Covalence, normal

This class is composed of simple inorganic compounds in which all available valencies are bound to azide groups hence, the ionicity of the element-to-azide bonds alone determines chemical reactivity. Accordingly, normal azides of the more electropositive metals are saltlike and may be made in aqueous solution and the rest in nonaqueous media some of the covalent normal azides require handling in closed systems. The explosive character may in this class likewise be derived from formal ionicities in fact, the normal azides are the only class to which the traditional concept of stable ionic and unstable covalent azides still applies. 

Both these molecules exist in the gaseous state and both are trigonal planar as indicated by reference to Table 2.8. However, in each, a further covalent bond can be formed, in which both electrons of the shared pair are provided by one atom, not one from each as in normal covalent bonding. For example, monomeric aluminium chloride and ammonia form a stable compound ... 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

Solid covalent dinitrogen pentoxide can be prepared by freezing the vapour with liquid helium. Normally, solid dinitrogen pentoxide exists as (NO2+) (NOj ), showing absorption bands in its Raman spectrum only at 1050 and 1400 cm the structure of this form has been determined by X-ray crystallography. ... 

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

Protonation of pyrido[2,3-f ]pyrazine occurs normally without covalent hydration, although the 2-hydroxy derivative did show such behaviour (63JCS5737). The pyrido[3,4-f)]pyrazine parent base does show the phenomenon, although the exact structure of the covalent hydrate seemed to be in doubt between protonated (392) and (397). The issue was resolved in favour of the former by NMR (79JHC301, 75AG356). The 3-hydroxy derivative also shows hydration effects, as does the 7-amino cation (63JCS5166). 

Considering the four potential monohydroxypteridines, pteridin-4- and -7-one 56JCS3443) behave normally whereas pteridin-2- and -6-one (25) form covalent hydrates. The reversible hydration of nitrogen heterocycles was actually discovered with pteridin-6-one (52JCS1620),... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Certain proteins are found to be covalently linked to lipid molecules. For many of these proteins, covalent attachment of lipid is required for association with a membrane. The lipid moieties can insert into the membrane bilayer, effectively anchoring their linked proteins to the membrane. Some proteins with covalently linked lipid normally behave as soluble proteins others are integral... 

---