82 



both crotoiiic acids, and dimethyl-acrylic acid (± 7 . 10^^', according 

 to preliminary deteiininations of Mi-. P. E. Vkhkadk, Dissertation 

 Delft 1915, 2'"^ Note, p. iy(i) is weaker still. Also citraconic acid is 

 much weaker than maleic acid. 



Still one ought to be cautious, for /.vo-butyric acid is somewhat 

 stionger than pro|)ionic acid (1.44 to 1,31 . lO"'') and the dimethyl 

 succinic acids are much stronger than succinic acid (1,9 and 1,3 to 

 0,6 . 10-4). 



2. Formation of an anhydride is not possible and therefore cannot 

 help us. 



3. Formation of comj)lex compounds: Biilmann has shown that 

 maleic, citraconic and allo-chumxmc acid form salts with mei-curic 

 acetate, which are soluble in sodium hydroxide, and tVoni which the 

 original acids could not be regenerated by elimination of mercury; 

 in this case /Miydroxy-acids were formed and in conse({uence B. 

 surmises that the salts were comj)lex mercury salts of these hydroxy- 

 acids, which are formed thus; 



H— C -COOH HO— CH— COOH 



il - I 

 H— C— COOH HC— COO 



I / 

 Hg . . 



In the same way «-crotonic acid remained in solution in the form 

 of a complex mercury salt, which could be precipitated with alcohol. 

 From this salt /?-hydroxy-butyric acid was obtained; in consequence 

 one deduces from this too the cis-configuration for ordinary crotonic 

 acid with the higher melting point. 



In order to corroborate this result we have subjected wo-crotonic 

 acid to the same operation and obtained an insoluble basic mercury 

 salt, which, after decomposition by means of H,S furnished a mixture 

 of ISO- and «-crotonic acid. 



It may be mentioned in passing, that z-crotonic acid must have been formed 

 during the eliminatio)) of mercury, for this acid — so far as it originally was 

 present in the iso-crotonic acid — was kept in solution as a complex salt and 

 because HgS does not, or at least extremely slowly, change free fso-crotonic acid 

 into a-crotonic acid. 



The coincidence of the conclusions from the dissociation constants 

 and from the researches of Biilmann gives some certainty to the 

 configuration of the crotonic acids. It follows that the formula of 

 these complex compounds is probably different from the one that 

 has been proposed by Bulmann. However, this method of discernment 

 is valid exclusively for «/?-non-saturated acids; other ethylene deri- 

 vatives, among which are the esters of isomeric acids, cannot be 



