Cyclohexadienyl cation, intermediate electrophilic aromatic substitution

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

Cyclohexadienyl cation (Section 12 2) The key intermediate in electrophilic aromatic substitution reactions It is repre sented by the general structure... 

How substituents control rate and regioselectivity in electrophilic aromatic substitution results from their effect on carbocation stability. An electron-releasing substituent stabilizes the cyclohexadienyl cation intermediates corresponding to ortho and para attack more than meta. 

Evaluation of pATR from measurements of rate and equilibrium constants for the protonation of carbon-carbon double bonds of alkenes suggests the possibility of a similar approach for aromatic double bonds. Protonated aromatic molecules are the parent structures of the arenonium ion intermediates of electrophilic aromatic substitution. For these cations the equilibrium constant Kk refers to equilibria with the corresponding aromatic hydrates, as is illustrated in Scheme 5 for the benzenonium ion (cyclohexadienyl cation) 9 for which the hydrate is cyclohexadienol 10. 

Cycloalkadienyl cations, particularly cyclohexadienyl cations (benzenium ions), the intermediate of electrophilic aromatic substitution, frequently show remarkable stability. Protonated arenes can be readily obtained from aromatic hydrocarbons244 251 in superacids and studied by 1H and 13C NMR spectroscopy.252,253 Olah et al.252 have even prepared and studied the parent benzenium ion (C6H7+) 88. Representative 1H NMR spectra of benzenium253 and naphthalenium ions25488 and 89 are shown in Figures 3.11 and 3.12, respectively. 

By stabilizing the cyclohexadienyl cation intermediate, lone-pair donation from fluorine counteracts the inductive effect to the extent that the rate of electrophilic aromatic substitution in fluorobenzene is, in most cases, only slightly less than that of benzene. With the other halogens, lone-pair donation is sufficient to make them ortho, para directors, but is less than that of fluorine. 

Increasing delocalization, as in the cyclohexadienyl cation, is further stabilizing. This structure is simply protonated benzene, and it serves as a model for the intermediate in electrophilic aromatic substitution (see Section 10.18). Similarly, benzyl ion is quite stable for a formally 1° ion. Aromaticity effects are clear, as in the much greater stability of the six it electron tropylium ion vs. the four tt electron cyclopentadienyl ion (see Section 2.4.1 for a discussion of aromaticity). 

A Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction that attaches a carbon-carbon bond to the ring.The electrophile is R, which will add to the aromatic ring to produce a cyclohexadienyl cation. Aromaticity is regained when that intermediate cyclohexadienyl cation is deprotonated. That s all there is to it—all the rest is details. Remember to watch out for rearrangements, because this Friedel-Crafts alkylation is especially prone to them. In the Friedel-Crafts acylation, an acid chloride is used to generate the acylium ion which is the reactive electrophile. No rearrangements are observed in Friedel-Crafts acylation. 

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