Disproportionation species

Chromates(V), (CrO ) ". Dark green species which hydrolyse and disproportionate to Cr III) and Cr(VI) (K2Cr04 plus KOH in melt). 

This mode of termination produces a negligible effect on the molecular weight of the reacting species, but it does produce a terminal unsaturation in one of the dead polymer molecules. Each polymer molecule contains one initiator fragment when termination occurs by disproportionation. 

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

The ground state distribution of electrons in the aluminum atom is lT2T2 3T3/). The oxidation state of aluminum is +3, except at high temperatures where monovalent species such as AIQ, AIF, and AI2 have been spectrally identified At lower temperatures, these compounds disproportionate... 

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

It is clear that, if synthetic routes could be devised which would mechanistically hinder disproportionation, such compounds might be preparable. Although univalent compounds of the Group 2 metals have not yet been isolated, there is some evidence for the formation of Mg species during electrolysis with Mg electrodes. Thus H2 is evolved at the anode when an aqueous solution of NaCl is electrolysed and the amount of Mg lost from the anode corresponds to an oxidation state of 1.3. Similarly, when aqueous Na2S04 is electrolysed, the amount of H2 evolved corresponds to the oxidation by water of Mg ions having an average oxidation state of 1.4 ... 

Pyrolysis of nido- or nrac/ino-carboranes or their reaction in a silent electric discharge also leads to c/o50-species either by loss of Ht or disproportionation ... 

The reverse reaction to give the gaseous species AlX(g) at high temperature accounts for the enhanced volatility of AIF3 when heated in the presence of A1 metal, and the ready volatilization of A1 metal in the presence of AICI3. Using calculations of the type outlined on p. 82 the standard heats of formation of the crystalline monohalides AIX and their heats of disproportionation have been estimated as ... 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Mino and Kaizerman [12] established that certain. ceric salts such as the nitrate and sulphate form very effective redox systems in the presence of organic reducing agents such as alcohols, thiols, glycols, aldehyde, and amines. Duke and coworkers [14,15] suggested the formation of an intermediate complex between the substrate and ceric ion, which subsequently is disproportionate to a free radical species. Evidence of complex formation between Ce(IV) and cellulose has been studied by several investigators [16-19]. Using alcohol the reaction can be written as follows ... 

A species disproportionates when it is oxidized and reduced at the same time... 

Sometimes, with a species such as C103, oxidation and reduction occur together, resulting in disproportionation ... 

In general, a species in an intermediate oxidation state is expected to disproportionate if the sum + °,d is a positive number. 

Disproportionation A reaction in which a species undergoes oxidation and reduction simultaneously, 547-548 cooper, 547-548... 

In termination, unsaturated and saturated ends are formed when the propagating species undergo disproportionation, head-to-head linkages when they combine, and other functional groups may be introduced by reactions with inhibitors or transfer agents (Scheme 1.2). In-chain defect structures (within the polymer molecule) can also arise by copolymerization of the unsaturated byproducts of initiation or termination. 

The transition state for disproportionation requires overlap of the p C—H bond undergoing scission and the p-orbital containing the unpaired electron.18 This requirement rationalizes the specificity observed in disproportionation of radicals 29 (Section 1.4,2) and provides an explanation for the persistency of the triisopropylmcthyl radical (33) and related species (Section 1.4.3.2).166 In the case of 33, the P-bydrogens are constrained to lie in the nodal plane of the p-orbital due to stcric buttressing between the methyls of the adjacent isopropyls. 

The most important mechanism for the decay of propagating species in radical polymerization is radical-radical reaction by combination or disproportionation as shown in Scheme 5.1. This process is sometimes simply referred to as bimolecular termination. However, this term is misleading since most chain termination processes are bimolecular reactions. 

Even though the absolute rate constant for reactions between propagating species may be determined largely by diffusion, this does not mean that there is no specificity in the termination process or that the activation energies for combination and disproportionation are zero or the same. It simply means that this chemistry is not involved in the rate-determining step of the termination process. 

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