Carbenium ions 3-scission

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. 

The initial products of beta-scission are an olefin and a new carbenium ion (Equation 4-9). The newly-formed carbenium ion will then continue a series of chain reactions. Small ions (four-carbon or five-carbon) can transfer the positive charge to a big molecule, and the big molecule can crack. Cracking does not eliminate the positive charge it stays until two ions collide. The smaller ions are more stable and will not crack, They survive until they transfer their charge to a big molecule,... 

Innumerable reactions occur in acid catalyzed hydrocarbon conversion processes. These reactions can be classified into a limited number of reaction families such as (de)-protonation, alkyl shift, P-scission,... Within such a reaction family, the rate coefficient is assumed to depend on the type, n or m cfr. Eq. (1), of the carbenium ions involved as reactant and/or product, secondary or tertiary. The only other structural feature of the reactive moiety which needs to be accounted for is the symmetry number. The ratio of the symmetry number of the... 

Hydrocarbons with up to 16 carbon atoms are detected in a typical alkylate (82). With the liquid acids, it was found that the oligomerization rate is higher for isoalkenes than for linear alkenes (49). The same is true for solid acids (14,83). Because of their tertiary carbon atoms, isobutylene and isopentene obviously react more easily with carbenium ions. This point can be inferred from the reverse reaction, (3-scission (see below), which is fastest for reactions of tertiary cations to give tertiary cations. In oligomerization experiments, the following pattern of... 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

Figure 13.44 Beta scission pathways for carbenium ions.

As shown, the ratio was very high on zeolite catalysts, while that on mesoporous silica was as low as those on AMS and quartz chip. The high ratio on zeolites can not be explained by classical mechanism of acid-catalyzed cracking supposing higher stability of tertiary carbenium ion and its cracking by P-scission, because this supposition predicts that the reaction (2) proceeds in preference to the reaction (1). Rather, a-scission of carbocation [12] may rationalize the higher C3/C4 ratio on zeolite catalysts. 

Olah and Hogeveen and co-workers The carbon-carbon cleavage in neopentane (7) yielding methane and the tert-butyl cation 4 occurs by a mechanism different from the /3-scission of carbenium ions [Eq. (5.56)]. 

This reaction is much faster than the carbon-carbon cleavage in neopentane, despite the initial formation of secondary carbenium ions. Norbomane is also cleaved in a fast reaction, yielding substituted cyclopentyl ions. Thus, protonation of alkanes induces cleavage of the molecule by two competitive ways (i) protolysis of a C—H bond followed by /3-scission of the carbenium ions and (ii) direct protolysis of a C—C bond yielding a lower-molecular-weight alkane and a lower-molecular-weight carbenium ion. 

Four principle cracking reactions, all of them being irreversible, have to be taken into account hydrogenolysis of the n-alkane feed /3-scission of straight chain carbenium ions ... 

The mechanistic background for such a comparison is illustrated in Figure 10 which represents in more detail the pathway of hydroisomerization and hydrocracking of two n-alkanes. Branched carbenium ions are formed via n-alkenes and linear carbenium ions. Then, either desorption or (3 -scission may occur in parallel reactions. Desorption (followed by hydrogenation) of a given carbenium ion yields an iso-alkane with the same carbon skeleton. f3 - scission, on the other hand, yields fragments of definite carbon numbers ( (3 -scissions which would yield or C2... 

Such a comparison is made in Table IV for all n-alkanes with a chain length ranging from 8 to 12 carbon atoms. Sums have been formed whenever necessary, i. e., if the same set of carbon numbers is formed by y3 -scission of different carbenium ions, or, if different sets of carbon numbers may be formed by -scission... 

Cracking patterns for cyclo-paraffins over US-Y have recently been reported [9] where products from the conversion of methycyclohexane include C2-C6 compounds with C3 + C4 hydrocarbons as the major yields. The authors of this study have assumed that isomerisation of the initial carbenium ion is relatively facile and that a combination of hydrogen transfer, isomerisation, and beta scission generates the products. This is envisaged in Scheme 2 which uses the classification A, B, C for cracking, A, B for isomerisation f8,91 and which presumes that intramolecular hydrogen transfer is rapid [8]. 

Similar fates can be postulated for isomers II and III (Scheme 2A) and also for C8 - CIO cyclo-olefins which would generate C4+ olefins. C5 and C6 olefins are also presumed to arise from cracking of methylcyclopentane [8] by the interaction of small carbenium ions with olefins to form larger carbenium ions which can subsequently undergo beta scission. Such reactions can clearly modify product distributions in the present more complicated system. 

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