C heterolysis

A number of reactions involving the C-NO2 group appear to contribute in aliphatic nitro compounds. Because NO2 is a major product upon fast thermolysis [28,39, 0,51,66], C-NO2 homolysis is an important reaction. It has been considered by others to be rate-determining [8]. Recently, a case for C-C heterolysis as the rate-determining step has been made for trinitromethylalkyl compounds [67]. Although the spectroscopic techniques that we have developed were not intended for kinetic measurements, the... 

Antal, M.J., Jr., Brittain, A., DeAlmeida, C., Ramayya, S. and Roy, J.C., Heterolysis and homolysis in supercritical water. In Supercritical Eluids Chemical and Engineering Principles and Applications, ACS Symposium Series 329, American Chemical Society, Washington, DC, 1987, pp. 77-86. 

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

Degradation initiated by chloride ions resulting from the heterolysis of the C—Cl dipole is shown in Eqs. (26) and (27). 

Simple mechanistic considerations easily explain why heterolytic dissociation of the C — N bond in a diazonium ion is likely to occur, as a nitrogen molecule is already preformed in a diazonium ion. On the other hand, homolytic dissociation of the C —N bond is very unlikely from an energetic point of view. In heterolysis N2, a very stable product, is formed in addition to the aryl cation (8.1), which is a metastable intermediate, whereas in homolysis two metastable primary products, the aryl radical (8.2) and the dinitrogen radical cation (8.3) would be formed. This event is unlikely indeed, and as discussed in Section 8.6, homolytic dediazoniation does not proceed by simple homolysis of a diazonium ion. 

The kinetic effect of increased pressure is also in agreement with the proposed mechanism. A pressure of 2000 atm increased the first-order rates of nitration of toluene in acetic acid at 20 °C and in nitromethane at 0 °C by a factor of about 2, and increased the rates of the zeroth-order nitrations of p-dichlorobenzene in nitromethane at 0 °C and of chlorobenzene and benzene in acetic acid at 0 °C by a factor of about 559. The products of the equilibrium (21a) have a smaller volume than the reactants and hence an increase in pressure speeds up the rate by increasing the formation of H2NO. Likewise, the heterolysis of the nitric acidium ion in equilibrium (22) and the reaction of the nitronium ion with the aromatic are processes both of which have a volume decrease, consequently the first-order reactions are also speeded up and to a greater extent than the zeroth-order reactions. 

Part C of the present procedure illustrates a mild method for effecting the elimination of thiophenol from thioacetals and thioketals under essentially neutral conditions. The reaction of simple thioacetals and thioketals with bis[copper(I) trifluoro-methanesulfonate] benzene complex in benzene-tetrahydrofuran at room temperature affords vinyl sulfides in high yield (Table I). The reaction presumably occurs by coordination of the thiophilic copper(I) reagent with sulfur, heterolysis to a phenylthio-stabilized... 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

Bordwell et al., 1988, 1989) and Amett (Amett et al., 1990a,b, 1992 Venimadhavan et al., 1992) have employed thermodynamic cycles consisting of heterolysis of a molecule and redox processes of the resulting ions to evaluate homolytic dissociation energies of C—H, C—C, C—N, C—O and C—S bonds in solution. In a similar way, knowledge of the A//het(R-R ) values allows determination of the heat of homolysis of carbon-carbon bonds [A/fhomo(R"R )] using (27). The results are summarized in Table 4. 

In nucleophilic substitution reactions, the C-X bond of the substrate undergoes heterolysis, and the lone-pair electrons of the nucleophile is used to form a new... 

Both If i 0i) s ted and Lewis acids are effective in coordinating with the hydroxyl oxygen to induce heterolysis of the C-O bond and cause formation of the necessary carbocation intermediate. The reactions are frequently conducted... 

This scheme differs from the previous one with respect to the heterolysis of the O—O bond, which occurs in the internal coordination sphere of the complex and is compensated by the formation of the C—O bond. 

The strained C,C bond in cyclopropanes is prone to heterolysis, particularly, in the presence of substituents stabilizing the resulting charges. These facts can be used to interpret easy methanolysis of cyclopropanes (385), resulting in regeneration of the starting functionalized AN (382) in high yields (Eq. 1) (493). 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Figure 28. Exploring hole transfer propagation through DNA.[481 (a) The hole is generated by photocleavage, followed by heterolysis and subsequent hole generation at a nearby C base, (b) Relative rates of HT, from G23 + to CCC, through various DNA duplexes. The shaded areas highlight the longest path between adjacent C bases.

Therefore any flexible acetal will undergo conformational changes to permit 2p(0) <-> 2p(C+) stabilizing interaction to intervene in the transition state of its heterolysis. This is also true for pyranosides for which the free energy difference between chair, boat and sofa conformers rarely surpasses 10 kcal/mol. 

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

Suitably protected glycosyl halides or acetates, upon Lewis-acid promoted SN1 heterolysis, generate glycosyl cation intermediates that can react with electron-rich arenes, heteroarenes, Me3SiCN, enoxysilanes, enamines, allyl silanes and stannanes, acetylenyl silanes and stannanes affording C-glycosyl compounds. 

Table 30 Structural features for the substrates in the gas-phsae heterolysis of several (CH3)2CH—X at 25°C, the calculated /3-deuterium-d6 EIEs and the hyperconjugative and inductive contributions to the EIE."... 

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