The Mechanism of Electrophilic Aromatic Substitution

Most of these reactions are carried out at temperatures between about 0°C and 50°C, but these conditions may have to be milder or more severe if other substituents are already present on the benzene ring. Also, the conditions can usually be adjusted to introduce more than one substituent, if desired. 

How do these reactions take place And why do we observe substitution instead of addition And what role does the catalyst play In the next sections, we will try to answer these questions. 

Much evidence indicates that all of the substitution reactions listed in the previous section involve initial attack on the benzene ring by an electrophile. Consider chlorination (eq. 4.6) as a specific example. The reaction of benzene with chlorine is exceedingly slow without a catalyst, but it occurs quite briskly with one. What does the catalyst do It acts as a Lewis acid and converts chlorine to a strong electrophile by forming a complex and polarizing the Cl—Cl bond. 

The reason why a strong electrophile is required will become apparent shortly. 

Electrostatic potential map of benzene illustrates the pi electron density (red-yellow) of the aromatic ring. 

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This led to the introduction of the concepts of inductive and resonance effects and to the establishment of the mechanism of electrophilic aromatic substitution. 

Section 12 2 The mechanism of electrophilic aromatic substitution involves two stages attack of the electrophile on the tt electrons of the ring (slow rate determining) followed by loss of a proton to restore the aromaticity of the ring... 

Diazo coupling follows the rules of orientation of substituents in aromatic systems in accordance with the mechanism of electrophilic aromatic substitution and the concept of resonance. 

The reason for this difference in selectivity of different electrophilic reagents between the 2- and 3-positions must be sought in the finer details of the mechanism of electrophilic aromatic substitution Melander and co-workers are studying this problem by means of isotope effects. 

Nixon model, have been abandoned in all modern views of aromatic structure. Thus, given our present views of the structure of benzene, and the mechanism of electrophilic aromatic substitution, the Mills-Nixon hypothesis has no meaning. Nonetheless, the legacy of this hypothesis doggedly persists in our research discussions and should be laid to rest. ... 

This chapter is concerned with reactions that introduce or replace substituent groups on aromatic rings. The most important group of reactions is electrophilic aromatic substitution. The mechanism of electrophile aromatic substitution has been studied in great detail, and much information is available about structure-reactivity relationships. There are also important reactions which occur by nucleophilic substitution, including reactions of diazonium ion intermediates and metal-catalyzed substitution. The mechanistic aspects of these reactions were discussed in Chapter 10 of Part A. In this chapter, the synthetic aspects of aromatic substitution will be emphasized. 

Write the mechanism of electrophilic aromatic substitution reactions of pyridine. 

These two brominations are examples of the mechanism of electrophilic aromatic substitution, which, in many different guises, will return again and again during this chapter. In its most general form the mechanism has two stages attack by an electrophile to give an intermediate cation and loss of a proton from the cation to restore the aromaticity. 

What is a likely structure for the yellow compound The isolation of this and related compounds is considered to be strong support for the mechanism of electrophilic aromatic substitution. Why should this be so ... 

Keep in mind that the mechanisms of electrophilic aromatic substitution reactions are all very similar. After the formation of the electrophile, attack of the electrophile on the aromatic ring occurs to give a resonance-stabilized cation intermediate.The last step of the mechanism is proton transfer to a base to regenerate the aromatic ring. The base in nitration is water, which was generated in the formation of the electrophile. 

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