ON DYNAMIC ISOMERISM. 211 
V. Optical Inversion. 
When optical isomerides become isodynamic the resulting mixture is 
inactive, for under normal conditions the d and / forms are equally stable. 
At least in the case of carbon it appears to be impossible directly to alter 
the point of attachment of the radicles, and the interchange on which the 
inversion depends can only be effected ‘by an indirect process. In many 
cases the inversion appears to be brought about by means of a double 
keto-enolic isomeric change, the inactive ‘enolic form being in equilibrium 
with both active forms. The proportion of enol is often exceedingly 
small, and isomeric change must then be slow even under the most favour- 
able conditions. If the mobile hydrogen atom is displaced by methyl the 
inversion is no longer possible. Thus in the case of phenylglycollic acid, 
which becomes inactive when heated alone,! or with dilute alkalies,? the 
inversion: is probably effected through the intermediate inactive form 
indicated in the equation 
C,H, O,H; 
Ho /CC CO.0H2HO °C: COW): 
d-acid (50 per cent.) inactive enolic acid (tra ae fe acid (50 per ile ) 
CH. 4 /CO.OH 
A similar explanation may be given of the conversion of maleic into 
fumaric acid, since this also involves the inter change of H and CO,H. 
Although changes of the keto-enolic type are exceedingly ceniiges to 
alkaline catalysts “the optical inversion of acids is often brought about 
more rapidly by acid catalysts, on account perhaps of the greater stability 
of the ketonic form in the metallic salts. The esters (see p. 206) would 
probably be racemised even more easily than the acids but for the fact 
that it is impossible to introduce either acids or alkaline catalysts without 
hydrolysing the ester. Kipping and Hunter ® have shown, however, that 
although benzylmethylacetic acid, C,H;.CH,.CHMe.CO,H, is unchanged 
when heated at 170° during two hours, the acid chloride is racemised 
slowly at 70° and rapidly at 100°, and this may be taken to indicate 
that the chloride, unlike the acid, contains an appreciable quantity of 
enol. 
d-Camphoric acid, which contains two asymmetric carbon atoms, 
undergoes a reversible isomeric change when heated with water or with 
a mixture of acetic and hydrochloric acids. Only one of the asymmetric 
carbon atoms carries a hydrogen atom, and this is readily inverted, giving 
vise to a mixture of d-camphoric and /-isocamphoric acids in unequal 
proportions. 
vy ,CO.OH 
CX C< 
mar : \co.0H Aer oe 
ao WP OE, Ph itee 
“ < oS 
een fie 
\cO0.0H \c0.0H 
d-camphoric acid l-isocamphoric acid 
' Lewkowitsch, Ber. 1883, 16, 2721. 
2 Holleman, Rec. Trav. Chim. 1898, 17, 323. 3 Trans. 1903, 88, 1008. 
P2 
