i2 4 CHEMISTRY 



point and molecular refraction. This has been connected with the occurrence of con- 

 jugated double linkings, a view confirmed by the fact that these menthadienes are able 

 to combine with only one molecule of halogen or halogen acid, whereas the other 

 isomerides can absorb two such molecules. 



Much work has been done lately in connection with rubber, and there is no doubt 

 now that synthetic caoutchouc, comparable in all respects with the natural product, 

 can be produced by the polymerisation of the hydrocarbon isoprene and its homologues. 

 This is best effected, as Matthews has found, by heating the hydrocarbon with metallic 

 sodium at about 60 for several days (see Perkin, J. Soc. Chem. Ind,, 1912, 31, 616. 

 Compare Harries, Ann., 1911, 383, 157). Isoprene is readily obtained from the iso- 

 amyl alcohol contained in commercial fusel oil. Iso-a,my\ chloride, obtained from the 

 alcohol by the action of hydrogen chloride, is chlorinated under special conditions, so 

 that the formation of products more highly halogenated than the dichlorides is avoided.. 

 When the mixture of dichlorides thus produced is passed over soda lime heated to 

 about 470, isoprene is obtained in 40 per cent yield. The possibility of turning out 

 synthetic rubber on the commercial scale depends on an abundant and cheap supply of 

 fusel oil. This problem appears to have been solved by Fernbach's work, which shows 

 that the fermentation of starch may be so directed as to yield either acetone or fusel oil. 



One or two other interesting compounds which have been prepared lately may be 

 enumerated here: viz. azomethane, CHs-NiN-CHs, a colourless gas (Thiele, Ber., 1909, 

 42, 2575), diphenylene, CeHjrCeHj, melting at 74.5-75 (Dobbie, Fox and Gauge, /. 

 Chem. Soc., 1911, 99, 683), cyc/o-octatetraene, CsHg, the eight-membered ring compound 

 analogous to benzene (Willstatter and Waser, Ber., 1911, 44, 3423), and epicamphor, 



/CO /CH 2 



CgHuy i isomeric with ordinary camphor, CsH]4<f i (Lankshear and Perkin, 



NCH 2 \CO 



Proc. Chem. Soc., 1911, 27, 166). 



Discussion has long ranged round the question whether acetoacetic ester (see E. B.. 

 i, 135) is to be represented as possessing the " keto " form CHs.CO.CH^.C^CaHj, or the 

 " enol " form CH 3 .COH:CH.CO2C 2 H 6 , and latterly the view has been held that the 

 liquid ester as ordinarily obtained is an equilibrium mixture of the two forms. Knorr 

 has now succeeded, by carrying out operations at 80 and in the complete absence of 

 moisture, in isolating both the ketonic and enolic isomerides in a pure state (Bcr., 19:11, 

 44, 1138). A comparison of the refractive indices of these two substances with that 

 of the equilibrium mixture usually obtained shows that the latter contains 2 per cent 

 of the enolic form (compare, however, Meyer, Ann., 1911, j$o, 212; Ber., 1911, 44, 2718). 

 At the ordinary temperature both the enolic and ketonic forms change spontaneously 

 into the equilibrium mixture, with great rapidity in the presence of certain catalytic 

 agents. The isomeric forms of other tautomeric substances, such as methylbenzoyl- 

 acetate and acetylacetone, have also been isolated (Ber., 1911, 44, 2767). 



In connection with optical isomerism much attention has lately been devoted to. 

 the Walden inversion. This term refers to a change of configuration occurring in an 

 optically active compound when one of the atoms or groups attached to the asymmetric 

 C-atom is replaced. Thus, for instance, the action of phosphorus pentachlori.de on 

 1-malic acid produces d-chlorosuccinic acid, from which in turn, on treatment with 

 silver oxide, d-malic acid is obtained. Hence it is possible to convert 1-malic acid into 

 d-malic acid, and the reverse change can be similarly effected. Recent investigation of 

 analogous cases has shown that change of configuration does not necessarily accompany 

 any particular substitution, but is dependent on the character of the reagent employed 

 to bring about the substitution. Thus the action of phosphorus pentachloride on 

 /-a-hydroxy-a-phenylpropionic acid leads to the corresponding d-chloro-acid, while if 

 the replacement is effected by thionyl chloride, the /-chloro-acid results (McKenzie, 

 J. Chem. Soc., 1910, 97, 1016, 2566). It was thought at one time that a Walden inver- 

 sion occurred only in cases where carboxyl was one of the groups attached to the asym- 

 metric C-atom, i.e. in the a-substituled acids, but recent work has shown that 0-amino- 



