Specific Substitution Mechanisms

At this point, attention can be given to specific electrophilic substitution reactions. The kinds of data that have been especially pertinent to elucidating mechanistic detail Include linear free-energy relationships, kinetic studies, isotope effects, and selectivity patterns. In general, the basic questions that need to be asked about each mechanism are (1) What is the active electrophile (2) Which step in the general mechanism for electrophilic aromatic substitution is rate-determining (3) What are the orientation and selectivity patterns  

A substantial body of data Including reaction kinetics, isotope effects, and structure-reactivity relationships has permitted a quite thorough understanding of the steps in aromatic nitratlon. As anticipated from the general mechanism for electrophilic substitution, there are three distinct steps  

Conditions under which each of the first two steps is rate-determining have been recognized. The third step is usually very fast. 

The existence of the nitronium ion in sulfuric-nitric acid mixtures can be demonstrated both by cryoscopic measurements and by spectroscopy. An increase in the strong acid concentration increases the rate of reaction by shifting the equilibrium of step 1 to the right. Addition of a nitrate salt has the opposite effect, by suppressing the preequilibrium dissociation of nitric acid. It is possible to prepare crystalline salts of nitronium ion such as nitronium tetrafluoroborate. Solutions of these salts in organic solvents rapidly nitrate aromatic compounds. The general features of the o--complex forniation step can be successfully modeled by ab initio MO calculations at the STO-3G level.  

With very few exceptions, the final step in the nitration mechanism, the deprotonation of the o- complex, is fast and therefore has no effect on the observed kinetics. The fast deprotonation can be confirmed by the absence of an isotope effect when deuterium or tritium is introduced at the substitution site. Several compounds such as benzene, toluene, bromobenzene, and fluorobenzene have been 

Hoggett, R. B. Moodie, J. R. Penton, and K. Schofield, Nitration and Aromatic Reactivity, Cambridge University Press, Cambridge, U.K., 1971 L. M. Stock, Prog. Phys. Org. Chem. 12 21 (1976) G. A. Olah, R. Malhotra, and S. C. Narang, Nitration, VCH Publishers, New York, 1989. 

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