Radical reactions additions

Arnaud, R., Barone, V., Olivella, S., and Sole, A., Ab initio mechanistic studies of radical reactions, addition of methyl radical to acetylene and ethylene, Chem. Phys. Lett. 118, 573 (1985). 

Radical reactions Addition reactions Elimination reactions... 

The addition of a triplet carbene to an alkene can be considered to be rather like a radical addition to a double bond. The concerted addition of a singlet carbene, on the other hand, is a pericyclic reaction, and from Chapter 3 5 you should be able to classify it as a [1 -h 2 ] cycloaddition, addition to alkenes of triplet carbenes is a radical reaction addition of singlet carbenes is a [1-h2] cycloaddition... 

The combination of Et3B and 02 is widely used to initiate radical reactions. The mechanism involves two unusual radical reactions. Addition of 02 to the empty orbital of BEt3 provides a one-electron bond between B and O, and a fragmentation follows to provide Et- and Et2BOO-. 

In 1992, the reported preparations of C-disaccharides included the utilization of free radical reactions. Additionally, the use of various polyol cyclizations were discussed. In the application of radical chemistry to the synthesis of C-disaccharides, Bimwala, et al.,35 expanded upon his earlier work illustrated in Scheme 8.9.5. As shown in Scheme 8.10.1, glycosidic radicals were added a,p-unsaturated carbonyl compounds producing yields ranging from 47% to 73%. Furthermore, the illustrated C-disaccharide precursors exhibited diastereomeric ratios of approximately 5.5 1. 

Addition mechanisms are broadly defined to be heterolytic, homolytic, or cyclic, which are processes that involve ionic or radical intermediates or are concerted, respectively. Concerted reactions will be discussed in Chapter 11. The emphasis here will be heterolytic (ionic) additions, although we will also consider some aspects of radical reactions. Addition reactions may be categorized further as being electrophilic or nucleophilic. In an electrophilic addition, a compoimd with a multiple bond reacts with an electrophilic reagent to produce an intermediate that subsequently reacts with a nucleophile. In nucleophilic addition, the unsaturated substrate reacts with a nucleophile to produce an intermediate that subsequently reacts with an electrophile to produce the final product. ... 

Sources of Radical Intermediates Introduction of Functionality by Radical Reactions Addition Reactions of Radicals with Substituted Alkenes Cyclization of Free-Radical Intermediates Fragmentation and Rearrangement Reactions... 

PAHs are very stable at ambient temperature, but undergo chemical reactions at appropriate conditions. These reactions can be grouped into seven main classes electrophilic substitution, nucleophilic and free radical reaction, addition reaction, reduction and reductive alkylation, oxidation reaction, rearrangements of aromatic ring s) tems, and complex formation. 

Benzene can undergo addition reactions which successively saturate the three formal double bonds, e.g. up to 6 chlorine atoms can be added under radical reaction conditions whilst catalytic hydrogenation gives cyclohexane. 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

Without other alternatives, the carboxyalkyl radicals couple to form dibasic acids HOOC(CH)2 COOH. In addition, the carboxyalkyl radical can be used for other desired radical reactions, eg, hydrogen abstraction, vinyl monomer polymerization, addition of carbon monoxide, etc. The reactions of this radical with chloride and cyanide ions are used to produce amino acids and lactams employed in the manufacture of polyamides, eg, nylon. 

Fig. 6. Coupling of polymer chains via (a) photoinduced hydrogen abstraction free-radical reactions and (b) nitrene insertion/addition reactions.

Addition. Chlorine adds to vinyl chloride to form 1,1,2-trichloroethane [79-00-5] (44—46). Chlorination can proceed by either an ionic or a radical path. In the Hquid phase and in the dark, 1,1,2-trichloroethane forms by an ionic path when a transition-metal catalyst such as ferric chloride [7705-08-0], FeCl, is used. The same product forms in radical reactions up to 250°C. Photochernically initiated chlorination also produces... 

Vinyhdene chloride polymeri2es by both ionic and free-radical reactions. Processes based on the latter are far more common (23). Vinyhdene chloride is of average reactivity when compared with other unsaturated monomers. The chlorine substituents stabih2e radicals in the intermediate state of an addition reaction. Because they are also strongly electron-withdrawing, they polari2e the double bond, making it susceptible to anionic attack. For the same reason, a carbonium ion intermediate is not favored. 

This reaction proceeds through a chain mechanism. Free-radical additions to 1-butene, as in the case of HBr, RSH, and H2S to other olefins (19—21), can be expected to yield terminally substituted derivatives. Some polymerization reactions are also free-radical reactions. 

Composite resins can be cured using a variety of methods. Intraoral curing can be done by chemical means, where amine—peroxide initiators are blended in the material to start the free-radical reaction. Visible light in the blue (470—490 nm) spectmm is used to intraoraHy cure systems containing amine—quin one initiators (247). Ultraviolet systems were used in some early materials but are no longer available (248). Laboratory curing of indirect restorations can be done by the above methods as well as the additional appHcation of heat and pressure (249,250). 

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