i6o 



NA TURE 



(June 15, 1905 



in the formation of benzene from hexane, 2 atoms of 

 hydrogen have been eliminated without influence on the 

 refractive increment of the product. 



But in the formation of benzene from hexane, 2 atoms 

 of hydrogen have been employed to close the ring (see 

 Fig. on p. 159). The withdrawal of these two atoms, and 

 the closing of the ring, have therefore taken place without 

 causing any optical anomaly. 



In the formation of the olefines and dioleiines from the 

 paraffins, however, there is no closing of the ring. These 

 substances are of open-chain structure, and every removaj 

 of 2 hydrogen atoms corresponds here to the creation of a 

 double carbon bond : — 

 H3C CH,/CH„CH.,CH2CH3 CH„ = CHCH.,CH.,CH2CH3 



Hexane Hexylene 



(CeH,^) (C,H,„) 



CH„ = CH CH2-CH2CH = CH, 



niallyl 

 (CeHi„) 



Hence, also, the refractive increment of the olelines and 

 diolefines is directly proportional to the number of hydrogen 

 atoms removed from the paraffin. 



From all this it follows that the removal of hydrogen 

 atoms causes optical anomalies only where double carbon 

 bonds are created by the process. The splitting-off of 

 hydrogen ivhich results in a closing of the ring is, on the 

 other hand, without abnormal optical influence, and pro- 

 duces no refractive increment. 



This latter principle, which has since been confirmed 

 many times by experiment, has proved of the same import- 

 ance as the first in the investigation of the chemical struc- 

 ture of bodies. 



.\ few examples will show how these two principles can 

 be utilised for the discovery of chemical structure. 



Besides the formula already mentioned for benzene — that 

 suggested by Kekuli — several others have been proposed, 

 e.^. those by Ladenburg and Claus : — 





Neither of these graphic formula; is reconcilable with the 

 results of spectrochemical investigation, because the neigh- 

 bouring carbon atoms contained in them are associated 

 only by single, cycloid, or ring-closing affinities, and not 

 by any so-called double bonds". Substances of this kind 

 should be optically normal, while benzene and its deri- 

 vatives are, as a matter of fact, abnormal. Kekul^'s 

 formula for benzene is really the only graphic represent- 

 ation of its structure in a single plane which is confirmed 

 by chemical optics. 



Thus it can be at once determined by optical methods 

 whether a given body belongs to the par'affinoid, olefinoid, 

 or cycloid products, whether these products contain double 

 bonds or not, and, if so, how many. 



Now, too, we can imagine why the diamond, i.e. pure 

 crystallised carbon, is, as already mentioned, optically 

 normal. We obtain an idea of the chemical constitution 

 of the mineral, and of the way in which the atoms of 

 carbon are perhaps combined in the sparkling gem. 



For the reasons already stated, the diamond cannot 

 possibly contain any double bonds ; a combination, sav, in 

 the form 





with one atom of carbon at each of the six corners, and 

 with each atom connected with its neighbour by a double 

 bond, is altogether impossible. 



Imagine, however, at each of the si.x corners of a regular 

 octahedron, a single molecule of marsh-gas, CHj, i.e. 

 altogether C^H,,, and then imagine all the 24 hydrogen 

 atoms successively removed, so that each carbon atom is 

 connected with each of its neighbours only by a single 

 bond, and thus all six atoms of carbon are united together 

 in a single whole. Then you obtain, as the most simple 

 representation of the molecule of the diamond, a regular 

 octahedron, with one atom of carbon at each of its six 

 corners, while the edges represent the mutual bonds : — 



Several simple molecules of this kind may be combined 

 into one crystallised particle of the spectrochemically normal 

 diamond. 



Thanks to the explanation of the optical behaviour of 

 benzene, with the resultant discoveries, it all at once be- 

 came possible to understand the causes of the spectro- 

 chemical abnormality of whole classes of bodies, such as 

 the olefines, diolefines, terpenes, aromatic compounds, &c. , 

 and light w'as cast on the chemical constitution of whole 

 classes of bodies. 



At the same time, however, it at once became apparent 

 why both Landolt and Gladstone had succeeded in observ- 

 ing complete optical normality in very numerous substances 

 of the most various types — alcohols, acids, ethers, hydro- 

 carbons, S.C. .\nd now it was understood why in such 

 bodies the moleculan refraction is determined solely by the 

 component elements, while the different grouping of the 

 atoms, i.e. the isomerism, remains without any appreciable 

 optical influence. 



All the bodies of this kind proved to be either paraffins. 

 I.e. saturated hydrocarbons, or simple derivatives of the 

 same. But the paraffins, as we now know, are always 

 optically normal, because they contain no double carbon 

 bonds. For this reason all such simple derivatives of the 

 paraffins must also be normal. Their molecular refrac- 

 tion will thus always correspond to the elements of which 

 they are composed, however the atoms may be grouped, 

 i.e. chemical isomerism is here also without influence. 



For the same reason, however, all cycloid (ring-shaped) 

 closed formations, if they contain no double carbon bonds, 

 must be optically normal, for those bodies also may be 

 conceived as originating in the simple replacement of 

 hydrogen by paraffin fragments, and may therefore be re- 

 garded as combined paraffins. 



Thus we can imagine the hexamethylene already men- 

 tioned not only as formed from hexane by removal of two 

 hydrogen atoms from the ends, but also as arising from 

 ethane and butane, i.e. from two paraffins, by the removal 

 of four hydrogen atoms and welding together of the 



H, 

 C" 



H 



xamelhyle 

 (CgHi.>) 



As a combined paraffin, hexamethylene must be normal, 

 as is also confirmed by experiment, and here we see again, 

 as in the case of the diamond, that a progressive removal 

 of h\'drogen and increase of carbon need not lead to the 

 slightest optical anomaly. 



NO. 1859, VOL. 72J 



