Electrophilic reaction-energy diagram

Reaction energy diagrams for an electrophilic addition to an alkene and ( ) an electrophilic... 

Reaction-energy diagram for the second step of the addition of HBr to buta-1,3-diene. The allylic carbocation (center) can react at either of its electrophilic carbon atoms. The transition state ( ) leading to 1,2-addition has a lower energy than that leading to the 1,4-product, so the 1,2-product is formed faster (kinetic product). The 1,2-product is not as stable as the 1,4-product, however. If equilibrium is reached, the 1,4-product predominates (thermodynamic product). 

A reaction energy diagram for the electrophilic addition of HCI to 2-methyl propene. The tertiary cation intermediate forms faster than the primary cation because it is more stable. The same factors that make the tertiary cation more stable also make the transition state leading to it more stable. 

Reaction energy diagram for the electrophilic addition of HBr to 1,3-buladiene. The 1,2 adduct the kinetic product because it forms faster, but the 1,4 adduct is the thermodynamic product ause it is more stable. 

A reaction energy diagram for the electrophilic bromination of benzene. The overall process is exergonic. 

Reaction-energy diagram for the second step of the addition of 1,3 butadiene to HBr. The allylic carbocation (center) can react at either of its electrophilic carbon atoms. The transition state (4) leading to... 

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

Figure 2.3 Reaction-energy diagram of electrophilic aromatic 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. 

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... 

Fig. 18 Energy diagram for the first step of an electrophilic substitution reaction illustrating the crossing of DA and D +A" configurations. The effect of a substituent that stabilizes the D + A configuration (e.g. by improving the arene donor ability) is indicated by the dotted line. The diagram illustrates the correlation between AAv, the difference in excitation energies for the perturbed and unperturbed systems, and AE, the difference in activation energy for the two systems. (Avoided crossing deleted for clarity)...

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). 

Winstein et al. [45] first presented evidence for the concept that different types of electrophilic species, each with distinct reactivities, may participate in reactions involving cationic intermediates. As shown in Eq. (36), Winstein et al. proposed that four species are in equilibrium, including covalent electrophiles, contact ion pairs, solvent-separated ion pairs, and free ions. In addition, ion pairs may aggregate in more concentrated solutions- According to this concept, electrophilic species do not react with a continuous spectrum of charge separation, but rather in well-quantified minima in the potential energy diagram. 

---