Carbocations, continued tertiary

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-... 

Having participated, the methyl group continues to migrate to carbon 1 because by doing so it allows the formation of a stable tertiary carbocation, which then captures water in a step reminiscent of the second half of an S l reaction. 

The tertiary carbocation can now act as an electrophile and attack the alkene to form another tertiary carbocation of similar stability and reactivity to the first. So the polymerization continues. 

Protolytic ionization of methylcyclopentane gives an equilibrium mixture of the tertiary 1-methyl-1-cyclopentyl cation (29) (more stable by about 40 kJ/mol) and the secondary cyclohe l cation (32) (Scheme 5). At low temperature, irreversible reaction of 29 with CO leads to ion 30, which, after reaction writh ethanol, gives the 31 ester. Product composition, in this case, reflects the difference in stability of the intermediate carbocations. Since the carbonylation step is reversible at higher temperature, and carbocation 32 has a much higher affinity for CO, the concentration of 33 in solution continuously increases to yield, after quenching with ethanol, the 34 ester. This is an example of a kinetically controlled product formation through a thermodynamically unfavorable intermediate. 

The more stable a cation is, the less reactive it is and, conversely, the less stable a cation is, the more reactive it is. This means that, although a methyl cation is very reactive if it forms, this higher energy carbocation has a higher activation barrier to formation, relative to a more stable ion such as a tertiary carbocation. As noted, this information leads to the conclusion that it is easier to form a tertiary cation by ionization of a tertiary halide than it is to form a secondary carbocation by ionization of a secondary halide. Continuing the trend, it should be very difficult to form a primary cation from a primary halide such as 1-bromopentane (72) and very difficult indeed to form a methyl carbocation from iodomethane or bromomethane. 

Initiation by protonation of an alkene requires the use of a strong acid with a noimudeophilic anion to avoid 1,2-addition across the alkene double bond. Suitable acids with nonnucleophilic anions include HF/AsF and HF/BF3. In the following general equation, initiation is by proton transfer from H+BF, to the alkene to form a tertiary carbocation, which then continues the cationic chain growth polymerization. 

The bromine radical then adds to the double bond to give the more stable radical. Radical stability broadly follows carbocation stability, so the stable species is the tertiary rather than the secondary radical (Figure 11.41). This tertiary radical then abstracts a hydrogen from HBr to regenerate the bromine radical. We describe this as a radical chain reaction—once started, the reaction should continue without further initiator. The steps of Figure 11.40 are described... 

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