Acidic and Basic Radicals

The classification of chemical substances as electrophilic and electrodotic according to their behavior in their reactions with other substances can be extended to radicals in a molecule. When this is done, a flood of light is thrown upon the nature of acid-base catalysis. Ingold,Robinson, and others have already done much to clarify many organic reaction mechanisms, but the electronic theory of acids and bases provides a measure of correlation and insight which so far is unobtainable by any other method. Generalized acid-base catalysis will be considered later. In this chapter we shall deal primarily with the effect of acidic and basic radicals in the benzene ring. 

Various terms have been proposed to describe what we here call acidic and basic radicals. The principal ones are (1) electrophilic and cationoid for acid radicals and (2) nucleophilic and anionoid for basic radicals. The reasons for preferring electrophilic and electrodotic to describe general behavior have been given in the preceding chapter. In this chapter we are concerned with the specific acidic and basic nature of the radicals, so the terms acidic and hade will be used. 

The electronic theory of acids and bases is very helpful in explaining the orienting effects produced by many functional groups attached to benzenoid compounds. The acidic radicals (electro- 

The acidic characteristics of these radicals are often ascribed to a displacement of an electron pair ( ) which results in an electron deficit on one atom (Chapter 2). This atom thus becomes acidic (electrophihc). 

The sulfur atom in the sulfo group is also made electrophilic (to realize its covalence maximum, as discussed in Chapter 2), by the electron displacements (v ) shown. 

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An interesting topic is related to the data of trace element contents in human hair. The results of analysis of trace element contents in human hair are useful for the diagnosis of some diseases. These data can also provide valuable information about the influence of environment on human health. Like the trace element analysis of serum or urine, hair analysis can provide valuable information. Contrary to the results of the analysis of serum or urine, the results of hair analysis can reflect metabolic process in the body over a period of several months. Metal ions, moreover, are permanently built into the a-helical structure of hair keratin, thereby creating chelate links with the acid and basic radicals of amino acids, as a result of which the content of some metallic elements in hair is much higher than that in other tissues. Up to now, 78 elements have been detected in human hair. 

The action of the acid and basic radicals in directing substituents will be considered in Chapter 5. 

Ingold,Robinson, and Price have laid the foundation of, and also elaborated on, an electronic interpretation of ring substitutions. The orientating influence of acidic and basic radicals can be adequately explained if it is assumed that the substituent is an acid radical. ... 

Holleman has made a careful study of the role played by a large number of acidic and basic radicals in the nitration of benze-noid compounds. 

On adding the hydroxyl substituent to the CH2CH2 unit, the barrier for C-C scission is lowered because of more favorable thermodynamics (Albini and Spreti 1987, Barton et al. 1996). However, the hydroxyl substituent becomes effective only after its deprotonation. Cation-radicals of 2-, 3-, and 4-arylalkanols, all of them, undergo C(l)-H deprotonation at pH 4. At pH 10, they display a different behavior. The 2-(4-methoxybenzene)ethanol cation-radical experiences C(2)-C(l) scission, resulting in the formation of formaldehyde and 4-methoxybenzyl radical the 3-(4-methoxybenzene)propanol cation-radical gives rise to 3-(4-methoxybenzene) propanal the 2-(4-methoxybenzene)butanol cation-radical behaves as the C -H acid both in acidic and basic solntions (Baciocchi et al. 1996,1999a). 

Photolysis degraded photolytically on soil thin films, t,/2 = 13-57 d in artificial sunlight (Tomlin 1994). Oxidation photooxidation t,/2 = 4.2 h in air, based on an estimated rate constant for the vapor-phase reaction with photochemically produced hydroxyl radicals in the atmosphere (Atkinson 1985 quoted, Howard 1991). Hydrolysis neutral hydrolysis rate constant k < 1.5 x lO 5 h 1 with a calculated t,/2 > 700 d in neutral solution and with faster hydrolysis rates in acidic and basic solutions to be expected (Ellington et al. 1987, 1988 quoted, Howard 1991). 

B-2,6-Dimethylarylborazines have not been obtained in a direct borazine synthesis, though they can be prepared by Grignard arylation of l,3,5-trimethyl-2,4,6-trichloroborazine 01.92), These compounds show high hydrolytic stability, even in acid and basic medium, and they can be subjected to substitution reactions at the aromatic sites 91>. For example, free radical bromination 93> or Friedel-Crafts acetylation 94> has been accomplished. 

Several methods have been applied to determinations of dissociation constants of radicals. Most of them are based on measuring a parameter which at any pH is a weighted average of the parameters of the acid and basic forms present at that pH. A plot of the observed parameter versus pH gives the typical sigmoidal curve with an inflection point where pH = pK. Several parameters can be different for the acid and basic forms to allow p/ -measurements. The most commonly used method utilizes differences in the optical absorption spectra of these two forms of radicals. Monitoring by... 

This type of effect may be observed also when Rj and R2 are the acid and base forms of the same species. For example, Wigger et al. (1967) monitored the decay of C6H5NOH as a function of pH and found the highest rate at the pA -value of this radical. They concluded that the reaction of Rj + R2 is about an order of magnitude faster than either that of Rx + R, or R2 + R2, where Rx and R2 are the acid and basic forms of C6H5 NOH. 

The ESR technique can be applied to the determination of accurate pA a values if the acid and basic forms of the radical undergo rapid exchange so that the ESR parameters at any pH are the weighted average of those of the two forms. This method is not dependent on the overall yield of the radicals and is not sensitive to chemical complications as is the optical method. The ESR method has been applied to measure the pA a for protonation of phenoxyl radicals in strongly acidic solutions ° ". The main results of these measurements are summarized in Table 7. 

Fluorine owes its unparalleled reactivity, on the one hand, to the ease of its homolytic dissociation into radicals (only 37.8 kcal mol , compared with 58.2 kcal mor for CI2) and, on the other hand, to its very high redox potentials of +3.06 V and +2.87 V, respectively, in acidic and basic aqueous media [8]. 

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