4 So 



NA TURE 



[September 12, 1895 



quinone oxygen atoms are present. But there is nothing in the 

 doctrine of x'alency which leads lis to suspect that these ortho- 

 <)uinon(' deri\-atives can exist while their parent compound 

 resists all attempts at is<:>lalion. I am aware that it is dangerous 

 I'l argue from negative evidence, and it would be rash to assert 

 that these orthoquinones will never be obtaine<l. But even in 

 the present state of knowledge it may be distinctly aflirmed that 

 the melhixls which readily furnish an orlhoquinone of naphtha- 

 lene completely fail in the case of benzene, and it is just on such 

 points as this that the inadequacy of the hyixithesis becomes ap- 

 I>arent. In other words, the doctrine fails in the fundamental 

 requirement of a scientific theory ; in its present form il gives us 

 no power of prevision — it hints at possibilities of atomic group- 

 ings, but it does not tell us <} //v't»r» which of these groupings are 

 nkcly to be stable and which unstable. I am not without hope 

 that the next great advance in the required direction may yet 

 come from the stereochemical extension of the hyiiolhesis, 

 although the attempts which have hitherto been made to supply 

 ks deficiencies cannot but be regarded as more or less tentative. 



The Xf.w Theory of .-VRSTRAcr Tvi'es. 



I will venture, in the next place, to direct attention to a modern 

 development of structural chemistry which will help to illustrate 

 still further some of the points raised. Kor many years we have 

 iKjen in the habit of abstracting from our structural formulx cer- 

 tain ideal complexes of atoms h hich we consider to represent the 

 nucleus or type from which the compound of known constitution 

 is derived. In other words, the hypothesis of valency which was 

 developed originally from (lerhardt's types is now leading us Ijack 

 to another theory of types based upon a more intimate knowledge 

 of atomic grouping within the molecule. In some cases these 

 types have been shown to be caj^ble of existence ; in others they 

 are still ideal. Used in this way the <loclrine of valency is most 

 suggestive, but at the same time its lack of prevision is con- 

 stantly forcing itself upon the attention of chemical investigators. 

 The jxirent com|x>und has sometimes been known before its de- 

 rivatives, .as in the case of ammonia, which was known long 

 before the organic amines and amides. In other instances the 

 derivatives were obtained before the type was isolated, as in the 

 case of the hydrazines, which were characterised l>y Kmil 

 Fischer in 1S75, and the hydrazo-compounds, which have been 

 known since 1863, while hydrazine itself was first obtained by 

 Curlius in 1S87. I'henylazimide was discovered by Gries-s in 

 1S64, and many representatives of this group have been since 

 prepared : bul the jiarenl compound, hytlrazoic aci<l, was only 

 iMilated byCurlius in 1S90. Derivatives of triazole and tetrazole 

 were obtained by Bladin in 1885 ; the types were isolated by this 

 chemist and by .\iidreocci in 1892, I'yrazole derivatives were 

 pre|>ared by Knorr in 1883 ; pyrazole itself was not isolated till 

 1S89, by Buchner. .\lkyl nilramides were discovered by 

 Franchimonl and Klobbie many years before the typical com- 

 P'lund, nitramide, NO._,.N'II^, which was isolated la.sl year by 

 Thiele and Lachman {Her., 27, 1909). Examples might be 

 multiplied tf> a formidable extent, bul enough have been given to 

 illustrate the principle of the erection of types, which were at 

 first imaginary, but which have since become real. The utility 

 •if the hyiMthesis is undeniable in these cases, and we are justi- 

 fied in pushing il 10 its extreme limits. Hut no chemist, even if 

 endowed wilh prophetic instinct, could have certainly foretold 

 six years ago that the type 01 Griess' " triazobenzene" would be 

 capable of free existence, and still less that when obtained il 

 would prove to be a strong acid. The fact, established by 



Curtius, that the group yN-funclions in chemical molc- 



riilcs like the atom of chlorine is certainly among the most 



t; of recent discoveries. Only last year the list of nitrogen 



iinds wa.s enriched by the addition of Cf)(Nj)„ the 



■ n anal'jgue of phosgene (Curlius, Her., 37, 2684). 



I -■- illuslralions, drawn from the c<im|>ounds of nitrogen, 



' ' bring out the womlerful development which our 



f ihe chemistry of this elemenl has undergone within 



' "- I mighl Ik- lempled here into a iligression on 



"g of ihe very striking fact thai an element 



nvc in ihe free .slate should be so remarkably 



' :■ diiin, bul I must keep lo ihe main topic, a.s by 



" I I ' iiiiixiuntls il i« possible lo iiluslrale slill further 



^ of riur niiKlern conceplionsof 



iiic of ihe undiscovered com- 



, '• ■■• ...... ;.) ihe pr^Kre.ss of ideal abstrac- 



NO. 1350, VOL, 52] 



tion of types. The azoxy-compounds contain the complex 

 - N - N - - N = N - UN - NH 



Xr-,/ or ■• . The types would be \r\/ 



^ O ' ^ 



UN = Ml 

 or •• . The first of these formula; represents the un- 



O 

 known dihydro-nitrous oxide. The azo-compounds are de- 

 rivatives of the hypothetical diimide UN : NH. An attempi to 

 prejwre this compound from azodicarbonic .icid (Thiele, .Aim., 

 271, 130) resulted in the formation of hydr.azine. The dielhyl- 

 derivalive may have been obtained by Harries (Her., 27, 2276), 

 but this is doubtful. It is at present inexplicable why compounds 

 in which the group . X : N • is in combination with aromatic 

 radicles should be .so remarkably stable, while the parent com- 

 pound appears to be incapable of existence. The addition of 

 two atoms of hydrogen converts this type .again into a stable 

 compound. There is nothing in the structural formula" to indi- 

 cate these facts. The aniidines are stable compounds, and the 

 so-called " anhvdro-bases," or imidazoles, are remarkably stable : 



the parent compound HC/ , has not been obtained, while 

 ^NH, 

 .NH 

 its amido-derivalivc, H„N.C/^ , is the well-known substance 



^NH. 

 guanidinc. The isodiazo-compounds recently discovered by 

 Schraube and Schmidt and by Bamberger [/hid., 27, 514, 679, 

 &c. ) are possibly derivatives of the hypothetical substance 

 0:N.NHj, which might be named nitros-amidc. Why this 

 compound should not exist as well as nitramide is another 

 question raised by the princijile of abstract types. The carbizines 

 were formerly regarded as derivatives of the compounds 



/NH /MI 



C0(^ • andCS^f- (Fischer, .^«h., 212, 326 ; Freund and 



^NH ^NH, 



Goldsmith, Her. ,21, 2456). .Mthough this structure has now been 

 disproved the possible existence of the types has been suggested. 

 Carbizinc and thiocarbizinc differ from urea and ihiocarljamide 

 only by two atoms of hydrogen. Those tyjies have not been 

 isolated ; if they are incapable of existence the current views of 

 molecular structure give no suggestion of a rea.sou. The 

 diazoamidcs are derivatives of the hyimthetic.al H.^N.NH.NH, 

 or IIN:N.NH2, compounds which Curtius speaks of as the 

 pro|)ane and propylene of the nitrogen series. The latter com- 

 plex was at one time thought lo exist in diazohippuramide (Her., 



24, 3342. This has since been shown lo be hippurazide, i.e. a de- 

 rivative of NjH, Her., 27, 779), and a bi.acidyl derivative of the 

 former type has also been oblained {Ihid., 3344). Both these 

 types await isolation if they are capable of existence. 

 I may add that several attempts to convert diazoamidcs 

 into dihydro-derivatives by mild alkaline reduction have 

 led me to doubt whether this nitrogen chain can exist 

 in combination with hydrocarbim radicles. The bisdia- 

 zoamides of H. v. Pechmann and Frobenius (Her.. 27, S()S) are 

 deiivatives of the 5-atom chain H.^N.NH.Ml.Nll.NI l.j or 

 IIN : N.NII.N : NH, a type which hardly .seems likely to be of 

 sufiicient stability to exist. The telrazones of Fmil Fischer 

 have for their type the 4-aloin ch.ain H5N.N:N.Nllj or 

 H.jN.NII.NH.NIlj, of which the free existence is ecpially 

 problematical, although a derivative containing the chain 

 -N:N.NIi.NH- has been obtained by Curlius (/hid., 26, 

 1263). Hydrazoic acid may be regarded as a derivative of 



NH 

 triimidc, HN;( | , but this type appears to be also incapable 



^NH 

 of isolation (Curtius, /ier., 26, 407). The hydrazidines or for- 

 mazyls of I'inner (/ier., 17, 182) and of II. v. I'echmann (/hid., 



25, 3175), have fur ihcir parent compound the hypnlhetical 

 substance HjN.N : CH.N ; Nl I. In 188S I.impiichi described 

 certain azu-compounds (ihid., 21, 3422) which, if possessing 

 the structure assigned by th.at author, must be regarded as 

 derivatives of diamidoletrimide : 



HN-NH IIjN.N-N 



II I II 



HN-NH H,N.N-N 



Both these ly|x:s arc at present im.iginary ; whether it ispo.ssible 

 for cyclic nitrogen .sysleins III exist we have no means i>f know- 

 ing— all that can be saiil is that they have never yet been (iblMined. | 

 It is pos-sible, .a.s I poinled out in 1890 al the Leeds meeting of 



i 



