Generalized mechanism for electrophilic aromatic substitution

The evidence for a o--complex intermediate is based on several lines of investigation. One particularly informative approach involves measurement of 

In general terms, the observation of an isotope effect will depend on the relative rate constants ki, k i, and 2 When k2 ki, k-i, no isotope effect will be 

The case for the generality of the cr-complex mechanism is further strengthened by numerous studies showing that such compounds can exist as stable entities under suitable conditions. Salts of substituted benzenium ions (an alternative name for the 

O -complex) can be isolated as crystalline materials or observed by spectroscopic techniques (especially NMR) in nonnucleophilic solvents  

At low temperatures in nonnucleophilic solvents, aromatic compounds can be protonated to give stable ions that can be characterized on the basis of their proton and carbon NMR spectra as being substituted benzenium ions  

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Now that we ve outlined the general mechanism for electrophilic aromatic substitution we need only identify the specific electrophile m the nitration of benzene to have a fairly clear idea of how the reaction occurs... 

Scheme 10.2. Generalized Mechanism for Electrophilic Aromatic Substitution...

At this point, attention can be given to specific electrophilic substitution reactions. The kinds of data that have been especially useful for determining mechanistic details include linear ffee-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 ... 

The simplest and most general mechanism for electrophilic aromatic substitution in solution is the so-called arenium ion mechanism, depicted in Scheme 2 [54,254]. 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

Section 15.9 General Mechanism for Electrophilic Aromatic Substitution Reactions... 

Substitution reactions allow the aromatic sextet of electrons in benzene to be regenerated after attack by the electrophile. We can see how this happens if we examine a general mechanism for electrophilic aromatic substitution. 

This mechanism would attribute position selectivity to a different structural feature than does the general mechanism for electrophilic aromatic substitution. If the radical pair intermediate were involved, position selectivity would be determined by the collapse to the a complex. The product distribution should then be governed by the distribution of unpaired spin in the aromatic radical cation. This would be expected to exhibit the normal orthOy para preference but might differ quantitatively from the usual mechanism. Detailed consideration of the electron transfer step has,... 

We account for these patterns by means of the general mechanism for electrophilic aromatic substitution first presented in Section 9.5. Let us extend that mechanism to consider how a group already present on the ring might affect the relative stabilities of cation intermediates formed during a second substitution reaction. 

FIGURE 14.119 The general mechanism for electrophilic aromatic substitution. 

THE GENERAL MECHANISM FOR ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS... 

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