Standard Data for Nitration and Hydrogen Exchange

Comparison of Standard Data for Nitration and Hydrogen Exchange 

This chapter is concerned with the electrophilic substitution of pyridaz-ine, pyrimidine, pyrazine, and their derivatives. Aspects of this topic have been reviewed previously [72AHC( 14)99 74AHC(16)1] and the general chemistry of the monocyclic azines has been surveyed in Comprehensive Organic Chemistry, Vol. 4 (1979) and Comprehensive Heterocyclic Chemistry, Vol. 3 (1984). 

The fact that none of these reports has emphasized the physical aspects of electrophilic substitution in the series reflects the paucity of quantitative studies, and the low reactivity of these compounds in the presence of electrophiles. Few kinetic studies have been reported and the regio-chemical effects of substituents have seldom been quoted in quantitative form. The present chapter brings together those quantitative results that are available, and collates data on substituent effects. One worthwhile field of study would appear to be the application to the azines of Taylor s method involving thermolysis of esters [75JCS(P2)277, 75JCS(P2) 1783]. 

The resistance of pyridine toward electrophilic substitution is well known, and this reluctance to react is enhanced in the protonated species. Incorporation of extra annular nitrogen atoms further decreases this reactivity with electrophiles each aza substituent has an electron-withdrawing effect similar to a nitro group. One estimate of the o value for an aza substituent (=N—) is 0.65, and 1.95 for the protonated species (=N + H—) [68JCS(B)1484] (see also Chapter 9, Sections 9.A-9.C). Val- 

---