2,6 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



can be cleft along the base (Fig. 32a). When, however, the chains 

 grow to great length, it becomes increasingly difficult to arrange the 

 terminal groups in fixed planes, and crystallization takes place as 

 pictured in Fig. 32b. Here no rigorous lattice order prevails, since a 

 lengthways displacement of one chain with respect to another, over 

 distances equal to some intramolecular spacings, i.e., only a fraction 

 of the chain length, does not affect the lattice structure. This is because, 

 owing to their multiplicity, the smaller spacings inside the molecule 

 (2.54 A in the case of paraffins) overshadow the periodicity of the end 

 groups. These arrangements of long chains are called c/yam lattice. It 

 is significant that the chains cannot revolve around their longitudinal 

 axis ; if they could, there would be no lattice order. The cross-section 

 of the chain lattice is, therefore, homogeneous, but inhomogeneities, 

 which are indicated in Fig. 32b by the end groups, occur lengthwise, 

 leaving only small homogeneous lattice regions. 



CH, 



.CH. 



I \ 



3CH2 



CH2 



CH2 H2C4 



H2C4 



H2/ 



CH2 



CHj 

 o) 



\ 



5CH2 



V 



H2 



.H, 



'CHj 



H3C6 



b) 



Fig- 33 



Fig- 34 



Fig. 33. Graphite lattice - Fig. 34. Hexane. a) Conventional structural formula; 

 b) morphologically correct formula; c) ring constellation, supplement to the 



valency angle v = 70°. 5. 



Aromatic compounds {layer lattice). Unlike the aliphatic compounds, the 

 aromatic ones cannot be derived from the structure of diamond. Their 

 structure is similar to that oi graphite. This modification of carbon crystallizes 

 in the hexagonal system and possesses a crystal lattice as represented in 

 Fig. 33. The carbon atoms form rings containing 6 atoms, which are linked 

 together in an uninterrupted plane. Thus at each lattice point 3 primary 

 valencies are engaged. The fourth valency is distributed among the neigh- 

 bouring atoms as in the benzene ring (Fig. 24, p. 25). Accordingly, as a 

 result of the larger bond energy, the C — C-distance is reduced to 1.45 A 

 (see Fig. 33). As all primary valencies are thus engaged in a plane, the 



