Reversible bases

The reactions are reversible based on the partial pressures of the acid gases. Potassium carbonate also reacts reversibly with COS and CS2. 

It is appropriate at this juncture to address some of the more useful transformations of 2,3-epoxy alcohols.913 A 2,3-epoxy alcohol such as compound 14 possesses two obvious electrophilic sites one at C-2, and the other at C-3. But in addition, C-l of a 2,3-epoxy alcohol also has latent electrophilic reactivity. For example, exposure of 14 to aqueous sodium hydroxide solution results in the formation of triol 19 in 79% yield (see Scheme 5). In this interesting transformation, hydroxide ion induces the establishment of an equilibrium between 2,3-epoxy-l-ol 14 and the isomeric 1,2-epoxy-3-ol 18. This reversible, base-induced epoxide migration reaction is a process known as the Payne rearrangement.14... 

Electron-deficient nitriles, such as for instance trichloroacetonitrile and trifluoroacetonitrile (A=B A = N B = CCC13, CCF3), are known to undergo direct and reversible, base-catalyzed addition of alcohols providing O-alkyl trichloroacetimidates (1,50). This imidate synthesis has the advantage that the free imidates can be isolated as stable adducts, which are less sensitive to hydrolysis than their corresponding salts. 

Figure 3.10 Reversible base pairing defines the rigid structure of a reversible polymer system.

The reduction of the Cu(R2Dtc)2 complexes often appears quasi-reversible (based on voltametric parameters), presumably owing to a structural reorganization of the CuS4 core of the Cu(R2 Dtc)2 product. Coulometric reduction of Cu(/-Pr2 Dtc)2 at 0.70 V gave an n value of 1.0 and was consistent with the simple reduction process shown by Eq. 4. Although the monoanion could not be... 

Several types of room temperature-operable molecular switches have been synthesized and demonstrated in nanopores and atop silicon-chip platforms. The functional molecular switches can be reversibly switched from an OFF state to an ON state, and/or the reverse, based on stimuli such as voltage pulses. The number of nanoparticles (usually metallic or semiconducting) and the number of the interconnecting molecular switches can vary dramatically based on the chosen size of the NanoCell and on the dimensions of the nanoparticles and molecules chosen.10... 

Cephalosporin esters are widely used as intermediates in cephalosporin synthesis and as prodmgs for oral administration of parenteral cephalosporins. These esters undergo reversible base-catalysed isomerisation according to the mechanism shown in Scheme 4.8. A proton in the 2-position is abstracted by a base (B) and the resulting carbanion can be repro-tonated in the 4-postion, giving a A -ester. On hydrolysis, A -cephalosporin esters yield A -cephalosporins, which are biologically inactive. 

Provided that the sweep is reversed at potentials well before the onset of the more anodic features, the process in (1.1) appears to be reversible. Based upon the magnitude of the charge, as calculated by a straightforward integration of the peak, Arvia and coworkers have concluded that this reaction involves only a few nickel monolayers. The divalent Ni hydroxide exhibits a finite, although very small solubility in this medium, and thus a... 

Many reactions can change direction. These reactions are called reversible reactions. Some reactions, such as the one shown in Figure 6.13, can be reversed based on the energy flow. 

For acylations with reactive esters, such as formate or oxalate (see Section 3.6.4.5), sodium alkoxides are still the bases of choice, but sodium hydride, dimsyl sodium, sodium or potassium amide or sodium metal have all been used for the in situ generation of the enolate anion. A typical example is shown in Scheme 47. Acylation by esters results in the production of 1 equiv. of the alkoxide ion, along with the p-dicarbonyl compound proton transfer then results in the production of the conjugate base of the dicarbonyl compound. This process normally leads to the more stable anion in the acylation of an unsymme-trical ketone. The acyl group thus becomes attached to the less-substituted a-position of the ketone. The less stable 0-acylated products are normally not observed in such reversible base-catalyzed reactions. Methyl alkyl ketones are normally acylated on the methyl group where both a-carbons are substituted to the same extent, acylation occurs at the less-hindered site. Acylation is observed only rarely at a methine carbon as the more stable p-diketone enolate cannot be formed. 

In Section 1.5 we used a graphical test for reversibility based on the Heyrovsky-Ilkovic equation. For a reversible reaction this gives a value of from an intercept on the plot. 

Figure 1 Schematic of the multi-step process of organic degradation in anaerobic systems, indicating the influence of H2. Boxes indieate major chemical pools, arrows with text indicate microbially-mediated process. Superscripts on arrow text indicate the mode of control by H2 (F)—facultative interspecies H2 transfer (O)—obligate interspecies H2 transfer (B)—H2 can be a by-product, shifting the product yield (U)— the reaction becomes thermodynamically unfavorable when H2 concentrations are too low (R)—the process is demonstrated or hypothesized to be reversible, based on H2 concentrations. Note ( ) important inorganic oxidants may include NO, Mn , Fe , and various oxidized sulfur speeies (e.g., S04 ) Xred corresponds to the redueed form (e.g., H2S). Adpated from Ref. [19].

Assignment could be reversed, based on McLachlan calculation of spin densities. Protons of t-butyl groups. One proton. Carbon atoms of r-butyl groups. Assignment could be reversed. McLachlan calculation of coupling constants using p- and d-model for sulfur. ... 

The first electron transfer to NAD+ is claimed to be reversible based on a log i-E plot [118] and cyclic voltammetric... 

Frey developed a pyrrole 338 synthesis starting from vinyldibromides 336 and enamines 337 [55]. In the presence of a strong base (KOtBu) an initial dehydrobro-mination of 336 led to an alkynylbromide 339. A consecutive equilibration was found to be crucial. Involving activating aryl substituents Ar (Ph, Naphthyl), a reversible base-induced H-shift, should have formed the corresponding allene 340. 

Esters (RCO2R) can be hydrolysed in aqueous acid (see Section 9.4.2) or base. In basic solution, this is known as a saponification reaction and this has an important application in the manufacture of soaps from vegetable oils or fats. Whereas acid hydrolysis is reversible, base hydrolysis is irreversible due to the formation of the resonance-stabilised carboxylate ion. 

Frequently branched, 1-1.5 mm in diameter, without rough feeling. Intemodes 1.5-3 cm long. Membranous scaly leaves 1-2 mm long, with 2 lobes (rarely 3), the upper part short-triangular, greyish-white, apex infrequently reversed, base brownish-red to brownish-black... 

BFj is a Lewis acid because the empty valence orbital in the boron atom accepts an electron pair from the oxygen atom in C2H5OC2H5, a Lewis base because it donates the electron pair. 9. F HPO/ HNO2 H3PO4 11. Acids HSO4 (forward) and HC2O4 (reverse) bases 304 (forward)... 

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