Alkene, energy diagram

Figure 7.8 Free-energy diagram for the hydrogenation of an alkene in the presenceof a catalyst and the hypothetical reaction in the absence of a catalyst. The free energy of activation [AG (i)] is very much larger than the largest free energy of activation for the catalyzed reaction [AG (2,].

Figure 8.1 Free-energy diagram for the addition of HX to an alkene. The free energy of activation for step 1 is much larger than that for step 2.

Figure 8.1 Energy diagram showing the vertical and nonvertical excited singlet states of an alkene...

This process seems to be contra-intuitive, since two nucleophiles lead to a third nucleophile, but is much easier than the allylzincation of simple alkenes. The reaction is profiled by ab initio calculations, and the energy diagram in Scheme 4 explains the rapid reaction of alkenyl metal with allylzinc halide12. A detailed and advanced computational study for this process by Nakamura and coworkers shows the existence of two possible pathways. One is the metalo-ene pathway whereas the second one corresponds to a metala-Claisen reaction (Scheme 5)13. In both cases, allylzinc and vinyl metal react via a six-membered transition state. 

Figure 2.1 Energy diagram tor an alkene, showing the vertical and non-vertical singlet excited states.

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

Having seen the energy diagram above, you will not be surprised to learn that the malonate radical adds readily not to electrophilic alkenes, but to nucleophilic alkenes, such as this vinyl ether, which carries an electron-donating oxygen substituent. This electrophilic radical can also be formed by H-abstraction and by oxidation. 

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). 

By assuming insertion of alkene into metal-hydrogen bond to be the ratedetermining step, draw a hypothetical free energy diagram for the catalytic cycle of Fig. 2.10. How many catalytic intermediates and transition states are there ... 

Figure 5.1 Energy diagrams for electrophilic addition to substituted alkenes. The cation adjacent to the electron-releasing group is stabilized.

When reversible addition and elimination reactions are carried out under similar conditions, they follow the same mechanistic path, but in opposite directions. The principle of microscopic reversibility states that the mechanism of a reversible reaction is the same in the forward and reverse directions. The intermediates and transition structures involved in the addition process are the same as in the elimination reaction. Under these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The reversible acid-catalyzed reaction of alkenes with water is a good example. Two intermediates are involved a carbocation and a protonated alcohol. The direction of the reaction is controlled by the conditions, which can be adjusted to favor either side of the equilibrium. Addition is favored in aqueous solution, whereas elimination can be driven forward by distilling the alkene from the reaction solution. The reaction energy diagram is show in Figure 5.1. 

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