Ions versus radicals

Termination - the coupling of two radicals to form only non-radical products. 

The chlorination of alkanes, such as methane (CEU), leads to the substitution of hydrogen atoms for chlorine atoms in a radical chain reaction (Section 5.2.1). 

Pericyclic reactions take place in a single step without (ionic or radical) intermediates and involve a cyclic redistribution of bonding electrons. 

The Diels-Alder cycloaddition reaction (Section 6.2.2.11). 4jt electrons 2jt electrons 

Heterolytic cleavage of bonds occurs at room temperature in polar solvents. The ions which are formed are solvated (i.e. a solvent shell surrounds them) and stabilised by polar solvents. 

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An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

Reactions of Radical Ion versus Doubly Charged Ion Disproportionation versus ECE Mechanisms. .. 

The details of a careful study conducted by Rathore and coworkers of the controversial arenium ion versus cation radical mechanisms of the SchoU reaction were reported only recently [52]. By using a dichlorodicyano-p-benzoquinone (DDQ)/H+ system and FeCls as oxidants, Rathore etal. obtained strong experimental evidence that the reaction proceeded largely via a cation radical mechanism. 

Interestingly, only 4-oxoretinoic add was deteded as a protonated species (versus radical molecular ions), even under LDI conditions. The authors suggested that this might be the result of a higher proton affinity of the oxo-retinoic acid compared to the other retinoic adds. 

Exploitation of time-resolved spectroscopy allows the direct observation of the reactive intermediates (i.e., ion-radical pair) involved in the oxidation of enol silyl ether (ESE) by photoactivated chloranil (3CA ), and their temporal evolution to the enone and adduct in the following way.41c Photoexcitation of chloranil (at lexc = 355 nm) produces excited chloranil triplet (3CA ) which is a powerful electron acceptor (EKelectron-rich enol silyl ethers (Em = 1.0-1.5 V versus SCE) to the ion-radical pair with unit quantum yield, both in dichloromethane and in acetonitrile (equation 20). 

It is noteworthy that both thermal and photoinduced electron-transfer activation of the [ArH, IC1] complex leads to the ion-radical triad. Consequently, iodination versus chlorination represents the competition between ion-pair and radical-pair collapse. This is confirmed by reactivity studies of dimethoxybenzene cation radical with chloride and iodine (atom), respectively,225 i.e.,... 

The explanation for the dichotomy between aromatic nitration versus de-alkylative oxidation in equation (84) lies in the dynamics of ion-radical triad (which is predictably modulated by solvent polarity and added inert salt). For example, the nonpolar dichloromethane favors aromatic nitration via a radical-pair collapse of ArH+, NO, 239 i.e.,... 

The electron-transfer formulation in Scheme 23 suggests that the efficiency of nitrosation is the direct result of the competition between deprotonation of the Wheland intermediate versus its breakdown to the original EDA complex via the ion-radical pair, i.e.,... 

Cyclizable radical-probe experiments have been extensively used in ET versus Spj2 investigations (see Ashby, 1988, and references cited therein). Attention has, however, been recently drawn to causes of possible misinterpretation, particularly in the case of iodides, where an iodine-atom-transfer chain mechanism is able to convert most of the starting linear iodide into the cyclized iodide, even if only a minute amount of linear-chain radical is present in 7-8 2 reactions (Newcomb and Curran, 1988). Rather puzzling results were found in the reaction of (CH3)3Sn ions with secondary bromides, which should not be involved in atom-exchange chain reactions... 

Chlorides RMe2CCH2Cl [(a) R = Me, R = Ph and (b) R = CH2Ph] reacted with diphenylphosphide ions in liquid ammonia, via a proposed 5rn 1 mechanism and their reactivities were measured. The higher reactivity of (a) has been attributed to efficient intramolecular electron transfer from the phenyl ring to the C—Cl a bond (intra-ET catalysis). The lower reactivity of (b) is ascribed to a decrease in the rate of the intra-ET by elongation of the bridge by one methylene unit. The relative reactivity of (a) versus (b) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds. ... 

An attempt to combine electrochemical and micellar-catalytic methods is interesting from the point of view of the mechanism of anode nitration of 1,4-dimethoxybenzene with sodinm nitrite (Laurent et al. 1984). The reaction was performed in a mixture of water in the presence of 2% surface-active compounds of cationic, anionic, or neutral nature. It was established that 1,4-dimethoxy-2-nitrobenzene (the product) was formed only in the region of potentials corresponding to simultaneous electrooxidation of the substrate to the cation-radical and the nitrite ion to the nitrogen dioxide radical (1.5 V versus saturated calomel electrode). At potentials of oxidation of the sole nitrite ion (0.8 V), no nitration was observed. Consequently, radical substitution in the neutral substrate does not take place. Two feasible mechanisms remain for addition to the cation-radical form, as follows ... 

Becanse of the high concentration of isomeric molecules (>0.1 mol dm ), this equilibrium is established instantaneously. The IP of trans-AtcaYm is 0.02 eV lower than the IP of cw-decalin (9.24 eV versus 9.26 eV). Therefore, the electron-transfer eqnilibrinm is shifted slightly to the left side. Thns, in terms of charge-transfer kinetics, the two ions behave as a single species. It shonld be worth noting that decalin has only two isomeric forms, cis and trans. On the contrary, n-nonane exists in the multitude of conformations. The rate constant of electron exchange between parent nentral molecules of nonane and its cation-radicals is much lower, namely, 2 orders lower than the diffnsion-controlled limit (Borovkov et al. 2007). 

Interaction between the radical-cations of primary alkyl halides and acetonitrile is on the verge of Sivj2 versus S l type reactivity. Some product is formed by direct substitutiorr, but rearrangement also occurs to give the most stable carbonium ion [20, 21], Thus 1-bromopentane yields a mixture of three N-pentylacetamides on... 

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