Acidic and basic substituents

These figures are broadly consistent with the average values of Andrews et al. which were in the range 34-48kJ/ mol for charged phosphate, amine and carboxyl groups. 

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Copolymers can be used to introduce a mixture of chemical functionalities into a polymer. Acidic and basic substituents can be introduced, for example, through comonomers like acrylic acid and vinyl pyridine. The resulting copolymers show interesting amphoteric behavior, reversing their charge in solution with changes of pH. 

What is the difference between the isoelectric pH and the isoionic pH of a protein with many different acidic and basic substituents ... 

The trends are easy to understand in qualitative terms. For instance, acidic and basic substituents that are nr-electron donors, such as OH and NHj, become more acidic and less basic in the excited state, respectively. The ability of the substituent to transfer electron density into the arene is greatly increased upon excitation, since it then can occur into one of the bonding orbitals that were doubly occupied in the ground state. (Cf. Section 2.4.2.) The increased positive charge on the substituent functionality increases its acidity and reduces its basicity. 

Fig. 1. Ionic liquid/water distribution coefficients compared to the octanol/water values (log scale). Crosses amino-aromatic compoimds, open triangles neutral compounds or compounds with both acidic and basic substituents, filled-diamond acidic and/or phenolic compounds ( http //www.mariecurie.org/annals/volumeS/berthod.pdf).

For a broad review of substituent and solvent effects on acidity and basicity, see R. W. Taft, Prog. Phys. Org. Chem. 14 247 (1983). 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

Hydroxy-l-alkenyl diisopropylcarbamates 2 (X = OCb), in this respect, occupy a medium position since they are stable in strongly acidic and basic protic solvents. For deblocking vinyl carbamates, the presence of catalytic amounts of mercuric or palladium(II) salts is required. Due to this stability, several reactions of homoallylic alcohols, proceeding with high diastereo-selectivity, e g., epoxidation, are applicable in order to introduce further hetero-substituents. 

Effect of Fluorine Substituents on the Acidity and Basicity of Compounds... 

It was also found that both the Lewis acidity and basicity depend not only on specific electronic properties of the central group 13 and group 15 elements, but significantly on substituent effects.24 Electron-withdrawing substituents increase the Lewis acidity but decrease the Lewis basicity, whereas electron-donating substituents decrease the Lewis acidity and... 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

The accurate determination of gas-phase basicities and gas-phase acidities opened the way to analyses of the effect of solvation on proton acidities, and on hydrogen-bond acidities and basicities, as well as on substituents effects. 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

On adding the hydroxyl substituent to the CH2CH2 unit, the barrier for C-C scission is lowered because of more favorable thermodynamics (Albini and Spreti 1987, Barton et al. 1996). However, the hydroxyl substituent becomes effective only after its deprotonation. Cation-radicals of 2-, 3-, and 4-arylalkanols, all of them, undergo C(l)-H deprotonation at pH 4. At pH 10, they display a different behavior. The 2-(4-methoxybenzene)ethanol cation-radical experiences C(2)-C(l) scission, resulting in the formation of formaldehyde and 4-methoxybenzyl radical the 3-(4-methoxybenzene)propanol cation-radical gives rise to 3-(4-methoxybenzene) propanal the 2-(4-methoxybenzene)butanol cation-radical behaves as the C -H acid both in acidic and basic solntions (Baciocchi et al. 1996,1999a). 

The condensation of acetone and 1,2-diarylbiguanides bearing identical substituents yields the same alkali-stable product (CXXXVI) under both acidic and basic conditions (95). 

Hydrazines with at least one hydrogen substituent on nitrogen are oxidised in aqueous solution to the substituted diazene. Oxidation takes place in both acid and basic solution. Phenylhydraziiie is oxidised by inorganic reagents such as iodine buffered by sodium hydrogen carbonate to form benzenediazonium ion. The latter combines with the substrate to form the isolated product [140]. 1,1-Disubstituted... 

The reactivity of carboxyhc acid derivatives depends on the basicity of the substituent attached to the acyl group. Therefore, the less basic the substituent, the more reactive is the derivative. In other words, strong bases make poor leaving groups. Carboxylic acid derivatives undergo a variety of reactions under both acidic and basic conditions, and almost aU involve the nucleophilic acyl substitution mechanism (see Section 5.5.5). 

Several additional studies have confirmed interannular electronic interactions in ferrocene. Early investigations by Nesmeyanov and coworkers and by others demonstrated the electron-donor properties of the ferrocenyl group (71). Aminoferrocene, for example, is approximately 20 times as strong a base as aniline, while benzoic acid is several times more acidic than ferrocenecarboxylic acid. Moreover, acidities and basicities of this sort are markedly affected by substituents on the opposite cyclopentadienyl ring. 

Alkyl cyanides with electron withdrawing substituents are more reactive and cyclotrimer-ize under both acidic and basic conditions as well as at high pressure (Table 14). Complexes of Lewis acids and hydrogen halides are particularly valuable catalysts. Two mechanisms for the acid-catalyzed cyclotrimerization have been postulated. Grundmann et al. suggest... 

Depending on the ratio of the reagents in the reaction, the complexes obtained may have different composition and structural type. Also the acidity of the reaction solution can cause drastic changes in the structure of the products. The complexes formed as a rule are polymeric, poorly soluble, and sometimes unfit for X-ray diffraction study. Apart from the conditions of the synthesis, the structure of the complexes is significantly affected by the type of the substituent attached to the endocylic carbon, and by the acidity and basicity of the ligand <2002RJC1457>. 

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