Beta-scissions

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 new carbocation may experience another beta scission, rearrange to a more stable carbonium ion, or react with a hydrocarbon molecule in the mixture and produce a paraffin. 

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 ... 

If R = H in the above example, then according to the beta scission rule (an empirical rule) only route b becomes possible, and propylene would be a product ... 

Aromatization of paraffins can occur through a dehydrocyclization reaction. Olefinic compounds formed by the beta scission can form a carbocation intermediate with the configuration conducive to cyclization. For example, if a carbocation such as that shown below is formed (by any of the methods mentioned earlier), cyclization is likely to occur. 

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 ... 

Free radicals are extremely reactive and short-lived. They can undergo alpha scission, beta scission, and polymerization. (Alpha-scis.sion is a break one carbon away from the free radical beta-scission, two carbons away.)... 

Beta-scission produces an olefin (ethylene) and a primary free radical (Equation 4-2), which has two fewer carbon atoms [1] ... 

The newly formed primary free radical can further undergo beta-scission to yield more ethylene. 

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 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 is splitting of the C-C bond two bonds away from the positively charged carbon atom. 

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.

A reaction scheme for PAH formation from propane shown in Fig. 14.8 illustrates the general discussion above. The abstraction of an H atom from propane can lead to the left branch, ultimately leading to acetylene and methane formation. The other possible initial abstraction reaction (right branch in Fig. 14.8) forms the isopropyl radical, which undergoes beta scission to form propylene. The sequence of abstractions (e.g., forming the... 

For a hydrocarbon species with a radical site on carbon number n, a beta-scission reaction breaks one of the chemical bonds on carbon number n + 1, simultaneously forming a higher-order bond (e.g., a single bond going to a double bond) between carbons n and n + 1. 

Beta-scission the rupture of a carbon-carbon bond two bonds removed from an aromatic ring. 

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 analysis of substituent effects on RSE values does not only aid our understanding, but also holds a degree of predictive power, allowing one to design and select species with optimal radical stabilities for specific practical applications. Indeed, provided due attention is given to the effects of substituents on the other species involved, RSEs can even provide a qualitative guide to the thermodynamic stability of radicals in other types of chemical reaction, such as addition and beta-scission. In this section, some practical applications of RSE values are illustrated using some selected case studies from the literature. 

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