BY €1. R. C.IK\-ETH
During the past !.ear there has been carried oii iii thi, 13110rator!' the esaiiiiiiation of a iiiiiiiher of bodies occiirriiig in tnw ~nodification.;,of n-hich the one with tlie higher iiieltiiig-poiiit \{-as the less stable. Iii elm-!. case esaiiiinecl i t has lieen proT'etl that tlie Ilulieiii theory of periiiatient change as de\-eloped b!. naiicroft aiid sj.steiiiatized by the iise of Phase Knle niethods i,s ;ihle to esplaiii tlie plieiioiiieiia obsers.ed. For a further test of this theor!. the writer lias esaminecl tlie conditiolis of equilibria preseiit with the ~araaiiisaltloxiiiiesn-hen the two iiiodificatioii.: are regarded as foriniiig a liiiiarj. systteiii and also wlieii liy tile iiitroductioii of aiiotlier sul)staiice, it is assunied that there is foriiied a teriinr!. s>.steiii. T h e first work 011 I)araaiiisalclosiinen-as done 11~.TYesteiilierger' ~ l i foriiied o tlie coiiipoiiiid li!. ad(liiig, to a17 aqiieous suliition of li!dros!~laiiiiiie li!-drocliloride, socliiiiii h!~droside and t 11e 11 tli e cal cul a tecl ai tio 1111t of ai1i sald el1ycl e i 11 a1cc)1101 i c so 111tion. T h e !.ellow oil resiiltiiig was taken iip in ether, the ether c\-nporated aiicl the solution cr~~stallizetl ox-er siilfiiric a c i d Kecr!-stallizatioii froiii alcohol aiid ligrc~iiigave white lea\-es of 11. P. 45' whicli \\ere but slightl!. sohihle i n cold hut quite solulile in hot wnter. Four !.ears later (;olclschiiiiclt ant1 Polonon-ska. made iise of V*esteiiberger's niethccl, and obtained as a prodiict long hard prisms of AI.!?. 61'. I n 1S891Iillerj precipitated ;I beiizol solution of the osiiiie b!- ligroin, ol~taii~itiga white p y der of 1I.P. f q G 7 soliihle iii liot x t t e r , ether atid l~ciizol. Other ~
1
I k r , c l i e i ~ i .Ges. Ilerliii, 16,299.3( ~ h S) j.
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Iliid, 2 0 , 210; I 18s;). 11)icl. 22. 2790 i1XS9 I .
references to tlie melting-point of the compound are given 1 ) ~ . Goldschmidt,i Reckma~in,'Hantzscli,; and by Goldscliiiiidt and Rietschoten,+ tlie latter giving as tlie A L P , 6 3 " , tlie former three observers 61 ". 111 1890 Keckiiiaiiiif discovered an isoiiieric form of tlie o s iiiie, tlie oiie iiow called tlie B modification. T h i s lie iiiacle bhforming the hydrochloride of the a osiiiie, treating this with sodium hydroxide aiid crystallizing. Since the p iiiodificatioii is less soliible than tlie a form in benzol, it n-as coiiiparatiT-elJ-easj. to effect a sharp separation bj- iisiiig benzol as the solvent. T h e prodact obtained is stated to have slion.ii a constant iiieltiiig1)oiiit of 1 3 0 ~ - 1 3 0 .even ~ ~ on slow heating: which, i t will be remembered, is in marked contrast \vitli the benzaldosinie which attains its inelting-point onlj. oii lieiiig heated ver!. rapid!.. IYheii boiled with ether for a f e n lionrs or \\-lien heated with alcohol at I ~ o ' ,the /3 foriii is changed into its isomer; but this reaction is said not to occiir when the substance is heated with alcohol to 100' for one hour. (;oldsclimidt' in esaiiiitiiiig the ,B foriii describes it as 1iai.iiig a meltiiig-point of 133' aiicl as being less soluhle than the a in ordiiiary solvents. aiid states that it may readily be chaiigecl into the a form b). heating above its 1I.P. Of 133'. Tliis slioi-t 1-6suiiii.slions conclnsivelJ- that the aiiisalclosiiiie does exist iii two iiiodifications wiiicli are capable of existence as solids at ordinary teniperatiirer ; that the)- possess iiieltiiigpoints aiid other properties nhicli are distinct but that they are nevertheless readilj- convertible one into the other. T o such cases as this tlie niilieiii theor!. may he applied with great acl\.antage, lvliile without its aid tlie esplaiiation gil-eii for tlie pheiioiiieiia must be insufficient if iiot inaccurate. T h e ailisaldehyde used as starting-point iii tlie research was the coiiiiiiercial product, aiibi.piiie, supplied to 11s h!. JIessrs. Rei-. clieiii. Ges. Derliii. 23, 2163 ( is90 , . Ihid. 23, 16s: (rSgo'i. I l i i d 26, 929 I rSq3 Ihid. 26, 2 0 s ; ( 1S93'. ' Ihid. 23, 216; 11Sg0'8.
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Fritzche Hros., of S e i v York. This was fractioiied, tlie part coiiiiiig o\-er a t 246"-247' (744i n i n ) beiiig retaiiiecl aiid refractioiled in ai1 atiiiosphere of hydrogen. The aldehyde obtained was shaken t i p n-itli coiiceiitrated aqtieons socia solution, and the theoretical amoutit of h\.drc.,s>-laniiiieh~~clrocliloride added. -1fter washing \\-it11 cold n a t e r the first product slion.ed a 1I.P. of 50'. This \\.as coiisequeiitl!. recr! stallized a great iiiiinber of tiiiies froiii etlier until tliere v e r e obtaiiietl beautifnl white crystals of 11.1). 63". T h i s n-lien iiielted tip in aiiioiiiits of three to five grams s h o i ~ e t ljnt l a \.er!. slight tinge of !.ellon., n-liicli subsecliteiit cr!-stallizatioii failed to re1iioi.e ; moreover it was not fonnci pssiible b>. repetition of the cr!-stnllizatioas to raise tlie 1I.P. of the product. It \\-asfound later that crystallizatioii froiii toluene remo\-ed the colored i i i i p r i t i e s more rapidl!, thnii \\.lie11 etlier iV&s used as .sol\.ent. Tlie ,d iiiodihcatioii u.as prepared froiii the osiiiie liydrochloride. T h i s salt \\-asitself iiiatle b y passing dr>-hydrochloric acid gas1 slo\vl!. into ai1 ethereal soliitioii of the oxiiiie at tt'iiiperatures rangiiig froiii o'-2o". After the precipitated h!drochloride \\-as filtered, ivasliecl \\-it11 etlier and driecl, it was pill\.erized, piit iiito ice-cold nxter aiid slo\\-l!- iieutralized b>-dilute cold soliitioii of potassiuiii carbonate. T h e solid osiiiie resiiltiiix \\-as \\-aslied with cold n a t e r , driecl and crystallized froiii benzene. Iii this w a ~ i.t was possiihle to obtain quite a large \.ield of the $ from the a osiiiie. Tlie foriiiei- crystallizes iii long iieedles aiid 011 fairlj. rapid Iieatiiig shon-ed a iiieltiiig-poiiit of I 3 2 " . l'lie slight discrepancies liet\\-een tlie melting-poiiits of 11otIi foi-ins as gil-eii b\- different o h s e r \ ~ r lias s caused tis to investigate lion, iiiiich importance must be attached to the meltiiig-points of this substaiice iii particular as a iiieaiis of coiifiriiiiiig its puri tj.. T h e esperinieiit \\-asperforiiiecl i n the fol1on.ing- manlier. The suhstance \vas put iiitu a capillar\. ineltiiig-poiiit tube which I The gas was tlrird hy passing through >.ulfuric acid, towers of glass ~ 1 0 mid calciuin chloride. Slight traces uf sulfuric acid carried over i n some of the first esperirneiits ga7-e rise to colored corrlpountls \\.hose nature was not iiives. tigatid
1
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T h i i was fastened b j a rubber band to a tlieriiioiiieter aiid the latter was then immersed in a bath of liquid kept a t its boiling teiiiperature. T h e Beckiiianii boiling-point apparatus s e n es the purpose of a constant temperature bath adiiiirabl) , while miuturei of the ordinar) organic 501vent5 \\ it11 each other or nitli water give oiie n series of liquids whose boiling-points ma) be \ ariecl a t M ill. T h e results which were obtained n ere as follon 5 . as then sealed.
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a osiiiie at 62.SL-iiielted a '* " 61.8~-iiielted
a t once. but iict coiiipletel> in 2 2 niinutes. a " " G~.o'-iiielted in j j minutes. a '' ' ' j8.9"-11ot iiielted in 135 iiiiiiutes. ,d oxime a t 134.o'-iiielted 111 30 secoiidi.
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slion.ii a iiidtiiig-poiiit of 62.8". It was heated to 6.4' for thirty iiiiiintes, then cooled a i d the freezing-point takeii : then heated at 87' for the saiiie time, cooled aiid freeziiig-poiiit noted. Proceediiig siiiiilarl!. with the initial teiiiperature a t roo",12 j',aiid 1 4 2 O , i t was foiiiid that for three sets of readiiix::, the teiiiperature at n-liicli tlie mass solidified \vas ,jq.zo nitli ;I iiieaii \-ariation of 0.1 5'.
To ascertain whether the final equilibrium attained \vas the .same n-hetlier one uses as first material tlie a or the 13 cr?.stals, we have heated the P form up to the point of fusion aiid iiiimediatel!. deteriiiiiied its freezing-point, \vliich was found to he 34.2'. Successive meltings and freezings have slio\vii this also to be coiistaiit at 54.20' 0 . 2 j'. 111not a siiigle case lias there been found an!- indicatic,ii of decoii~position,either b!. change of coloi- or by loss of n-eight. Froiii these experiments tlie oiil!. conclusions which Cali possibly be dranii are that the initial temperature of heating-:at least up to 142'lias no effect oii tlie final conclitioii of eqiiililxitiiii, aiid that the same eyuilibriuiii is attained \vhetlier oiie starts with tlie a or the form. T h e change of one iiioclification into the other is therefore accoiiipaiiied by little or no lieat efYect, a plienomeiion \vliicli has dread\. been obser\.ed in the case of acetaldosiiiie and Iienzaldoxime, ancl n.liich ma!- possiiblj. applj, to all tlie oximes. It lias been Iiopecl that the complete eyiiilibi-iuai ciin-e for the two foriiis might be determined! siiice the work of Cameron' implies this as a possibility. T h e n aiid P foriiis \vere taken iii known aniouiits, xt-liicli were then iiitiiiiatelj- inisecl aiid pot into small capillary tubes. T h e eiids of the tubes \\-ere sealed and the melting-points takeii iii tlie ordiiiary manner. I n lint fexv cases \vould a repetitioii with a second tube similarly prepared give results \vhicli n e r e a t a11 in liariiiony \\it11 the first determination. T h e diaiiieter of tlie melting-point tiihe especiall!. had a great deal to do n-itli tlie iiieltiiig-point o b s e r l d , this occurring at a l o n e r teiiiperature a h e n the diameter of the tube was increased, if the heating were not too rapid. T h i s is probably dne to the influence of tlie \valls in preventing the 13 form from dissolviiig up rnpidl!. iii the liquid tiiisture. T h e method used b!- Caiiiei-on is not ai1 ideal one in aiiy case, siiice tlie effect of tlie tiiiie factor is oiie which caiiiiot he estimated. JIoreover the iiiiiiiediate taking of tlie freeziiig-point ivill gi1.e bnt a ~ ) o o r check on the melting-point determinations, since there is, at present, no n a y of estiniating. the extent of displaceinent nliich
occiirs n-hen the solid fuses, or of ascertaining whether the mass iii the capillary is coiiipletel!. solidified. T h a t Caineroii \vas able with tlie heiizaldosimes to obt.iiii resiilts n-liich agree with each other must have k e i i due to tlie coniparatix-el\. d o n change of tlie P iiito the a form. IYitli tlie aiiisaldosiine we ha\-e slion.11 that e\-en considerably below its lI.P., it changei ol-er iiito the n form, while after it has heeii once heated to its iiieltiiig-poiiit, its freezing-point falls iminecliatelJ- to 54.20". I t has, therefore, been fotiiicl ahsolutel?- irnpo.~.~ihle to obtaiii with an\- degree (2f accurac!- the conipositioii oi the triple point. T h a t at this point there is present a iiiisture aiid iiot pure a has been verified in the follon-ing \\-a!.. Tlie ec1uilibriiiiii mixture foriiied after iiieltiiig the /3 ci-J.stalshas been crJ-stallized froiii henzeiie after it \vas sho\vii that beiizeiie liad but a sniall effect on the eqiiilibriniii relations. There n-ere deposited at first a ver!. fe\v CrJ-stals \\-hose crJ-stalliiie form aiid high melting-point shun-et1 theiii to be the /3 iiiodificatioii. Tliis ofYers, therefore, coiifiriiiatory ei-idence of tlie stateiiient that, at the stable triple point. hot11 of the stereoisomeric forms co-esist iii the iiiclt. I t n-as iiow deciclecl to ascertain lvliat \\-as the position of tlie entectic point rrlatii-e to the triple point. .Jll that can be said is that i t lies aboiit 0 . 2 " l o n e r tliaii the latter. Tliis \\-a> proved 11)- aclcliiig the /3 itiodificatioii in \-erj. siiiall aiiiouiits to the triple-point mixture, dissol\-iiig at teiiiperatures below 56' aiid noting the ireeziiig--poiiit. The iiiasiiiiiiiii depression \vas, as stated, 0.2'. Tlie difference in coiiipositioii between these t\vo poiiits iiiiist also be estrenielJ-siiiall, since oii cooling a l a y ? amount (If the melted oximes it \vas iiiipossihle to observe tlit. second-i. e. the eutectic poiiit--bJ. plottiiig tiiiie-teniperature res111ts. Here again the teiiiperatures ohser\.ed nlien different anioiiiits of ,h' \\-ere addecl coiild not be plotted satisi:wtorilJ-owing to the change of one form iiito the o t h e r : for this reasuii all 1111iiierical resnlts oii this point are omitted. .After reaching the eutectic poiiit it n-as possible 11~.addition of the /3 iorni to 1iai.e the freezing-point rise aeyin, but tie\-er to
444
H. h'. C'ni-wth
a greater extent than about 1.5'. T h e esperiineiit requires a temperature below 60" for the complete solution of /3 in the liquid ancl a rapid deterniiiiatioti of the freezing-point. Of coiirse continued heating above 60" caiises the freezing-point to fall rapidly to that of the triple point. I t was iio\v thonglit desirable to exaiiiiiie the relations when a third coiiiponeiit \vas present. I n tlie first instance, a substance \vas chosen \\-liicli coitld form a coiiipoiind with one of tlie coiiiponeiits, viz. h ~ ~ d r o c h l o racid. ic T h e osiiiie 11)-drocliloride is usually obtained as a white crjstalliiie powder b!- passing dr!- li~-drocliloric acid gas into an ethereal sollition of tlie oxime, but i t iiiaj- also lie obtained as fibres by evaporation of an ethereal solution. Ai1 attempt \vas macle to see if the coinpoiiiid, like lmth forms of the oxime, nould melt helolv tlie socalled iiieltiiig-point. T h e method used is tlie same as was used with the oxime and the results are given below : /3 oxime Ii!~drocliloride at "
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I n e\.er!- case the li!-drochloritle iiieltecl with decomposition, so that, from these results, 110 co1iclilsioi1s as to the existence or iioii-existence of a second niotlification would he justified. nniiiljer of esperiiiieiits \Yere iiiade in the hope of obtaiii, + A
iiig- aiiotlier liydrochloricle. Carcfull~,dried li!drochloric acid gas \vas passed into soliitioiis of tlie osiiiie in ether, benzene, and alcoliol at teiiiperatnres rangiiig from - 20' to ~-~ 60",but in all of these experiments the one prodnct alone was obtained, \.iz. the /3 Iiydrocliloride. Finally tlie gas was passed into the melted osiiiie a t a teniperature of 6,". T h e precipitate formed \vas washed with benzene ancl clriecl. I t gave a melting-point of I 17'. On boiling n-itli benzene or on recrystallizatioii from ether, however, it gave the iiieltiiig-point of the hydrochloride,
the other product of crystallization being tlie original osiiiie. It is ver!- probable, therefore, that there is but oiie form of oxime 1ij.d rocliloride, T h i s is not in accord n-ith the viexv-s of Hantzscli' from whose paper the follon-itig translation is made : If n o ~ vordiiiar!~anisantialdosime and c i i r n i n a i i t i a l d o s i ~ ~i~ne ether solution are changed bj. the action of hydrochloric acid gas into the li!.di-ochloricles, and the dried precipitates are covered with excess of an ethereal solution of aiiimonia, then, after filtering off tlie aniiiioniuin chloride formed. there will be ohtaiiietl the original aiiisantialdosiiiie of X P .61 ' or tlie original c u ~ n i ~ i ~ n t i a l d o ~ iiiie of 1I.P. 58' ; or, converselj-, if oiie starts with the stereoisomeric s~-iialclosit~ies, then under quite the same conditioiis tliei-e is obtained the original oxime, aiiissynalclosinie of 11, I]. I 30' or cuniins!~naldosi~iieof 31.1'. I IO". There result, therefore. the different stereoisomeric hjdroclilorides b '
\yhicIi i n external appearance and iii chemical hehax-ior ma!. scarcelj- be distiiigiiished from each other s n d n-hicli show their individiiality only in the inr.ariahle regelleration of the osiiiie from n-hich they are derived. T/lr c h ~ i i ~ qqf'c thC, ~ i z t t ' ~ ~ l ( ~ ~ ) . i - i ~ ~ i ~ ~ . , iiito the s?iiznln7il.viiirL'.\. Fbtlzich /Ins bct'ii itzciition(,d docs iioi thcrcj o i - c takc j l n c c , / I C I T , 2;f; in .y!dittt'iig of the lzj8drochloril.(?(.id, ;('t~tt:i, i.7 pi.Jy>Ct(l'L'.cClll dc'd. Moreover these deril-atives of penta\-alent nitrogen are ~ ~ i u cmore h labile than those of tlie trivalent element, that is\ than t h e oximes themselves. If. for example, the Ii!drochloride of anisantialdosiiiie be covered with not too great an excess of ethereal aiiiiiioiiia, or, lxeferablj., if dry ainmonia gas he led into absolnte ether in n-hich the h!drocliloride is suspeiided, tlieii the filtrate lea\-es behind almost piire atiiss~-iialdosiiiie ; the ciiiiiins\-naldosime under the same coiiditioiis leaves behind tlie cutiiiiiantialdosiine ; finally from either j - i i i trobenzantialdosiiii~ or 'L
~
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!Her. cheni. Ges. Berlin, 26, 9-29(Is93 I .
p-iiitrobenzsj-iialdoxiiiie is obtained a mixture of both stereoisomers." T h e results given above were sufficieiitl\. strange to warrant a repetition. I t was found that in ever\- case n-here the hydrochloride was precipitated from absolute ether--n-hether there was used a solution of tlie pure a , pure io, or of a mixture-and where dry aiiiiiionia gas !vas passed into an amount insufficient to precipitate all the hjdrocliloric acid as aiiinioiiiiiiii chloride, t h a t the crystals which appeared first were those of the sjxaldoxime. IYheii, however, an excess of the aiiiinonia was used, the crystal product was a mixture, the n form predominating. It wonld therefore appear tliat the action of amnionia is directly opposite to that of the acid-the tendency of the former being to make the a the stable form, x-liile the acid favors the formation of the 13. T h i s is in direct confirmation of the results of Goldschniidt atid Roder' on the benzaldoximes, if we assume, as seems positive, that no ainiiioiiiiini salt of the oxime is possible. Perhaps also Haiitzscli has laid too great stress on tlie presence of water as preventing the change of the anti- into the syn- forms when tlie experiments lie quotes show that the change niaj. take place even when water is entirely abseiit, if the ammonia is not present i n excess. I n his work on the equilibria of stereoisomers, Bancroft2 has iiicluded in his general treatment a discussion of the results which are to be expected iii the system, a arid ,8 aldoxiines and hydrochloric acid, if the alone form the compound. It had been hoped that i t would be possible to pass along the curve of stable eqailibriuiii for the three-component sj.steiii. T h i s has not been realized owing to the tendency of the ,B hydrochloride to decompose. Starting with the composition represented by the triple point, the writer added the oxime hydrochloride to this to the amount of two percent. This dissolved readily a t about j O a and gave a freezing-point of j 5.60". Heating a t 65" for some time ~
I ~
Rer. chem. Ges. Ikrlin, 28, 2013 (1594). Jour. Ph!s. Cliem. 3, 144 (1S99'.
lowered the freezing-point to 54.9S": but this \vas the lowest freezing-point obtaimble. I t \vas e\-ideiit, therefore, that the curve of stable eyuilihriniii, if it rises toward the stable triple poiiit as required 11~.the theoreni of van Ryii \-ai1 .llkeniade, inlist have eiided before the coiicsiitratioii of the hydrocliloride 112s reached t w s p2rc:iit. It is proh:ible that the first addition of the salt Iiad talieii the system a t once into the field n-here t h e p h~~clrocliloride exists as solid phase. Further measurements confirmed this view but it was impossible to can!. tlieiii far enough to 1: of sen-ice. "lie possibilit!. of seciiriiig qiiaiititative resiilts in systeiiis siich as these is not very liopefiil and until there are foniid cases where tlie cliange from one isomer into another takes place so slon.l!, as to he ~iieasurable,qiinlitatil-e data alone must suffice. Esperinieiits were iioiv iiiade t u tleteriiiine tlie effect 011 tlie equilibriam of a third coiiipoiieiit \vhicli does not react nit11 either of tlie modifications. T h e sol\wits choseii were alcoliol, acetone, benzol, toluol, s ~ . l o laiicl petroleuni ether. In all of tliese tlie ,d is the less soluhle, and in eI-ery case it has hecn fotind that after heating the /3 forin with these soll-eiits in sealed tuljt-s, the teiiiperature of lientiiig lj.iiig abol-e that of the boiling-point of t h e solvent, aiid the time of heating being iie\.er less tliaii an lionr, tlie iiieltiiig-poilit of the cr!.stals left invariabl!. la!. loner than that of the /3 osiiiie. T h i s proves that in all these sol\.e-ntr tlie reactioii proceeds iii such a iiiaiiiier as to form a osiiiie, It \vas noticed in repeatiiig the Ivork of (;oldscliiiiidt I)!. Iieatiiig three tubes of t h e i.3 dissol\-ed in ahsolute alcohol for oiie Iiour. that the ~ i i e l t i n ~ - p o i iofi t tlie cr\.stal protlilct i nliicli, of coiirse, \vas not crystallized fractionall!.) had fallen to I I 7', I I 8" :inti I 20'. Further lieatiiig for an hour a n d a lialf g-zi1.e iiieltingpoints of 83'. Sj' and 86.5" shonilig that tlie i-eactioli lvliicli C;oldscliiiiiclt noticed :it I 50' also occiirs at I oo",altlio~ightlie \-elocit\. of cliaiige is loner. I n benzol and toluol soliitions, tlie rate of chaiig-e is quite lo\\-. The /3 boiled for three hours n-it11 benzol shoned a llieltiiig poiiit of I I I ; lvlien boiled for sel-ent!. minutes wit11 toluol
n iiieltiiig-point of 127”; and when a solution in acetone was
boiled for two hours a melting-point of 62’ was obtained. T h e temperature of heating when a sol\-ent is present is not the only factor in determining the eqnilibriiiin, but the specific nature of the third component in respect to its solvent action on tlie two different forms mnst also play a 1-ery great r61e. Tlie experiments with the benzol and toluol solutions, n h i c h haye just been mentionecl, explain why these solvents are so effective in the separation of the /.3 from the a form-not merely is there a great difference in the solability of the t v o forins in these solvents, lint the rate of change of one form into the other is so slow as to allow of practicall?. complete separation. I t n-ould have been iiiterestiiig to have determined, further, whether the m e of a solvent in which the /3 was the more soluble form would have reversed the stability of the niodificntions. t7xifortunately there has been disco\-ered no sol\-ent which does not react with tlie ,B where tlie latter form was inore soluble. .ittention ma>.now be directed to the manner in which the results of tlie consideration of these different stereoisomeric forms may lie adapted to laborator>. experience. (I If one has in hand a product from which the different fractional crystallizations shosv a1n-a)-s the same percentage composition, and the same melting-point, and which has a freezingpoint different from the initial melting-point (decomposition excluded) there is then very strong evidence for the existence of two modifications of this particular compound. If the freezingpoint lies liiglier than the melting. point, the product in hand is instable: if it lies lover, either the stable or instable forms may form tlie initial material. Liider ordinary circnmstances the stable form will be the one first disco\-erecl. ( 2 ) Tlie instable modification may not be discovered without making iise of n third component. T h i s may reverse the stabi1it)- relations either b>-mere sol\-ent action or by forming a compoiiiid. T h e first case is readily tested by examining the whole crJ.stal mass left after complete evaporation of the solvent. Should the nielting-point ha\-e chaiiged at all, this is a proof
that the continued action of tlie solL-eiit iiiaj. produce a separation. T h a t it does not change is 110 proof of tlie lion-existence of the isomer. Should no separation lie possible i n this !yay, make, if possible. a n additive coiiipoiind from the initial product. aiid ascertain if this also has a nielting-point n-liicli 7.aries with the time of heating, or n.liicli is diffei-eiit from the freezing-point [ decoiiiposition again excluded ). If either of the latter plienoriiena are obserl-ed, the existence of a secoiid coiiipound is verj- probable. In this coiiipo~indthe stahilitj- relations ma!- be displaced. In this case, the choice of correct conditions as to lion- to split off the third coinponeiit caii be determined reatlilj-, tlie iiiaiii poiiit n-liicli is to be obserx-ed being that the reaction inust take place at a teiiiperatnre n-here the change of one form into :iiiother is iiot iioticeable. T h e formation of the /3 1iioclificatioii.i of lienzaldosiiiie aiid 1,araanissldoxiiiie illustriitss this case, the j-ields being greater tlie lon-er the temperatiires at ~,-liiclit1ie.w forms are isoliitecl. T h e correct choice of solvent is essential, h i t at present there are n o data n.liich caii be of an>-general serl.ice on this point. It is possible that in the coiiipoiind tlie stability relations may not be reversed. Elmi in this case it ma:. lie possible li!, workiiiq rapidly aiid at l-er!. low temperatures to obtain the instable conipoiind and, a s in the IireT-ions case. the instahle nioclification of the original iiiaterial. TTe shall gix-e o n l ~ two . references to illustrate these rules. Hill aiid Lllleii' in speaking of the melting-points of certain procliicts state that the)- were taken ( ' a t ordinary speed," and to explain this refer back to a n article b\- Hill aiid Palmerz, n-liere t 11ese sc ie ii tis t s not ed t 11at 111o nobroiii maleic a11d di broin iii a1ei c acids gi1.e different melting-points, according to the time of heating. For instance iiionohroiiiiiialeic acid melts iii 0.7 iiiiniite, in a bath a t 133', in 1.3 minutes a t 133'~in 4.5 iiiiiiiitesat IZ~', in 8 iiiiiiutes a t I z I ', aiid, according to Kekiil6,T hegins to softeii _ _ _ _ ~________ I .kn. Chem. Jour. 19, 666 1 1897 ) . Ihid. IO, 4 2 2 ( ISSS). Liehig's .Inn. 130,6 ,1 1864 8 .
a t roo'. Taken " a t ordinar)- speed the ~nelting-pointis gij-eii at I Z ~ ' - - I ~ O ~ . If decomposition does not take place, one w h o knows tlie Diilieiii theory coulcl at once state that such beha\.ior points plaiiilj. to the existence of an isomer. T h e probable e s plaiiation is t k t tlie other isoiner is broinfuaiaric acid of 1I.P. 190') that it is stable a t the higher temperatures, and that the eutectic point lies below IOO'. Similar statemelits iiiig-lit be made with respect to tlie mono- and dichlorinaleic acids which also s l i o ~ melting-points varying with the tiiiie of heating. T h a t the sol\.eiit ma!- be of service in disco\.eriiig another forni is sho\vii very plainly by tlie work of (;oldschmidt aiicl Rietschoten' who examined the action of \-arious reagents on the oximes. T h e latter were dissol\~etl,usnall!- in etlier, and the reagent added. In tlie case where it was iiecessarj- to let the mixtiire stand for .s0111e time i n order to obtaiii a solid product-u-lien paraaiiisantialdositiie \\.as treated Tvith plieiiylcyaiiate or with ortliotol!.lcj-atiate ; \\.lien orthoanisaldosiiiie or cnniiiiantialdosirne reacted v i t h phen!.lcyaiiate or nitli ortho-orparatolylcyanate ; paranitrobeiizaiitialdosiiiie with ortliotol!.lc!-anate and metaiiitrobeiixaiitialdoxiiiie with ortliotol~~lc!.anate-iiot once is i t recorded that a second iiiodificatioii was discovered. But in almost ever!. case nlicre the con~poiiiidn-as precipitated at once when the reagent \vas added-plienylcyaiiate 01- ortliotolylcyanate to i~araanissyiialdosiiiie~ pxratolylc\.aiiate to both the anisalcloximes, there was al~\-a!,s foniicl a11 isomer. This latter \vas always discovered in tlie saiiie na!-, 1%. by hoiliiig the precipitate Ivith ;1 sol\-eiit. In regard to the action of paratol).lcyaiiate 011 nietaiiitroheiizs).naldosiiiie \Ye inust note that a precipitate was foriiiecl hilt no liieiitioti is made of an!- attempt to discover the other forni. I t appears strange that (;oldschaiidt and Kietsclioteii did not note the relations we ha\-e just discovered from a consideration of their own work. Had they chosen other solvents for their oximes, the!. iindoubtedl!. woulcl ha\-e discovered more of the isoiiiers for which they were looking. For their paper "
2
Der. cheni. C k s . Berlin, 26, 20S7 [ 1893 )
proves coiicliisivelj. that sudden precipitatioii favors the instable iiiodification : atid from this the stable foriii may be .i.er!. readily d eriI.ed. It follons, therefore, that papers such as the one jiist discussed offer absolute11- iio proof as to the noli-existence of isomers of certaiii conipounds, and all argninents in regard to constitution which ha\-e as their basis experiments showing tliat uiitler one .set of conditions certain isomers cannot exist, must be regarded as useless. T h i s means that, sooner or later, space foriiiiilae a,
