Homolytic bond cleavage process

Pyrolytic Source Pyrolytic sources can be utilized if radical precursors are available that have very weak bonds so that a thermally induced homolytic bond cleavage process generates two radical species (Fig. 11.3). 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

Processes accompanied by a decrease in volume, such as C—C bond formation, in which the distance between two carbon atoms decreases from the van der Waals distance of ca 3.6 A to the bonding distance of ca 1.5 A, are accelerated by raising the pressure and equilibria are shifted toward the side of products (AV < 0, AV < 0). The reverse reaction, a homolytic bond cleavage, leads to an increase in volume (AV / > 0, AV > 0). Pressure induces a deceleration of such a process and a shift in equilibrium toward the side of reactants. However, in an ionization, such as an ionic dissociation, the attractive interaction between the ions generated and the solvent molecules leads to a contraction... 

DR. JAMES ESPENSON (Iowa State University) I should like to raise a slightly different aspect of substitution reactions, an aspect which relates to a different kind of mechanism. This involves a change from the conventional heterolytic process of metal ligand bond cleavage to one dealing with homolytic bond cleavage. 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

The free-radical reaction is ordinarily initiated by a homolytic bond cleavage (Equation 9.1). The radicals may then undergo various processes, for example... 

Homolytic bond cleavage (Section 21.1) A bond-breaking process in which one electron of foe bond remains with each of the atoms radicals are produced. 

The fragmentation of even-electron anions has been reviewed by Bowie [13,14], Observed reaction pathways include homolytic bond cleavage, the loss of one or several H radicals being a common process. Alkyl radical loss is also observed [15,16] ... 

Okada, Okamoto and Oda [97] have recently described a novel method of decarboxylation through a PET bond cleavage process in IV-acyloxyphthalimides. A photoexcited electron donor such as l,6-bis(dimethylamino)pyrene (BDMAP) transfers an electron to the phthalimide to produce an anion-radical that is protonated to form a radical prior to homolytic N—O bond cleavage. Bond... 

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

Thermal degradation of the oligomers as a model for the transfer process indicates that the process is homolytic bond cleavage and not a retro-ene reaction.458... 

A volume expansion is expected for homolytic bond dissociations as already pointed out in the Introduction. This expectation has been confirmed for several homolytic bond cleavages showing positive activation volumes near AV - +10 cm mol [123]. The analysis of the pressure effect on the cleavage of azo compounds is however, complicated by the possibility of one- and two-bond scission processes [124]. The benzylic and benzhydrylic 1,4-shifts in the substituted pyr-idiminiumoxides (Scheme 2.29, entry (1)) [125] illustrate the utility of high pressure for the distinction between a pericyclic and dissociative mechanism comparable to the rearrangement of l,3,4,6-tetraphenyl-l,5-hexadiene which has already... 

When a bond breaks such that both of its electrons stay with one of the atoms, the process is called heterolytic bond cleavage or heterolysis. When a bond breaks such that each of the atoms retains one of the bonding electrons, the process is called homolytic bond cleavage or homolysis. Remember that an arrowhead with two barbs signifies the movement of two electrons, whereas an arrowhead with one barb— sometimes called a fishhook—signifies the movement of a single electron. 

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