Electrophilicity haloalkyl radicals

The halocarbons react more rapidly with nucleophilic radicals than with electrophilic radicals. Thus, values of Cir with S and VAc are substantially higher than those with acrylic monomers ( fable 6.4) where the transfer constant is close to ideal (Clr=l.0). The haloalkyl radicals formed have electrophilic character (Section 2.3,2). 

Heating or photolytic treatment of A,A-dialkyl-A-haloamine in sulfuric acid or trifluoroacetic acid, followed by neutralization with a base, generates a pyrrolidine or piperidine skeleton. This is the Hofmann-Loffler-Frey tag reaction, and the reaction comprises of the formation of an electrophilic aminium radical, 1,5-H shift (6-membered transition state) or 1,6-H shift (7-membered transition state), formation of 4-haloalkyl ammonium or 5-haloalkyl ammonium, and its polar cyclization by neutralization with a base. Eq. 6.16 shows the formation of A-alkyl pyrrolidine (31) from A-chloro-A-alkyl-A-butylamine (30) in sulfuric acid [46, 47]. 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilicity of radicals is by way of a Hammett o p correlation. Thus, the reactions of radicals with substituted. styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character " while haloalkyl radicals" and oxygcn-ccntcrcd radicals" have electrophilic character (Table 1.4). It is anticipated that electron-withdrawing substituents (e g. Cl, F, CO R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

For nucleophilic radicals like styrene, the reactivity is much higher than for electrophilic radicals like acrylates. The haloalkyl radicals that are formed in the transfer reaction are, in turn, electrophilic in nature. 

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See also in sourсe #XX -- [ ]

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