34 CHEMISTRY OF PLANT LIFE 



Comparisons of the above formulas will show that the differ- 

 ence between the formulas for d- and Z-glucose lies in the arrange- 

 ment of the H atoms and the OH groups around the two asymmet- 

 ric carbon atoms next the aldehyde end of the chain; while the 

 d- and Z-galactoses differ in that this arrangement is in the reverse 

 order around all four of the asymmetric carbons. By similar 

 variations in the grouping around the four asymmetric atoms, it 

 is possible to produce the sixteen different space arrangements 

 shown on page 37 for the groups of an aldohexose. Sugars 

 corresponding to fourteen of these different forms have been dis- 

 covered, three of which are of common occurrence in plants, either 

 as single mono-saccharides or as constituent groups in the more 

 complex carbohydrates; the remaining two forms have only 

 theoretical interest. 



explained on the basis of the open-chain arrangement represented by the 

 Fischer formulas used here. A closed-ring arrangement, showing the alde- 

 hyde oxygen as linked by its two bonds to the first and the fourth carbon atoms 

 of the chain, thus forming a closed-ring of four carbon and one oxygen atoms, 

 instead of being attached by both bonds to a single carbon atom, as in the 

 above formulas, is undoubtedly a more nearly accurate representation of the 

 actual linkage in the molecule than are the open-chain formulas used above. 



The differences in conception embodied by these two types of formulas may 

 be shown by the following formulas for glucose: 



CH 2 OH CH 2 OH 

 CHOH CHOH 

 CHOH CH CHOH i O 



ivy i 

 __ _._ _ H- CHOH- CHOH. CHOH 



!H-C] 



CHOH CH CHOH 

 CHO OH 



Fischer's Closed-ring formulas 



formula 



It will be observed that in the closed-ring formula there are five asymmetric 

 carbon atoms, and the asymmetry of the terminal one forms the basis for the 

 explanation of the existence of the so-called a and /3 modification of d-glucose 

 (see page 46). However, the ordinary aldehyde reactions of the sugars are 

 more clearly indicated by the open-chain formula. Some investigators are 

 inclined to believe that these sugars actually exist in the open-chain arrange- 

 ment when in aqueous solution, and in the closed-ring arrangement when in 

 alcoholic solution. The closed-ring formulas will be used in this text in the 

 discussions of the birotation phenomena and of biochemical properties, but 

 for the explanations of the stereo-isomeric forms and similar phenomena, the 

 open-chain formulas are just as useful in conveying an idea of the possibilities 

 of different space relationships, and are so much simpler in appearance and in 

 mechanical preparation, that it seems desirable to use these rather than the 

 more accurate closed-ring formulas. 



