Acceptor reaction equilibria

Greaney and coworkers have introduced the conjugate addition of thiols to Michael acceptors as an effective adaptive DCL strategy [46,47]. The reaction is well suited for biological DCL synthesis, taking place in water with no requirement for external reagents. As with disulfide bond formation, the reaction is subject to simple and effective pH control. Under mildly basic conditions, the thiolate anion adds rapidly to Michael acceptors under equilibrium conditions. Acidification effectively switches the reaction... 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

The Brpnsted and Lowry theory states that an acid is a proton donor and a base is a proton acceptor. Since equilibrium exists between what are considered the unionized (neutral) and ionized forms of a compound, a constant can be determined. This is termed the equilibrium acid ionization (Ka) and expresses the ratio of concentrations for the reaction ... 

We shall study the donor acceptor reactions concerning chemical equilibrium in the next chapters correlating drawings and mental models will be provided for better comprehension. This way, comprehension of chemical equilibrium can be intensified and completed. 

Since dehydrogenase catalysis is reversible, the production enzyme can be used to perform the regeneration reaction of NAD(P)+ using a suitable cosubstrate as electron acceptor. In this case, the regeneration enzyme in Fig. 16.2-2 is identical with the production dehydrogenase. However, conversion rates in this set-up tend to be low because of a given reaction equilibrium, which requires an efficient method to withdraw the products and coproducts. 

Table II notes that in the process two important equilibria in acceptor reactions are involved. These were checked experimentally since the literature data are contradictory. No data at all were available on the H2S acceptor reaction, which is important in the gasification step, and the equilibrium was also determined for Reaction 5.

Equilibrium Studies of Acceptor Reactions. The equipment used for equilibrium studies in the CO2 acceptor reaction (Reaction 1) is illustrated in Figure 2. 

Pdi-Clusters as electron acceptors - Reaction of [Pd3(//3-CO)(//-dppm)3] with CO gave the adducts [Pd3(/i3-CO)(CO)(yU-dppm)3] + [Pd3(j 3-CO)(/i3-CO)-(//-dppm)3], it was proposed that fluxionality involving this equilibrium is rapid even at -90 When cyanide reacts with [Pd3( 3-CO)( -dppm)3] + the adduct... 

The HSAB principle is qualitatively useful, but lacks a satisfactory quantitative basis. Pearson has pointed out that the hard-hard or soft soft matching of add and base represents a stabilization that is additional to other factors that contribute to the strength of the bonds between donor and acceptor. These factors include the sizes of the cation and donor atom, their charges, their electronegativities and the orbital overlap between them. There is another problem. Complex formation usually involves ligand substitution. In aqueous solution, for example, ligands displace H2O and this is a competitive rather than simple combination reaction (equilibrium 7.85). 

In sorption-enhanced reforming (SER) reactors, one of the products is extracted from the reaction zone, thus shifting the reaction equilibrium to the product side. In SER, the methane steam-reforming catalyst is mixed with a CO2 sorbent ( acceptor ). The CO2 produced during the reaction is adsorbed and the reverse... 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

It follows from the electrochemical mechanism of corrosion that the rates of the anodic and cathodic reactions are interdependent, and that either or both may control the rate of the corrosion reaction. It is also evident from thermodynamic considerations (Tables 1.9 and 1.10) that for a species in solution to act as an electron acceptor its redox potential must be more positive than that of the M /M equilibrium or of any other equilibrium involving an oxidised form of the metal. 

The equilibrium between copper and cuprous and cupric ions is disturbed by the presence of oxygen in solution, since the reaction shown in equation 4.3 is facilitated, the oxygen acting as an electron acceptor. 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

Whereas SHMT in vivo has a biosynthetic function, threonine aldolase catalyzes the degradation of threonine both l- and D-spedfic ThrA enzymes are known [16,192]. Typically, ThrA enzymes show complete enantiopreference for their natural a-D- or a-t-amino configuration but, with few exceptions, have only low specificity for the relative threo/erythro-configuration (e.g. (122)/(123)) [193]. Likewise, SHMT is highly selective for the L-configuration, but has poor threo/erythro selectivity [194]. For biocatalytic applications, the knovm SHMT, d- and t-ThrA show broad substrate tolerance for various acceptor aldehydes, notably induding aromatic aldehydes [193-196] however, a,P-unsaturated aldehydes are not accepted [197]. For preparative reactions, excess of (120) must compensate for the unfavorable equilibrium constant [34] to achieve economical yields. 

The water equilibrium illustrates the amphiprotic nature of H2 O. In this reaction, one water molecule acts as a proton donor (acid), and another acts as a proton acceptor (base). 

The Oppenauer oxidation makes use of ketones (typically acetone) or alkenes as hydrogen acceptors and this absence of a strong oxidising agent allows to overcome some potential NHC oxidative instability. Reactions consist of an equilibrium between an alcohol and its oxidised form (Scheme 10.9). 

The microscopic mechanism of these reactions is closely related to interaction of the reactants with the medium. When the medium is polar (e.g., water), this interaction is primarily of electrostatic nature. The ionic cores of the donor and acceptor located at fixed spatial points in the medium produce an average equilibrium polarization of the medium, which remains unchanged in the course of the reaction and does not affect the process of electron transfer itself. The presence of the transferable electron in the donor induces additional polarization of the solvent around the donor that is, however, different from polarization in the final state where the electron is located in the acceptor. 

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