Homolytic splitting

In certain highly energetic collisions with any molecule M in the system, a bromine molecule may be dissociated in a homolytic split of the bond joining two bromine atoms. 

Alcohols are formed as a result of reactions with the homolytic splitting of the O—O bond of hydroperoxides, for example (see Chapter 4) ... 

The limiting step of the reaction between two peroxyl radicals is the homolytic splitting of tetroxide. Such reactions occur with high pre-exponential factors (1014 to 1016 s-1). 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

Parallel reactions of homolytic splitting and heterolytic isomerization were observed for triphenylcumylperoxysilane (anisole, T= 433-463 K) [11] ... 

Alkyl peroxyphosphates also undergo heterolytic isomerization simultaneously with homolytic splitting [12] ... 

The phenomenon of acid catalysis of peroxide homolytic splitting will be discussed in Chapter 10. 

In the past this was referred to as a homolytic reaction [23], It is clear that it would be highly unlikely that dihydrogen would be split into a metal hydride and a highly energetic hydrogen radical in this sense the term homolytic splitting is misleading. 

Two plausible mechanisms for the reaction of Co(CN) - with H2 invoke either homolytic splitting of H2 ... 

Heterolytic splitting would lead to an initial Co-D/Co-H ratio in the product of 1.0. (R = 0.53). It is apparent that this is not the case. Since D2O does not enter into the homolytic splitting, R should equal 0. At early times this appears to be so. Why might the value slowly increase ... 

The radical-forming reactions are suggested to take place mostly after an Si T type ISC the reactions have nonactivated character. The homolytic split to H atom and alkyl radical has a considerable yield in the photolysis of n-alkanes and cycloalkanes, while the scission to two radicals is characteristic of the decay of excited branched alkane molecules. 

Alternative possibilities, however, involving the formation of HgH+ as the initial product of the rate-determining step, through heterolytic splitting of hydrogen by Hg++ or homolytic splitting by Hgf, cannot be excluded. This species, if it is formed, would probably dissociate in aqueous solution into Hg and H+. 

A similar experiment using platinum as catalyst yielded D2 as the initial product suggesting homolytic splitting on this surface with both adsorbed atoms exchanging rapidly with the water. 

Reaction (26) is followed by a first order process, which has been interpreted as an homolytic splitting of the N-S bond, Eq. (27), with formation of the one-electron reduction product of NP, [Fe(CN)sNO]3, ... 

Fig. 21. Proposed scheme for the reactions of NP with thiolates, RS"-, showing the formation and spontaneous homolytic splitting of the adduct, [Fe(CN)5N(0)SR] I + 2 ), followed by autocatalytic redox reactions proceeding with thiolate excess, in anaerobic medium. The influence of oxygen is also included for some of the reactions.

The following reaction scheme of the formation of the polymeric carbon can be assumed (according to [12]). The iodobutadiyne radical forms after the photochemical homolytic splitting of the C-I bond ... 

Photolysis is also widely applied to generate the Ura-5-yl radical from 5-halo-uracils. In aqueous solution, the reaction is a homolytic splitting of the C-X (X = Br, I) bond (Chap. 10) in DNA, the reaction seems to be much more complex (Chap. 12). 

Initiation produces radicals by homolytic splitting of a bond. A rule of thumb (Problem 6.2) suggests that C—C splits more readily than C—H, which in turn splits more readily than C—O or C=0, and a good first guess is... 

Free-radical halogenation of an alkane occurs by a very different mechanism. The first step in these reactions is the homolytic splitting of a bond to give a pair of free radicals. 

If the photolysis takes place in an inert gas matrix, both the homolytic splitting of the metal-metal bond and the breaking of a metal-nitrogen bond will be followed by a fast backreaction to the parent compound. The radicals formed by homolysis of the metal-metal bond can not diffuse from the matrix site and will recombine to the parent compound. Moreover, the photoproduct obtained by breaking of a metal nitrogen bond, will not be stabilized by a coordinating solvent molecule and therefore react back to the parent compound. Because of this the photochemistry of some of these complexes has also been studied in a Cl-k-matrix at 10K and for comparison in a PVC film, which is a less rigid medium than the matrix especially at room temperature. 

The formation of these two photoproducts can be explained with a homolytic splitting of the metal-metal bond. Most radicals will react back to the parent compound. Some of the M(CO)s radicals will however react, presumably associatively (33,34), with the R-DAB ligand of the M (CO)3 (R-DAB) radical or with THF in the PVC film with formation of these photoproducts and release of CO. 

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