Energy diagram for electrophilic aromatic substitution

FIGURE 12.1 Potential energy diagram for electrophilic aromatic substitution. 

Figure 2.3 shows a reaction-energy diagram for electrophilic aromatic substitution, occurring in four stages (see page 14). 

A reaction energy diagram for eiectrophiiic aromatic substitution. The eiectrophiie adds to benzene in an endothermic first step, with ioss of aromaticity. A proton at the carbon attacked by the electrophile is iost in an exothermic second step, with restoration of aromaticity. 

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... 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

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. 

This completes our preliminary survey of the most important reactions in aromatic electrophilic substitution. We shall switch our attention to the benzene ring itself now and see what effects various types of substituent have on these reactions. During this discussion we will return to each of the main reactions and discuss them in more detail. Meanwhile, we conclude this introduction with an energy profile diagram for a typical substitution. 

PROBLEM 4.11 Electrophilic aromatic substitutions to benzene and electrophilic additions to alkenes both involve a slow first step and a fast second step. Using Figure 4.5 as a guide, draw a reaction energy diagram for the reaction shown in eqs. 4.14 and 4.15. 

Figure 15.3 The free-energy diagram for an electrophilic aromatic substitution reaction. The arenium ion is a true intermediate lying between transition states 1 and 2. In transition state 1 the bond between the electrophile and one carbon atom of the benzene ring is only partially formed. In transition state 2 the bond between the same benzene carbon atom and its hydrogen atom is partially broken. The bond between the hydrogen atom and the conjugate base is partially formed.

A simplified energy diagram is presented in Fig. S based on that of Olah and associates [12, 13]. In a similar way R. D. Brown [14] considered that an electrophilic substitution begins by the formation of unstable charge-transfer complexes. R. Taylor [15] observed anomalously high and solvent dependent o/p ratios for the nitration of biphenyl and rationalized it that a tr-complex between NO and biphenyl is formed initially and rearranges to a more stable a-complex at the ortho position of one of the aromatic rings of biphenyl. The final experi-... 

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