Cracking beta scission

Cracking, or beta-scission, is a key feature of ionic cracking. Beta-scission is the splitting of the C-C bond two carbons away from the positive-charge carbon atom. Beta-scission is preferred becau.se the energy required to break this bond is lower than that needed to break the adjacent C-C bond, the alpha bond. In addition, short-chain hydrocarbons are less reactive than long-chain hydrocarbons. The rate of... 

The secondary free radical can crack on either side of the carbon carrying the unpaired electron according to the beta scission rule, and a terminal olefin is produced. 

The carbon-carbon beta scission may occur on either side of the carbocation, with the smallest fragment usually containing at least three carbon atoms. For example, cracking a secondary carbocation formed from a long chain paraffin could be represented as follows ... 

When liquid hydrocarbons such as a naphtha fraction or a gas oil are used to produce olefins, many other reactions occur. The main reaction, the cracking reaction, occurs by a free radical and beta scission of the C-C bonds. This could be represented as ... 

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,... 

Because beta-scission is mono-molecular and cracking is endothermic, the cracking rate is favored by high temperatures and is not equilibrium-limited. 

Isomerization reactions occur frequently in catalytic cracking, and infrequently in thermal cracking. In both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions they shift around and crack to produce branched molecules (Equation 4-10). (In thermal cracking, free radicals yield normal or straight chain compounds.)... 

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. 

A previous report indicates that the conversion of 10-8 at 450°C gives mainly C4 products with little C3 + C5 [8]. This suggests that the increased rate of beta scission associated with the cracking of the C8 cation iscmer (III, Scheme 1) is the preferred route to cracked products. This is in keeping with the accepted order of reactivity for cracking (A > B > C), and similar cracking schemes can be postulated for the other C6f olefins. [8,9]. 

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. 

The Fig. 15 indicates the change in the number of species generated (paraffins, olefins, ions, in Fig. 15a) and the number of reactions (hydrogenations, pro-tonations, HS, MS, ethyl shiftes (ES), PCP branching, PCB branching, beta scissions, in Fig. 15b) according to the number of carbons of a single hydro-cracked normal paraffin. 

Mechanisms of hydrocracking of paraffins have been studied extensively (27-33). A carbonium ion mechanism is usually proposed similar to the mechanisms previously proposed for catalytic cracking except that hydrogenation and hydroisomerization are superimposed. The paraffins are first dehydrogenated to an olefin, then are adsorbed as a cation on an acidic site, isomer-ized to the preferred tertiary configuration, and undergoes beta scission. Virtually no methane and ethane are formed. The reaction becomes more selective for isoparaffin production as the temperature is decreased. 

Reaction Mechanisms, In catalytic cracking, the basic reaction mechanisms involve the formation of carbonium ions and include beta scission cracking, olefin isomerization, dealkylation, transalkylation and hydrogen transfer (Venuto and Habib, 1979). The rate of cracking of paraffins increases with increasing carbon number although the olefins (formed from... 

The most important cracking reaction, however, is the carbon-carbon beta bond scission. A bond at a position beta to the positively-charged carbon breaks heterolytically, yielding an olefin and another carbocation. This can be represented by the following example ... 

In the FCC process using heavy feedstocks, light olefins are probably produced by secondary cracking of primary olefins in the FCC naphtha fraction. The reaction proceeds readily over ZSM-5 zeolite-containing catalysts. It is widely accepted that olefin eracking over catalysts with Bronsted acidity involves initial protonation of the double bond to form a tricoordinate carbenium ion, with subsequently scission of a carbon-carbon bond in the beta position, to form a free olefin and a smaller carbenium ion. 

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