Carbonium ions continued

The third mode of termination which occurs in some carbonium ion polymerizations involves rearrangement of the active carbonium ion into an inactive one which cannot continue the propagation. These reactions can be avoided to a great extent by working at sufficiently low temperatures, and on the whole, they only contribute significantly to the termination reaction in a few systems. 

Indeed, the acidity of the reaction mixture was found to increase upon continued photolysis, in accord with the carbonium ion mechanism. 

Five years ago a brief review focused on the applications of nuclear magnetic resonance (nmr) as a method for determining charge density in carbonium ions and pointed out some of the precautions required (Fraenkel and Famum, 1968). Since then, proton nmr (pnmr), which was emphasized in that review, has continued to attract primary attention as a probe into the structure and charge density of organic cations and anions (Olah and Schleyer, 1968,1970, 1972, 1973 Oth ef al., 1972 Takahashi et a/., 1973 van... 

The larger carbonium ion thus formed cannot continue to exist, but may depolymerize, unite with the catalyst, or stabilize itself by the attraction of an election pair from a carbon atom adjacent to the electronically deficient carbon (C+) with its proton. This establishes a duuble bond involving the formerly deficient atom. Thus a proton is expelled to the catalyst or attracted to the catalyst. If this takes place with one of the methyl groups, the product... 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

To continue with the Kolbe reaction, it has been shown that carbon anodes strongly favour the carbonium ion pathway (Koehl, 1964) at least for simple alkanecarboxylic acids. Also, for phenyl-acetic acid and 1-methylcyclohexylacetic acid the same tendency towards carbonium ion formation on carbon anodes was observed, the phenomenon being explained as due to the presence of paramagnetic centres in carbon. These would bind the initially formed radicals, impede their desorption and hence promote the formation of carbonium ions via a second electron transfer (Ross and Finkelstein, 1969). However, cases of Kolbe oxidations in which no dependence on anode material was noticeable have been found more recently (Brennan and Brettle, 1973 Eberson and Nilsson, 1968a Sato et al., 1968). Actually, the nature of the carbon material determines the yield of products formed via the radical versus carbonium ion pathway (Brennan and Brettle, 1973). Yields of the... 

The first two steps are identical with those of the dimerization reaction. In step (3) a carbonium ion abstracts a hydrogen atom with its pair of electrons (a hydride ion, essentially) from a molecule of alkane. This abstraction of hydride ion yields an alkane of eight carbons, and a new carbonium ion to continue the chain. As we might expect, abstraction occurs in the way that yields the tert-butyl cation rather than the less stable (P) isobutyl cation. 

It has been suggested that there is a continuous spectrum of mechanisms for nucleophilic substitution ranging from the idealized S l reaction (called Lim, for limiting) at one end, to the idealized Sn2 reaction (called N) at the other. On progress-of-reaction plots, the energy minimum for the carbonium ion becomes shallower and shallower as We move away from the SnI end at the 8 2 end the minimum has disappeared, and we have a single maximum. 

The regenerated carbonium ion can of course continue the process, a key feature being that under alkylation conditions this active species is formed from saturated alkane, not an olefin as required by polymerization. Different alkenes, such as propylene, 1-butene, or the 2-butenes may also form carbonium ions in a similar manner to the process of Eq. 18.25. However, neither /7-butane nor /7-pentane can replace an isoalkane for the hydride transfer since an /7-alkane is not capable of forming a stabilized carbonium ion. Nevertheless, this is one advantage that the alkylation process has over polymerization as a route to gasoline it is able to use both light hydrocarbon alkanes (as long as they are branched) and alkenes. Alkylation and polymerization both produce branched products, but the alkylation products are saturated (Table 18.5) whereas the polymerization products are alkenes. 

In the presence of strong acids such as aqueous H2SO4, carbonyl compounds may react with olefins to form unsaturated alcohols and other products, depending on the reaction conditions. Using H-mordenite as catalyst in a continuous-flow system, 10% conversion of formaldehyde to isoprene was observed at 300° using an isobutylene-to-HCHO (molar) ratio of 3.7. A carbonium ion-type reaction scheme, involving a Prins reaction (1,2) and a subsequent dehydration-rearrangement step... 

In Scheme 2 we d ict the activation of n-hexane involving direct protxsnation to the carbonium ion. However, this is not to imply that these species are stabilised cn zeolitic surfaces but rather to provide possible mechanistic pathways. In fact, at this time, the nature of sorbed activated hydrocarbon species is not known and for example, a role for radicals or radical ions cannot be e olixied (28) nor can the involvement of Lewis acid sites vhich continue to attract interest... 

Interest in aromatic chemistry continues at the same high level as in recent years. Substitution and cyclization processes are two areas which have attracted considerable attention over the past year. Pertinent here, as elsewhere in this volume, are the reviews on photochemistry with circularly polarized light by Buchardt,1 and the photochemistry of carbonium ions by Cabell-Whiting and Hogeveen.2 Current literature describing light-induced reactions of pyrroles has also been briefly reviewed.8... 

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