182 HEPORT— 1903. 



Stereochemidnj. — A graphic method of demonstrating the nuliiber of 

 different stereoisomeric forms in which a substance can exist, has been 

 brought forward by Aschan,' as being preferable to the use of models. 

 It is shown in detail that the possibilities of isomerism in ring-systems 

 are more truly seen when the symmetry of the molecule is ,alone con- 

 sidered ; and further, it is demonstrated on these lines that optical activity 

 becomes possible in certain ring-systems in the absence of an asymmetric 

 carbon atom. 



If one imagines the plane of a carbon ring of an alicyclic compound as 

 standing vertical to the plane of the paper, it can, provided the ring atoms 

 lie in one plane, be represented by a straight line on the paper. The sub- 

 stituents (omitting hydrogen atoms and unsubstituted methylene groups) 

 are then written, according as to whether they lie on the upper or lower 

 half of the ring, above or below the projected line. Only such forms 

 are identical as can be superimposed either directly or after turning 

 through 180° in the plane of the paper. 



The simplest example is afforded by trimethylenedicarboxylic acid, 



,CH . COOH 

 CH., ! 



CH . COOH 



which can exist in the three following forms : — 



COOH : COOH COOH COOH 



COOH COOH 



A plane of symmetry can be drawn through form 1, which is not 

 possible with 2 and 3, these being mirror images of one another. Aschan 

 defines as optically isomeric only those substances which are mirror 

 images of one another, Avhilst the term geometrical isomerism applies to 

 all those stereoisomeric foims, active or inactive, which show a dissimi- 

 larity in all their physical properties. 



On Dihydrohenzenes and on Aromatic Compounds derived from 

 Hydro-aromatic Siihstances. By Dr. A. W. Crossley. 



Dihydrohenzene. — It has been shown that the dihydrobenzene obtained 

 from dihydroresorcin ^ has the formula 



.CH = CH 

 CH, \CH 



\CH.,-CH 



that is, the double bonds are in the 1 : 3 position. Up to the present 

 time it has not been found possible to prepare the hydrocarbon in a pure 

 condition, as it is contaminated with tetrahydrobenzene ; but further 

 experiments are being conducted in the hope of obtaining the pure 

 substance by this method. 



A second means of producing this same dihydrobenzene seemed to 

 consist in the removal of two molecules of hydrogen bromide from 



> Ber., 1903, 35, 3389. * J.C.S., 1903, 83, 494. 



