Electrophilic aromatic substitution sigma complex

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

Electrophilic Aromatic Substitutions via Sigma Complexes ( Ar-SE Reactions )... 

The electrophilic aromatic substitution via sigma (Wheland) complexes, or the Ar-SE reaction, is the classical method for functionalizing aromatic compounds. In this section, we will focus on the mechanistic foundations as well as the preparative possibilities of this process. 

Electrophilic aromatic substitution by the nitronium ion gives nitrobenzene. Step 1 Attack on the electrophile forms the sigma complex. 

This alkylation is a typical electrophilic aromatic substitution, with the ferf-butyl cation acting as the electrophile. The ferf-butyl cation is formed by reaction of ferf-butyl chloride with the catalyst, aluminum chloride. The ferf-butyl cation reacts with benzene to form a sigma complex. Loss of a proton gives the product, fm-butylbenzenc. The aluminum chloride catalyst is regenerated in the final step. 

Phenoxide ions, easily generated by treating a phenol with sodium hydroxide, are even more reactive than phenols toward electrophilic aromatic substitution. Because they are negatively charged, phenoxide ions react with positively charged electrophiles to give neutral sigma complexes whose structures resemble quinones. 

An intermediate in electrophilic aromatic substitution or nucleophilic aromatic substitution with a sigma bond between the electrophile or nucleophile and the former aromatic ring. The sigma complex bears a delocalized positive charge in electrophilic aromatic substitution and a delocalized negative charge in nucleophilic aromatic substitution, (p. 756)... 

In a proton transfer to an aromatic carbon atom, a so-called sigma complex is formed in which the configuration of the valence electrons of the carbon has been changed from sp2 to sp3. In the next step, the other electrophilic atom or group bonded to the same carbon may be split off. This leads to an electrophilic aromatic substitution. Examples are aromatic hydrogen isotope exchange, aromatic decarboxylation, deboro-nation, and deiodination (see Sect. 9 Chap. 2, and Vol. 13, Chap. 1). 

Usually the attacking electrophile is more reactive than a proton, and therefore the rate-determining step is electrophile attack. Proton loss from the sigma-complex will be an easier process than loss of a reactive electrophile. Figure 4.51 shows a typical energy diagram for an electrophilic aromatic substitution. 

Media pH errors and media pH span errors are common. Since electrophilic aromatic substitution is almost exclusively an acidic media process, do not make any strong bases during the mechanism. The proton on the aromatic ring becomes very acidic after the electrophile attaches and forms the carbocationic sigma-complex. However, before the electrophile attacks, the aromatic H is not acidic at all, p Ta = 43, so do not get your steps out of order and try to pull the H off first. 

Naphthalene undergoes electrophilic aromatic substitution at the position next to the second ring. The sigma complex leading to the obtained product has more resonance structures, indicating a more delocalized charge than that of the alternative. 

When benzene is treated with I2 in the presence of CUCI2, iodination of the ring is achieved with modest yieids. it is beiieved that CuCi2 interacts with i2 to generate i+, which is an excellent electrophile. The aromatic ring then reacts with 1+ in an electrophilic aromatic substitution reaction. Draw the mechanism of the reaction between benzene and 1. Make sure that your mechanism has two steps, and make sure to draw all of the resonance structures of the sigma complex. 

The intermediate Electrophilic aromatic substitution proceeds via a sigma complex, nucleophihc aromatic substitution proceeds via a Meisenheimer complex, and elimination-addition proceeds via a benzyne intermediate. 

This intermediate should remind us of the intermediate in an electrophilic aromatic substitution reaction (the sigma complex), but the main difference is that a Meisenheimer complex is negatively charged (a sigma complex is positively charged). Let s take a close look at the Meisenheimer complex, and let s focus our attention on one particnlar resonance structure ... 

The formation of the sigma complex in electrophilic aromatic substitution has a higher activation energy than the formation of a carbocation in electrophilic addition to an alkene (Figure 13.1). Therefore, the rates of electrophilic aromatic substitution reactions are slower than the rates of electrophilic addition reactions to aUtenes for the same electrophile. For example, bromine reacts instantly with alkenes, but does not react at all with benzene except in the presence of a strong Lewis acid catalyst. 

Then, one of the rings attacks the complex to give a resonance stabilized intermediate (a sigma complex), which then loses a proton to restore aromaticity (electrophilic aromatic substitution) ... 

In acidic conditions, 2-methylpropene is protonated to give a tertiary carbocation, which can function as an electrophile in an electrophilic aromatic substitution reaction. The aromatic ring attacks the carbocation to give a resonance stabilized intermediate (sigma complex), followed by deprotonation which restores aromaticity. In aqueous acidic conditions, the proton source is H3O (in the beginning of the mechanism), and the base is H2O (at the end of the mechanism). 

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