Primary radicals, stability

A similar explanation lies behind the diminished strength of the sp —sp carbon-carbon bond in ethylbenzene. The general trend toward weaker C—C bonds with increased substitution that can be recognized in Table 1.3 reflects the increased stability of substituted radicals relative to primary radicals. 

The radical stabilization provided by various functional groups results in reduced bond dissociation energies for bonds to the stabilized radical center. Some bond dissociation energy values are given in Table 12.6. As an example of the effect of substituents on bond dissociation energies, it can be seen that the primary C—H bonds in acetonitrile (86 kcal/mol) and acetone (92kcal/mol) are significantly weaker than a primaiy C—H... 

Stabilizer molecule Monomer molecule Initiator molecule Primary radical Oligomeric radical... 

In the mass spectrum (Figure 8) of the corresponding ketal of 5-deoxy-D-xt/Zo-hexose, 5-deoxy-l,2-0-isopropylidene-D- rt/Zo-hexofuranose (11), the peak from C-4-C-5 cleavage, m/e 159, is of minor relative intensity. Since the ions at m/e 159 are the same from both isomers, 10 and 11, the intensity difference must be attributable to the lower stability of the primary radical formed from C-5 of 11 compared with the secondary radical from 10 ... 

Figure 10.2 Energy diagram for alkane chlorination. The relative rates of formation of tertiary, secondary, and primary radicals are the same as their stability order.

Heterolytic (two-electron, ionic) oxidation of 1, or alternatively further one-electron loss from the primary radical 2, affords chromanoxylium cation 4 with its positive charge mainly localized at C-8a. Cation 4 is stabilized by resonance so that a positive partial charge results also at C-5 and C-7, where nucleophilic attack is... 

The effects of substitutents on the y-carbon on the efficiency of the type II cleavage are presented in Table 3.15.<89) These data indicate that the rate constant of cleavage increases as the strength of the y C—H bond decreases, that is, from a primary to a secondary to a tertiary hydrogen atom. The substitution of groups capable of radical stabilization, such as — or... 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

On the oxidation side, the primary radical is symmetrically stabilized by addition of a base, or nucleophile, or by expulsion of an acid, in the general... 

The calculations also suggested that 5 was favored over 6 by 2.4 kcal mol 1 as found experimentally. The explanation was based on the higher stability of a tertiary alkoxide compared to a primary alkoxide [15], which outweighed the opposite trend for radical stabilization. Epoxide opening was irreversible [16]. 

Quite surprisingly, 28a and 29a are formed from 28 and 29 with about the same reaction energy (A E -4.0 kcal mol" ), even though secondary radicals are more stable than primary radicals by approximately 3 kcal mol-1 based on their bond dissociation energies. This must be due to steric interactions with the cyclopentadienyl ligand in 29a, which fully counterbalances the radical s increased stability. A similar trend of product stability is observed in the formation of the less favored primary radicals 29b and 30b. The formation of 30a is more favorable by 4.5 kcal mol 1 compared to 29a. This is even higher than the stability difference between a tertiary and a secondary... 

In the case of asymmetrical ketones, two different modes of a-cleavage can occur, with the major products being formed via the more stable pair of initially-formed radicals. For alkyl radicals, the stability of the radical increases as its complexity increases and radical stabilities are tertiary > secondary > primary. 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

Although a radical is neutral, it is an electron-dehcient species that will be very reacdve as it attempts to pair off the odd electron. Because radicals are electron dehcient, electron-releasing groups such as alkyl groups tend to provide a stabilizing effect. The more electron-releasing groups there are, the more stable the radical. Thus, tertiary radicals are more stable than secondary radicals, which in turn are more stable than primary radicals. 

The selectivity of radical bromination reactions depends, in part, on the increased stability of secondary or tertiary radical intermediates compared with primary radicals. In Section 9.2 we noted that allyl and benzyl radicals were especially... 

Bromination occurs exclusively at the benzylic position, i.e. adjacent to the benzene ring. The radical formed at this position is resonance stabilized, whereas no such stabilization is available to the primary radical formed by abstraction of one of the methyl hydrogens. 

There are many excellent books and reviews on the structure and reactions of secondary radical ions generated in radiolytic and photolytic reactions. Common topics include the means and kinetics of radical ion production, techniques for matrix stabilization, electronic and atomic structure, ion-molecule reactions, structural rearrangements, etc. On the other hand, the studies of primary radical ions, viz. solvent radical ions, have not been reviewed in a systematic fashion. In this chapter, we attempt to close this gap. To this end, we will concentrate on a few better-characterized systems. (There have been many scattered pulse radiolysis studies of organic solvents most of these studies are inconclusive as to the nature of the primary species.)... 

The latter proposal would lead one to conclude that radical 56, having a cyclopropane ring and a resonance-stabilized secondary radical, is more stable than is the isomeric form 55, which has a conjugated ketone and a primary radical. The product, therefore, is one derived from the intermediate 56. In the case where the 19-oxime is formed without any rearrangement, the initially formed intermediate 57, having a double bond and a primary radical, would be more stable than is the isomeric form 58, which contains a cyclopropane ring and a secondary radical. 

Various substances can reduce the rate at which a monomer is converted to polymer. Inhibitors completely suppress polymerizations whereas retarders only reduce the rate. The former deactivate very readily the primary radicals so that growth of polymer chains cannot begin the latter deactivate growing polymer radicals so causing premature termination. Inhibitors are commonly used to stabilize monomers during storage. Many nitro compounds and quinones act as inhibitors and retarders. 

Tables 9.3 and 9.4 list selected bond dissociation energies and radical heats of formation. Note particularly that the decrease in energy required to remove hydrogen in the series methane, primary, secondary, tertiary, parallels increasing radical stability, and that aldehydic, allylic, and benzylic hydrogens have bond dissociation energies substantially lower than do alkyl hydrogens.

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