86 



crotoiiic acids are formed, one of which is volatile with steam. 

 This one has a dissociation constant ^9,5.10^^; the other acid 

 has k = 14,4 . 10~^. From this it may be concluded with some 

 certainty, that in the first mentioned acid the chlorine atom lies 

 farther from the COOH group than in the other. Now the relatively 

 weaker acid on reduction furnishes the relatively stronger /^o-crotonic 

 acid; on the other hand the relatively stronger /i-chloro-crotonic 

 acid gives rise to the relatively weaker or-erotonic acid ; in both 

 cases an inversion must have occui-red and we come to the conclu- 

 sion, as well as in the series maleic acid — ^ wo-dibromo-succinic 

 acid — » bromo-fumaric acid, that an inversion has taken place at the 

 attack of the valency, which governs the configuration. 



By catalytic hydrogenation only of phenylpropiolic acid in the 

 presence of colloidal platinum 80 "/n of the theoretically [)ossil)le 

 amount of «//o-cinnamic acid was formed ; on the other hand, 

 by the action of zinc dust and acetic acid, resp. alcohol, ordinary 

 cinnamic acid was almost exclusively obtained '). As the catalytic 

 hydrogenation of acetylene compounds appeared to warrant some 

 certainty, we applied it to tetrolic acid, which ought to give chiefly 

 «-crotonic acid. 



However during a microchemical investigation, which was executed 

 some years ago with the collaboration of Miss (). B. van der Weide, 

 «-crotonic acid could not be found among the reduction products of 

 the sodium salt of tetrolic acid. 



By hydrogenation of the free acid under the influence of pailadium- 

 sol, crotonic and /.yo-cro tonic acid are formed in the proportion of 2 : 1. 



We see, therefore, that no more than with tiie reduction of phenyl- 

 propiolic acid, this chemical method is capable of giving us sufficient 

 certainty about the configuration. 



4. To cyclic cis-trans diols also this unsafe mode of determining 

 the configuration is generally not applicable, because the correspondent 

 saturated diols cannot be obtained. 



The hydro-aromatic glycols form an exception, as they can be 

 obtained from aromatic di phenols, which may be considered as cis 

 diols. Of course this case is not quite to be compared to the hydrogen- 

 ation of acetylene compounds; it is known with rather great certainty 

 that the OH groups of the phenols are situated in the plane of the 

 benzene nucleus; on the other hand it is to be supposed, considering 



1) HoLLEMAN and Aronstein B. 22 1181 (1889): Liebermann and Truchsüss B. 42 

 4674 (1909); E. Fischer, Ann. 386 385 (1912). 



