Free radical addition initiators

FIGURE 6 7 Initiation and propagation steps in the free radical addition of hydrogen bromide to 1 butene... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Free-Radical Addition. In free-radical addition polymerisation, the propagating species is a free radical. The free radicals, R-, ate most commonly generated by the thermal decomposition of a peroxide or aso initiator, 1 (see Initiators, free-RADICAl) ... 

Examples of perfluoroalkyl iodide addition to the triple bond include free radical addition of perfluoropropyl iodide to 1 -heptyne [28] (equation 21), thermal and free radical-initiated addition of lodoperfluoroalkanesulfonyl fluorides to acetylene [29] (equation 22), thermal addition of perfluoropropyl iodide to hexa-fluoro 2 butyne [30] (equation 23), and palladium-catalyzed addition of per-fluorobutyl iodide to phenylacetylene [31] (equation 24) The E isomers predominate in these reactions Photochemical addition of tnfluoromethyl iodide to vinylacetylene gives predominantly the 1 4 adduct by addition to the double bond [32] Platinum catalyzed addition of perfluorooctyl iodide to l-hexyne in the presence of potassium carbonate, carbon monoxide, and ethanol gives ethyl () per fluorooctyl-a-butylpropenoate [JJ] (equation 25)... 

Free-radical addition of hydrogen bromide to the double bond can also be initiated photochemically, either with or without added peroxides. 

Acrylamide polymerization by radiation proceeds via free radical addition mechanism [37,38,40,45,50]. This involves three major processes, namely, initiation, propagation, and termination. Apart from the many subprocesses involved in each step at the stationary state the rates of formation and destruction of radicals are equal. The overall rate of polymerization (/ p) is so expressed by Chapiro [51] as ... 

The free radical additions of sulfonyl halides to alkenes, catalyzed by light or typical chemical radical initiators (In), were first investigated in the 1950s69. The products which are / -halo sulfones (22) were obtained via a chain reaction in which RSO j acts as the chain carrier, namely61-62,70,71... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

The compounds CCI4, BrCCla, ICF3, and similar simple polyhalo alkanes add to alkenes in good yield. These are free-radical additions and require initiation, for... 

A variant of the free-radical addition method has been used for ring closure. For example, treatment of 75 with the free-radical initiator hexamethylditin gave a mixture of cis- and tmns-76, with a small amount of cis- and trans-11 (total yield 83%). The reaction has been performed with a-iodo esters, ketones, and malonates. 

The recognition of anti-Markownikoff orientation when HBr was added to alkenes in the presence of traces of peroxides or air lead to the discovery of the large and important class of free radical addition reactions to unsaturated systems89). The ant Markownikoff orientation of these reactions i.e., the preference of initial radical at-... 

Free-radical additions of trichlorosilane to acetylenes initiated by benzoyl peroxide were stereospecific trans additions, giving only cis adducts. The same workers observed that tri-n -butyl amine also catalyzed addition of trichlorosilane to phenylacetylene but gave a mixture of cis- and trans -l-phenyl-2-trichlorosilylethene, 1,1-phenyltrichlorosilylethene, and 1-phenyl-l,2-bistrichlorosilylethane (54). No stereospecificity was observable with the amine as catalyst. 

The /-BuO abstracts H- from malonate in the initiation part. A free radical addition mechanism like the one in problem 5.3 ensues. 

Intermolecular free-radical addition reactions almost always proceed by chain mechanisms. Here light photoexcites acetone, and O then abstracts H- from the a-position of another molecule of acetone to complete the initiation. 

According to free radical addition polymerization theory, the rate of polymerization of monomer M is proportional to the square root of the initiator I concentration (Equation 3) when termination is bi mol ecular (18). 

Figure 2.19 Addition polymer free radical formation, initiation of the monomer, and propagation...

Write the name and structure of one of the common Initiators used In free radical addition pol5mierIsatIon. 

---