Carbocations, continued structure

Cationic polymerization of XII may therefore be visualized in terms of Figure 9 according to which the ir complex initially formed between the active site and the monomer is converted into a carbocation with rupture of a C—C bond in the cyclopropane. This cation may be Xlla, b, or c, but only the latter can give rise to Structure M, alone compatible with the experimental data. This change necessitates the transfer of a hydride ion to transform the primary cation XIIc into the more stable tertiary cation Xlld. On this assumption, the termination reaction probably occurs as the result of the displacement of a proton in the alpha position with respect to the C+, which is relatively easy, whereas the steric hindrance around the active site does not favor continued poly-... 

Once the transition state is reached, the reaction can either continue on to give the carbocation product or revert back to reactants. When reversion to reactants occurs, the transition-state structure comes apart and an amount of energy corresponding to -AG is released. When the reaction continues on to give the carbocation, the new C-H bond forms fully and an... 

Carboanionic Intermediates.—Whereas characterization of the carbocations active in cationic polymerization has been difficult, identification and quantification of the carboanionic centres in anionic propagations have been somewhat easier, largely as a result of a considerably reduced tendency to rearrange and isomerize. Nevertheless, detailed investigations continue in particular areas. Thus Bywater and Worsfold ° have used n.m.r. spectroscopy to probe the structure of various model compounds which closely resemble those presumed to be responsible for anionic propagations. Whereas in tetrahydrofuran solvent 2,5-diphenyl-2,5-dipotassiohexane and 2-lithio-4,4-dimethyl-2-phenyl-pentane display coupling constants consistent with the carbon atoms... 

With due regard for the difference of opinion between the Japanese and American chemists, on the one hand, and the Soviet, on the other, the latter continued their efforts to prove the correctness of the two-stage scheme of the process. With this aim in view study was made of nucleophilic substitution reactions in a wide range of media — from strongly nucleophilic to superacidic. Also, the authors include the comparative research of structure and properties of carbocations generated from the same precursors in different media. 

In the early sections of this chapter, alkenes reacted with acids to form a car-bocation. Once the carbocation is formed, it reacts with other electron-donating species, including the alkene itself. Continuous reaction of an alkene to form a new carbocation allows an alkene to continue reaction until a large molecular-weight material known as a polymer is generated. Alkenes also react with radicals to give new compounds. (See Chapter 7, Section 7.4.3, for the structure of radicals.) When an alkene reacts with a radical, the product is another radical. This process can be controlled in many cases to produce polymers. This section will serve as a brief introduction to the chemistry of radicals. 

Finally, despite use of solvents (or mixtures of solvents) to solvate the intermediate carbocation and the anion, the structure of the substrate continues to play the criticalrole and,for example,while,as noted above,at50°C,2-bromo-2-methylpropane (f-butyl bromide [(CH3)3C-Brj) dissociates approximately 105 times faster than 2-bromopropane (isopropyl bromide [(CH3)2CHBrj) in water at 50°C, the difference is only slightly diminished (from 105 to 104) in 60% ethanol 40% water (v/v) at 55°C. 

If the concentration of either bromomethane molecules or hydroxide ions is doubled then the rate of reaction is also doubled. We can therefore deduce that the reaction is first order with respect to both bromomethane molecules and hydroxide ions, and that both these species are involved in the rate-determining step - that is the step in the overall reaction that is the slowest and so limits the overall rate of reaction. The reaction is thought to be a continuous, one-step process. The chemical structure shown in square brackets in Figure 20.3 is not an intermediate (as the carbocation in the Sj l reaction discussed later is) but a transition state. It is a halfway stage in the reaction, where covalent bonds on the carbon atom are being simultaneously broken and made - each of these half-formed bonds has a bond order of 0.5. The transition state is believed to have a trigonal bipyramidal shape (see Chapter 14). 

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