THE EQUILIRR1:l O F STEREOISO3IERS, I
BY WILDER D. B h S C R O F T
Duhenil has recently published a series of papers i n wliich h e lias del-eloped a theor!. of permalietit changes ' and has also showii tlie waJ-in which this theory can be applied to explain tlie behavior of supercooled sulfur. There are many facts i n organic chemistry 11-hich become more intelligible when coiisidered from this same point of \-iew. It will be better, for the piirpose in hand, to substitute the graphical for the analJ-tical iiietliocl. Since the phenomena are all qualitative, no qu:uititative test of the theor!- is possible aiitl the graphical method ha; the advantage of enabling one to take in the whole field a t 3 glance. TT-hat \ve have then to consider is the form of the diagram for a s!-stem in n h i c h there are two modifications in eqiiilibriuni in the liquid phase. Suppose re have the two constituents a and p. If n-e introduce tlie liiiiititig condition aE we are dealing with a one-component system. If we take into account the eleinent of time there are then three possibilities. Equilibrium is reached practicallj, instantaneousl!.. So approach to equilibriiun can be detected except under special conditions. Equilibriiiiii is reached in a iiieasurahle length of time. In tlie first case the system n-ill behave in all respects like an!- other one-component sJ-steni. T h e second case presiippose.s a state of false equilibrium and will lie discussed in a subsequent paper. In the third case the system will appear to contain two coiiipoiieiits provided n-e act rapid!. enough atid one coiiipoiieiit provided we act slon-ly. T h e concentration-temperatiire diagram Zeit. phys. Chem.
22, j l j
: 23, 193.497 rS9;
I.
1.14
IIi'ldcr
n.Bnlzcl-off
for such a system is given i n Fig. I. If the two constituents are the only solid pliases \x-liicli can occur and if two liquid pliases are impossible, tlie diagrani ~ v o u l dconsist solely of the boiuidary curves ,ID and ED provided neither c o n d t u e i i t could change a t all into tlie other. T h e curve AD shows the coexisting concentrations and temperatures for the a modification in eqnilihriuiii as solid phase lvitli solution and 1-apor. T h e cun-e IED gives the correiponding \-dues when the p modification is solid phase. Since the two constituents can change one into the
Y
B
Fig. I
other, these two cur\ es can be realized experinlentall! onlj \I! g i i es the working v, itli a certain degree of speed. T h e line relative concentrations of the t n o ~iioclificationsa t different temperaturei after equilibrium is reached. T h i s line cuts .ID a t S. AItthis temperature, therefore, the a iiiodificntion can exist in stable equilibrium \T itli liquid and T apor. T h i s line represents, for liquid aiici vapor? what Duheiii calls the '' natural state." From the diagram we can make the follon itig predictioiic. If we take the a inoclificatiori and lieat i t quickly n e shall find that i t melts iiiore or less sharpl! at the temperature denoted b j A, On cooling tlie melt i t ivill be found not to begin to solidif~ until a lower temperature is reached. TTliat this temperature will be, clepends i n general OII the temperature to n.hich the
liqiiid is raised and the length of time during which i t is kept at that temperature. T h e cause for this change of freezing-point is the con\.ersion of some of the a modification into tlie p modification, this latter lowering the freezing-point of the foriiier jnst as an!. other snbstance woulcl. If the a modification be just melted and cooled at once, the concentration will have changed lint little, the freezing-point will lia\-e been lowered iiot more tliaii a fraction of a degree in some cases and the sJ-steiii, solid, solution and vapor, will be represented by a point on ,AD lj-ing x-erj-near &A. nj- longer heating or by heating to a liiglirr temperature and thus iricreasitig the reaction 7-elocity, tlie percentage of the p modification will be increased and tbe freczingpoint lowered correspondingly. TTe will suppose n e s t that n-e cool the s!.steiii suddenly when the relative coilcentrations in the liclnid phase are those represented bj- the point S. T h e a modification nil1 separate a t the temperature correspotidiiig to S and the iiiass will solidify without change of temperature provided the fnrtlier cooling be done n i t h snfficient slowiiess. If the system be cooled rather rapicllj- aiicl if the reaction velocity is low, the a modification will crJ-stallize faster than the p modification can change back and the solution will become more concentrated with respect to tlie 1.3 modification. T h e freezing-point will fall and the system will pass along S D . One of tu-o things is possible. Either tlie solution will solidify completely before the point D is reached, owing to tlie complete conversion of the p into the a modification or the /3 iiiodification will appear as solid phase a t the point 1). If n-e do iiot cool tlie solution so niucli : bnt stop, for instance, at a temperature represented by the point 2,n-e sliall find that tlie freezing-point of this soliltion rises n.itli time owiiig to the ,8 modification cliaiiging back into the a modification, cliluting the solution. T h i s rise of freezing-point Ivi11 cease when the temperature of the point S is reached, the natural freezing-point of tlie system. If \ve fuse the a modification and keep the liquid at some temperature until eqiiilibrium is reached, we shall then have the composition of the soliltion represented by the point a t which that particular isotherm cuts the line T X . If w!cool the system X-erS- slowly, there n-ill be I'
a gradual displacement of tlie equilibrium, the sj-stem will pass through the states represented by tlie line I'S and the a modification will begin to crjstallize at tlie temperature of the point S. If n-e have kept the sjsteiii a t the temperature of the point I-untile quilibrium is reached aiid then cooled snddeiilj , the solid phase will iiot appear riiitil the temperature of the point Z is reached. On standing, the freezing-point will rise more or less slowly to the temperature of the point S, If the line ST slant to the right, as in the diagram, raising the temperature to which tlie sj-stem is heated will lower tlie apparent freezingpoint or temperature a t which the solid phase first appears. If the line ST slants to tlie left, raising the temperature to which tlie system is heated raises the apparent freezing-point n hile this point will be independent of the temperature if the line S T is lertical.' I t is assumed esplicitlj- in each of these three caSes that the sj.Steni is kept heated at coilstant temperature until equilibrium is reached and that the cooling is sudden. I n order to obtain some of these changes i t is iiot necessar!. to fuse the a modification. A4tanj- temperature abol-e that of the point X the final state of stable equilibriuin is represented 11-: a point on tlie line XI'. If tlie a riiodificatioii be kept long enough a t an! temperature between that of the point X a11d that of tlie poiiit I'-i t will liquef! spontaneously. If we start , ! modification, melt it a t the temperature of the with the piire E point B :ind cool i t suddenly, we shall find a lowering of the freezing-point clue to the foriliation of some of the a iiioclificatioii. €3)- prolonging tlie time during which the liquid is heated aiid then cooling suddenlj-, the s j stem can be made to pass aloi2g the cur1-e I:D, tlie freezing-point falling continually. Ai little longer heating will cause the apparent freezing-point to rise, the s!-steiii passing from D to X. -It temperatiires between those of tlie points D and €3 tlie p iiioclificatioii will liquefy spontaneously. If the temperature is belon that of tlie point X, the liquid will solidifj- graduallj. to ci-Jstals of the a iiiodificatioti. If the tern-
' If soluhle sulfur lie taken as the a modification and insoluhle sulfur as the , j niotlification. tlie line X Y \vi11 slant first to the right and then to the left, the change of direction comiiig at about I i o " .
perature is above that of the point X, liquid and vapor is stable state and no crystals will be formed. A t temperatures below that of the point D the ,8 modification will change into the a modification n.it.hont liquefaction. T h e a modification is therefore the stable one. T h e temperature of the point S would be tlie freezing-point and tlie meltingpoint of the crystals provided the equilibrium were reached instantaneouslj-. In such a case i t \vould be iiiipossible to realize any of the curves except the point X. In the case that we are actually considering, in which a measurable time is necessary before equilibrium is reached, the crystals which separate as the system passes along the curve ,411 will melt a t the temperatare of the point -Iif heated rapidly n-liile the crj-stals ~ l i i c l separate i as tlie sj-stem along the curve BD will melt at the temperature of the point R if heated rapidly. If we define the triple point as the temperatiire a t which solid, liquid and vapor are in stable equilibrium, n-e have in oiir hypothetical a modification a case in which a solid can be heated above the triple point. I t is to be noticed that no assumption has been made as to the relati\-e temperatures of the points A and R. If the teniperatnre of tlit: point B is higher than that of -Iwe shall have a case in n.hich the more stable modification is the oiie with the lower apparent melting-point. OstwaldI has receiitl:. called attention to tlie fact that, of two solid modifications, the higher melting oiie will be tlie more stable if only one liquid modification is formed. T h i s conclusion ceases to hold if the assuiiiption. made explicitly by Ostwald, is not applicable. In the discassion, i t has been slio\vn what is to he expected when t x o liquid niodjficatioiis are possible. It is possible, of coiirse, to reverse the reasoning and specify what phenomena n.il1 suffice to enable lis to predict the existence of two liquid modifications. If a solid substance melts a t one temperature and freezes at another, this is proof positive of the existence of two liquid modifications provided no decomposition takes place. T h i s test is applicable to all cases. There is another test which is 'Zeit. p l i ~ s Cliem. .
22,
31;
[
1S97).
148
11 i'ln'C?'
n.11072Cl*cft
absolute as far as i t goes. If the more fuiible of two soiicl modifications be tlie more stable, \\-e are justified in deducing t h e existence of two liquid ~nodifications. T h e difficulty with this is that the converse is not necessarily true. If the less fusible of two solid modifications he the more stable, i t does not follow t h a t only one liquid modification is possible. In the preceding pages I have pointed out some of tlie conclusioiis which may be drawn when one applies to the general case the principles laid dowii hj- Duhein i n his paper on the beha\-ior of sulfnr. I t now remains to be seen lion- far these results can he T erified esperiinentally. T h e first case that we n-ill take up is that of stilbene cliclilorid, C,Hi.CHC1.CHC1.C,H-, studied b>- Zincke.' T h e a modification melts a t 192-193' and can be sublimed betvieen watch-glasses n-ithout deconiposition. If heated above the melting-point, the freezing-point falls and can be dropped to 160-16 j', but no lower. T h i s is in exact accordance with the theor!, T h e temperature of 192-193' is that of tlie point -1,while the point S would then be denoted by 160165". From the experiments of Zincke, i t is iiiipossible to cleternline whether this range of five degrees a t the point S is clue to differences i n the temperature of heating and a large displacement of the equilibrium with the temperature or whether Zincke was actually realizing part of the curve S D owing to the slow rate of change of one modification into the other. T h i s however is immaterial as far as the general theory is concerned and is of interest merelj- when considering this particular case. T h e ,8 modification melts a t about 93". I I I e i i heated for a short time above its melting-point, there seeins to be no change until the initial temperature of heating is about I 60". T h e n the freezing-point rises rapidly ancl soon reaches 160-165', beyond which i t cannot be forced. Here we have onlj- partial confirmation of the theory. Our hypothesis reqnires that the freezing-point should first fall and then rise, reinainiiig constant a t the temperature 160-165".On11 the latter requirement, of tlie rise of freezingpoint, \vas realized by Zincke. Fortunatel>- this gap has been
Equ ilih r i n of St e r r n i s o u i crs
'39
filled in 11~- I,ehiiiann.l H e found t h a t if tlie a modification n-ere just melted aiicl cooled at once, tlie freezing-point \\-as lowered some five degrees. On heating for a longer tinie, the freezing-point rose again. Leliniaiin assiiiiies the existence of a third niodificatioii in tliis case : lmt, under the circuinstaiices, tliis is not prohalile. Tlie explanation of Ziiicke's experiments would seem to be not that the substance remained iincliaiiged on heating above the nieltiiig-poi tit, hut that lie failed to cool the melt iuitil after tlie freezing-point had begun to rise again and that the crj-stals which first separatetl in the neighborhood of 94" \\-ere really the other modification. It \vas onlj- when lie heated tlie siihstance to a higher temperature that tlie reaction \-elocitJ- increased sufficiently to enable him to detect a noticeable rise of temperature. T h i s esplaiiatioii is macle reasonablj- certain bJ. tlie esperiineiits of Lehiiiann, jnst referred to ; hut it is, of course, desirable that tlie experiments should be repeated by somebod!-. 'I'lie fact that the a modification caii lie sublimed betiveen n-atchglasses seems to lie a proof that two iiiodificatioiis exist iii the i-apor as well as in the liquid phase. T h i s has lieen slio\vii to lie true in other cases. Each of the two stilbene diclilorids lias been isolated as solid phase and the one with tlie higher melting-point has been found to lie the more stable. IYitli acetaldoxime, CH-CH : S O H , the higher iiielting modification is the more stable : but the other or ,8 iiiodificatioii lias not !-et been obtained as a solid. - ~ c e t a l d osinie has lieen studied rather carefiilly by Dunstan and D ~ - i i i o t i d . ~ I quote from their secoiicl paper, p. 209: T h e pure acetaldoxiiiie lioils a t I 14. jo icorr). and, Jvlieii supercooled in ice after having been recentlJ- distilled, i t almost entirelj. solidifies to a iiiass of long, stout, acicular crJ-stals, n-liich, if pressed quite free from adhering liqnid, melt a t 46.5' (corr.); tlie entire niass melts several degree.+ lower. . . . IYlien these crystals are melted, and the liquid is kept at the melting-point for a short time, it does not solidify until i t 113s lieen cooled se\.eral degrees below the teinpei-atnnt a t n-hicli the crj.stals melted, e\.eii if a '(
' 3Iolekularpliysik. 1 , 2 0 6 . 'Jour. Cheni. SOC. 61, 4;o \ 1592 i : 65,
206
f 1S94l.
crystal of the original substance is introduced into it. T h e longer the liquid is heated in this x a y , the greater is the reduction of the freezing-point, but the masiinuin reduction it has been found possible to effect is to 13') that is, 23.5' below the melting-point of the original substance. T h e fall to this point occurs slowly when the temperature is near that of melting, b u t if the original liquid is heated a t IOO", the change is rapidly brought about and freezing cannot be effected above 13', by "touching off " with a crystal. S o clieniical decoiiipositioii accompanies the change ; the liquid freezing a t I 3 ' distils constantly a t I 14. j', and fnrnislies, 011 combustion, the same data as the origiiial crj-stals. T h e liquid thus produced does iiot, however, consist entirelJ- of a new modification of acetaldosime ; i t behaves as a solntion of the solid oxime in a liquid modification. T h e liquid does not completely solidify, and the crystals which separate from it a t 13' consist of the original inodification : when pressed quite free from adhering liquid, they melt a t 46.5'. T h e residual liquid, when further cooled, J ields more of these crystals, and by repeating the process, the whole of the liquid may be obtained as the crystalline modification. A4tall temperatures a n equilibriuin seems to be established between tlie tn-o inodifications ; the lower the temperature, the more of the crystalline foriii is produced and vice-versa.' -4lthough i t is possible to separate the solid modification in a pure state, no method we have tried has been successful in leading to the isolation of the piire liquid." T h i s quotation shows that the theory is applicable to this particular instance. Referring back to the diagram Fig. I , the point denotes the melting-point of the crystals 36.5" ; tlie melt is found to begin to solidify a t temperatures yarying between 46. j o and 13', the point X of the diagram, depending on the lcngth of heating and the highest teiiiperature reached. T h e liquid does not solidify entirely at the temperature of X and the I This conclusion. though a very natural one. is iiot really justified h !- any of the facts disco\-ered 111; Dunstan a n d I I y n ~ o ~ i d Subsequent . work in I I I ~ laboratory b y 3Ir. Carveth has slioii-ti that the equiiil~riuniis practical!>- inde-
pendent of the temperature.
Eq it ilihrirr qf .Sfpi*coiko n i r rs
151
authors have realized a small portion of the curve S D . The!. lia\,e not noticed that: solutions which had been cooled, say to 0') \\-oulcl freeze some daj-s later at a liiglier temperature ; h i t the!. do record that tlie n-hole of the liquid mass can be coni.ei-tec1 into crystal in time. Here the lowering of the freezingpoint of the crystals takes place so slowly at 46. j " that no precautions are necessan- to deterinine the melting-point of the cr!.stals. TYe shall see later that this is not al\\-a~.stlie case. Freezing-point cleterminations in benzene and i n acetic acitl sliowed that tlie crJ-stals and tlie liquid had the same niolecular n-eight. I t \\-asalso noticed that the solid acetaldosime n-ill liquefy spontaneously on standing at ordinary temperatures. Propionaltlosiiiie behayes like acetaldosime.' Tlie a modification melts a t 2 I. j " ; the temperature at which solid, liqnid and vapor are in equilibrium is I I . j' ; the crystals liquefy spontaneousl!. a t 0"; the ,8 moc1ific:ition has never been obtained i n tlie pnre state. JYith benzaldosinie, C,HjCH : S O H . we have the liiglier melting form the less stable. Our knowledge of this substance is due in large part to Reckmann' and to Lusiiioore.3 T h e a modification melts a t 34" and the ,f3 modification at 130'. Tlie stable triple point has not been determined ; hiit the crystals obtained are the a modification. Since the teniperature of the stable triple point is lower than room temperatiire, both of the modifications should liquefy spontaiieousl!.. This is the case experimentallj-, though Reckinanii states that if the ,8 inodification be obtained absolutel!- pure, it may be kept for a very long time at ordinar\- temperatures withoiit liquefj-ing. T h e peculiarity of forming two niutually convertible isomers is characteristic of most of tlie oximes, and it is very often the lower melting form which is the more stable. *Is !.et aiiother example, we may take the $-anisaldoxime.4 T h e a inodificatiori ~
I Dunstan and D\-rnoncl. Jour. Cheni. SOC.65! 22 I ' rS94). 'Ber. cheni. Ges. Berlin, 20,2;66(1QS;i; 22,~29(rSSg); 23, 16~%(1S9oi. Jour. Chem. SOC.69, I;? 11S961. ' Becktriann. Ber. cheni. Ges. Berlin, 25, 16So ( 1 S90) ; Goldscl~tnidt. Ibitl. 23, 2163 (1S90j.
melts at 61-62O and is tlie more stable. T h e ,8 modification melts a t 133' a i d changes over slowlj. if kept a t its nieltingpoint, I t woulcl be easj- to fill pages with accoants of other oximes each furnishing further proof of the applicabilitj. of Dnhem's tlieorj-; but i t will be more profitabIe not to confine ourselves to one class of compounds. Phenylisonitrometliaiie, C,H5CH2K02,melts at 84"if heated rapidly.' On standing i t changes spontaneouslg- into an oil. In a benzene solution i t \vas fonnd that the iso-compound at first gave values for the reacting weight much higher than tlie normal. On standing, the freezing-point fell slowly till the normal depression was reached. From this the atitliors conclude t h a t the reacting weight of the iso-componnd is double that of tlie other compound. T h i s is rather unusual. In the 1-ast majority of cases the reacting weights of the two modifications have proved identical. In this particular case the result is still more surprising because the normal compound does not conduct while the iso-modification is an acid about as strong as acetic acid. Of course, it is possible that the explanation of tlie anomaly is due to this very fact since organic acids give abnormallj- high yalnes in benzene solution. Bromplieng-lnitrometliaiie melts a t 60" and is stable. T h e iso-compound melts at 89-90" and changes spontaneously into tlie lower melting form. T h e so-called a dimeth>-lketo1 or ketoxybutane,' CH3C0.CHOH.CH3, is a n oil which crj-stallizes after long standing. T h e crystals are knon.n as tlie ,8 modification and melt a t 12;-128O. If tlie ,8 niodificatioii be kept heated for a vliile and then cooled, the original oil is obtained which, in the coiirse of time, solidifies as before. If tlie freslilj- cooled oil be stirred \-igoronsljwith something rough, such as a piece of zinc, crystals are formed n-liich melt a t 96-98' and are kno~v-nas tlie y niodification. These y crj-stals liquefy spontaneousl>- to an oil which finallj- solidifies to tlie ,8 crj-stals. ,411 three forms give a liquid
' Haritzsch
arid Sclrultze. Ber. clieiri. Ges. Berlin, 29, 699, 2 2 j 1 1S97). v. I'echinann and Dahl. Ber. chem. Ges. Berlin, 23, 2 4 2 j ( r S 9 0 ) .
boiling a t about I ~ o - - I ~ z ’ .I t is obvious that the aiiioclification is merely a mixture of the ,8 and y modifications. T h e p moclification is the stable one and the stable triple point (X i n Fig. I ) lies betlveen the ordinary temperature and 127’. ITith the y modification we have a striking instance of the case which was illustrated rather misatisfactorily by the p modification of stilbene chlorid, namely of a spontaneous lowering of the freezingpoint followed by a spontaneous rise. n’ith the p stilbene chlorid, the lowering was only some four or five degrees. ITitli tlie y dimethyl ketol it is at least a hundred degrees. T h e crystals liquefy spontaneous1)- and then the liquid solidifies to tlie other k i d of cnstals. l y e must also have a marked change of tlie equilihriuin with the temperature. If this were not the case, the freezing-point of the p modification could be lowered by heating only to about the neighborhood of the stable triple point while, as a matter of fact, the equilibrium is displaced to such an extent that tlie y crystals are the first to appear. If, it1 Fig. I , the line XX7 had been slanted still more so that the projection of X7,the point 2, had fallen on the ciirve RD we should have the state of things which has been found by \-. Pechmann and Dahl. ITe may draw yet another interesting conclusion from these experiments. I t is no uncommon thing to read that ‘an oil was obtained which gradually crystallized after standing three‘ or four weeks in a v x i i m n desiccator ’. I t would advance the science s-ery niiich if, under these circumstances, people would distinguish between the two sets of conditions which may have existed. I t is possible that, in any given case, the oil dicl not cr).stallize because no crystals were present to start the reaction. Under these circuinstaiices one must wait until what seems like chance starts the crj-stallization. T h i s is the state of the freshly cooled dimethyl ketol and the y crystals. T h e solution is supercooled with respect to them and yet the crystals do not form readily. This is tlie state of things which is tacitly assiiined to exist in almost all cases n-here one obtains oils which crystallize later. T h e second possibility is that two liquid modifications occiir and that the oil cannot crystallize at once. Since the reaction velocity may be very low, it might well be weeks be-
154
I [ i7dt’t.l- D. h’uuoqf)
fore the oil n-as in a state of superfiision. T h i s can be realized also with the freslil!. cooled dinietli!.l ketol. In the coiirse of time cr!.stals of the p modification do separate ; but it n.ould he 110 use to add one of these cr).stals at the beginning because i t \vould dissolve, the relative concentration, at that time. being such that the solution is siipercooled with respect to the y cr\.stals and not with respect to those of the p modification. I t woiild he a very simple matter for an organic chemist to decide between these two alteriiatiires. -111 lie lias to do, after his oil lias solidified, is to take out three or four crJ-stnls, lieat t h e rest of the iiiaw, cool it quickl>. and, as sooii as i t is cold, throw in one of the crystals. If the ci?-stal dissoll.es lie has two liquid modifications. If the crj-stal gron.s lie call draw no conclusion. If his crystal disappears, all lie has to do is to wait a suitable length of time and then add m o t h e r crystal. Retiiriiiiig from this digression we will next consider the esperiinents of Griinliagen’ 011 the beha\-ior of diorthotoluidin meth~-lene,( CH.C,H4.SH2)2CH2. =2ii oil is obtained that, 011 standing. changes slowly into the ,8 iiiodificatioii n h i c h melts at I 3j O . T h e reverse c1iaii:ire was not noticed, presuniably because it v a s not looked for. IXparatoluidin methylene separates also as an oil which solidifies slowly to the ,8 modification melting at I 56”. 111 all probability these two iiistaiices differ from that of the climethy1 ketol chiefly in that tlie a conipoiiiids ha\-e not yet been obtained as solids. It sliould he noticed, liowe\-er,that the el-idence is indirect and not coiiclusil-e. A% more interesting case is that of foriii!.lplieii!.lacetic ester,‘ H C 0 . C H . C , H ~ . C 0 2 C Z H -T. h e solid modification riielts at 69joo. At oi-clinary temperatures tlie solid liquefies spontaneousl!-, while at lower teniperatures ( 110 figiires given) tlie liquid changes back into crystals. T h i s is precisel!- aiialogoiis to tlie heliaviol- of acetaldosinie. Kriickehergj has found that a l~eiizazoc!.aiiacetic ester, 1,iehig’s ,\iiiialen, 256, zSj ( rS9o). IVisliceiius. Her. cheni. Ges. Iierliii. 2 0 , 2930 1 rSS;): 28, 76; i rS9j 1. Jour. prakt. Chenl. ( 2 I 46, 579 I is92 I : 4 7 , 591 ( 1793); 49, 321 (1894 /. Cf. Haller. Coniptes reiitlus, 106, 1 1 7 1 ISSAj.
” \T.
C,H:S2HC.CS.C02C :, H-> , iiielting a t 1 2 j'* cliaiiges into tlie p iiiodificaticm, iiieltiiig a t 8 2 ", coiitiniiec1 Iieatiiig to I 30'. Tlie two iiiodifications have practicall!, the saiiie solubilitJ- in alcoliol, Ivliile i i i lieiizeiie tlie ,8 modification is about tn-el\-e tiines as soluble as tlie other. third isoiiier iiielting at 10610s" is iiierel!- a iiiisture of the other t n o . T n o iiiodificatioiis of c)-tolueiieazoc!-aiiacetic ester are kno\vii. Tlie a cr!.stals iiielt a t 85' and change readily into the p crystals iiieltiiig at 733". Here the forni wit11 the higher melting-point is the more stable. C h i the other liaiid the a crj.stals of p-toliieiieazocyaiiacetic ester iiielt at 116-1 rti", n-liile the iiiore stahle p cr!% tals iiielt at 74-75'. IYitli //r-s!-leiieazocyaiiacetic ester aiid pseudoc!.iiieiitazocyaiiacetic ester the a iiiodifications iiielt at 74-75' aiid 100' respecti\.el\-, ivhile the more stable p iiiodificatioiis melt, oiie at 16tic, and the other at 136'. I t has lieen slio~viiby v. Baeyer and 17illigcr1that the (.islactoiiic acid of the os!.diinetli!-ltricnrhallylic acid, (CH3 )~.C.CO,,H.CH.CO~€~.CHOH.C( )zH, melts at 2c):', aiid tliat after fifteen hours heating at 150' ninetenths of it is conT-erted into the ti-triis-iiiodificatioii iiieltiiig at 2 1 1-212'. Oti heatiiig, the latter goes back to a certaiii csteiit iiito the former. Kjelliii' fomid that diazoiiiuniacetacetic ester, C,H.T~.HC.COCH~.C)OzCzHS. . . melts at 80-84' if heated rapidly aiid illat the freezi&-poiiit drops to belolv 50". He did not siicceed in obtaining the second isomer as a solid phase. T h e less stable form of ci-clilordiazoiiiuiiiacetacetic ester melts a t S3'~ the more stable form at 62-63', while the temperature of tlie stable triple point semis to be aboiit 59". I t is of interest to note tliat the more stable iiiodification is aboiit four times as soluble i t i ligroln as the other. 0 1 i l ~one solid isomer of s-triI)ronidiazoiiiiuiiacet3c.etic ester has heen isolated, h i t its melting-point is I 2 1-123(', while the freezing-point iiiay drop as lo\\. as 95-107".
' k r . c:lietti. Ges. R w l i i i , 30, r9jS 11S97') ? R e r . cherii. Ges. Berlin, 30, 1965 (~ 1S9;).
R a i k o w describes two forms of a-methyl cinnamic acid, C,Hj.CH CCH-.COOH, the more stable melting a t 82', the other a t 74'. Claisen' reports that dibenzoyl acetone, ( C,H-C0)2HC.C0.CH3,
80-81 ', crystallizes from hot alcohol as the isomer which melts a t 109-112' on rapid heating and a t I O T - I I O ~ when heated more slon-ly. T h e reverse change can be brought about by heating with carbon tetrachlorid. Here we are dealing with the effect of the solvent and we are handicapped by having very little information in regard to the behavior of the pure substances. For this reason i t is possible merely to call attention to this as a case well worthy of careful stud!-. According to TTalther3 P-diazoamidobenzene.C,H j S 2 K H C , Hj, melts a t SI' and, on heating, changes into the a modification which melts a t 98-100'. Kipping4 says of benzylidenehydrindone, C,H4 (CH2)JCO)C.CHC,H. : T h e crystals obtained from a solution of benzj-lidenehydrindone in a mixture of cliloroform and light petroleniii melt sharply at 109-1 IO' ; if, however, after the substance has solidified, it is reheated, i t melts a t about 90°,but not always completelj-, the remainder melting a t 109I IO'. In some cases the portions which have been melted once, begin to liquefy a t 90' and then, although the temperature is rising, gradually become completel-: solid, melting for the second time a t 109-110' ; i t is also a curious fact that the presence of the liquid formed by the crystals melting a t 90' does not lower the inelting-point of the inodification which liquefies a t 109-1 IO' ; on keeping the crj-stals of lower melting-point a t ordinary temperatures, they are transformed into the more stable modification." T h i s is not entirely clear, but the case would seem to be analogous to that of dimethyl ketol. Onheating thestable or I 10' modification above its inelting-point the equilibrium is displaced so much that, on cooling, some of the other modification c (
~
' Ber. cheni. Ges. Berlin, 2 0 , 3396 ( I S S ~ ) Liehig's .lnnalen, 277, I S A (1Sg3 , ' J o u r . prakt. Chem. ( 2 ) 5 5 , 545 (1897). 'Jour. Cheni. SOC. 65, 499 (1894).
Eyui/ibrin of S f r r ~ o i s o r ~ n * s
157
separates. I a m inclined to consicier 90' as the eutectic point of the two inodifications, corresponding to the point D in Fig. I . T h e temperatiire of the stable triple point el-itlentlj. is abont 109-1IO' a i d coiiicid.es very closely with the melting-point of the stable compoiiiid, unless, as is possible, the stable iiiodification could be made to melt higher lij- more rapid heating. T h e less stable modification, according to this h>.potliesis, n-ould melt a t some temperature above 90°, and i t is a pitj- that i t was not obtained piire. Kippiiig recognized that a second iiiodificatioii I find a statement' was formed. T h i s is not aln.a~-sthe case. that the methyl ester of iiiucopheiioxj-bromosime, C,,H2(oC,H-)BrS03CH~, melts at 168-170"when heated 1w-y rapidlj- and a t 150-165' n-hen heated slowlj-. ~lucop1ieiioxj.broiiioxiiiieanh>-drid melts a t 124-126' or IZI", depeiidiiig on the rate of lieatiiig. T h e corresponding figures for plieiios!-broinmaleiiiiiiiid are I 48-1 50" and 144-14j" ; for the metliyl ester of mucoplieiiosj-chlorosiiiie Since 110 inentioii is inacle the\- are 162-166" and 156-160'. of any decomposition, i t seems certain that each of these substances exists in a t least two moclifications. I7nfortiiiiately this was not perceived bj- the authors and they did not investigate the plienomena. Sudhorough* has noticed some very curious facts n-hich lie has not studied with care. Chlorstilbene, C,H- to guide him, LVislicenus did not notice that the freezing-point of tlie fused maleic acid dropped a t first arid began to rise only when the fumaric acid begun to crystallize. There can be no doubt that this takes place. Tlie tolanedibromid, C,H-.CI3r : CRr.C,H5, melting at 2 0 5 O , is tlie more stable form, the one melting at 68.5"the less stable. liquid mass is obtained if either modification be kept long enough at ISO' ; a solid mass is obtained if either modification be kept long enoiigli at I 60". T h e teinperature of the stable triple point cannot be far from 170'. ;It each temperature at which liquid is possible, a definite equilibrium is reached.* T h i s was easily shown experimentally, because the lower melting form is about one hundred and twelve times as soliible i n alcohol as the other. Tolanedichlorid occiirs i n two forms, one melting a t 1 4 3 " ~the other at 63'. On heating, each changes partly into tlie other. Experiments are also given to show tlie rate at which each of the a-chlorcrotonic acids, CH3.CH: CCl.COOH, changes into tlie other, also for the two P-chlorcrotonic acids. CH3.CC1: CH.COOH. These instances are siifficient to sliow the varied nature of substances to which Duheni's theory is applicable and the way in which i t is to be applied when we are considering the pure substances only. In the second paper I shall take up the question of stability when another component is added, which major may not form compounds with tlie two modifications. Coriieil C I i iver-sity 1 If fumaric acid is suhlitr~eti at temperatures helow that of the stable , 32 triple point, it passes over ~ i ~ i c h a n g e d .Cf. Tanatar. Liehig's ~ i n n a l e n273, j1S92). This innst he the case because we have one con~ponentantl two phases.
