Protic hydrogen bond

The proportion of the non-planar form of TINS in films of poly(vinylalcohol) (PVA) a protic, hydrogen-bonding polymer and poly(vinylpyrrolidone) (PVP) an aprotic, hydrogen-bonding polymer, is approximately 10% and 90% respectively (12). These results demonstrate that the polymer environment can have a profound effect on the preferred conformation of the stabilizer and thus influence its effectiveness in photostabilizing the substrate. 

The fluorescence emission spectra of TINS in PVA and PVP also show only a single band near 400 nm which is attributable to emission from a non proton-transferred excited state. The similarity between the values of the fluorescence quantum yield, < >fnp, for the non proton-transferred form of TINS in PVA and PVP (12.) indicates that the PVA polymer is unable to behave in an analogous manner to protic, hydrogen-bonding solvents and suggests that no complexation which can facilitate ESIPT occurs in the excited state as a result of the restricted motion of the PVA chains. 

The excited-state lifetime calculated for TINS in PVA film is found to be 1.3 + 0.1 ns compared with 44 4 ps found in the case of water (H). This supports the earlier proposal that complexation, which is proposed to occur in protic, hydrogen-bonding solvents, does not occur in this polymer. In the PVP film an intense fluorescence and a long excited-state lifetime, similar to that found for TINS in PVA, is observed and is consistent with the ESIPT process being prevented in this aprotic medium. 

The increase in the rate of these three analogous bimolecular nucleophilic displacement reactions on transfer from methanol (a typical protic, hydrogen-bond donor type solvent) to DMF (a typical dipolar aprotic solvent) varies from a factor of 10 to 10 , being faster in the aprotic media. The medium has little effect on the CHgAT free energy, these molecules being slightly better solvated in DMF. This stabilisation is almost independent of X, The increase in rate clearly reflects the substantial destabilisation of the N on the transfer to DMF, modified by a small destabilisation of the transition state positive for Z = Cl... 

Different behavior is observed in polar protic (hydrogen-bonding) solvents (e.g. ethanol). Both (82) and (94) give the same uv spectrum, with maxima at 329 and 390 nm which indicates that the abovementioned equilibrium is displaced toward the anhydronium base and/or ammonium... 

The nucleophilicity of anions, in general, depends very much on the degree of solvation. Much of the data that form the basis for quantitative measurement of nucleophilicity is for reactions in hydroxylic solvents. In protic, hydrogen-bonding solvents, anions are subject to strong interactions with solvent. Hard nucleophiles are more strongly solvated by protic solvents than soft nucleophiles, and this difference contributes to the greater nucleophilicity of soft anions in such solvents. Nucleophilic substitution reactions often occur more readily in polar aprotic solvents than they do in protic solvents. This is because anions are weakly solvated in such... 

Conversion from a protic, hydrogen-bonding solvent to a polar, aprotic one accelerates the reaction enormously by reducing solvation of the negatively charged oxygen atom (compare Table 6-6). See Problem 57 for a similar exercise. 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

Water (HOH), alcohols (ROH), and carboxylic acids (RC02FI) are classified as polar protic solvents they all have OH groups that allow them to for-rn hydrogen bonds... 

Hydrogen bonding with protic solvents or reagents occurs widely in azines even when they are not appreciably basic and the protic compounds are very poor acids. The latter do not have to be present... 

A similar effect is produced by cocrystallization with protic solvents capable of forming a hydrogen bond-stabilized environment. Thus, dihydropyrimidine 47 (R = R = aryl,R = R = COOR, R = H) cocrystallizes with water (1 1) exclusively as the 1,6 tautomer (98T9837). 2,4,6,6-Tetraphenyldihydropyrimidine 47 (R = R = R = R = Ph, R = H) exists as the 1,6 tautomer in its solvate with... 

Protic solvents, such as methanol and ethanol, slow down SN2 reactions by solvation of the reactant nucleophile. The solvent molecules hydrogen bond to the nucleophile and form a "cage" around it, thereby lowering its energy and reactivity. 

Another marked physical difference between sulphides and sulphoxides (or sulphones) is that sulphoxides (and lower alkyl sulphones) are hygroscopic and dissolve quite readily in water or protic solvents such as alcohols, and even more so lower alkyl or alkyl aryl sulphoxides are almost freely miscible with water. This can be accounted for by the formation of the strong hydrogen bond between the S—O bond in the sulphoxides and water molecules. Moreover, lower alkyl sulphoxides and sulphones such as dimethyl sulphoxide (DMSO) or sulpholene can dissolve a number of metallic salts, especially those of alkali and alkaline earth metals, and hence these compounds have been widely utilized as versatile and convenient solvents in modern organic chemistry26 (Table 3). 

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

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