylanilincwith aromatic aldehydes. These base.; furnish the subject iiiattcr of the ;)rcscnt piper. ?'lie!- d l show a 1~ havior toward hydrochloric acid, and (so far as has h e n tested) also tolvard other acids, entirely analogous t o that described in thc w!+ oi the piperonal conipound. .\ iiiinor exception has t o be noted in t h case of anisaldehydc. When its condensation product with ,/-n:iiino dimethylaniline is treated with hydrochloric :wid. thc sanie color clianges are obsery-ed as in thc otlivr c:~scs, h t analysis of thc pr-odttcts ~ I O ~that V a maxiinuni of ncaarly t 1irr.i: il!olc,culrs of thi. acid is Iicrc. al~sorI)c.ti. This 1)ehal-ior will be cliscusscd iiiorcx fully in t h c y expcrinitmtal part.
COLORGD SALTS OF SCHIFF’S BASE&
395
Subsequent papers will deal with other compounds similarly constituted. Thus we have prepared a number of bases of this class by condensing aromatic aldehydes with +-aminodiethylaniline. Of these it ma?- be said t h a t as far as investigated, they show the same reactions toward acids as the corresponding dimethyl compounds. The salts are, however, less stable. One of us, in collaboration with Mr. R. G. Woodbridge, Jr., is also studying the bases formed by condensing +-aminodiphenylamine with aldehydes. These bases unite with one molecule of hydrochloric acid t o form dark red salts like the monohydrochlorides already described. These salts, however, do not add a second molecule of the acid. From the above it is clear that we are dealing with a quite general law which may be stated thus: Bases of the general formula
\-N/
CH, R--CH=Ndiatc.ly turns tiark red, obviously owing to the formation of the red nionoli!.drocliloride. The color then slowly fades out as this salt splits into the hydrochloride of mino no dim ethylaniline and piperonal. Sow the weak spot in the hydrochloride of piperonylidene aniline is obviously the iiiolccule of acid attached to the cciitral nitrogen. If the red silt 11:;s t h e n a constitution analogous t o t h a t shown in formula (4), i t is not, a t first sight, quite clear why it should hydrolyze a t all. The abox-e argument, however, against (4) docs not seem as serious as t h a t which can lie brought against (3). If we give the latter formula to the red hydrochloride, we can onl!- ascribe the intensity of its color t o salt formation-halochroniy. \Ye should then expect a further intensification of tlic color when the second niolecule of hydrochloric acid is ndcled. 21s wc have already seen, this is not the case. There remains the quinoid formula, (j). This also offers a consistent explanation. &:e have only to assume t h a t thc red hydrochloride exists in the quinoid, the yellow dihydrochloride in t h e benzoid form. I t seenis a t first a little difficult t o account for the hydrolysis into aldehyde and amine salt on this basis, but the shifting of double bonds involved is not of a particularly revolutionary character. I t would also seem possiblc t h a t the dihydrochlorides might also exist in a quinoid form. Perhaps, when niore cornpounds have been studied, such salts will lie found. 111 the iiieantiiiic, sotiic work is under way in this laboratory designed t o throw light upon the probability of a quinoid formula for the red hydrochlorides. .in explanation, difieriiig from any of the above, has occurred t o us since reading ;t recent article by Anseltiiino' which calm t o our notice after the experimental work described in this paper was practically conipleted. Xnseltnino found that when p-homosalicylic aldehyde is coliderised with aniline the product forniecl exists in two different forms, a red and a yellow. That this is not simply a case of diniorphism is shown by thc fact that the two forrns difler in chcniical bcha\-ior and give different derivatives. The most probablc assumption concerning their constittition would sceiii t o be t h a t the!. :ire stereoisonxric in the same sense as the isomeric oximes of unsymmc.tric:d ketones. If we try t o explain the behavior of the compounds wc are studyiiig upon a siiiiilar basis? we should ha\-e t o say t h a t the salts x-ary i n color liecause they Lire derived onc rccl thc- othcr yellow. That in fro111 tliffcrent sterc.oiso!iic.ric lxi. 1
uct.., 40, 346.5 (1907) ; :&o
Z b ~ d . ,38, 3989 (1905).
COLORED SALTS OF SCHIFF’S BASES.
399
the case of the bases themselves, the yellow form is alone stable; t h a t when the bases add hydrochloric acid, the yellow salts first formed are unstable and go over to form salts of the other-red base. Finally, when the saturated salts are formed, t h a t a transformation in the opposite sense takes place. This explanation disregards the question as t o which nitrogen holds the hydrochloric acid. Now, in this connection, we have t o remember that previous t o this recent work of Anselmino, no well attested cases of stereoisomerism had been observed among benzylidene compounds, though they have long been looked for; and, further, that Anselmino himself observed no such isomerism in the case of the salts. I n fact he points out t h a t his red and yellow bases gave identical hydrochlorides. This, of course, is no conclusive argument against a stereochemical explanation of the facts which we have observed. It does, however, furnish a reason for not accepting such an explanation hastily. I t will be seen in the experimental part of this paper that the salts of salicylidene p-aminodimethylaniline undergo change in color on standing, and i t has been already pointed out that compounds of this class containing the diethylamino group show similar behavior. From what has already been said, it will be seen in what a variety of directions, an explanation of this fact might be sought. Any discussion of these possibilitieswould, however, be premature until more work upon these ethyl compounds has been done. For the present also we wish t o reserve any opinion as t o the probable formulae of the red hydrochlorides.
Experimental Part. The condensation product of p-aminodimethylaniline with benzaldehyde was first prepared b y Calm.‘ I t is interesting t o note his observation t h a t this base adds two molecules of hydrochloric acid t o form a ‘‘ white” salt. This illustates the light color of the saturated hydrochloride, and a t the same time shows that Calm attached no significance t o the formation of the red intermediate product. He gives a determination of chlorine in the saturated salt. The other bases studied in this paper were prepared by Xuth’ a t the suggestion of Calm. He did not prepare salts. We have little t o add t o his description of the bases, except for some melting points already corrected by more recent observers. We prepared the bases by mixing molecular quantities of the amine with the various aldehydes, sometimes warming a little on the water bath. The reaction then proceeded at once and gave good yields. The product was recrystallized once from alcohol. It was usually found t h a t further recrystallization did not raise the melting-point. The bases range in color from a light yellow t o a light orange. Ber., 1 7 2938 ~ (1884). Ibid., 1 8 ~ 573 (1885).
400
F. J. MOORE AND K. D. GALE.
I n studying thc salts, we at first allowed hydrochloric acid t o act upon the bases in the drj, state. When piperonylidene p-aminodimethylaniline.. for example, is spread upon :L piecc of porcelain (a crucible cover) and a current of hydrochloric xcid passed o w r i t , the inass suddcnly turns a deep rcd. This color 110 sooiler appears than it begins again t o fade, owing to the formation of the light yellow saturated salt. A similar effect is produced wlien the base is dissolvcd in dry ether and hydrochloric acid gas introduced. Here the liquid first turns dark red ; then a precipitate of the same color appears, and, finally, the titnc varying with the amount of substance present, the color of the solution grows rapidly lighter in shade, the precipitate becomes bright yellow, and at. the end, the supernatant liquid is colorless, the satiiratetl salt being practically insoluble in ether. I t will frcquently 1 ) ~noticed that a t the mouth of t h e tube where the gas enters, and whcrc: consequently the acid is always in excess, a yellow deposit of thc dihydrochloridc forills a t once. This makes it probablc that t h e red salt, liowe\-cr it is prepared, contains a good deal of the yellow enclosed. 'l'his shows itself in tlic analytical data furnished further on. \Vhen the salts were prepared for analysis, the following procedure was eniployed in order t o avoid this kind of contamination as much as possible. First. thix irce basc was dissolved in sodiun-dried ether and then a solution ol hydrochloric acid in dry ether was run in from a burcttc. \\-hen preparing the saturated salt, the addition of the acid solution as continued as long :?s m y precipitate formed. To prepare a n unsaturated salt, a little less than half as much acid was added ;IS had been found iiecessary in preparing the saturated one. The precipitates so obtained$re filtered by suction, washed repeatedly with dry ether, arid finally dried in a vacuum desiccator containing both concentrated sulphuric acid and sticks of caustic soda. If lumps formed during the drying, these were g-round up and the desiccation continued. -4s far as our observation goes. these salts melt only with decomposition, and the teniperature obseryed depends largely upon the time of hcating. In the case of the saturated salts, the point of decomposition lies quite close t o zoo0 in almost all cases. The chlorine determinations were in all cases rnade by the Carius method. The snlts precipitated and dried as a b o w described were uscd for ana1ysi.s without recrystallization. Alcohols would have 1x.cn the only practicablc soll-ents for this purpose, but some experiments inadc in this direction led us t o fL%arthe contaminating effects of hydrolytic pction. As w e ha\^ poiriteti out above, the salts were probably more or less contaniinatccl with each othcr, and pcrhaps, also with the free bases. Gnder the circumstances, KC' have not thought it desirable t o make a large nuiiihcr of dctcrniinatioiis. in the hopc that sonic of them might agree ii!or(, closel!. with tlir rc:sults ol tlieorctical computation. In spite of the wide divergcmcc. t o he observed in many cases, n e think that
COLORED SALTS OF SCHIFF'S BASES.
401
t h e numerical results make plain the one point which we wish to emphasize a t this time, namely, t h a t the light colored salts contain about one molecule of acid more than the dark ones. The results follow: Benzylidene p-Aminodimethylani1ine.-Red hydrochloride : calculated for C,,H,,N,HCI, C1, 13.60; found, 17.01. Saturated hydrochloride : calculated for C,,H,,N,. zHC1, C1, 23.09 ; found, 20.48. Cinnamylidene p-AminodimethyLaniline.-Red hydrochloride : calculated for C,,H,,N,HCI, C1, 12.36 ; found, 15.33. Saturated hydrochloride : Calculated for C,,H,,N,.~HC1, C1, 21.95 ; found, 21.34. Pieeronylidene p - Aminodimethylani1ine.-Red hydrochloride : calculated for C,,H,,O,N,HCl, C1, I 1.63; found, 15.19. Saturated hydrochloride: calculated for C,,H1,O,N,.2HCl, C1, 20.78; found, 21.12, 21.05. Salicylzdene fi-Aminodimethylani1ine.-Red hydrochloride : calculated for C1,Hl,ON,HCl, C1, 12.81 ; found, 13.07. Saturated hydrochloride : calculated for C,,H,,ON,.~HCl, C1, 22.65; found, 22.47. The color of the salts seemed less permanent in this case than in any we had previously studied. Nuth records that the free base turns red on standing in the air, and we notice that the red hydrochloride has a tendency t o grow lighter in color. What is perhaps more curious is t h a t the saturated hydrochloride which, when first precipitated, is almost colorless, grows considerably darker in color. We have since met with even more marked changes of this kind in the study of the bases formed by condensing p-aminodiethylaniline with aldehydes. We consider any theoretical discussion of this phenomenon t o be premature until these diethyl compounds have been more thoroughly studied. The fact should be borne in mind, however, t h a t , according t o Anselmino, salicylic acid is one of those which forms isomeric aniles, one red, t h e other yellow. Anis ylidene p- A minodimethylani1ine.-Red hydrochloride : calculated for Cl6Hl,ON2HC1, C1, 12.20; found, 12.20. C,,H,,ON,.~HCl, C1, 21.68; found, 18.34. Saturated hydrochloride : calculated for C1,H,,ON,.3HC1, C1, 29.27; found, 28.78, 26.16, 26.61. I n view of the fact t h a t in the case of the red hydrochlorides high results for chlorine are usually obtained, probably owing t o the tendency of these salts t o enclose some of the saturated compounds, the numerical results given above leave it a little in doubt whether the red salt, in this case, contains, when pure, one or two molecules of acid. It is quite evident, however, t h a t the saturated salt contains nearly three molecules. This seems t o be one of those cases referred t o by Baeyer' where a base shows itself capable of combining with more acid than its formula accounts for. It might be supposed t h a t in both salts the additional molecule of hydrochloric acid attached itself t o the methoxyl group to Ber., 38, 1157 (1905).
v.
402
J . MOOKI.:
mr)
K . D. GALE.
forni a n oxonium salt. This iiiay be the correct explanation, hut it is not necessarily so; for we find t h a t the most simply constituted nienibcr of the whole group, benzylidene aniline, though it conains but one nitrogen and no oxygen, yet adds two molecules of hydrochloric acid, as shown by the following analysis: Calculated for Cl,HllNHCl, C1, 16.29; for C,,HllN.2HCl, C1. 2 7.90; found, 27.68, 27.62. We were much surprised a t this result, as it raised the questioii whether all of the benzylidene compounds containing one nitrogen might not bchave similarly, and whether in those containing two atoms. all of the acid might not be held by one nitrogen. The analyses of the salts of the three following compounds, however, show that the addition of more molecules of acid than there are nitrogen atoms in the base, is the exception; though from what has gone before, it may well he of more frcquent occurrence than has hitherto been supposed. Pifieronylidenc .4 ~ziline.-'l'his base was first prepared by Lorenz. ' It is absolutely colorless, the first of these compounds which we hax-c. been able to prepare in t h a t condition. The hydrochloride is bright yellow, and this niakcs perhaps the most striking example of simple halochromy " which we have observed in the group. The hydrochloride contains but one molecule of acid, as is shown by the following analytical data : Calculated for C,,H,,O,NHCl, C1, 13.55; found, 13.76, 13.j1. Pipwonylidene fi-Toluidine.-This substance crystallizes from alcohol in creaiii-colored prisms which melt a t 98 '. Calculated for Cl5HI3O2N,C, 75.26; H , j.48; X , 5.87. Found, 75.28; "
j.jj,
6-11.
The hydrochloride is light yellow. The percentage of chlorine was determined. Calculated for C,,H,,O,NHCl, C1, 12.86; found, 13.03, 12.98. P,ifievonylidene $-Chloraniline,-The base melts a t 78 '. I t has not been analyzed. It forms a light yellow hydrochloride. Calculated for C,,H,,,O,NCl.HCl. C1, 23.95 ; found, 23.43. We wished to learn something of the color of salts analogous to those formed by condensing aldehydes with fi-aminoclirnethylaniline, but which contained an amino group not substituted by alkyl radicles. We had some hopes t h a t a t least a small yield of such compounds might hc obtained by condensing one molecule of aldehyde with one of a diamine. Accordingly we treated piperonal with $-phenylenediamine, and also with benzidine. In both cases the only products we ohtained wcre thosv formed by the condensation of one molecule of diamine with two molecules of piperonal. These bases are soluble with difficulty in alcoBer., 14, 792 (1881).
COLORED SALTS OF SCHTPP'S BASES.
403
hol, ether, or the aromatic hydrocarbons. They can be crystallized from nitrobenzene, from which they separate in bronze yellow scales of a semi-metallic luster. The compound with p-phenylenediamine melts Calculated for C,,H,,O,N,: C, 70.94; H, A.33; N , 7 . 5 5 . Found, a t 216'. 70.30, 4*52J 7*9O* The compound formed by condensing benzidine with piperonal melts not quite sharply a t 241'. The liquid formed is not transparent, and it is possible that it is crystalline in character, as liquid crystals are not infrequently met with among compounds of similar constitution. The composition of the base was verified by a nitrogen determination. Calculated for C,,H,,O,N,, N. 6.50; found, 6.43. We add some incomplete data concerning some substances prepared during the present investigation, but which, as they have no further theoretical interest for us, will probably not be worked with further. Piperonylidene p-Aminoethy1benzoate.-This substance was prepared by condensing piperonal with paminoethylbenzoate. The base melts a t 109O, is almost colorless, and forms a yellow hydrochloride. Calculated for C1,H150,N, C, 68.64; H , 5.09; N, 4.72. Found, 68.34, 5.08, 4.84. Piperonal condenses readily with in-nitraniline t o form a base which melts a t 119'. This forms a yellow hydrochloride. We have a nitrogen determination in the free base. Calculated for Cl,Hl1O,N2, N, 10.35; found, 10.56. Piperonal condenses with p-bromaniline t o form a product melting a t 109'. This forms a hydrochloride of a light canary-yellow color. We have analyzed neither the base nor the salt. Summary. When p-aminodimethylaniline is treated with aromatic aldehydes, condensation products are formed which have the general formula
R-CH
N-
/-\
LJ-\CH;
These bases add one molecule of hydrochloric acid t o form dark red salts of a much deeper color than the free bases. The addition of more hydrochloric acid produces salts of a light yellow color, lighter than t h a t of the free base. Three explanations of this behavior are considered: (I) That the first molecule of acid adds t o the auxochrome nitrogen augmenting the color, while the second adds t o the chromophore nitrogen, changing its valence and consequently destroying its chromophore character. (2) That the monohydrochloride has a quinoid structure, while the saturated salt and the free base are benzoid.
404
WM. LLOYD EVANS AND HENJ. T. BROOKS.
(3) That there is a double series of stereoisomeric red and yellow bases and salts, the red form of the monohydrochloride being the stable one, the yellow form being stable in the other cases. A decision as t o which of these explanations is most applicable must be deferred until more experimental material can be collected. BIASSACHUSETTS I N S T I T C T E O F
January
I.
TECHNOLOGY.
190%
[CONTRIBUTION FROM THE
CHEMICAL LABORATORY OP THE
OHIO
STATE
UNIVERSITY. ]
ON THE OXIDATION OF META-NITROBENZOYL CARBINOL. BY WILLIAJI LLOYD
EVANS.4ND
BENjAMIX
T . BROOKS.
Received J a n u a r y 15. 1908.
The work of Nef' on the oxidation phenomena exhibited by many series of organic compounds has made it possible t o follow by experiment the exact course taken by such reactions. The recent work of Denisz on the behavior of various aldehydes, ketones and alcohols towards oxidizing agents is an excellent example of this kind of experimental stud),. In a previous paper by one of US,^ it was shown that reactions of this type in the benzoyl carbinol series lend themselves admirably to this kind of treatment. Zincke' and his students were the first to show that benzoyl carbinol, when acted upon by various oxidizing agents, gives mandelic, benzoylformic and benzoic acids in varying amounts according to the agents used. I n a further study of this same substance by one of it has been shown t h a t benzoyl-formaldehyde is also one of the products of oxidation of bexizoyl-carbinol. When benzoyl- formaldehyde is acted upon b~7alkalies,6 and copper salts,' a t IOO', it undergoes a benzilic acid rearrangement, giving mandelic acid exclusively. It has been shown by Denis that acetyl formaldehyde undergoes a similar rearrangement with dilute solutions of sodium hydroxide, and even with water alone a t 100' it suffers a pal.tial transformation. * I t is a well-known fact that many orthodicarbonyl compounds, s-c =0
I
Y-c=o Ann. Chem., 318, 137; 335, 191; 357, Am. Chern. J., j8, 561. Evans, Ibid., 35, 1 1 j. 4 Ber., I j , 6 3 5 ; Ann. Chem., 216, 311. L O C . cit. fl Pechmann, Ber., 20, 2904; 22, 2 5 5 6 . Evans, Am. Chem. J., 35, 124.
' I h i d . , 38, 584, , j 8 j .
214.
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