Carbocations ethoxy

The y8-silyl effects are much greater for the purely aliphatic systems than the vinyl ether or phenylacetylene, indicating that the stabilization in these latter systems is attenuated by the carbocation-stabilizing ability of the ethoxy and phenyl groups, respectively. 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

The rearranged product, 2-ethoxy-2-methylbutane, results from a hydride shift, the movement of a hydrogen atom with its bonding pair of electrons. A hydride shift is represented by the symbol H. In this case, the hydride shift converts the initially formed secondary carbocation to a more stable tertiary carbocation. Attack by the solvent gives the rearranged product. 

The preparation of block copolymers by combination of thermally radical and photoinduced cationic polymerization processes has also been reported [151], Indeed, styrene/cyclohexene oxide (CHO) copolymers have been synthesized by using a bifunctional azobenzoin initiator such as ABME, previously described, through a two-step procedure. In the first step, thermal Iree radical polymerization of styrene in the presence of the above azobenzoin initiator gives poly(styrene) prepolymers with benzoin photoactive end groups, as reported in Scheme 38. These prepolymers, upon photolysis and subsequent oxidation to the corresponding carbocations in the presence of l-ethoxy-2-methylpyridinium hexafluoro phosphate (EMP+PFg ), finally give block copolymers by cationic polymerization of cyclohexene oxide (Scheme 45). 

Nucleophilic Substitution Reactions Section 8.4D.2 This first reaction is an Sn2 reaction as a result of the strong nucleophile. Sn2 reactions proceed with inversion of configuration so the product will be trans 1-ethoxy-2-methylcyclopentane. The SN1 reaction involves a carbocation intermediate that can be attacked from either side and consequently the cis and trans products are formed. 

Is this correct It depends on how long the reaction is allowed to proceed. If the reaction is stopped after 1 or 2 hours, the answer is, indeed, no reaction. If the reaction is heated for several days or several weeks, two different products may be observed 2-iodo-2-methylbutane and 2-ethoxy-2-methyl-butane. Both products arise by an SnI process. Given sufficient time, slow ionization to a tertiary carbocation is followed by trapping with iodide ion or with ethanol. This process is known as solvolysis (see Chapter 11, Section 11.6). This contradicts the assumption that ionization can only occur in water because it is an assumption, as pointed out in the earlier section and thus not always correct. Indeed, ionization occurs in protic solvents such as ethanol, but it is slow relative to ionization in water. If 2-bromobutane is heated with KI in ethanol, the Sn2 product (Chapter 11, Section 11.2) is formed, indicating that the Sn2 reaction is much faster than the very slow ionization reaction in ethanol. 

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