Carbon radicals, determination

The rate-determining step in the formation of the x-lithio ethers is the formation of a carbon radical as a precursor to the anion. The intermediate radical in the tetrahydropyranyl system is expected to be nonplanar, to be capable of rapid equilibration between the quasiequatorial and quasiaxial epimers, and to exist largely or entirely in the axial configuration at — 78 °C. However, treatment of the a-phenylthio ether 4 with LDMAN at higher temperature in the presence of A, A, lV, ./V -tetramethylethylenediamine leads to the more stable equatorial epimer of the lithio ether 5 and, after addition to benzaldehyde, the axial- and equatorial-substituted products were obtained in a ratio of 13 87. 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

In this paper, the kinetics and polymerization behavior of HEMA and DEGDMA initiated by a combination of DMPA (a conventional initiator) and TED (which produces DTC radicals) have been experimentally studied. Further, a free volume based kinetic model that incorporates diffusion limitations to propagation, termination by carbon-carbon radical combination and termination by carbon-DTC radical reaction has been developed to describe the polymerization behavior in these systems. In the model, all kinetic parameters except those for the carbon-DTC radical termination were experimentally determined. The agreement between the experiment and the model is very good. 

Preparative electrochemical reduction of aryltrimethylsilanes in methyl-amine in the presence of LiCl gives the Birch-type products, 1,4-cyclohexan-dienes (Scheme 34) [6], A mechanism involving the electrochemical formation of lithium metal which chemically reduces the substrate has been suggested. The hydrogen atom is introduced on the carbon adjacent to the silicon preferentially. This regioselectivity is consistent with the spin density of the anion radical determined by ESR spectroscopy (Sect. 2.2.1). 

Here it should be mentioned that the chemical structure of carbonate radical is still under debate. The reaction of the carbonate radical has been intensively investigated by pulse radiolysis [154-156] and laser photolysis [157], and the pAa of HCO3 radical was determined to be 7.6 [158] and 9.6 [157,159,160] from the change of the reactivity as a function of pH. However, recent studies of Raman spectroscopy [160] and pulse radiolysis [161] of carbonate solution proposed that the carbonate radical is a strong acid and keeps a chemical form of CO3 even at below pH 0. Furthermore, dimer formation mechanism has also been proposed [78]. 

The formation of RSSR from RS and RS species is particularly relevant in the present context because it is the reverse of the electro-induced radical anion cleavage (equation 76). Actually, the formation of RSSR from reaction (79) is as well studied as the reaction between aryl carbon radicals and anionic nucleophiles, the fundamental step of the SrnI. Equilibrium constants in the range 10 -10 M for reaction (79) were determined for a number of alkyl-type systems in water, although the corresponding values for aryl-type systems are smaller. The rate constants... 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

The PTOC carbamate method for efficient and controlled generation of aminyl radicals allows kinetic studies that previously were not possible with tetrazene precursors. As is the case with carbon radicals, optimum synthetic utility of chain reaction sequences is found when absolute rate constants or ratios of rate constants for competing reactions are known, i.e., Scheme 8, step D vs step E. If an absolute rate constant is known for one reaction, then other absolute rate constants can be determined for other reactions from the product distributions in competitions of the reactions of interest with the reaction with a known rate constant. 

Electron spin resonance determinations of g-values, linewidths, radical densities and saturation properties have been performed on carbon radicals in samples of coal macerals isolated by density gradient centrifugation techniques. These data are compared with elemental analyses and density measurements. Each maceral type exhibits a different ESR signature" which can be understood in terms of the nature of the starting organic and the extent of coalification. 

To continue with the Kolbe reaction, it has been shown that carbon anodes strongly favour the carbonium ion pathway (Koehl, 1964) at least for simple alkanecarboxylic acids. Also, for phenyl-acetic acid and 1-methylcyclohexylacetic acid the same tendency towards carbonium ion formation on carbon anodes was observed, the phenomenon being explained as due to the presence of paramagnetic centres in carbon. These would bind the initially formed radicals, impede their desorption and hence promote the formation of carbonium ions via a second electron transfer (Ross and Finkelstein, 1969). However, cases of Kolbe oxidations in which no dependence on anode material was noticeable have been found more recently (Brennan and Brettle, 1973 Eberson and Nilsson, 1968a Sato et al., 1968). Actually, the nature of the carbon material determines the yield of products formed via the radical versus carbonium ion pathway (Brennan and Brettle, 1973). Yields of the... 

Other reactive species are or may be formed upon irradiation of CDOM, but their production rates were determined (or are considered) to be much lower than for the species represented in Scheme 1. Nevertheless, these species can be important for the transformation of organic contaminants that are not susceptible to reaction with the main reactive species. They comprise the hydroxyl radical, the carbonate radical and all the radicals derived from the DOM (carbon-centred, oxyl, peroxyl radicals). The following subsections describe in more detail the role that each reactive species plays in the transformation of aquatic organic contaminants. Hydrogen peroxide, although considered a reactive oxygen species (ROS) and an important player in aquatic photochemistry [7], is neglected because there is no evidence that... 

Two types of substrate probe, cis- and trans-l,3-dimethylcyclopentane and ethyl- and 2-propylcyclopropane, were used to shed light on mechanishc details of the hydroxylahon step [30]. In the use of the first two probes, the participation of Ti—OOH species in a concerted mechanism would predict either the retention or the inversion of configurahon at the chiral center, while the stereochemistry of a homolytic mechanism would be determined by the compehhon between the epimerization of the transient terhary carbon radical and C—O formation (Scheme... 

Huie et al. 106) have also determined rate constants by pulse radiolysis techniques for the oxidation of the above-mentioned cyano complexes by the carbonate radical at pH > 11.2, thus ensuring that only the anionic form in the acid/base equilibrium... 

In the case of methane, there was only one type of hydrogen that the chlorine radical could attack, but in a larger alkane there is often a choice. One of the principal factors that determines which hydrogen will be abstracted is the stability of the resultant carbon radical. Suggest what will be the order of stability of primary, secondary and tertiary carbon radicals. 

The shape of the intermediate carbon radicals is important in determining the stereochemical consequences of radical substitution. For the simple alkyl radical, CR3 , suggest the various... 

Huang and Mabury [154] used a radical trapping approach to determine steady state concentrations of the carbonate radical in a variety of fresh water environments. In their study, carbonate radical concentrations ranged from 5 X 10 to 10 M, and were a function of DOM and nitrate concentrations (which are the main sources of the OH radical in these waters), and alkalinity. The wavelength dependence and quantum yields for carbonate radical production are not known, but it is expected that they will mirror the action spectra for OH radical production, and therefore will largely be confined to wavelengths in the UV-B extending out to approximately 330-340 nm [47]. 

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