Neopentyl radical

The unimolecular decomposition of the neopentyl radical has also been investigated. Neopentyl was generated by the reaction of Cl atoms with neopentane, and the dissociation to isobutane and methyl was followed by photoionization mass spectrometry over the temperature range 560 to 650 K, and as a function of bath gas [126] ... 

These results suggest an ET reaction giving the radical intermediate 95, which reacts, for instance, with PhS ions to give 93, or cyclizes to 96 to give finally 94 (equation 72). The rate constant for the coupling of neopentyl radicals with PhS" ions in DMSO52 is about 1.2 xlO8 M"1. 

Secondly, processes (2), (5) and (6) are quasi-non-determining, because both k3 [(CH3)4C]/fe2 and (fe6/fe4)1/2 fe3[(CH3)4C]/fe2) are small, though not absolutely negligible, with respect to unity. From a mechanistic point of view, that means that process (2) is much easier than (3) and, accordingly, that neopentyl radicals are less concentrated than methyl radicals. Practically, accurate values of k2 and k6 are not needed, because they play a role only in correction terms. 

Structural factors are important, for example, 2-C3H7 and f-C4H9 both give 99% alkene because alternative reactions are very limited. Neopentyl radicals with no H atom on the carbon atom adjacent to the carbon with the free valency are unable to form conjugate alkene. The formation of products through oxidation of alkyl radicals is potentially very complex because at least three routes are available, the direct bimolecular process... 

The data for the neopentyl radical showed that the rate coefficient is close to the high pressure limit under the experimental conditions employed, allowing an accurate extrapolation to be made. Analysis gives ... 

Time resolved measurements of the equivalent reaction for the neopentyl radical, using LIF of OH as a probe, are fully compatible with gas chromatographic analysis of the products (see Section 2.7). For this radical, the analogue of channel (lb), to form the alkene, is not possible and the reaction proceeds exclusively via the peroxy/hydrop-eroxy route. 

However, direct studies of the R + O2 equilibrium, of the type described in Section 2.6.2, produce values for the equilibrium constant over an order of magnitude different from the thermochemical estimates affecting the absolute values of the isomerization rates. Pilling and coworkers [122, 123] devised a time resolved equivalent of Baldwin and Walker s experiment (Fig. 2.37b) to directly measure the isomerization rate for the neopentyl radical. This technique is described in Section 2.7.2. The results of the direct studies are shown to be compatible with the earlier indirect work and can be used to scale the results of Baldwin and Walker. The relationship between isomerization rate and structure is discussed in Section 2.7.3. 

Unless the position of the T atom can be determined, reactions 64 and 65 give the same product. The yield of reaction 66 is about 1 % of that of the sum of 64 and 65. This is probably mainly due to the short half-life of the neopentyl radical which, due to lower C-C bond strength, can be easily decomposed to CH2T + CH2=C(CH3)2. 

The proposed mechanism for this reaction included the formation—by a one-electron transfer —of a benzophenone ketyl and a neopentyl radical in a solvent cage, which then could either collapse to produce the normal product or from which the radicals could escape to form different products, such as benzopinacol and neopentane (Scheme 6) ... 

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

The fact that the electrophilic alkylation, which is very important in the homocyclic aromatic series is not applicable with the heteroaromatic bases and in any case would cause a completely different orientation also contributes to the S3uithetic interest of the homolytic alkylation. Moreover, the homolytic substitution occurs without rearrangement even in the case of the neopentyl radical and without isomerization of the reaction products, which take place frequently in electrophilic alkylation. 

Chemaly and Pratt [86] have recently described the oxygen promoted decomposition of neopentylcobalamin which was ascribed to the occurrence of an equilibrium mode II cleavage to cob(II)alamin and neopentyl radicals (Eqn. 51). 

This mechanism involves a closed sequence, or chain, consisting of reactions (52) and (53). The two free radical chain carriers are the neopentyl radical and the... 

The neopentyl radical is a very reactive p radical at negligible concentration with respect to the methyl radical of type B the main termination is therefore of the BB type. The ethyl radical is a Bp radical, therefore of the B type at "low" temperatures, but much less reactive than the H atom the main termination is therefore of the Bp-Bp type. 

The first case of radical isomerization was established by Urri and Kharash in 1944 for the neopentyl radical. Many other examples of radical isomerization were discovered during next 40 years. In most cases, it was proved by the composition of formed molecular products of multistage radical transfoimation. Quantitative data on this class of reactions are yet insufficient. Various isomerization reactions of radicals can be grouped as follows isomerization of hydrogen atom abstraction, with group migration, with closure and breakage of the cycle, and cis-trans- isomerization of allyl radicals. 

Alkyl radicals are oxidized by ion-oxidants to olefins and alcohols or esters. In many cases, carbocations are formed as intermediates, which proves the composition of the oxidation products. The oxidation of the neopentyl radical with copper acetate in acetic acid results in the formation of isomeric 2-methylbutenes and /ert-amyl acetate, which agrees with reactions involving intermediate carbocations... 

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