Radical attack

AH components of the reaction mixture, whatever their source, are subject to the same kind of radical attacks as the starting substrate(s). Any free-radical oxidation is inevitably a cooxidation of substrate(s) and products. The yields of final products are deterrnined by two factors (/) how much is produced in the reaction sequence, and (2) how much product survives the reaction environment. By kinetic correlations and radiotracer techniques, it is... 

Between 6 and 30% of the radical attack on butane may occur at the primary hydrogen atoms (213). Since ca 6% of the butane goes to or through butyric acid (22), the middle of this range does not seem unreasonable. Because it is much more resistant to oxidation than its precursors or coproducts, acetic acid (qv) is the main product of butane LPO. 

Free-Radical Reactions. Free radicals attack isoprene, and two competing mechanisms, at the double bond or involving C—H bonds, are postulated ... 

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

Most solvents for hydroperoxides are not completely inert to radical attack and, consequendy, react with radicals from the hydroperoxide to form solvent-derived radicals, either by addition to unsaturated sites or by hydrogen- or chlorine-atom abstraction. In equation 15, S—H represents solvent and S is a solvent radical. 

Product identification does not distinguish OH versus hole oxidation, because the products are identical. For example, the products identified in the photo oxidation of phenol (qv) (Fig. 7) may originate either by OH radical attack of the phenol ring, or by direct hole oxidation to give the cation radical which subsequendy undergoes hydration in solvent water. 

Substitution Reactions. Substitution reactions can occur on the methyl group by free-radical attack. The abstraction of an aHybc hydrogen is the most favored reaction, followed by addition to that position. 

Pryolysis of soHd Cp2Ti(CD2)2 yields CD H but not CD. Pyrolysis of (C D )2Ti(CH2)2 yields CH D. These results show that the radical attacks the Cp rings (301,302). Pyrolysis of Cp2Ti(CgH )2 proceeds via a ben2yne intermediate, as shown by trapping experiments involving cycloadditions (293,303-306). 

Free-Radical Addition. Free-radical attack on a butylene occurs so that the most stable radical carbon stmcture forms. Thus, in peroxide-catalyzed addition of hydrogen haUdes, the addition is anti-Markovnikov. 

Whereas polyisobutylene and butyl mbber exhibit chain cleavage on free-radical attack, halobutyls, particulady bromobutyl and CDB, are capable of being cross-linked with organic peroxides. The best cure rate and optimal properties are achieved using a suitable co-agent, such as y -phenjiene bismaleimide. This cure is used where high temperature and steam resistance is required. 

Electrophilic attack Nucleophilic attack Free radical attack Photochemical reactions Oxidative and reductive reactions... 

The concept and use of free radical attack on pyrimidines has been little developed. However, pyrimidine does react slowly with p-nitrobenzenediazonium chloride to yield some 2- and 4-p-nitrophenylpyrimidines (51JCS2323) in addition, 2,4-and 4,6-dimethyl-pyrimidine are converted by hydroxymethylene radicals (from ammonium peroxydisul-fate/methanol) into 6- and 2-hydroxymethyl derivatives, respectively (77H(6)525). Certain bipyrimidine photoproducts appear to be formed from two similar or dissimilar pyrimidinyl radicals (see Section 2.13.2.1.4). 

There appear to be no reports of direct radical attack on the pyridopyrimidine ring system, but radical bromination of methyl substituents in the 7-position of the pyridine ring has been utilized in the synthesis of deaza analogues of natural products (62JCS4678, 79JHC133). 

It is estimated that thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (112) are treated with persulfate (8UCS(Pt)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

Free radical attack at the ring carbon atoms... 

Phenyl radicals attack azoles unselectively to form a mixture of phenylated products. Relative rates and partial rate factors are given in Table 7. The phenyl radicals may be prepared from the usual precursors PhN(NO)COMe, Pb(OCOPh)4, (PhC02)2 or PhI(OCOPh)2. Substituted phenyl radicals react similarly. 

Reactions with Radicals S.06.3.7.1 Radical attack on sulfur... 

Oxaziridines substituted in the 2-position with primary or secondary alkyl groups undergo decomposition at room temperature. In the course of some weeks, slow decomposition of undiluted compounds occurs, the pattern of which is analogous to that of acidic or alkaline N—O cleavage (Sections 5.08.3.1.3 and 4), Radical attack on a C—H bond in (109) effects N—O cleavage, probably synchronously (57JA5739). In the example presented here, methyl isobutyl ketone and ammonia were isolated after two hour s heating at 150 °C. 

When )3-scission can occur in the radical, further reactions compete with acid amide formation. Thus oxaziridine (112) with iron(II) ion and acid yields stabilization products of the isopropyl radical. If a-hydrogen is present in the Af-alkyl group, radical attack on this position in (113) occurs additionally according to the pattern of liquid phase decomposition. 

The diaziridine ring exhibits a surprising stability towards strong oxidizing agents. Diaziridines unsubstituted at both N atoms can be transformed into diazirines by dichromate in acidic solution or by chlorine (Section 5.08.4.3). Radical attack by decomposing peroxide converts (136) to the diaziridinyl radical (137), as evidenced by ESR spectroscopy (76TL4205). 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

The simplest method for obtaining selective fluonnation is to conduct reactions under conditions that invigorate the electrophilicity of fluorine In practice this method entails the creation of anionic or strongly nucleophilic reactive centers on substrate molecules while suppressing or reducing the tendency toward radical attack Numerous examples of seleetive fluorine attack on carbanionic, amido and carboxylato species are documented Especially abundant is alpha fluonnation of nitroalkanes in polar solvents [42 43, 44, 45 46] (equations 10-14)... 

In an unusual example of displacement of fluonne by hydroxyl, hydroxyl radicals attack fluorinated benzenes Hexafluorobenzene is the least reactive The hydroxyl radical generates the pentafluorocyclohexadienonyl radical from it [13] (equation 13) These unstable species are detected spectroscopically Their disap-... 

TFifluorothiolacetic acid adds to a senes of olefins under ultraviolet irradiation The addition appears to start by a CFjCOS radical attack, giving the more stable radical intermediate [14] (Table 2)... 

In the reaction of 2,3,3-triethyloxazirane (25), three radicals are involved 26, 27, and 28. Radical 26 (Fig. 1) corresponds to the chain reaction propagating radical of the previously mentioned decomposition [Eqs. (20) and (21)]. From 26 hy fragmentation an ethyl radical (27) is formed together with the acid amide. Finally, by radical attack on the oxazirane, 29 can be formed which rearranges to the... 

The mechanism proposed by Emmons thus corresponds in part to the decomposition of the trialkyl-oxaziranes by ferrous salts. By radical attack on the 7V-alkyl group of the oxazirane, the radical 32 is formed which rearranges with ring opening to 33. Radical 33 propagates the chain by attack on a further molecule of oxazirane. It takes up an H-atom and is decomposed to ketone and ammonia. The aldehyde produced from the M-alkyl group is converted to tar. 

The reaction described here is probably responsible for the slow decomposition of many oxaziranes at room temperature, 2-fert-Alkyl-oxaziranes are stable at room temperature for unlimited periods radical attack on the a-C-atom of the iV-alkyl group is not possible. By contrast, oxaziranes containing a C—H group which is alpha to the N-atom are unstable at room temperature on keeping they are largely decomposed within a few weeks. 

This interpretation for acridine is consistent with the finding of Waters and Watson that benzyl radicals attack the meso-carbon but not nitrogen, but it is possible that methyl radicals, like benzyl radicals, also react at the nitrogen centers of phenazine (cf. Section... 

Vapor phase brominations have given rise to varying products dependent on the reaction temperature. At 300°C bromine converted quinoline in the presence of pumice into 68 (25%) at 450°C 2-bromoquinoline (25%) became the major product at 500°C the yield of the 2-bromo isomer increased to 53%, but there was some dibrominated material [77HC(32-1)319]. The absence of 3-bromoquinoline at the higher temperatures could be accounted for in terms of radical attack, or it could be due to thermal instability of that isomer [59CI(L)1449]. 

In carbonyl compounds the aryl radical attacks the carbonyl carbon atom, whereas in thiocarbonyl compounds the sulfur atom reacts. Petrillo et al. (1988) obtained various S-arylthioacetates in 40-60% yield by treating arenediazonium... 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

One of the steps in the radical chain selenosulfonation of multiple bonds (vide infra) is equation 10, in which the R radical attacks via a SH2 reaction the Se-aryl areneselenosul-fonates to regenerate a sulfonyl radical, namely. 

Once it has been initiated, the chain reaction can propagate as the radicals attack the monomer molecules CHX=CH2 (with X = H for ethene itself) and form a new, highly reactive radical ... 

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