HO/ 



THE ISOMERIC TARTARIC ACIDS i;5 



same structural formula, and behave similarly from a chemical poipt of view, but 

 differ in a physical sense (crystalline habit, solubility, behaviour towards polarised 

 light). Such compounds, whose different behaviour can only be explained by the 

 assumption of a difference of grouping in the molecule, are designated stereo- 

 isomers. Of the compound Caked there may exist one modification capable of 

 deviating polarised light towards the right hand, and a second producing a left- 

 handed rotation ; and one part by weight of the dextro-rotatory modification 

 gives rise to just as great a deviation towards the right hand as one part by 

 weight of the levo- modification does towards the left hand. 



If now a compound containing such an asymmetric carbon atom be artificially 

 prepared from substances devoid of asymmetric atoms, the probability is that 

 just as many levo- rotatory as dextro-rotatory molecules will be produced ; and 

 assuming that each molecule of the one kind coalesces with a molecule of the 

 other class to form a double molecule, then an optically inactive compound will 

 result. There may therefore be presumably obtained from one combination, 

 Cabcd, containing a single asymmetric carbon atom, three different modifications, 

 two of which (one dextro-, the other levo-rotatory) are optically active and the 

 third inactive. An example of such a compound is afforded by ethylidene lactic 

 acid 



CH 3 H CH 3 



C = CH.OH 



I 

 COOH COOH 



of which one optically inactive form is known, and in chemical text-books 

 is generally named fermentation lactic acid. The dextro-rotatory modification 

 has also long been known to chemists under the name of sarco-lactic or para- 

 lactic acid, whilst the third modification, levo-lactic acid, was only discovered 

 and produced a few years ago by fermentation physiologists. This will be again 

 referred to later on. 



138. The Isomeric Tartaric Acids. 



Of the compounds containing two asymmetric carbon atoms in the molecule 

 we will refer to only that group in which these two atoms are connected by a 

 single bond, as is represented in the subjoined typical formula 



I C C a 



/ ^ 



c 7 



As regards the positional arrangement of the two groups of substitutes 

 (a, b, c, or a, ft, y) in the molecule, four possibilities are conceivable : (i) both 

 grouped in the dextro- position; (2) both in the levo- position; (3) the one 

 dextro- and the other levo- ; (4) or, finally, vice-versd. 



Assuming the group C to possess a (numerically) greater optical 



rotatory power (D or L) than C ft, which may be expressed in letters 



1 



thus 



D > d L > I, 



