Radical, generally

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Reactions between carbon-centered radicals generally give a mixture of disproportionation and combination. Much effort has been put into establishing the relative importance of these processes. The ratio of disproportionation to combination (kt /k]t ) is dependent on the structural features of the radicals involved and generally shows only minor variation with solvent, pressure, temperature, etc. 

In the absence of heteroatom containing substituents (e.g. halo-, cyano-), at or conjugated with the radical center, carbon-centered radicals have nucleophilic character. Thus, simple alkyl radicals generally show higher reactivity toward electron-deficient monomers (eg. acrylic monomers) than towards electron-rich monomers (e.g, VAc, S) - Table 3.6. 

Cyanoisopropyl radicals generally show a high degree of specificity in reactions with unsaturated substrates. They react with most monomers (c.g. S, MMA) exclusively by tail addition (Scheme 3.4). However, Bcvington et al.11 indicated that cyanoisopropyl radicals give ca 10% head addition with VAc at 60 °C and that the proportion of head addition increases with increasing temperature. 

Primary and secondary aikoxy radicals generally show a reduced tendency to abstract hydrogen or to undergo 3-scission when compared to the corresponding i-alkoxy radical.1"3 402 This has been correlated with the lesser nucleophilicity of these radicals.427... 

The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other... 

Free radicals generally undergo one-electron transfer processes in homogeneous solution. Two-electron transfer processes, in which two radicals participate, are often highly exoergic. Typical examples are... 

Closs and Trifunac, 1970 Baldwin and Andrist, 1971 Lepley and Closs, 1972 Bethell and McDonald, 1977). The formation of free radicals from aromatic carbenes is often easily detected by the fast laser spectroscopic techniques discussed earlier. The radicals generally have characteristic absorption spectra and reactivity patterns that make their identification certain. The direct insertion reaction of singlet carbenes is not expected to generate free radicals. 

Electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR), has rightly been called the NMR of unpaired electrons . A sample of a radical (generally in solution) is irradiated with a beam of microwave radiation and the proportion of the radiation that is absorbed is then determined. 

B. Giese, C-Radicals General Introduction, in Methoden Org. Chem. (Houben-Weyl) 4th ed. 

In reactions of free radicals or atoms to form molecules the electronic partition function may not be negligible, since atoms or radicals generally have odd numbers of electrons and hence a multiplicity of electronic states, while the molecules will not. 

The alcohol radicals contain a dissociable proton on the hydroxyl group so that the basic form will begin to be present when the pH of the solution approaches the pKa of the radical. Generally, the basic form of an alcohol radical is a stronger reductant than the undissociated form. The relevant data are collected in Table 6. 

Pericyclic Reactions of Anion Radicals General aspects of anion radical pericyclic additions... 

However, since free radicals generally lack selectivity in attacking substrates or vinyl monomers, grafting efficiencies lower than 100% are to be expected. 

Free radicals generally exhibit ESR intensities that obey the Curie law (7=CT ). Although ESR intensities of coals have been reported to increase with decreasing temperature as required by Curie s law (12, 23),... 

Mixtures with perchloric acid Reactions of free atoms or radicals General considerations Final remarks Storable liquids Cryogenic liquids Literature... 

Certain radicals, generally acid, such as the sulpho-group, SO3H, and the carboxyl group, if introduced into a chromogen, confer upon it acid properties, hut without increasing the colour appreciably, indeed in many cases the dyeing power is considerably decreased. 

---