Carbocations, continued protonation

Now we finish the El route by path Dg to produce the most stable trisubstituted alkene. The most basic group present is water, so it removes the proton from the carbocation. The protonated water would be expected to protonate the next molecule of alcohol, continuing the acid catalysis and freeing up water as a product. 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

The Lewis acid-base complex of water and boron trifluoride donates a proton to an isobutylene monomer to produce a carbocation. This carbocation adds to another isobutylene monomer to produce a laiger carbocation, and the process continues, producing the polymer ... 

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-... 

The path leading to II follows a more circuitous route that may not become directly apparent unless the central cation VIII is assumed to participate. One has to figure out first, however, how a methylene of the side chain is activated properly for C-C bonding, since it is now this carbon that is involved in the production of II. (Before continuing, the reader must be convinced of this.) Its activation may be accomplished by rather standard means, namely, El elimination that would yield olefin IX followed by anti-Markovnikov addition of a proton. The secondary carbocation thus formed then would be trapped by the enol function in X, configured now in the opposite direction to that in IV (see Scheme 52.2). 

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. 

Cationic Poiymerization Electophilic Addition in the Absence of a Reactive Nucleophile. In the presence of catalytic amounts of acidic materials (protic as well as nonprotic) lacking nucleophilic counterions, the attack of the electron-rich alkene on the electrophile appears to occur with the formation of a carbocation as expected but, as a nucleophilic anion is missing, another alkene attacks the carbocation (Table 6.1, entry 19). The result is the formation of a new carbocation, larger than the original by one alkene unit (Scheme 6.32 R H).The process continues until termination is effected (e.g., by proton loss). 

Do not give the OH another proton, as that pathway will not quickly lead to the ketone. Students who attempt to protonate the OH group invariably continue by losing water as a leaving group to form a vinylic carbocation. That pathway is too high in energy and simply does not lead to the product. 

The reaction continues as the carbocation is captured by the solvent, often water, as in Figure 7.53, in what is called a solvolysis reaction. Solvolysis simply means that the solvent, here water, plays the role of nucleophile in the reaction. Next, any base present, here likely to be bromide or water, can remove a proton (deprotonate the protonated alcohol) to give the alcohol itself (Fig. 7.53). 

The final example of an electrophile that we will consider here is the carbocation, derived from the alkene itself. For example (Figure 11.24), styrene is readily protonated to give the secondary benzylic carbocation. If no other nucleophile is available, this may be attacked by the x-bond of another molecule of alkene. Continuing with this process ultimately gives a polyalkene, in this case, the familiar polystyrene. We will discuss this type of reaction in more detail in Chapter 21. 

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