614 



NA TURE 



[April 26, 1894 



FURTHER LIGHT UPON THE NATURE OF 

 THE BENZEISE NUCLEUS. 



AN important memoir, containing an admirable compendium 

 of the data now accumulated bearing upon the much-dis- 

 cisised question of the nature of the fundamental hydrocarbon 

 oi the aromatic compounds, together with the results of new 

 spectrometric observations of creat value, is contributed to the 

 current issue of the Joiirnalfiirfraktische Chc7nie, by Prof. J. 

 \V. Bri'hJ, 01 Heidelberg. I'he main question now at issue is 

 whether benzene is best represented by the well-known structural 

 formula of Kekule, 



H 



C 



/^^ 

 HC CH 



li I ' 



HC CH 



\/ 



C 



H 



in which the carbon atoms are linked together by alternate 

 double and single linkages, or by a formula in which there are 

 no double linkages, and each carbon atom is attached to three 

 others, the three extra linkages being diagonal, 



H 

 C 



HC/|^CH 



HC 



■\l/ 



C 



H 



I 

 CH 



a view which has latterly received some support at the hands of 

 Prof, von Baeyer, A full discussion of the valuable experi- 

 mental work of the latter chemist is given, together with the 

 thermochemical work of Thomsen, Dieffenbach, Horstmann 

 and Stohmann. 



Prof. Briihl has recently determined the specific gravities and 

 optical constantsof thethree compounds benzene dihydrideCgHg, 

 benzene tetrahydride C^H^f,; ^n<i benzene hexahydride CgHjo, the 

 two former of which were prepared some time ago by Prof, von 

 Baeyer, together withthoseofhexylene. Therehavenow beenfully 

 investigated as regards the spectrometric constants eight com- 

 pounds which are so closely related as to enable most important 

 deductions to be derived from their comparison. These com- 

 pounds are : benzene itself CgHg, benzene dihydride C^Hg, 

 benzene tetrahydride CyHj^,, benzene hexahydride C,;Hj2, hex- 

 ane C^H^, hexylene C^Hj,, diailyl C«Hj,,, and dipropargyl 

 CgH,;. The second, third, and fourth are the graduated pro- 

 ducts of the addition of hydrogen to benzene, two atoms at a 

 time ; the fifth, hexane, is the open chain six carbon paraffin, 

 hexylene the six carbon olefine with one double linkage, diailyl 

 the six carbon fatty compound containing two double linkages, 

 and dipropargyl the six carbon compound containing two acety- 

 lene triple linkages. 



Upon comparing the specific gravities at 20° of these eight 

 compounds it is observed that the density steadily decreases 

 from benzene to benzene hexahydride; there is then a sudden 

 large fall upon the disruption of the ring and formation of the 

 open chain compound hexane. The density then slightly in- 

 creases through hexylene and diailyl, and again a sudden break 

 of continuity, a large rise, occurs upon the passage to the 

 acetylene derivative dipropargyl. Siiil more striking are the 

 changes exhibited by the molecular volumes. There is an ex- 

 ceptionally large rise of twenty-three units between benzene 

 hexahydride and hexane, and a similar large decrease between 

 diailyl and dipropargyl. Passing to the molecular refraction, it 

 is observed that upon the graduated addition of hydrogen to 

 benzene, this constant becomes gradually larger as far as the 

 hexahydride, then there is a great leap upon the breaking of the 

 ring and production of hexane. Similarly as the hydrogen is 

 again removed step by step a continuous decrease occurs until 

 diailyl is reached, when upon removal of 2H.1 and formation of 

 dipropargyl, the two ethylene groups being changed into 

 acetylene radicles, the molecular refraction falls precipitately. 



The whole of these physical properties thus exhibit a 

 break of continuity on passing from benzene hexahydride 

 to hexane, that is upon the opening of the ring into a straight 



NO. 1278, VOL. 49] 



chain, and also when the ethylene derivatives are converted into 

 derivatives of acetylene. Moreover, if the isomers among these 

 eight compounds are compared, benzene with dipropargyl, 

 benzene tetrahydride with diailyl, and benzene hexahydride 

 with hexylene, it is found that the physical constants differ very 

 materially. From these considerations Prof. Briihl concludes 

 that the Kekule structural formula is most in accordance with 

 the facts. 



In addition to the above new observations, the spectrometric 

 constants of the ethyl ester of phthalic acid have been deter- 

 mined. Prof, von Baeyer considered that he had proved that 

 phthalic acid cannot be constituted according to Kekule's con- 

 ception of the aromatic nucleus, but that a nucleus with three 

 diagonal single linkages must be present. Upon comparing, 

 however, the observed molecular refraction and dispersion of the 

 ethyl ester with the values for these constants calculated upon 

 the assumptions of the two hypotheses, they are found to corre- 

 spond closely with those demanded by the Kekule structural 

 formula, and are very far removed from those calculated upon 

 the basis of three diagonal linkages. 



Prof Briihl considers it to be well founded that in benzene 

 tetrahydride and in hexylene there is one ethylenic double link- 

 age, and that in benzene dihydride and in diailyl there are two 

 such linkages. Now the continuity in the entire physical pro- 

 perties as hydrogen is removed step by step from hexane to 

 diailyl on the one hand, and from benzene hexahydride to benzene 

 itself upon the other, points conclusively to a continuity in the 

 nature of the alteration of the constitution in both series. Benzene 

 must therefore, according to Prof. Biiihl, likewise contain 

 ethylenic double linkages, three in number, if benzene dihydride 

 contains two such linkages and the tetrahydride one. The view 

 that three effective diagonal linkages, or, as has recently been 

 surmised by certain chemists, three central potential linkages, 

 can bring about in benzene the same physical action as three 

 ethylenic bonds, appears to Prof. Briihl to be out of the question. 

 He shows, moreover, that the values for the whole of the eight 

 substances agree most remarkably with the numbers calculated 

 upon the basis of the Kekule formula, and further, that the 

 thermodynamical data all point to the same conclusion. 



The relation of the atoms to one another in the benzene 

 nucleus is not, however, ideally expressed by Kekule's struc- 

 tural formula. This can only be achieved by a spacial repre- 

 sentation. The happiest conception of the spacial configuration 

 j of benzene, according to Prof Briihl, is that of Sachse. This 

 j model is constructed by taking a cardboard octahedron, removing 

 1 two parallel sides, and upon each of the six remaining ones 

 placing a regular tetrahedron. The six tetrahedra represent 

 the six carbon atoms, and the hydrogen atoms are supposed to 

 be attached at the six apices. The six carbon atoms then lie in 

 two parallel planes, as do likewise the six hydrogen atoms. 

 j The properties of such an arrangement would be such as accord 

 1 with the observed facts. The gradual addition of hydrogen 

 ; would cause a regular and continuous movement of the tetra- 

 hedra, corresponding with the observed continuity in physical 

 properties. The best representation of this model in one plane 

 is the structural formula of Kekule. 



A. E. TUTTON. 



THE FACE OF THE EARTH} 



A T the present time we all acknowledge the value of the 

 ■'^ accepted classification of the relief-forms of the earth's ' 

 surface in continents and islands, mountain chains, plateaus, 

 plains, &c. ; into ocean-basins, seas, lakes, and the like. But 

 lew of us ask ourselves the very natural question'^, "What is j 

 the fundamental unit among all these morphological individuals, 

 great and small ? Is there any surface unit existent among them I 

 which, like the species of the biologist, once identified, will not 

 only be found lo group its individuals rank over rank into the | 

 genera, the families, the orders, and the kingdoms of the surface j 

 world ; but the study of whose life-history and necessary ' 

 interactions with its fellow-species will eventually afford us 

 some clue to the relationships and the natural classification of 

 the whole?" 



This aspect of the subject perhaps excepted, there is probably 

 no theory possible upon the matter of the grouping of the forms 

 of the earth's surface which has not, either as a whole or in part, 



J A aper read by Prof. Chas. Lapworth, F.R.S., at the Royal 

 Geographical Society, on April 23. 



