Carbon acids ionization, solvent effects

A study of acid-catalysed enolization and carbon-acid ionization of isobutyrophenone has combined the solvent isotope effect k /kv = 0.56 and substrate isotope effect kH/kD = 6.2 determined for the enolization in H2O and D2O with literature information in order to estimate the solvent isotope effect on the enolization equilibrium, A e(H20)/A e(D20) = 0.92, and on the CH ionization of butyrophenone, kf (R20)/kK(D20) = 5.4.130 This is the first report of an isotope effect on AY forketo-enol equilibrium of a simple aldehyde or ketone. 

A study of acid-catalysed enolization and carbon-acid ionization of isobutyrophenone has combined the solvent isotope effect — 0.56 and substrate isotope effect... 

It should be mentioned that a solvent change affects not only the reaction rate, but also the reaction mechanism (see Section 5.5.7). The reaction mechanism for some haloalkanes changes from SnI to Sn2 when the solvent is changed from aqueous ethanol to acetone. On the other hand, reactions of halomethanes, which proceed in aqueous ethanol by an Sn2 mechanism, can become Sn 1 in more strongly ionizing solvents such as formic acid. For a comparison of solvent effects on nucleophilic substitution reactions at primary, secondary, and tertiary carbon atoms, see references [72, 784]. 

Many reactions become possible only in such superbasic solutions, while others can be carried out under much milder conditions. Only some examples of preparative interest (which depend on the ionization of a C—H or N—H bond) will be mentioned here. The subsequent reaction of the resulting carbanion may involve electrophilic substitution, isomerization, elimination, or condensation [321, 322]. Systematic studies of solvent effects on intrinsic rate constants of proton-transfer reactions between carbon acids and carboxylate ions as well as amines as bases in various dimethyl sulfoxide/ water mixtures have been carried out by Bernasconi et al. [769]. 

Salts of weak bases or weak acids are hydrolyzed in aqueous solution (see Chap. XI) and they behave as if they contained excess of strong acid and strong base, respectively. According to the law of mass action the presence of one acid represses the ionization of a weaker one, so that the effective conductance of the water, which is due mainly to carbonic acid, is diminished. The solvent correction in the case of a salt of a weak base and a strong acid should thus be somew hat less than the total conductance of the w ater. For solutions of salts of a w eak acid and a strong base, vhich react alkaline, the correction is uncertain, but methods of calculating it have been described they are based on the assumption... 

Certain types of traditional LC mobile phase additives should be avoided due to nonvolatility and ion suppression effects. Mobile-phase related ion suppression will not depend on the analyte proximity to the solvent front, or capacity factor. These additives include detergents surfactants ion pairing agents inorganic acids such as sulfuric, phosphoric, hydrochloric, and sulfonic acids nonvolatile salts such as phosphates, citrates, and carbonates strong bases and quaternary amines. Complete suppression of ionization as well as interferences in both positive and negative ion mode will occur when these agents are utilized. 

Substitution of CO by phosphines 145 The Dotz reaction 149 Rearrangement reactions with loss of CO 151 Photochemical reactions 153 Reactions at the carbene carbon 158 General features 158 Amine nucleophiles 159 Phospine and phosphite nucleophiles 167 Alcohols and alkoxide ion nucleophiles 171 Thiol and thiolate ion nucleophiles 179 Intramolecular nucleophilic reactions 191 Hydroxide ion and water as nucleophiles 194 Insertion reactions initiated by nucleophilic attack Acid-base reactions at the a-carbon 207 General features and methods 207 Kinetic and thermodynamic acidities 209 Effect of structure on pKa values 210 Intrinsic rate constants for proton transfer 219 Thermodynamic acidities in organic solvents 223 Hydrolysis of ionizable carbene complexes 228 Acknowledgments 232 References 233... 

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