Reactions of radicals

Radicals are very reactive reaction intermediates and their half-life period is very short. Some of the reactions of radicals are given below. 

The reaction of methane with chlorine in the presence of ultraviolet light gives a mixture of methyl chloride, methylene chloride, chloroform and carbon tetrachloride. When excess of chlorine is used and the time of the reaction is prolonged, the final product is predominantly carbon tetrachloride. 

The substitution of a hydrogen atom in the benzylic position by a bromine atom on reaction with N-bromosuccinimide in the presence of catalytic amounts of AIBN (2.37) is known as the Wohl-Ziegler process k 

Dehalogenation Dehalogenation of haloalkanes (R-X) is often carried out with trib-utyltin hydride (2.43) in the presence of AIBN (2.37). The reactivity of R-X is in the order ofR-I R-Br R-Cl (R-F being inert) tertiary secondary primary aryl or vinyl. 

The overall reaction results in the reduction of carbon-halogen bonds to carbon- hydrogen bonds. 

Radical reactions are frequently found to occur as chain reactions composed of three types of processes  

An initiation step, such as one of the generation reactions discussed in the previous section. 

One or more termination steps that stop the chain reaction. 

Each propagation step in a radical chain involves the reaction of a species with one unpaired electron to produce another species having one unpaired electron. Moreover, the reactant in one propagation step is a product in a subsequent propagation step. In the chlorination of methane, the first propagation step is the abstraction of a hydrogen atom by a chlorine atom (equation 5.13), and the second step is the abstraction of a chlorine atom from CI2 by the methyl radical (equation 5.14). 

A carbon-centered radical can also abstract an atom from another molecule (or from another atom in the same molecule) to fill its outer shell if the free energy change for the abstraction is favorable. One example of this process is radical trapping, in which a radical abstracts a hydrogen atom from a 

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For the reaction of radical i with monomer j, Price and Alfrey assume that the cross-propagation rate constant can be written as... 

The mathematical model was based on the scheme utilized in chemiluminescent method that was supplement with the reactions of radicals, formed of inhibitor molecules - AO. 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

In their thermal stability the diaziridines approximate to the oxaziranes. As with most oxaziranes, they are stable at 100° C for short periods they are decomposed by heating at 200°C 1,2-di-n-butyl-3-ri-propyldiaziridine thus eliminates butylamine. The thermal decomposition has not yet been investigated in detail. Similarly no information is available on the reaction of radical reagents on diaziridines. 

As early as 1940 it has been established9 that diketene does not polymerize by a radical mechanism. It has, however, been shown later10 that it undergoes reactions of radical copolymerization with many vinyl monomers11. In this reaction the double bond is involved and the lactone ring is preserved in the copolymer. 

Tabic 1.3 Relative Rate Constants for Reactions of Radicals with Alkyl-Substituted Acrylate Esters CHR CFEcOaCHs"... 

Outcomes from the reactions of radicals with substituted acrylate esters depend on the attacking radical (refer Table 1.3 and Scheme 1.4). The results may be summarized as follows (the methyl substituent is usually considered to be electron donating - Section 1.2.2) ... 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilieity of radicals is by way of a Hammett op correlation. Thus, the reactions of radicals with substituted styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character38,39 while haloalkyl radicals40 and oxygcn-ccntcrcd radicals " have electrophilic character (Tabic 1.4). It is anticipated that electron-withdrawing substituents (e.g. Cl, F, C02R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

The efficiency of these inhibitors may depend on reaction conditions. For example the reaction of radicals with stable radicals (e.g. nitroxides) may be reversible at elevated temperatures (Section 7.5.3) triphenylmethyl may initiate polymerizations (Section 7.5.2). A further complication is that the products may be capable of undergoing further radical chemistry. In the case of DPPH (22) this is attributed to the fact that the product is an aromatic nitro-compound (Section 5.3.7). Certain adducts may undergo induced decomposition to form a stable radical which can then scavenge further. 

Analysis of the products from the thermal decomposition of the mixed azo compound 6 showed that in the cross-reaction of radicals 5 and 7 ld/A tt(90oC) is 0.61.179 This study also found that in disproportionation, hydrogen transfer from 5 to 7 is ca 2.2 times more frequent than transfer from 7 to 5. Both self-reactions involve predominantly combination (Scheme 7.14). The values of Ar1j/Aru.(80°C) are 0.16 and 0.05 for radicals 5 (Section 5.2.2.1.1) and 7 (Section 5.2.2.1.3) respectively. It is clear that values of kJkK for homotermination cannot be used as a guide to the value for kjkyt in cross-termination. 

The authors did not give much quantitative information in their papers. The G values for decomposition are almost twice as large in ACSO (8.5) as compared to PCSO (4.6). The value in ACSO is higher than the yield of the radicals in aqueous solution and should be attributed to reactions of radicals produced from ASCO with other ASCO molecules, for... 

Fig. 2. Fomiation and reactions of radical pairs. M = precursor (multiplicity, S or T) SX=solvent molecule or radictil scavenger.

Many chemical reactions can be classified by either abstraction or addition-elimination mechanisms. Abstraction mechanisms are common in the reaction of radicals with closed-shell species, such as the reaction... 

General Considerations Regarding Reactions of Radicals with the DNA Bases... 

Absolute rates have been measured for some carbene reactions. The rate of addition of phenylchlorocarbene shows a small dependence on alkene substituents, but as expected for a very reactive species, the range of reactivity is quite narrow.119 The rates are comparable to moderately fast bimolecular addition reactions of radicals (see Part A, Table 11.3). 

Addition Reactions of Radicals with Substituted Alkenes... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

The reactions of radicals with oxygen are diffusion-controlled it-S-S. Moreover, as has been previously shown, the isobutyryl radical a could readily be captured by a nitroxide. It is therefore not easy to see why reaction between oxygen and the species a does not also occur. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

He argues that solute reaction does not contribute to /), but creation of a species by reaction of radicals or a solute does. With this understanding and comparing with numerical calculation for the 2-radical, 1-solute case, he finally suggests... 

B. Reactions of Radical Anions of Aromatic Hydrocarbons with... 

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