Alkenes reaction coordinate diagrams

As we saw in Section 3.7, the addition of the electrophile to the alkene is relatively slow, and the subsequent addition of the nucleophile to the carbocation occurs rapidly. The reaction of the carbocation with a nucleophile is so fast that the carbocation combines with whatever nucleophile it collides with first. In the previous hydration reaction, there were two nucleophiles in solution water and the counterion of the acid (e.g., CF) that was used to start the reaction. (Notice that HO is not a nucleophile in this reaction because there is no appreciable concentration of HO in an acidic solution.) Because the concentration of water is much greater than the concentration of the counterion, the carbocation is much more likely to collide with water. The product of the collision is a protonated alcohol. Because the pH of the solution is greater than the pTTa of the protonated alcohol (remember that protonated alcohols are very strong acids see Sections 1.17 and 1.19), the protonated alcohol loses a proton, and the final product of the addition reaction is an alcohol. A reaction coordinate diagram for the reaction is shown in Figure 4.5. 

To answer this question, we must determine which of the alkenes is formed more easily—that is, which is formed faster. The reaction coordinate diagram for the E2 reaction of 2-bromobutane is shown in Figure 11.1. 

Reaction coordinate diagram for the El reaction of 2-chloro-2-methylbutane. The major product is the more substituted alkene because its greater stability causes the transition state leading to its formation to be more stable. 

The reaction coordinate diagram in Figure 15.4 shows that the reaction of benzene to form a substituted benzene has a AG° close to zero. The reaction of benzene to form the much less stable nonaromatic addition product would have been a highly ender-gonic reaction. Consequently, benzene undergoes electrophilic substitution reactions that preserve aromaticity, rather than electrophilic addition reactions (the reactions characteristic of alkenes), which would destroy aromaticity. 

Figure 2 illustrates several reaction coordinate diagrams that allow exothermic oxaphosphetane formation. Option a (four-center process) and the kinetically equivalent b (transient betaine precursor of the oxaphosphetane two-step mechanism) are consistent with the observation that oxaphosphetanes are formed rapidly and decompose slowly when R = alkyl. Since the barrier AGjJgc decomposition to the alkene is smaller than AGJ y, there will be little reversal or loss of stereochemistry in option a. Reversal should become less likely if the a-substituent R is unsaturated (CH=CH2 or aryl), a situation that would decrease AG g by weakening the P—Cj bond (reaction profile c). If substituents are present that retard the rate of decomposition relative to reversal (as in options d or e), then oxaphosphetane reversal and equilibration of stereochemistry become possible, as discussed in a later section. However, this behavior has not been demonstrated for members of the Ph3P=CHR ylide family in the absence of lithium salts. 

A reaction coordinate diagram can be drawn for each step of a reaction (Figure 5.5). In the first step of the addition reaction, the alkene is converted into a carbocation that is higher in energy (less stable) than the reactants. The first step, therefore, is endergonic (AG° is > 0). In the second step, the carbocation reacts with a nucleophile to form a product that is lower in energy (more stable) than the carbocation reactant. This step, therefore, is exergonic (AG° is < 0). 

What factors dictate which of the two alkenes will be formed in greater yield In other words, what causes the regioselectivity of an E2 reaction We can answer this question by looking at the reaction coordinate diagram in Figure 10.1. 

The relationship between AG and AG is normally presented in a diagram, where free energies of the reactants, products, transition state, and intermediates are plotted against the extent of reaction, or more precisely the reaction coordinate. This is shown in Fig. 2.9. Even a simple homogeneous catalytic reaction such as alkene hydrogenation involves many intermediates and transition states. The free energy diagram thus resembles (c) rather than (a) or (b). 

Fig. 11. State correlation diagram for radical addition to alkenes showing the variation in energy of the reactant (RA), the product (RA ), and the charge-transfer configurations (R+A and R A+) as a function of the reaction coordinate. The dashed line represents the overall energy profile of the reaction.

---