4 DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



ing differences in chemical properties, it is necessary to represent the arrangement of the 

 atoms in the three dimensions of space. Wlien substances have the same kintls of atoms 

 and the same number of each of the kinds of atoms, they are isomers and have tlie same 

 molecular formula. If they differ in any of their properties there is a difference in the 

 linkage of their components which is expressed in differences in their structural formula; 

 or, if they have the same structural formula but differ in their properties, while the com- 

 ponents have the same linkage the units differ in their positions in the three dimensions 

 of space, and are then distinguished as stereoisomers. 



Now, it is an extremely important fact, as pointed out by Fischer, that stereoiso- 

 mers may show far greater differences in their properties than may be obser\'ed among 

 related isomers. As shown by van't Hoff, substances which contain one or more asym- 

 metric carbon atoms are optically active, and it has since been found that every optically 

 active carbon compound contains at least three carbon atoms, one of which is asymmetric. 

 If a compound containing one asymmetric carbon atom be so modified that one of its 

 attached atoms, groups, or masses is substituted by another so as to give rise to a plane 

 of symmetry, as in the case of isoamyl alcohol to form amylen hydrate, when the hydroxyl 

 group is replaced by hydrogen, and the hydrogen by a hydroxyl group, optical activity 

 disappears for the obvious reason that asymmetry no longer exists in the molecule: 



Isoamyl alcohol. Amylen hydrate. 



CH3 CH3 



I I 



C2H5 C H C0H5 C OH 



I " I 



CHoOH CH3 



Each stereoisomer, whether it contain one or more asymmetric carbon atoms, is 

 enantiomorphous, or, in other words, there are two complementary forms which are char- 

 acterized by having opposite but equal effects on polarized light, and also by other differ- 

 ences, all of which are due to modifications in the space relations of the components of the 

 molecules. There are therefore two types of optically active stereoisomers which are desig- 

 nated the dextro and lajvo forms in respect to their optical effects. Moreover, inactive 

 isomers may exist if the two enantiomorphous forms be mixed in equal proportion to form 

 a mechanical mixture, as in racemic tartaric acid; and also when the units of the mole- 

 cules of the two forms combine in the molecule in equal (luantity to form a true compound, 

 as in meso-tartaric acid. In the first instance, the substance is inactive because of external 

 compensation, the effect of the dextro-molecule being compensated for by that of the 

 la;vo-molecule; in the second instance the compensation is internal and due to the units 

 which constitute one half of the molecule neutralizing those of the other half. Obvi- 

 ously, those of the first kind can be separated into the two enantiomorphous forms, 

 while those of the second can not. The first are known as racemic substances, and the 

 second as ?/teso-substances. There are, therefore, in the case of tartaric acid, four distinct 

 stereoisomeric forms: dextro-rotatory, lajvo-rotatory, racemic, and meso-tartaric acids, 

 respectively. The dextro-active, the inactive, and the te\'o-active forms may be likened 

 physically to the differences in the three images seen when the head is viewed in the 

 three-sided mirror. 



When a substance contains a number of asymmetric carbon atoms, the number of 

 possible stereoisomers increases with each addition, and soon becomes considerable, so 

 that there may be many of each of the dextro, Isevo, and inactive forms. When the 

 number is relatively small, as in the members of the aldohexose group which have the 

 formula CH20H.(CHOH)4.COH, in which there are only four such atoms, the theoretical 



