Intermediate, transient

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

A number of proteins are known to pass through a transient intermediate state, the so-called molten globule state. The precise stmctural features of this state are not known, but appear to be compact, and contain most of the regular stmcture of the folded protein, yet have a large side-chain disorder (9). 

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

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 has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

The sulfur analogue of the Hauser ortho-substitution rearrangement provides access to an arylacet-ic NSAID. Reaction of the aminobenzophenone 176 with ethyl methylthioacetate and tert-butyl hypochlorite gives the intermediate 178. The reaction probably proceeds by way of formation of the S-chlorinated sulfonium derivative 177 displacement on sulfur will lead to the salt 178. Treatment with triethylamine leads initially to the betaine 179. Electrocyelic rearrangement of that transient intermediate leads, after rearomatization, to the homoanthranilic acid 180. Internal ester-amine interchange leads then to indolone 181 [45]. The thiomethyl group is then removed with Raney niekel. Saponifieation of intermediate 182 affords bromfenac (183) [46J. 

In redox initiation, the free radicals that initiate the polymerization are generated as transient intermediates in the course of redox reaction. Essentially this involves an electron transfer process followed by scission to give free radicals. A wide variety of redox reactions, involving both organic and inorganic components either wholly... 

Benzyl radicals and a- and 3- substituted derivatives also undergo unsymmetrical coupling through the aromatic ring (Section 2.5). The formation of the (i-o and a—p coupling products is reversible. Consequently, these materials are often only observed as transient intermediates. 

A characteristic property of most diazotizations of aminoazoles is the occurrence of a relatively stable transient intermediate (probably the A-nitrosoamine), in contrast with the diazotization of carbocyclic aromatic amines, where A-nitrosoamines have been considered to be unstable intermediates. This problem will be discussed in the context of the mechanism of diazotization in Section 3.4. 

The addition of acetate ion to an arenediazonium ion is also an O-coupling. As the equilibrium of this reaction lies very much to the side of the starting ions (Suschitzky, 1967), and since diazoacetates play an important role as transient intermediates in homolytic reactions of A-nitrosoacetanilides, we will discuss diazoacetates in that context (Sec. 10.10). 

In the last two decades a number of phenomena found many years ago in azo coupling and other substitution reactions have been elucidated with regard to their structural and mechanistic basis. These include charge-transfer complex formation, radical pairs as transient intermediates, and changes in product ratios due to mixing effects — a phenomenon which was not understandable at all only a few years ago (see Secs. 12.8 and 12.9). 

An especially important application of this concept allows the determination of the reactivity of transient intermediates. More will be said about this in Chapter 5 here we shall consider one simple but pertinent example. 

Elegant methods for determining the relative reactivity of transient intermediates have been developed.6 Consider the reactions of benzyne with water and hydroxide ions ... 

Pj release occurs at a relatively apparent slow rate (kobs = 0.005 s" ), so that the transient intermediate F-ADP-Pj in which P is non-covalently bound, has a life time of 2-3 minutes (Carlier and Pantaloni, 1986 Carlier, 1987). While the y-phosphate cleavage step is irreversible as assessed by 0 exchange studies (Carlier et al., 1987), the release of Pi is reversible. Binding of H2PO4 (Kp 10 M) causes the stabilization of actin filaments and the rate of filament growth varies linearly with the concentration of actin monomer in the presence of Pi (Carlier and Pantaloni, 1988). Therefore, Pi release appears as the elementary step responsible for the destabilization of actin-actin interactions in the filament. 

Turecek F (2003) Transient Intermediates of Chemical Reactions by Neutralization-Reioniza-tion Mass Spectrometry. 225 75-127 Ublacker GA, see Maul JJ (1999) 206 79-105 Uemura S, see Nishibayashi Y (2000) 208 201-233 Uemura S, see Nishibayashi Y (2000) 208 235-255... 

A limited amount of information is available on vinyl cations in the gas phase. These mass spectral data suggest that the heat of formation and stability of simple alkylvinyl cations, such as CH2=8h and CH3CH=6h, is in between those of methyl and ethyl cations (2). The bulk of the evidence for the existence of vinyl cations comes from mechanistic studies in the liquid phase. Although vinyl cations have not yet been prepared in solution with lifetimes adequate for direct spectral observation, sufficient, increasing evidence has been presented for the existence of such species as transient intermediates. 

Lu, C. Lin, W. Wang, W. Han, Z. Yao, S. Lin, N. (2000). Riboflavin-(VB2) photosensitized oxidation of 2 -deoxyguanosine-5 -mono-phosphate (dGMP) in aqueous solution A transient intermediates study. Physical Chemistry Chemical Physics, Vol.2, 0anuary 2000), pp.329-334, ISSN 1463-9076. 

The effects of aluminium, zinc, iron, nickel and copper powders on the thermal degradation of waste PS were studied. The results showed that the catalytic effects of metal powders were related to their activities. The catalytic effects increased with increasing activities of metals. It was suggested that PS degraded through a transient intermediate in the presence of metal powders and that the degradation of the transient intermediate was the rate-determining step. 10 refs. 

Physical properties of carbon black-filled EPR and EPDM elastomers have been found to be comparable with the suUur-cured analogues [372]. Aromatic oils increase the optimum dose requirement for these compounds due to the reaction of the transient intermediates formed during radiolysis of the polymer with the oil as well as energy transfer which is particularly effective when the oil contains aromatic groups. The performance and oxidative stability of unfilled EPDM as well as its blend with PE [373], and the thermal stabdity and radiation-initiated oxidation of EPR compounds are reported by a number of workers [374,375]. 

Developments in computer techniques making it possible to solve complicated fluid motions in a combustion environment that are affected by diffusion and involve complicated chemistry (large numbers of elementary reactions, which individually are not "complex" but quite simple, i.e., most of them involve two reacting species, sometimes three, and the formation or breaking of just one bond), and with a large number of transient intermediates formed in the course of fuel oxidation and pollutant formation. 

Turecek F (2003) Transient Intermediates of Chemical Reactions by Neutralization-Reionization Mass Spectrometry. 225 75-127... 

Midway through the displacement, the bond between R and L has weakened but has not yet been completely severed, and the new bond between E and R is as yet incompletely formed. This transient intermediate—in which neither free substrate nor product exists—is termed the transition state, E R L. Dotted lines represent the partial bonds that are undergoing formation and rupmre. 

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