2 PRINCIPLES OF STRUCTURE 37 



resulting main valency layers are united into a lattice by weaker secondary 

 valencies. The distance between the layers (3.41 A) is therefore considerably 

 larger than that in the rings. A structure in which the lattice forces and 

 spacings within a plane are so different from those in a direction perpen- 

 dicular (or nearly perpendicular) to this plane is called a layer lattice. Com- 

 pounds of this lattice type always crystallize in the form of flakes and are 

 as a rule easily split along the base (mica, serisite). Many benzene derivatives 

 and other aromatic compounds (naphthalene, anthracene, etc.) belong to this 

 class. The division into aliphatic and aromatic substances is therefore not 

 only based upon their chemical behaviour, but it also has a morphological 

 background in that the one tends to crystallize into a chain lattice, while the 

 other shows a strong tendency towards the development of a layer lattice. 



Cyclic compounds. The structural formulae of aliphatic chemistry are 

 found to be very similar to molecular models if the valency angle 

 between two successive C-C bonds are taken into account. A chain 

 such as hexane should therefore be kinked instead of straight (Fig. 34a, 

 and b). Molecules which do not form part of a crystal lattice, but can 

 freely move about in the gaseous or dissolved state, are subject to the 

 so-called free rotation of the groups around the direction of the valency 

 lines. In Fig. 34a rotation would not give rise to a new structure. 

 In kinked chains, however, the free rotation means that, for instance, 

 group I in Fig. 34b need not necessarily lie in the plane of drawing 

 with 2 and 5 ; it can be located anywhere on the perimeter of a cone 

 which has its apex in group 2 and whose apical angle is the supplement 

 of the valency angle. Among these possibilities there is one special 

 case in which groups 4 and 5 are turned through 1 80°, thus resulting 

 in a ring-shaped model. It is not difficult to see that this can easily lead 

 to cyclic compounds. Fig. 34c shows why rings of 5 or 6 atoms are 

 formed preferentially: the supplement (70°. 5) of the valency angle is 

 contained somewhat less than 6 and somewhat more than 5 times in 

 360° (5 -70°. 5 = 3 5 2°. 5; 6 -70°. 5 = 423°). The different forms which a 

 molecule can assume are called its constellations; so Figs. 34b and c 

 represent two different constellations of the same molecule hexane. 



Other atoms besides carbon can also occur in the ring (heterocyclic 

 rings). Let us here briefly discuss the example of sugar, which is so 

 important in biology. The monosaccharides, which formerly were 

 considered as "open" chains (Fig. 35a), have been shown to contain 

 a heterocyclic ring with an oxygen bridge. In glucose this is usually 

 a T-5 bond, often represented in the manner of Fig. 35 b. The formula, 



