Alpha-scission

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.)... 

Alpha-scission is not favored thermodynamically but does occur. Alpha-scission produces a methyl radical, which can extract a hydrogen atom from a neutral hydrocarbon molecule. The hydrogen extraction produces methane and a secondary or tertiary free radical (Equation 4-3). 

Alpha-scission the rupture of the aromatic carbon-aliphatic carbon bond that joins an alkyl group to an aromatic ring. 

Oxidation. AH polyamides are susceptible to oxidation. This involves the initial formation of a free radical on the carbon alpha to the NH group, which reacts to form a peroxy radical with subsequent chain reactions leading to chain scission and yellowing. As soon as molten nylon is exposed to air it starts to discolor and continues to oxidize until it is cooled to below 60°C. It is important, therefore, to minimize the exposure of hot nylon to air to avoid discoloration or loss of molecular weight. Similarly, nylon parts exposed to high temperature in air lose their properties with time as a result of oxidation. This process can be minimized by using material containing stabilizer additives. 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

This radical can undergo beta-scission. The products will be an alpha-olefin and a primary free radical (Equation 4-4). 

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 silyl radicals formed in the initial scission appear to undergo further reactions, which may be complex. A possible secondary reaction is hydrogen transfer from an alpha carbon atom to give Si-H and a silicon carbon double bond (21)... 

POLYAMINO ACIDS Aliphatic polyamino acids irradiated in the solid state have been reported to undergo N-Ctf, main-chain, bond scission on gamma radiolysis (9) and the stable radical intermediates formed following radiolysis at 303 K are alpha carbon radicals, as observed in the N-acetyl amino acids. 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

Another common method for determination of alpha-samylase involves measurement of the diminution of the iodine-stainability of whole starch or amylopectin )8-limit dextrin. These methods are useful, as very small quantities of alpha-amylase may be estimated in this way. They are comparative, and cannot be used to determine rates of bond scission, unless they are carefully calibrated by a method that measures reducing end-groups. Such a calibration has been made for the action of hog-pancreas alpha-amylase on waxy-maize starch, although the validity of the reducing-power determinations, made with iodine and thiosulfate, has been confirmed for maltose only thus, it is not yet known whether the calibration is actually valid for the experimental system in which larger oligomers, for example, the triose and tetraose, are also formed. 

A third technique for determination of alpha-amylase is the viscometric method. This is the most sensitive method for studying the initial attack on amylose, but it is too time-consuming to be used as a routine method for enzymic assay. If the relation between viscosity and molecular weight is known for the substrate, an estimate of the rate of bond scission may be made. (This estimation would be valid only if completely random attack were occurring.)... 

Estimations of and inhibition studies are, however, more valid, because determinations of absolute rates of bond scission are not necessary. The Michaelis constants for several alpha-amylases have been determined, and are shown in Table XI it may be seen that wide variations occur. For hog-pancreas alpha-amylase, it has been claimed that the value of depends on the degree of hydrolysis of the substrate this is probably true for all alpha-amylases when high degrees of hydrolysis are reached. 

Oxabicyclic ketones have also been further derivatized to the alpha-oxidation products which are in turn cleaved, offering still another option for carbon-carbon bond scission. For example, hydroxyketone 114, available in > 98 % ee from the parent ketone, was cleaved by lead tetraacetate to afford an excellent yield of the hydroxyester 115, a key intermediate in Noyori s synthesis of showdomycin, Eq. 86 [121,129]. In this case, the ozonolysis of the silyl enol ether of the parent ketone led to complex mixtures, demonstrating the complementarity of these approaches. 

Figure 6 shows the uncorrected thermochemical excitation distributions corresponding to energetic substitution reactions at the fluorinated alpha carbon atom positions in CH3CHF2 and CH3CF3. From the left the respective fractional yield segments include the undecomposed primary species and the decomposition products from HF-eliminations, from secondary carbon-carbon scissions to alkyl radicals, and from tertiary radical dissociations. Following these alpha substitution processes product decomposition occurs principally via HF-elimination. 

GC-MS). Initial reactions involved abstraction of a hydrogen from a methylene group alpha to an ester group. This was followed by oxidation, formation of hydroperoxide, and homolytic scission of the hydroperoxide to form an alkoxy radical. This can be summarised as ... 

A combination of gas chromatography and either electron-impact or chemical ionization mass spectrometry has been used to analyse the products of thermal degradation of poly(vinyl fluoride) and of a number of other polymers [poly(vinyl chloride), aromatic polyimides, polyurethane]. The degradation of poly(vinyl-idene fluoride) has been related to its crystalline form. It is claimed that dehydrofluorination may take place preferentially in crystalline segments containing trans sequences. Thermo-oxidative breakdown is modified if vinylidene fluoride is copolymerized with tetrafluoroethylene or hexafluoroacetone. Dehydrofluorination occurred in both copolymers, but in the latter it was preceded by cleavage of the H from the CHj group in the alpha position to the ether bond followed by scission of the C-0 bond. ... 

Hafi et al. [2] studied the effect of 11 MeV protons and 26.6 MeV alpha particles on chain scission and cross-linking of polyether ether ketone. 

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