Page 793 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 9 E + +. E . +
Aromatic Substitution
H E
π complex
σ complex

This mechanism implies that a considerable change in the structure of the electrophile
occurs prior to -bond formation. These structural changes could account in large
5
part for the energy barrier to formation of the complex. Moreover, this mechanism
implies that the cation radical–radical pair might play a key role in determining the
isomeric (ortho, meta, para) product composition. These issues have been investigated
most closely for nitration and bromination and are consider further when those reactions
are discussed.
Formation of the complex can be reversible. The partitioning of the complex
forward to product or back to reactants depends on the ease with which the electrophile
can be eliminated, relative to a proton. For most electrophiles, it is easier to eliminate
the proton, in which case the formation of the complex is essentially irreversible. The
electrophiles in group A of Scheme 9.1 are the least likely to be reversible, whereas
those in group C are most likely to undergo reversible -complex formation. Formation
of the complex is usually, but not always, the rate-determining step in EAS. There
may also be a complex involving the aromatic ring and the departing electrophile.
This would be expected on the basis of the principle of microscopic reversibility, but
there is little direct evidence on this point. 6
Let us now consider some of the evidence for this general mechanism. Such
evidence has, of course, been gathered by study of specific reaction mechanisms. Only
some of the most clear-cut cases are cited here. Additional evidence is mentioned when
individual mechanisms are discussed in Section 9.4. A good example of studies that
have focused on the identity and mode of generation of the electrophile is aromatic
nitration. Primarily on the basis of kinetic studies, it has been shown that the active
+
electrophile in nitration is often the nitronium ion, NO , which is formed by the
2
reaction of nitric acid with concentrated sulfuric. Several other lines of evidence
support the role of the nitronium ion. It can be detected spectroscopically and the
freezing-point depression of the solution is consistent with the following equation:
2H SO + HNO 3 NO + H 3O + 2HSO 4 –
+
+
2
4
2
Solid salts in which the nitronium ion is the cation can be prepared with unreactive
+
−
+
anion such as for NO BF 4 − and NO PF . These salts act as nitrating reagents.
6
2
2
Two types of rate expressions have been found to describe the kinetics of
many aromatic nitration reactions. With relatively unreactive substrates, second-order
kinetics, first order in the nitrating reagent and first order in the aromatic, are observed.
This second-order relationship corresponds to rate-limiting attack of the electrophile
on the aromatic reactant. With more reactive aromatics, this step can be faster than
formation of the active electrophile. In these cases, the generation of the electrophile
5 S. V. Rosokha and J. K. Kochi, J. Org. Chem., 67, 1727 (2002).
6
For additional discussion of the role of and complexes in aromatic substitution, see G. A. Olah,
Acc. Chem. Res., 4, 240 (1971); J. H. Ridd, Acc. Chem. Res.,4, 248 (1971).