Reaction energy diagram electrophilic addition reactions

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. 

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... 

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... 

The diagram above refers to thermodynamic stability. When we discuss addition reactions you will see that the most stable alkene when mixed with an electrophile is the most reactive according to this diagram. This paradox is due to the intermediate, usually a carbocation. Since a tertiary carbocation is more stable, the energy of activation is lowered and a reaction with a tertiary intermediate proceeds more quickly in general, to predict the alkene product, use the above diagram as a reference, but to predict the most reactive alkene to an electrophile, the order is based on cation formation and is nearly reversed. 

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. 

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. 

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