INDEX nucleophiles, reactions with

Orientation in electrophilic and nucleophilic reactions of aromatic compounds can be predicted with the aid of the reactivity index of MO theory. Electrophiles will attack positions of higher electron densities, larger superdelocalizability (electrophile), and the lower localization energy (electrophile). On the other hand, nucleophilic attack is preferred at positions of lower electron densities, larger superdelocalizability (nucleophile), and lower localization energy (nucleophile). Table XXIII shows reactivity indexes of some aromatic nitrogen cations. 

TABLE 8.6 Relative Rates log(fc mcoh) Nucleophilic Reactions (8.22) and (8.23) at 25°C [71] Compared with the Normalized Polarity index Ej... 

On the basis of this equation, an index of nucleophilicity pt can be assigned to each nucleophile Y (see Table 4.13). It is found, moreover, that a plot against pt of logfcy, for reaction of Y with another Pt(II) neutral substrate, is also often linear. Thus, Eq. (2.168) applies, and 5 is termed the nucleophilic discrimination factor (Sec. 4.7.1). Some of the departures from linearity of plots of Ary vs p, which have been observed, disappear if the Pt reference substrate chosen is of the same charge as the Pt reactants. The value of p, for a bulky nucleophile has also to be modified to allow for steric hindrance features. 

The relationships of oxidation potential to radical reactivity index Sr and nucleophilic reactivity index Sn illustrated in Figure 4 are very similar to those with antioxidation and antiozonization, where the maximum values were observed at 0.4 and 0.25 volt. Therefore, antioxidation seems to proceed by a radical mechanism in contrast to the nucleophilic type of antiozonization. Indeed, the antioxidation effect of amines toward NR, SBR, BR, and HR is well correlated with radical reactivity as shown in Figures 5-8. The protection of SBR solution by amines from oxidative degradation and the termination of chain reaction in the oxygen-Tetralin system are also shown as functions of Sr in Figures 9 and 10. 

Apart from these indexes, the present volume contains four chapters spanning a wide range of heterocyclic chemistry. 1,5-Diazocines by Perlmutter, continues his coverage of important eight-membered heterocycles (cf. Azocines in Volume 31, and 1,4-Diazocines in Volume 45). Charushin, Alexeev, and Chupahkin from the Soviet Union, and Van der Plas from Holland cover reactions of 1,2,4-triazines with nucleophiles, a subject to which they bring much expertise. 

The product matrix (Table 8.3) serves as a summary of most of Chapter 8. It is the source and sink matrix that acted as an index to Sections 8.2 through 8.8 filled in with the most common reaction product in generic form. It cannot be stressed too much that everything depends on your ability to recognize the generic class of the sources and sinks that are present in the reaction mixture. However, sometimes a species has a dual reactivity and therefore may fit into more than one generic class. A common example of this is that most anions can behave as a nucleophile or as a base. Chapter 9 discusses the common major decisions. 

Using their CNDO results Helland and Skancke calculated indices of reactivity (frontier electron density, FED, for electrophilic substitution frontier orbital density, FOD, for nucleophilic substitution frontier radical density, FRD, for radical substitution) for the thienopyridines. It was indicated that the FED index has its highest value for C-3 in the [2,3-]- and 3,2- -fused systems and for C-2 in the [3,4-1-fused isomers. As far as the former group is concerned, the predictions are in agreement with experimental observations (see Section IV,A.). Little experimental evidence is available for the [3,4-]-fused systems, but it seems highly probable that they would have a considerable tendency to undergo addition reactions at the 1,3-positions, since the product would contain a normal rather than a quinoid pyridine ring [Eq. (23)]. 

B3LYP/6-31 lG(d) calculations have used frontier molecular orbitals, chemical potential, and Pearson s electrophilicity index co to study the reactions between allylic and aliphatic alcohols and ethylacetoacetate. All three methods predict the correct product substitution by the R group of the alcohol at the methylene carbon when the alcohol is electrophilic, and transesteriflcation of the ethylacetoacetate when the alcohol is nucleophilic. The results agree with the existing experimental evidence. 

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