ISOMERIC CHANGE 



239 



As inversion does not take place if the a-hydrogen atom is 

 displaced by methyl it is clear that the mobility of this atom 

 is an essential feature in the process of change, and there can 

 be little doubt that the mechanism suggested above is that by 

 which the inversion is actually accomplished. 



The transference of a mobile hydrogen atom is usually 

 promoted to a greater extent by alkaline catalysts than by 

 acids, but as the sodium salts are transformed less readily 

 than the acids from which they are derived optical inversion 

 is more frequently effected by the help of acids than of 

 alkaline agents. Mandelic acid can, however, be racemised 

 by heating it either with water 1 or with dilute alkali 2 



C S H 



6 1J 5 N 



'H 



HO/ 7 '\C0.0H 4 



rf-Acid 



[ 



C*H 5 \ -1 



>C = C(OHX, ^ 

 HO/ J 



Inactive enolic acid 



C 6 H 5 \ /COOH 



ho/ '\oh 



/-Acid 



It may further be pointed out that the same series of changes 

 would account for the interchange of H and COOH in the 

 conversion of maleic into fumaric acid, although this explanation 

 differs from that commonly given of this isomeric change. 



Tartaric Acid, which contains two )C( groups, can 



7 X CO.OH 



undergo optical inversion in two stages, the a'-acid giving rise 

 first to the inactive (internally compensated) ra^so-acid and then 

 to the (externally compensated) racemic or d + / acid. These 

 are both optically inactive, but are of unequal stability, the 

 production of the meso-acid from the racemic acid in 

 presence of hydrochloric acid at 140 being nearly twice as 

 rapid as the converse process. 3 The final equilibrium (meso- 

 acid 66 per cent., d + I acid 34 per cent.) is, therefore, 

 represented by the scheme 



HO.C 



HO.C 



-COOH 

 ,H 

 'H 

 \C00H 



/-Tartaric Acid 

 17 per cent. 



1 Lewkowitsch, Ber. 1883, 16, 2721. 



" Holleman, Rec. Trav. Chim. 1898, 17, 323. 



3 Ibid. 66. 



