Protic character

Within the monohydridic route, apart from the already explained inner-sphere mechanisms, there is another possibility involving the concerted outer-sphere transfer of one hydride and one proton to the corresponding substrate (Scheme 4b). This mechanism is very common to the so-called bifunctional catalysts. This term was proposed by Noyori for those catalysts having one hydrogen with hydridic character directly bonded to the metal center of the catalyst, a hydride ligand, and another hydrogen with protic character bonded to one of the ligands of the metal complex (20). In Scheme 9, examples of bifunctional catalysts that are synthesized... 

The Physicochemical Properties of Solvents and Their Relevance to Electrochemistry. The solvent properties of electrochemical importance include the following protic character (acid-base properties), anodic and cathodic voltage limits (related to redox properties and protic character), mutual solubility of the solute and solvent, and physicochemical properties of the solvent (dielectric constant and polarity, donor or solvating properties, liquid range, viscosity, and spectroscopic properties). Practical factors also enter into the choice and include the availability and cost of the solvent, ease of purification, toxicity, and general ease of handling. 

Protic character. The protic character of the solvent is an important consideration because electrochemical intermediates (particularly radical anions) frequently react rapidly with protons. The classification of solvents into protic or aptotic solvents is somewhat arbitrary. A simple classification1 is that protic solvents (such as hydrogen fluoride, water, methanol, formamide, and ammonia) are strong hydrogen-bond donors, exchange protons rapidly, and in-... 

The reversible one-electron transfer to form an anion radical (R ) is followed by an irreversible chemical protonation to form /f H, which is subsequently reduced itself (the reduction potential of the species / H, has been shown to be more positive3 than that of the parent, R) and then undergoes another irreversible protonation reaction. In a protic solvent, the reactions proceed rapidly to the final product, / H2. In a rigorously purified aprotic solvent, the intermediate anion radical R , has an appreciable lifetime and reacts only slowly, principally with adventitious impurities in the solvent. Thus, the stability of aromatic anion radicals can be taken as a measure of the protic character of a solvent. 

Both kinetic and thermodynamic factors are important in determining protic character. Although the equilibrium concentration of solvated protons in a protic solvent such as water or ethanol may be very small, there is a low activation energy for dissociation or exchange of protons, and these labile protons can rapidly react with any species having an appreciable proton affinity. Aprotic solvents usually have a lower equilibrium concentration of solvated protons (perhaps by a factor of as much as 1010) and the activation energy for... 

Bordwell et al [135] have pointed out that solvents referred to as dipolar aprotic are in fact not aprotic. In reactions employing strong bases their protic character can be recognized. Therefore, instead of dipolar aprotic the designation dipolar nonhydroxylic or better dipolar non-HBD solvents is strongly recommended. Cf. Section 2.2.5 and 3.4 (footnote). In order to avoid confusion, the nomenclature proposed by Chastrette et al [138] is retained in Fig. 3-6. 

The transfer free energy (as measured by the slope value) for any solute in an alcohol/water system should decrease as the chain length of the solvent alcohol is shortened. Of course the same effect ought to be observed if we hold the oil phase constant and make the aqueous phase less hydrophilic. The equations in Table V show that for a limited set of barbiturates partitioned between diethyl ether and a 50-50 mixture of water and dimethylformamide, the slope, compared with the octanol standard, is only 0.4 whereas the ether/water system gives a slope for the donor solute group of 1.13. Thus, a 50% reduction in the protic character of the polar phase reduces the sensitivity of the system by a factor of 2.8. 

Table V. Transfer Free Energy Reduced by Decreasing Protic Character of Aqueous Phase...

If Z also has a protic character (e.g., CsHg), an amine elimination reaction may... 

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