Grafts et al. — 22 — Water in Plants 



affinity drew a sharp distinction between primary and secondary or residual 

 affinities. They failed to realize that chemical affinity has dimensions of 

 both quantity and intensity. Organic chemists recognized coordination 

 groups or spheres within which valence forces became identical and they 

 recognized conjugation as a means of satisfying unsaturated valence force. 

 It was not however until the discovery of the electron and the demonstra- 

 tion of its relation to valence that a clear picture of chemical affinity became 

 available. In 1916 Lewis presented his electronic theory of valence postu- 

 lating complete transfer of electrons to explain the formation of ionic, or 

 valence bonds, and mutual sharing of electrons to explain covalence. 



Examples of these types of bonding are given in the following re- 

 actions. 



Ionic: :Na: • + -Ci: ^ :Na: :Ci: (7) 



H 

 Covalent : 4 U ■ + ■ C ■ ^ U -.C-.U (8) 



H 



Covalences may be of two types, normal and coordinated. In the nor- 

 mal type (equation 8), both of the reacting atoms contribute an electron 

 to complete the octet and thus form a stable bond ; in the coordinate type 

 both of the electrons are supplied by one of the atoms, which results in a 

 somewhat weaker bond. These two types of covalent linkage may be gen- 

 eralized in the following way : 



Normal covalent: A- +B_»A:B (9) 



Coordinate covalent : A : -|- B — > A : B (10) 



The hydrogen bond, a somewhat different type of linkage, which is 

 partly ionic and partly covalent in character, is of great significance in 

 biology. This is found only between atoms of high electronegativity {i.e., 

 F, O, N, and CI) and the strength of the bond is determined by the degree 

 of electronegativity and the ionic radii of the groups joined. Hydrogen 

 bonds are involved in many biological systems. As explained they are re- 

 sponsible for coordination of water in the liquid and solid states. They 

 are responsibe for formation of chelate rings as in the following dimer of 

 formic acid vapor. 



H (11) 



They account for the hydration of colloids such as cellulose and pro- 

 teins. And they form the bridges responsible for the 3-dimensional archi- 

 tecture of many bio-colloids, possibly of the living protoplasm. For a de- 

 tailed treatment of chemical affinity see Remick (1943). 



In aqueous solutions in which the biologist is interested practically 

 every form of bond known to the chemist may be involved. The nature of 

 the forces of coordination between water molecules has been pointed out 

 in Chapter II. The values of 1.04 at 100° C. and 0.60 at 150° C. (page 16) 

 given by Ewell and Eyring indicate the coordination of water vapor. 

 At 0° C. the number is variously estimated from 2.68 (page 16) to 4. 



Water molecules are pictured as small spheres having two positive and 

 two negative residual valence charges. The separation of these charges 

 gives the molecule dipole character and water has a dipole moment of 1.85. 



