Reactive species basicity

Thiazolium derivatives unsubstituted at the 2-position (35) are potentially interesting precursors of A-4-thiazoline-2-thiones and A-4-thiazoline-2-ones. Compound 35 in basic medium undergoes proton abstraction leading to the very active nucleophilic species 36a and 36b (Scheme 16) (43-46). Special interest has been focused upon the reactivity of 36a and 36b because they are considered as the reactive species of the thiamine action in some biochemical reaction, and as catalysts for several condensation reactions (47-50). 

Mercury and tin in complexes (68 or 69) (Scheme 32 (154 mav behave as electrophilic centers (155. 156). Under basic conditions, the reactive species is an ambident anion (70) (Scheme 33). 

The mechanism of the C02 transfer reaction with acetyl CoA to give mal-onyl CoA is thought to involve C02 as the reactive species. One proposal is that loss of C02 is favored by hydrogen-bond formation between the A -carboxy-biotin carbonyl group and a nearby acidic site in the enzyme. Simultaneous deprotonation of acetyl CoA by a basic site in the enzyme gives a thioester eno-late ion that can react with C02 as it is formed (Figure 29.6). 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

The Co(III) complexes Co(NH3)6 " and Co(NH3)sOH bring about oxidation of stannate(II) ion in strongly basic solution . The rates were found to be independent of the concentration of the Co(III) complex. It is proposed that stannate(Il) exists as a dimer, and that the monomer is the reactive species, the rate being close to half-order in stannate(II). Cyanide and thiosulphate catalyse the reaction but Co(CN)g is immune to attack by stannate(II) ion. The experimental difficulties encountered in this study preclude a full analysis as regards mechanism. 

The pKR+ value for the parent tri(l-azulenyl)methyl cation (2a+) is 11.3. Hydrocarbon-based carbocations, which comprise only of carbon and hydrogen, are generally very reactive species. Some extremely stable hydrocarbon carbocations, which exist even under basic conditions, were reported in the literature (5). However, most of these examples are cyclic cations, such as cyclopropenylium or tropylium ions (Figure 8). The tropylium ion 8+ annelated to three bicyclo[2.2.2]octane units is one of the most stable hydrocarbon-based carbocation ever reported (9). 

Although there is no controversy about the basic definition of stability constants, physical chemists and biochemists handle the concepts involved and the resulting calculations differently. Physical chemists think in terms of reactive species and biochemists in terms of total concentrations of components, A further source of confusion is the differing definitions of apparent constant. To a physical chemist the stability constant for MgATP formation... 

The ionization and excitation may lead to chemical bond cleavage and production of highly reactive species, free radicals, ions and molecular fragments, which subsequently interact with each other and at last stable degradation products are created. This complex sequence of processes can deliberately be divided into two basic phases, the initial physical phase, in which the ion energy is dissipated to electrons and atoms, and the chemical one comprising interaction of the reaetive species and production of the final stable products. 

The single most crucial piece of information necessary for the interpretation of the behaviour of a specific acid-catalyzed reaction is the concentration of the protonated, reactive species this depends on a knowledge of the basicity of the substrate. Until very recently this information has not been available for carboxylic esters, and consequently most work has necessarily been done using relatively weakly acidic media, where it can be assumed that the concern ration of the protonated species is small. 

If the current ideas of APS surface interactions are to be supported, then the highly-oligomerized APS molecules must first revert to more reactive species such as silanol monomers. Perhaps the surface acts as its own acid/ base catalyst to break up the oligomerized fragments into monomers. The newly formed APS silanols are then free to form surface bonds and eventually a two-dimensional surface network [3]. Many experiments done in this laboratory and some discussed above are supportive of the labile nature of the siloxane bond. At or near the surface, this bond may be cleaved by water at elevated temperatures or at basic pH s, for instance, to form silanols or siloxanols. Subsequently, these molecules can react to form siloxanes and release the water molecule. 

All types of nitro compds react easily with bases, forming diverse types of products (Ref 36) In the case of TNT, 2,4,6-trinitrobenzyI anion (I) is formed initially and rapidly. It is a highly reactive species thought to be intermediate in the many reactions of TNT conducted under basic conditions discussed above (Refs 32 61). The anion is formed without side reactions by the action of 1,1 f,3,3f-tetramethylguanidine in dimethylformamide solvent (Ref 106). Based on... 

Computational studies showed that the nature of the reactive species in the oxidation of trimethylamine, iodide ion, and dimethyl sulfide with lumiflavin is a C4 a-hydroperoxide complexed with water. The other two species, C4 a-hydroperoxide and C4 a-peroxide, yielded higher activation energies.237 Kinetic and spectroscopic studies on the effect of basic solvents, ethers, esters, and amides, on the oxidation of thianthrene-5-oxide with substituted peroxybenzoic acids indicated the involvement of the basic solvent in the transition state of the reactions. A solvent parameter, Xtc, based on the ratio of the trans to the cis form of thianthrene-5,10-dioxide, has been introduced.238... 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

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