Radicals changed recommendations, 336

Table 83 presents the recommended AatH°(RX+, 0) values. Calculation of AatH°(RF" ", 0) (parenthesized values in the fifth column of Table 83) from these lE(RF) values gives the atomization enthalpies that exhibit a clearly pronoimced double periodicity. The introduction of such corrections seems therefore to be justified. Indeed, there are no convincing arguments to believe that the substitution of one ligand (Cl ) for another similar one (F ) leads to a radical change in the bond energy for some 4f elements. 

Nevertheless it must be emphasized that, for any system for which the radical concentration changes rapidly from point to point in the reaction vessel, the use of comparative rates when one of them depends on a different power of the radical concentration from the other is not recommended. Thus mercury sensitization is not a particularly useful method for determining such rates and this is even more true for the 1849 A line than it is for the 2537 A line. 

The visible n n absorption band is shifted hypsochromically by A1 = 53 nm on changing the medium from n-hexane to water. Based on the negative solvatochrom-ism of this aminyloxide radical, a spectroscopic solvent polarity scale, called the scale of Eewis acidity has been proposed [336]. Because of its pronounced negative sol-vatochromism, 4-nitropyridine-l-oxide has been recommended as an empirical indicator of the HBD acidity of solvents [330] cf. Section 7.4. The solvatochromic range of this A-oxide, measured in 48 different solvents, amounts to Av = +2840 cm (A1 = —31 nm) for the solvent change n-hexane water. 

Official lUPAC names of these and other acyl groups have been designated by the Cormnission of the Nomenclature of Organic Chemistry in Pure and Applied Chemistry 10, 111 -125 (1965). In a number of cases lUPAC inserted an o in the traditional name, e.g., pahnityl became pafmitoyl and crotonyl became crotonoyl. However, acetyl was not changed. In many cases the systematic names, e.g., hexadecanoyl (from hexadecanoic acid), are preferable and lUPAC-IUB recommends that alkyl radicals always be designated by systematic names, e.g., hexadecyl, not palmityl alcohol. The older use of pahnityl for both acyl and alkyl radicals was one reason for lUPAC s adoption of new names for acyl radicals. Systematic names are not often used because of their complexity, e.g., hnolenic acid is cis,cis,cis-9,12,15-octadecatrienoic acid. 

This chapter describes in general terms the types of reactivity found in the typical six- and five-membered aromatic heterocycles. Considerable space is devoted to radical substitution, metallation and palladium-catalysed reactions, since it is in these areas that there has been most progress and change since the earlier editions of this book, and we feel that for a proper appreciation of their importance in the heterocyclic context, an introduction to these topics is required, their being only poorly covered in general text-books. Emphasis on the typical chemistry of individual heterocyclic systems is to be found in the summary/revision chapters (4, 7, 10, 12, 16 and 20) and a more detailed examination of typical heterocyclic reactivity, and many more examples for particular heterocyclic systems are to be found in the chapters - Pyridines reactions and synthesis etc. For the advanced student, it is recommended that this present chapter should be read in its entirety before moving on to the later chapters, and that the introductory summary/ revision chapters, like Typical reactivity of pyridines, quinolines and isoquinolines should be read before the more detailed discussions. 

The legislative changes proposed by the Robens Committee were as radical as the organizational changes it had suggested. The Committee recommended... 

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