Heterolytic process

Photochromism Based on Dissociation Processes. Both heterolytic and homolytic dissociation processes can result in the generation of a photochromic system. An example of an heterolytic process is the reversible formation of triphenylmethyl cation, by photolysis of... 

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. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Heats of reaction and bond dissociation energies allow the estimation of the feasibility of homolytic processes, as these are largely — but not solely — governed by thermochemical effects. The quantitative treatment of heterolytic processes, however, presents a far more difficult problem. Basic electrostatic considerations indicate that the dissociation of a covalent bond into positive and negative ions is inherently a highly endothermic process. It will be facilitated by any mechanism that allows dissipation or stabilization of the incipient charges. Chemists have come to differentiate these... 

It is clear that these solvolytic reactions of ipso-adducts are markedly dependent on reaction conditions strongly acidic conditions and electron withdrawing substituents will favour a heterolytic process lower acidity, higher temperatures and electron releasing substituents probably favour homolytic processes. With so many factors involved, each substrate under each set of conditions gives rise to its own particular behaviour. 

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. 

The splitting of hydrogen is presumably a heterolytic process (Krasna 1979). The conversion of para-Hi to ortho-H2, a reaction apparently not involving any redox reactions, is inhibited in H20. This was interpreted as ... 

Radicals undergo both homolytic and heterolytic versions of many reactions, whereas heterolytic processes predominate in reactions of closed-shell species. A major difference between radical and closed-shell molecule reactions is the rates of reactions. In general, rate constants for radical reactions are much larger than the rate constants for equivalent reactions in closed-shell molecules, in part due... 

An interesting and mechanistically unsettled problem of organocobalt chemistry is the 1,2-shift of a /1-carboxy or -hydroxy group in the homolytic Co—C bond fission-recombination process. The evidence for cobalt participation in the rearrangement is substantial, but its role is not yet clarified (Scheme 103). The heterolytic process also appears to be viable in the migration of a /1-hydroxy or -alkoxy group. 

The synthetic procedures available to the carbohydrate chemist have been largely dominated by standard reactions proceeding by heterolytic processes within a chiral matrix. The preparative utility of radical-mediated reactions has, however, been amply demonstrated in recent years. The chapter contributed here by L. Somsak (Debrecen) and R. J. Ferrier (Wellington), on bromination reactions of carbohydrates proceeding by radical processes integrates the literature related to Ferrier s pioneering work in this area and underscores its excellent potential in synthesis. 

The salient feature of the homolytic process is the presence of the free-radical intermediate. The- lack of charge of radicals and the high reactivity of nearly all of those that become involved in a typical organic reaction lead to important differences between homolytic and heterolytic processes. 

It is sometimes possible to combine a radical reaction with a heterolytic process to achieve a synthetically useful result, as in the Hofmann-Loffler reaction (Scheme 16).200... 

Heterolytic processes make up a large proportion of organic transformations because most bonds are somewhat polarized. Heterolytic cleavage is merely an increase of this polarity to the limit at which diere is no bond remaining that is, election movement follows in the direction established by the bond polarity to give a cation-anion pair. 

Many convenient methods for the introduction of carbon-carbon double bonds into a saturated carbon chain involve the removal of two atoms or groups from adjacent carbon atoms. Usually, but not invariably, one of these groups is hydrogen (i.e. the removal of HX). Two main types of elimination reactions are recognised - heterolytic processes in solution and pyrolytic reactions in the gas phase. A detailed discussion of the mechanisms of these reactions may be found in all standard and advanced textbooks in each of the reactions discussed below the probable mechanism is noted in relation to the aim of obtaining good yields of regio- or stereoisomerically pure compounds. 

Such is the case for halogen derivatives. Cleavage of the C-X bond may occur as a homolytic process, usually with low quantum yield for aryl chlorides (27) and more efficiently for bromides and iodides (Sch. 10) (28), or as a heterolytic process with a high quantum yield for some (hetero) arylfluorides, notably fluoroquinolones (Sch. 11) (29). Likewise, sulfa drugs are usually susceptible to homolytic fragmentation of the C-S02 bond (Sch. 12) (30). 

The Two B12 Cofactors Influence of the trans Nitrogen Ligand on Homolytic and Heterolytic Processes... 

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

The thermal and chemical stability of C—M bonds (M = Si, Ge, Sn, Pb), and therefore of all organic compounds of the silicon subgroup elements, decreases, both in homolytic and in heterolytic processes, when the atomic number of the M element increases. For example, the thermal stability of tetraalkyl derivatives R4M diminishes essentially when M is changed consecutively from Si to Pb151 l52. The ease of oxidation of R4M compounds and the ease of cleavage of C—M bonds by halogens, protic and aprotic acids, etc.,... 

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

In contrast to the homolytic autoxidations discussed in the preceding section, there is no heterolytic process known for the oxidation of organic substrates... 

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

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