Beta scission reaction

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. 

For example, at the roughest level of approximation, all carbonyl carbons can be treated as identical they all have similar thermochemistry and all can undergo certain types of reactions (e.g. they all can be formed by beta-scission reactions of the corresponding alkoxy radicals). However, if one looks at the situation more carefully, the carbonyl groups in ketones, ketenes, aldehydes,... 

When the temperature is high enough, which is the case of flames, the p and Bp radicals easily decompose by beta-scission reactions, so that their concentrations become negligible with respect to those of the B radicals. 

The alkoxy radical is usually described as a typical product of the thermal decomposition of hydroperoxides. Nevertheless, in the post-irradiation oxidation process at room temperature, it cannot originate from this reaction because all the formed products follow a kinetic similar to that of ketone formation [21]. The reaction between the alkyl macroradical and the peroxy macroradical forms peroxides (Scheme 9, Reaction 20), but we can also hypothesize Reaction 21, Scheme 9. Literature studies demonstrate that the alkoxy radical can give beta-scission (Reaction 28) forming a primary alkyl radical and CO, a product that is found during the irradiation of PE (Scheme 10, Reaction 29) [24]. The activation energy of this reaction is around 50kJ/mole. 

ViUano SM, Huynh LK, Carstensen HH, Dean AM. High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cychc ether formation, and beta-scission reactions of hydroperoxy alkyl radicals./Phys Chem A. 2012 116 5068-5089. ViUano SM, Huynh LK, Carstensen HH, Dean AM. High-pressure rate rules for alkyl + 02 reactions. 1. the dissociation, concerted elimination, and isomerization channels of the alkylperoxy radical. / Phys Chem A. 2012 115 13425-13442. 

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 ... 

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,... 

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.)... 

HEATRO = heat of reaction for the polymerization, cal/mole TF = reactor inlet temperature, °C DM0 = reactor fluid density, mole/l BETA = 3-scission reaction rate constant... 

P-parinaric acid, physical properties, 5 33t P-pentenoic acid, physical properties, 5 3 It P-peroxylactones, 18 484 Beta phase titanium, 24 838 in alloys, 24 854-856 properties of, 24 840, 941 P-phellandrene, 24 493 P-picoline, 21 110 from acrolein, 1 276 uses for, 21 120 P-pinene, 3 230 24 496-497 major products from, 24 478 /-menthol from, 24 522 as natural precursor for aroma chemicals, 3 232 terpenoids from, 24 478-479 P-propiolactone, polymerization of, 14 259 P-quartz solid solution, 12 637—638 Beta ratio, in filtration, 11 329—330 Beta (P) rays, 21 285 P-scission reactions, 14 280-281 P-skytanthine, 2 101 P-spodumene solid solution, 12 638-639 P-sulfur trioxide, 23 756 P-sultones, 23 527 P-tocopherol, 25 793 P-tocotrienol, 25 793 P-vinylacrylic acid, physical properties, 5 33t... 

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... 

Fig. 14.7 General reaction steps leading up to PAH formation. The reactions include abstractions (Abs.), beta-bond scission reactions (p), and addition reactions (Add.).

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. 

As a result of these oxidation reactions polymeric materials gradually lose their useful mechanical properties. During the oxidation degradation (mainly by beta-scission processes) and crosslinking reactions occur. 

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. 

The reaction of ODC6 DO to form a five member ring ODY(C5 )CDO radical has the lowest barrier of only 15 kcal mol This ODY(C5 )CDO radical can beta scission (eliminate) a formyl radical with a 37 kcal mof barrier to form cyclopentadiene-one (ODY(C5)). It is important to note that resonantly stabilized radicals of cyclopentadiene and cyclopentadiene-ones are formed in the oxidation steps of aromatic systems. Their thermochemistry, reaction paths and kinetics all need to be analyzed for the inclusion of aromatics in combustion mechanisms. 

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. 

Any free radical enters into the reactions of addition to oxygen (ad), of isomerization (is), of decomposition by beta-scission (bs) or by cyclization (cy), of oxidation (ox) and of metathesis (me). Some of these propagation reactions (bs, cy, ox, me) contribute to the formation of the primary products of the reaction. 

The addition reactions to oxygen of the R free radicals coming from the starting alkane RH, can be neglected. This causes the disappearance of the OOR, QOOH, QOQOOH, 11(0011)2 radicals and of their reactions (Figure 4). The only reactions left for the R radicals are those of beta-scission, of oxidation and of metathesis. 

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