Carbonium ion theory

Many theories have been proposed but three have received considerable attention Whitmore s carbonium ion theory (26) postulates that a carbonium ion (positive hydrocarbon ion) adds to an olefin to form a higher molecular weight carbonium ion which then yields the olefin polymer by elimination of a proton (H+). With acid catalysts—for example, sulfuric acid—the initial carbonium ion is formed by addition of a hydrogen ion from the acid to the extra electron pair in the double bond of the olefin. A second pro-... 

Of the several theories proposed for the mechanism of the alkylation reaction ( , 4, 7), the carbonium ion theory is probably the most widely accepted. Although a complete discussion of this theory is beyond the scope of this paper, an outline of the mechanisms it proposes as applied to known alkylation products is presented. 

Pure activated aluminas are also capable of catalyzing the skeletal isomerization of olefins (104, 105), but at considerably higher temperatures (350°-400°C) than those required for double-bond isomerization. The results obtained by Pines and Haag (105,106) leave little doubt that this type of isomerization is acid catalyzed. They found that (a) skeletal isomerization of cyclohexane or 3,3-dimethylbutene-l over pure alumina was poisoned upon ammonia addition and (b) the order of appearance of products from 3,3-dimethylbutene-l isomerization as contact time is increased was that predicted from carbonium ion theory. They also used indicator tests to show that the seat of acid activity in -y-alumina consists of Lewis, not Br0nsted, acidity. Independent infrared studies of pyridine chemisorbed on pure alumina have verified the existence of Lewis acidity and the absence of Brpnsted acidity in pure alumina (23, 107). 

Dilution of the catalyst with reaction by-products may reduce the conductivity and the activity of the catalyst, but this has no bearing on the carbonium ion theory (400b). They preferred a mechanism postulating the similarity of these catalysts to aluminum chloride. 

There is some evidence that carbonium ions are also formed on clay surfaces, and the carbonium ion theory has been used to explain cracking reactions. Evans (121) reports that a solution of 1,1-diphenylethylene,... 

Most of the chemistry of the cracking of hydrocarbons can be explained in terms of carbonium ion theory (Greensfelder, Voge, and Good, 123). The problem is the mode of formation of the complexes, and there have been a number of attempts to correlate catalytic activity with acidity (Tamele, 124) and structure (Milliken, Mills, and Oblad, 125). 

Historically, it was the occurrence of rearrangements that was chiefly responsible for the development of the carbonium ion theory. Reactions of seemingly... 

We find that the products expected on the basis of our mechanism are jiist the ones that are actually obtained. The fact that we can make this prediction simply on the basis of the fundamental properties of carbonium ions as we understand them is, of course, powerful support for the entire carbonium ion theory. 

It is possible, however, to write a mechanism for the reaction based upon the principles of carbonium ion theory which will embrace many of the experimental facts concerning the reaction. The structure of the... 

Probably the best-known examples of Wagner-Meerwein rearrangements are the conversions of camphene to isobornyl chloride and pinene to bomyl chloride by hydrochloric acid. In terms of carbonium ion theory these reactions may be interpreted as shown on pages 57 and 58. 

Reactions 3b and 3c illustrate principles of carbonium ion theory (Chapter 3). Reaction 3a has been beautifully demonstrated by Bartlett and his coworkers.16a When, for example, aluminum bromide in isopentane is treated with /erJ-butyl chloride at room temperature, after about 0.001 second the principal product is tert-amyl bromide ... 

The catalytic cracking of hydrocarbons is a chain reaction that is believed to follow the carbonium ion theory, involving three steps initiation, propagation, and termination. The initiation step is represented by the attack of an active site on a reactant molecule to produce the active complex that corresponds to the formation of a carbo-cation. Chain propagation is represented by the transfer of a hydride ion from a reactant molecule to an adsorbed carbonium ion. Finally, the termination step corresponds to the desorption of the adsorbed carbenium ion to give an olefin while restoring the initial active site. ... 

On this basis, then, it appears that the stereoselectivity of olefin isomerization can be adequately explained within the framework of classical carbonium ion theory without invoking such dubious species as the pi-complex or cyclic carbonium ion. The attractiveness of this mechanism is further enhanced by the fact that it is similar to the allyl carbanion mechanism proposed for the base catalyzed isomerization (145). Perhaps it would not be too great an extrapolation to presume that a similar mechanism, involving allyl radicals, may obtain over reduced metal catalysts. 

The carbonium ion theory originally proposed by Frank Whitmore (11) is thus as applicable to the mechanism of catalytic cracking as it is to other acid-catalyzed reactions like isomerization or alkylation. 

The original carbonium ion theory (now called the carbenium ion theory by modern purists) was applied in a more quantitative way by Charles L. Thomas (12) of UOP and in particular by Bernard Greensfelder and Hervey Voge of Shell Development Company (13). Its success has endured few challenges now for some 35 years, and it seems unlikely that a better theory as applied to catalytic cracking will ever replace it. 

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