Addition and Fragmentation Reactions

The anti-Markovnikov addition of HBr to alkenes was probably the first free-radical addition reaction to be discovered. The discovery was inadvertent around the turn of the twentieth century, scientists studying the regiochemistry of addition of HBr to alkenes found that the proportion of Markovnikov to anti-Markovnikov addition products varied inexplicably from run to run. Eventually, it was discovered that impurities such as O2 and peroxides greatly increased the amount of anti-Markovnikov addition product. The results were later explained by a free-radical addition mechanism. The anti-Markovnikov regiochemistry derives from the addi-ton of the Br- radical to the less substituted C of the alkene (steric reasons) to give the lower energy, more substituted radical (electronic reasons). In a polar reaction, Br- would add to the more substituted C of the alkene. 

In contrast to HBr, the acids HC1 and HI do not undergo free-radical addition to alkenes, even in the presence of peroxides or O2. Abstraction of H- from HC1 is too endothermic, and addition of I- to an alkene is too endothermic. However, thiols (RSH) add to alkenes by a free-radical mechanism exactly analogous to the addition of HBr. The initiator is usually AIBN or (BzO)2- The alkene may be electron-rich or electron-poor. Note that the conjugate addition of thiols to electron-poor alkenes can occur either by a free-radical mechanism or by a polar, nucleophilic mechanism. 

Problem 5.3. Draw a free-radical mechanism for the following addition reaction. 

Tributyltin hydride adds across C=C 77 bonds by a free-radical mechanism. Addition of BusSnH across alkynes is one of the best ways of making alkenyltin compounds, which are useful reagents in organic synthesis. The mechanism is exactly the 

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Unlike radical addition and fragmentation reactions, atom abstraction has no common counterpart in carbocation chemistry. 

Summaiy In this short review, selected experimental approaches for probing the mechanism and kinetics of RAFT polymerization are highlighted. Methods for studying RAFT polymerization via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerization and j,s-time-resolved electron spin resonance (ESR) spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerization. 

RAFT polymerization process uses dithioester as the mediator. Propagating radical reacts with the dithio compound and becomes adduct radical that is not active for monomer propagation. This is an addition reaction that is equivalent to radical deactivation. The adduct radical is not stable and undergoes S-scission in either direction to generate a propagating radical, which is a fragmentation reaction. These addition and fragmentation reactions are frequent and reversible ... 

Two approaches have been used in the synthesis of these types of compounds. Small boron-phosphorus ring compounds can serve as building blocks, and addition and elimination reactions with other main group elements can then extend the cage structure (see Schemes 23 and 24, Section 12.12.6.4.5). Alternatively, an unsaturated carbenoid fragment can be added to the bicyclic fragment as illustrated in Scheme 31 <1998IC490>. 

Fig. 1. Pressure effect on the yield of addition and fragmentation products in the reaction of oxygen atoms with propylene.

Fig. 2. Pressure effect on the yield of addition and fragmentation products in the reaction of oxygen atoms with 1,3-butadiene. (Filled circles show lack of effect of the presence of 3 mm. 02 on the yields of addition products.)...

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

This is a quasi-equilibrium, which is valid when the rates of termination reactions (e.g., and k terms in Eq. (Pll.20.2)) are negligibly small compared with those of addition and fragmentation ikadd and kfr terms inEq. (Pll.20.2)). 

Chapter 1 provides a historical viewpoint (perspective) on the study of ion/mol-ecule association (cationization) MS as well as explanation on the evolution of developments of the instmmental methods. In addition to serving as an introduction for the subject of cationization MS as it pertains to ion chemistry, this chapter briefs thermochemistry and chemical dynamics (and analytical application) of metal ion association reaction. The fundamentals for iorr/molecule association reaction are described in Chapter 2, providing a basic introduction to the mechanism and dynamics of termolecrrlar association reaction, dissociation and fragmentation reaction of associated ion and ion/molecule association mechanism in the corrderrsed-phase. 

Propagation radical - molecule reactions generate the characterizing reaction products and include abstraction, substitution, addition, and fragmentation. 

Characteristic products of free radical reactions are generated through abstraction, substitution, addition, and fragmentation (5). A common feature of these reactions is that radical reactants lead to radical products. This feature is key to the autocatalytic nature of radical reactions. 

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