Termolecular addition, with alkenes

The kinetics of bromine addition to alkenes can be more complex than the simple model in Figure 9.2 would suggest. Early studies revealed considerable kinetic variation with solvent, temperature, and the presence of additives such as water. The kinetic investigations can also be complicated because of the involvement of HBr3 (from Br2 and HBr) or of termolecular processes involving an alkene, Br2, and Br . The Br can be produced if the bromo-nium ion is intercepted by solvent or solvent anion (e.g., acetate ion). The equilibrium constant for dissociation of Bra" to Br2 and Br was determined to be 5 X 10 M in water and 2 x 10 M in methanol. Thus, a general kinetic expression for the addition of bromine to alkenes can be represented as " ... 

The reactions of OH with alkenes are in general addition, and the reaction of OH and C2H4 is expressed by termolecular reaction such as,... 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

It is interesting that syn addition to a 1,3-diene was considered, in the early 1950 s, to be the result of two coupled anti processes (Eliel, 1956). This useful, intuitive notion is contained in the general orbital symmetry scheme of Fig. 21. X and Y, with one bond or electron pair between them, react with LYMO of the tt chain. For concerted reactions, this could be termolecular anti for an alkene, bimolecular or termolecular syn for a 1,3-diene, termolecular anti for a 1,3,5-triene, and etc. For an all-ds triene, bimolecular anti addition of molecular bromine becomes possible —the Mobius chain of eight atoms mentioned earlier would also be appropriate here. 

The kinetics of the reaction reveals three competing pathways. The syn addition of HCl is found to be a second-order reaction, first order in alkene and HCl. Syn addition occurs via a contact ion pair, where the H and Ch necessarily add from the same face of the alkene. The kinetics of anti addition of chloride and acetic acid is third order—first order in alkene, first order in HCl, and first order in chloride or acetic acid, respectively. These additiorrs are ter-molecular. This explains the dependence of the product distribution on chloride. Because the reaction is termolecular, electrophilic and nucleophilic attack on the alkene would be expected to occur on opposite faces of the alkene due to steric considerations, and anti addition products result. Termolecular collisions are rare, so that the actual reaction likely occurs by collision of the nucleophile with a weak complex between the alkene and the acid. 

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