General Chapters on Reactivity

The steric and electronic effects of substituents on the electrophilic attack at the nitrogen atom have been discussed in the general chapter on reactivity (Section 4.02.1.3). All the conclusions are valid for pyrazoles and indazoles. The effect on equilibrium constants will be discussed in detail in the sections dealing with values (Sections 4.04.2.1.3(iv) and (v)) and the kinetic effects on the rates of quaternization in the corresponding section (4.04.2.1.3(vii)). 

Three general chapters on structure, reactivity, and synthesis precede the monograph chapters in each of Parts 2 to 5 of CHEC. The purpose is to introduce each family of ring systems and to emphasize the logical correlations within them, and so to help in the understanding of known reactions and in the prediction of new ones. These 12 general chapters thus provide an overview of the whole subject of heterocyclic chemistry and they should be of particular interest to students and teachers. These general chapters also appeared in the 1985 version of the Handbook of Heterocyclic Chemistry and it is planned that they will appear in a new revised version of the Handbook. 

In Chapter 4, we will discuss the relative importance of inductive effects and field effects on reactivity. Generally, field effects appear to be the dominant mechanism for the transmission of electrostatic effects of polar bonds to other parts of a molecule. 

Examples of effects of reactant stmcture on the rate of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. The general trends of reactivity of primaiy, secondary, and tertiaiy systems and the special reactivity of allylic and benzylic systems have been discussed in other contexts. This section will emphasize the role that steric effects can pl in nucleophilic substitution reactions. 

Throughout the remainder of the chapter it should be understood that any rate constants that are presented are apparent rate constants unless otherwise indicated to be those for the free ion or ion pair. In general, the available data are used to point out certain trends (e.g., the effect of solvent on reactivity) without necessarily accepting the exact value of any reported rate constant as that for the ion pair or free ion. Comparison of data from different investigators should be done with caution. 

Outside this chapter on general aspects of structure, bonding, syntheses, and reactivities the material will be presented according to the position of the metals in the periodic table. For heteronuclear clusters the compounds will be treated as belonging to the metal later in the periodic table. Compounds mentioned in this review without giving a reference have been dealt with in former reviews. 

Since 02 behaves as an electrophile, one can assume that electron-donating substituents on an organic compound will, in general, increase its reactivity, while electron-withdrawing substituents will have the opposite effect. In the case of phenolic compounds, the effect(s) of the substituent(s) on the pKa (see Chapter 8), and thus on the concentration of the reactive phenolate species present at a given pH, may be more important than the effect(s) of the substituent(s) on kp, 0r In this case, the overall transformation rate is dominated by the rate of transformation of the anionic species (A-) ... 

However, note that the values of pAR in Scheme 29 were measured in 50-50 v v TFE-water mixtures rather than water. In general, for anionic nucleophiles pAR is expected to be highly sensitive to solvent. Results of Pham and McClelland222 indicate that pAR pA 1 increases by 8 log units between water and 2% aqueous acetonitrile. The effect of a change from water to TFE-water will be much less than this, but a comparison for the /)-mcthoxybcn/yi cation shows that pAR decreases by 1 log unit.223 Thus neglecting any difference between pAR values in the two solvents the estimate of pAR for the a-trifluoromethyl-substituted /7-methoxybenzyl cation is increased to —22.5. This value has been considered at some length because equilibrium measurements for the ions summarized as 58 are relevant to the effects of a-trifluoromethyl substituents on reactivity discussed later in the chapter (p. 80). 

The first three chapters constitute a review of bonding and an introduction to organic compounds. Functional groups are introduced. Resonance is covered extensively, and numerous examples are provided. Acid-base chemistry is discussed in Chapter 4, and this reaction is used to introduce many of the general features of reactions, including the effect of structure on reactivity. Nomenclature of all of the functional groups is covered in Chapters 5 and 12. In this edition, stereochemistry is covered in two chapters to break up the material Chapter 6 discusses cis trans isomers and conformations, and Chapter 7 addresses chiral molecules. 

This chapter deals with chemical and physical properties other than ones for which the nature of the hydration products must be considered, which are treated in Chapters 5 to 8. In general, properties of the whole clinker or cement are alone considered, those of the constituent phases having been dealt with in Chapter 1, but factors affecting the reactivities of these phases are included as a link with the following chapters on hydration. 

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