Anions, proton transfers

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

Keywords Nucleic Acid Bases, Low Energy Electron, Strand Break, Stable Anion, Proton Transfer... 

The nature of the vaporized ILs was a topic of some controversy until a follow up study was published [126], Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was used at distillation conditions (4.0 x 10 6-1.3 x 10 5 mbar, > 200 °C). Aprotic IL vapors were found to be made up of neutral ion pairs, not free ions or clusters (neutral or ionic). It was also confirmed that protic IL vapors consisted of neutral molecules resulting from cation to anion proton transfer. Several studies have suggested that IL volatility is likely related to the extent of ion pairing present in the liquid phase [123, 127],... 

Solvation effects have been incorporated into the calculations of anionic proton transfer potentials in a number of ways. The simplest is the microsolvation model where a few solvent molecules are included to form a supermolecular system that is directly characterized by quantum mechanical calculations. This has the advantage of high accuracy, but is limited to small systems. Moreover, one must assume that a limited number of solvent molecules can adequately model a tme solution. A more realistic approach is to explicitly describe the inner solvation shell with quantum calculations and then treat the outer solvation sphere and bulk solvent as a continuum (infinite polarizable dielectric medium). In this way, the specific interactions can be treated by high-level calculations, but the effect of the bulk solvent and its dielectric is not neglected. An ej tension of this approach is to characterize the reaction partners by quantum mechanics and then treat the solvent with a molecular mechanics approach (hybrid quantum mechanics/molecular mechanics QM/MM). The low-cost of the molecular mechanics treatment allows the solvent to be involved in molecular dynamics simulations and consequently free energies can be calculated. In more recent work, solvent also has been treated with a frozen or constrained density functional theory approach. ... 

Step 4 Proton transfer from ammonia converts the alkenyl anion to an alkene ... 

Step 4 Proton transfer from methanol to the anion gives 1 4 cyclohexadiene H H H... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

Step 2 Proton transfer to anionic form of tetrahedral intermediate... 

Step 4 Proton transfer steps yield an alcohol and a carboxylate anion... 

Aromatic pyrazoles and indazoles, in the broad sense defined in Sections 4.04.1.1.1 and 4.04.1.1.2, will be discussed here. Tautomerism has already been discussed (Section 4.04.1.5) and acid-base equilibria will be considered in Section 4.04.2.1.3. These two topics are closely related (Scheme 10) as a common anion (156a) or a common cation (156b) is generally involved in the mechanism of proton transfer (e.g. 78T2259). For aromatic pyrazoles with exocyclic conjugation there is also a common anion (157) for the three tautomeric forms... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Which proton transfer is easier energetically, Q —> QH or QH- QH2 (The energy of H is given at right.) Use electrostatic potential maps of Q and QH anion (QH ) to explain this difference. 

The second proposed mechanism involves initial ring opening of the phthalimide. Alkoxide attack on one of the imide carbonyls furnishes amide anion 26. Proton transfer affords enolate 27, which undergoes Diekmann type condensation followed by aromatization to afford the requisite isoquinoline 23. 

Finally a proton transfer leads to formation of carboxylate anion 3. Of particular interest is the benzilic acid rearrangement of cyclic diketones such as 4, since it... 

Acrylamides represent still another interesting class of monomers.6 Their anionic polymerization may be initiated by strong bases, like, e.g., amides. The growing chain contains the unit —CH2—CH —CO—NH2 and intramolecular proton transfer competes efficiently with its carbanionic growth. Since the rearrangement... 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

CT) complex with absorption maxima at 470 and 550nm, was produced. These species were formed only in polar solvents with relatively high proton affinity. The data suggested an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion... 

Challis and Rzepa (1975) observed kinetic deuterium isotope effects in the azo coupling of 2-methyl-4,6-di-tert-butylindole (12.139) and its anion. The origin of this effect must also be attributed to steric hindrance of the proton transfer step in the substitution proper, since 2-deuterated methylindole and unsubstituted indole (Binks and Ridd, 1957) do not give isotope effects. 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

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