Substituted systems electronic structure

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

Authors [143] suppose that sulfur atoms are not affected by oxidation and only act as activating double bond substituents. However, the analysis of the electronic structure of tetrathiofulvalene shows that, resulting from the interaction of tt CC) and 7t(S) orbitals, HOMO of this molecule is an antibonding combination of jr and riji orbitals with the predominant contribution being from the latter. Indeed, the substitution of one or more S atoms with Se results in a less conjugated system and, as a consequence, in higher oxidation potentials [144]. 

An unique result of the electronic structure of the nitrosamine function is that it can apparently stabilize a positive or negative charge at the carbon adjacent to the amino nitrogen in a manner similar to the benzylic system. Hie stereoelectronic control of electroj ilic substitutions at the Or-position, to be discussed later, requires an orbital interaction of the nitrosamine function with the anionic electrons. Hie reactiv-... 

At the same time, Snell and coworkers used model systems to achieve most of the reactions of the pyridoxal enzymes (Metzler and Snell, 1952a,b Olivard et al., 1952 Ikawa and Snell, 1954a,b Metzler et al 1954a,b Longnecker and Snell, 1957). They too developed the modern mechanisms for the series of reactions and demonstrated the role of the coenzyme as an electron sink by substituting alternative catalysts for pyridoxal phosphate. In particular, they showed that 2-hydroxy-4-nitrobenzaldehyde (Ikawa and Snell, 1954) functioned in their model systems just as did the vitamin its electronic structure is really quite similar (3). 

Thus, the displacement of a K-band to longer wave-lengths by structural modifications of the terminal groups (VII, A—C or C—B) can be due to two factors. (1) An electron-shift in the ground state of the substituted system which will increase its polarizability and facilitate the charge migration in this direction. (2) An increase of the electron polarizability of the terminal group itself. 

The substitution system of nomenclature should be viewed as showing only how atoms are connected and not as indicating the precise electronic structure. Thus -adsorbed ethylene is one representation of 1,2-diadsorbed ethane. 

Alkali and alkaline earth metals dissolve in liquid ammonia with the formation of solvated electrons. These solvated electrons constitute a very powerful reducing agent and permit reduction of numerous conjugated multiple-bond systems. The technique, named for Birch provides selective access to 1,4-cydohcxiidicnes from substituted aromatics.8 In the case of structures like 21 that are substituted with electron-donating groups, electron transfer produces a radical anion (here 22) such that subsequent protonation occurs se lectively in the ortho position (cf intermediate 23) A second electron-transfer step followed by another protonation leads to com pound 24... 

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