I3 E S 2IL0IIT H0C.IR R 0 S Y 1,I C -4C I II
B Y C. -1.SOCEI
Renzilorthocarbosylic acid, C,HjC0.C0.C,H4COOH, occurs in two iiiodificatioiis, one white aiid the other !.ellon-.I Determinations iii acetic acid showed that the two have the same molecular weight. In alcohol, in fiftj- percent aqueous alcohol, and in chloroform, the yellow c n s t a l s are said to he abont twice as soluble as tlie white. The >.ellow modification inelts at 141. jot while the white forin melts at I 2 jC-1303aiicl cliang-es or-er to tlie yellow foriii. T h i s is esactl!. what shoiilcl happeii in case the two modifications existed in equilihriuni in the melt.’ T h e yellon- iiioclificatioii \voulcl then be the more stable arid the white the less stable foriii. 011 the other hand Graebe states that the white form can be obtained 11~. cr!dallizatioii at orclinary temperatures, even when starting froiii the ellon on- crystals. T h i s could be interpreted in one of two \rays. T h e white crystals might he more stable a t lo\v teniperatnres aiid the yellonabove IOO’. If this were the case. there n.onld iiecessaril!. he an ins-ersion point solnewliere between 20’ and 100‘ \vliicli nould be very interestiiig since 110 case is !.et recorded of ai1 iii\-ersioii point for two sabstances u-liicli melt to diff ereiit liquids. I t \vas also quite possible that the re\,ersal of stability inight be due to the sol\-eiit, the white crystals being stable at ordinary temperature oiilj- in presence of certain solvents. There is nothing in Graebe’s papers to enable one to distiiipisli hetn-eeii these two hypotheses. I have not been able to find an!. mention of the \vliite crj.stals cliaiiginx into the yello\v at ordinary teiii-
’ Graelx aii(1 Juillarrl. Ber. clit.111. (;e$. Iierliii. Ihitl, 23, 1 1j4, rS9w. B:incroft. Jour. I’hys. Chrrn. 2 , 142 I\ iSgS J ,
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ISSSJ: Graelw.
L?cI I Zilorth o m r h o q /ic ~ XI cid
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peratures nor of the reT-erse reaction. *At the suggestion of Professor Raiicroft I haye studied the conditions of eqliilibriu1n for tlie two iiioclifications. In preparing the acid I first made beiizilideiieplitlialicl aiicl tlieii u-ortliodesos!.beiizoiiicarbos!-lic acid accordilig to the (lirectioiis of Gabriel and AIicliael.l T h i s latter slibstance T ~ coii\-ertecl into henzilorthocarbosylic acid according to Graebe's niethod. I t n-as found that the !-ellow iiioclificatioii does change into the white form at ordinar!- temperatiires but so slow1~-that the claaiige might easily lie o\-erlcoked. T h i s is probabl!. the reason that Graehe did not mention the fact. To cleteriiiine the inversion temperature soiiie of tlie n-liite modification, together \vitli a trace of the re ell on- crJ-stals, were sealed i i i a glass tube, the !.ellon cr!.stals being added to guard against a persistence of a iiietastable state. Iii another tiil)e was placed some of the !.elion- iiiodification together with a trace of the white cr!.stals. Tlie third :tiid fourth tubes differed froiii the first aiid second in liai-iiig a little cliloroforiii added. I t was thouglit that this \\-auld increase tlie reaction 1-elocity' aiid possihlj- cliange the inversion temperatiire. 0 1 1 1 ~ .the first of these t\vo aiiticipatioiis \\-asrealized. - i t 50' the !-ellon. iiiodification cliaiiges to tlie Tlie iiiyerwhite \vhile the re\-erse reaction takes place at 75'. sioii temperature was -nest found to lie het\veen 60" aiid 7 0 ' . It \vas not possible to bring these limits nearer together n-itliout spending more time 011 the qiiestion than i t was n-ortli, since the change takes place \-el-!- slon-1)- near the inversion teiiiperatiire. T h e teiiiperature of tlie triple poiiit may be taken as 65" 3". T h e rate of change iii the tubes coiitaiiiiiig cliloroforin was roughly twice as g-reat as in the dry tiihes. 111 one esperiiiieiit a t 75" only the lower part of the tube was in the bath and some of tlie !.ellon- solid subliiiied into the cooler end, precipitating as white needles. T h e n e s t point was to deteriiiiiie the changes of equilibrium at higher temperatures in order to see whether the>-met the reciiiireiiieiits of Diihein's theory. I 1ial-e succeedecl in coiifir-ining ~.
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' Ber. cliem. Ges. Berlin. 1 1 ) 101; (rS;S) ;
18)3470 Reiclier. Recueil Trav. Pays-Bas, a, 246 (rSS3).
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all tlie resnlts obtained by Graebe in regard to the freezing-points though, a t one time, I was misled by a rather iuifortunate phrase of his into believing that in!. results did not agree with his. In the first paper' Graebe says that if the white crystals are heated to T I j" the!- become j.ellow and then melt at 131.j'. In the second paperz he says : T h e white modification becomes liquid at 1 2 5 ~ - 1 3 0 but, ~ after long heating a t this latter temperature, solidifies to the yellow form." T h i s was taken to mean that the melting-point of the white modification conld easily be determined and that the change to tlie >-ellow crystals was slow a t this temperatiire. -qs a matter of fact it \vas fouiicl that 011 lieating the white crystals they became !-ellow a t about IIO"--II j " and iiielted at 141.j". T h e rate of change \vas so great that the white crystals changed entirely into the ).ellon. before the melting-point of tlie former could be reached. Since tlie yellow crystals iiielt at 141.j", but begin to freeze a t a lower temperature, there must be two modifications in eyuilibriiuii in the iiielt. Viider these circiuiistances i t seemed probable that the second liquid modification was a third, hitherto ~ i i i k n o n . ~forni. i, This seemed the more probable because a similar case had already been realized with sulfnr.3 T h e white crystals n-ould then correspond to rhombic sulfur, tlie yellow to monoclinic sulfiir, arid the unknon.ii modification to tlie insoluljle siilfiir.4 Subsequent investigation slion-ed that this view \\-as wrong and it must be withdrawn. Graebe's resnlts are right with the exception that he lias 01-erestiiiiated the time necessary for tlie cliange. -1slias been said, it was found impossible to get a melting-point for the white crJ-stals iii tlie ordinary way. By plunging the capillary tube containing tlie white crystals into a bath heated to 1 2j c I 30" the crystals iiielted entirelJ- and then solidified imniedinteljto the >-ellowtiiodification. T h i s slio\\-s that the melting-point of the white crj-stals is about 130'. possilil~'a little higher, and '(
that the temperature of the stable triple point is certainl!. above 130'. Experiments were then made to determine the effect of the initial temperature of heating upon the apparent freezing-point. Some of the acid was placed in a thin-sr-alled glass tube and heated in a sulfnric acid bath. One thermometer was placed in the melt and another in the bath. T h e test-tube with its contents \vas heated to a definite temperatiire, kept there for awhile and then allowed to cool in the bath. T h e contents of the tube \!-ere stirred vigorously with the thermometer and the temperature noted at which the crystals began to appear. T h i s is not an ideal arrangement because there is nndoubtedly a change of equilibrium while the sJ-stem is cooling in the bath. Hal-ing hiit a sinall quaiitit!. of the acid at in>- disposal, sudden coolin,o did not seem advisable since the error due to supercooling v-oultl probably exceed the error due to the change of equilibrium. Since the error is a real one, the relation between the initial temperatiire of heating and the apparent freezing-point is qaalitative and not quantitative. T h e following results were ohtained : I . Heated to 141"-142~. Crystals began to appear at 128' ant1 the temperatiire remained constant a t I 27. j' while the liquid n-as solidifying.' In a second experiment cr\.stals began to appear at 128' and the mass was solid above 126'. 2. Heated to 160'. Crystals began to appear a t 1 2 0 3 ant3 the Inass solidified at 119.5'. In a second experiment the solidification temperature was I I 5.5c . 3. Heated very gracliiall\-, keeping the temperature of the k i t h and the tithe as nearly equal as possible. T h e contents of the tithe melted at abont 132'. Raised the teinperature to 173'174' and then allowed to cool. CrJ-stals began to appear at I 10' and the thermometer rose to I 12', reniaining coiistant there. _~_______ ' Siiic? it is itnpossi'5le. uiitler the circuiiistances. to tell when the last trace of liquid disappear'. this is to be taken as a statement of the facts i1.i they appear to be. It is neither 1)eliel.c-(1 iior ertrrl that the ternperaturcreally remains cotistaiit uiitil the last drop o f liquid has ilisappearetl. ~
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4. Heated to IQo'. Crystals began to appear at 115' and the therniometer rose to I 15.5". 5. Heated to 190'. Crystals began to appear at 118' and the teiiiperature rose to 118.S". 6. Heated to zoo". Partial decompositioii, the iiiass freezing at 1 1 4 ~ . A l l l the facts obser\.ed are in IiariiioiiJ- with 12ulieiii's theor!- as applied to stereoisomers. T h e iiielting-point of tlie yellow modification is 141.5'! that of tlie white cr!.stals about 130". T h e teinperatui-eof the stable triple point is about I 32O, tlie phases being !.ellon. cr!-stals, solutio11 and \.apoi-. T h e teiiiperature of the iiistable eutectic point is I IO"-I 12', prol.)abl\- about I I z'. TTitli rising teiiiperature. tlie eyuili1irit:iii is displaced to tlie side of tlie n-hite modification. Tlie theory foresees tlie possibilit!. of the freezing-point heiilg lon-ered and tlieii raised h>raising tlie initial temperature of lieating ant1 tliis has beeii realized expel-iiiientall!.. Tlie freezin:,.-point is I z7 wlieii the s!-stem is heated to I ~ z " ,I I C ) " after lieatiiig to rGo", 1 1 2 " after heating to 173"~115.5" after heating to ISO",1 1 ing to 190'. These last two points undoubtedl!. represent a solution saturated ivitli respect to the n.liite niodification. S o attempt vas iiiacle to slion tliis espei-inientnl1)- because the rapidit). n-it11 n-hicli the n-liite nioclification clianges to tlie yellow at these temperatures ~irecludedall hopes of a successfnl result unless one worked \\.it11 a ver!. large qnantit!. of tlie acid. T o show tlie presence of the n-liite iiiodificatioii in the melt I heated soiiie of the acid to 180" aiid tlieii cooled it veq. suddeiil!.. 011examining the inass under tlie microscope hotli !.ellow and n-hite crystals could he seen. Tlie experiment above 190' shon-s nothing because the acid had hepiti to clecoiiipose. TYhile all these facts are very satisfactor)., there is one point that, at first sight) seems not to be right. Tlie n.liite ci-j.stals change a t about I 15' to the yellow modification, thougli the theory as developed would lead one to espect that, at all temperatures between I I z aiid I 30' tlie white cr!.stals shculd _________ ' Bancroft.
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Jour. l%ys, Chem.
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143 I\ iS9S).
1iqiief:- and then solidif!. in tlie !.ellow forin. T h e explanation of this seeming aiionialy is probably that the rate of change of solid into solid is so rapid at this temperatiire that the intermediate state of solution does not appear, provided iic) solution be added. T h i s iii itself is not improbable. I t is k i i o ~ lthat the rate of change of solid into solid increases as the temperature rises above the iiir-ersioii point. I n this particular case the temperature is some fift>-degrees above that a t which the two solid inodificatioiis are in equilibrium. I t is kiiown also that very often a new phase does not appear when it slionld and that iiietastable states can be realized experiiiientally. li-liile this beliavior of the Jt-hite cr!.stals at I I 5" is verj. interesting, there is iiothiiig in i t to necessitate a modification of the theory. I t is to be noticed that although at I Z ~ ' - - I ~ O " tlie rate of change of solid into solid is necessarily greater than a t I 1 5 " ~the tendenc!- to forin a solution is also iiiore marked aiid it is therefore not surprising that \\-e get a passing formation of solution at these teiiiperatures. One interesting conclusioii can he drawn froin the esperiiiieiital data. in regard to the lieat of transfoniiatioii of tlie two niodificatioiis. -\hove 65 the !-ellon- iiiodificatioii is tlie iiiore stable and therefore it. is formed from the white with absorptioii of heat. On the other hand, i t has been slio\\-ii that, aho1.e 1 3 2 O , the eqnilibriuni is displaced n-itli rising temperature to tlie side of the white inodification and therefore the cliaiige of this latter into the !-ellon- form a t these temperatures must lie accoiiipaiiied b!- el-olutioii of lieat. From this, it is safe to coiiclitde that there is m i l e temperature between 65' aiid 132" at nliicli the heat of traiisforination is zero. iTliile the object of this iii\,estigation \vas to study the equilibriurii phenoiiieiia for pure ~ieiizilortliocarhos!-lic acid, I u-ish to call attention to a poiiit bearing on the preparation of the acid. Graehe found that he obtained both foriii5 11). rapid cr!.stallization froin chloroforiii hiit only the white cr-~.stals n-heti cr!.stallizatioii took place slon-lJ-. A t mi). giiwi temperature t 11ere \vo 111d be eq ui 1i b r i 11111 he t ween the two 111o di fi ca t ioiis i i i the solntioii. If tlie chloroforni e\.aporated l-er!. slowl!.. the
equilibrium x-ould continnally be restored and only tlie form stable a t that temperature would separate. If the chloroform were evaporated rapidly then the change of one form into the other might not keep pace with the loss of solvent and both kinds of crystals ~ v o u l dseparate, as was found experimentally by Graebe. T h e general results of this paper are : I . T h e white modification of benzilorthocarboxylic acid is stable below 65". 2. T h e yellow crj-stals are stable between 65" and 132'. 3. T h e temperature of tlie instable eutectic point is about 112".
4. Raising the initial teinperature of heating first lowers and then raises the apparent freezing-point. 5. T h e percentage of the white iiiodificatioii in the melt increases with rising teinperature. 6. =it some teiiiperatiire between 65" and 132' the heat of transformation is zero. COY11ell
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