Potential energy electrophilic aromatic

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Figure 12.1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. For electrophilic aromatic substitution reactions to... 

FIGURE 12.1 Potential energy diagram for electrophilic aromatic substitution. 

Several descriptions of the process of addition of the electrophile X" " to aromatic substrates, based on kinetic and other evidence have been given and most versions agree that the potential energy surface does not consist of a simple barrier, but involves details relating to metastable intermediates. 

Two of three nitrofluorobenzene isomers react with methoxide, but the third is unreactive. Obtain energies of methoxide anion (at left), ortho, meta and para-nitrofluorobenzene, and the corresponding ortho, meta and para-methoxide anion adducts (so-called Meisenheimer complexes). Calculate the energy of methoxide addition to each of the three substrates. Which substrate is probably unreactive What is the apparent directing effect of a nitro group Does a nitro group have the same effect on nucleophilic aromatic substitution that it has on electrophilic aromatic substitution (see Chapter 13, Problem 4) Examine the structures and electrostatic potential maps of the Meisenheimer complexes. Use resonance arguments to rationalize what you observe. 

By use of especially selected aromatic substrates—highly hindered ones—isotope effects can be detected in other kinds of electrophilic aromatic substitution, even in nitration. In certain reactions the size of the isotope can be deliberately varied by changes in experimental conditions- and in a way that shows dependence on the relative rates of (2) and the reverse of (I). There can be little doubt that all these reactions follow the same two-step mechanism, but with differences in the shape of potential energy curves. In isotope effects the chemist has an exceedingly delicate probe for the examination of organic reaction mechanisms. 

The potential to describe the chemical reaction path is emerging from the DF theory. Pearson [69] and Parr et al. [70] have proposed a principle of maximum hardness stable molecules arrange themselves as to be as hard as possible. Zhou and Parr introduced the activation hardness parameter for the electrophilic aromatic substitution [71], The same authors have shown a correlation between the absolute hardness of a molecule and aromacity [72]. Nalewajski et al. studied the protonation reaction and described the relation between the interaction energy and charge sensitivities hardness, softness, Fukui function [28, 38]. 

This polarization, in turn, causes the ring carbons to bind the it electrons more tightly, decreases their availability to an approaching electrophile, raises the activation energy for electrophilic aromatic substitution, and decreases the reaction rate. Figure 12.6 illustrates this effect by comparing the electrostatic potential maps of fluorobenzene and benzene. 

Xu et al. used the conical intersection concept applied earlier in computations on aromatic nitration. They maintain that the initial interaction of benzene with a nitronium cation could either involve an initial single-electron transfer or a polar conventional two-electron-transfer electrophilic mechanism, involving a vr-complex, since both intermediates are minima on the potential energy surface. The rate-determining step could be either the formation of the first complex or the ET, depending on the system and experimental conditions. They believe that the latter... 

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