Strength bases effect

The above reactions have been generalized by Basolo (in equation 32)107 who considers that the presence of base is of fundamental importance. From a kinetic study on the oxidation of CoL4(NO) species (L4 = quadridentate Schiff base), the same author has proposed a mechanism in which the first step is shown in equation (33). He finds that base strength effects reaction rate. This suggests that electron density, driven onto the nitrosyl group by base addition, facilitates oxidation. Conversion to the nitro group then occurs directly at the NO ligand. 

The steric bulk of ligands also introduces an influence that can act counter to a pure base strength effect. This has been discussed earlier for NR3 compounds (Chapter 2.2.2). Substituted pyridines present another example of this influence 2,6-dimethylpyridine is a poor ligand due to the location of the two methyl groups either side of the pyridine N-donor atom, despite a similar base strength to unsubstituted pyridine. 

THE EFFECT OF STRUCTURE BASE STRENGTH A base must be capable of accepting protons for this, at least one... 

The 40% aqueous solution of TRIS AMINO is nonirritating to the eyes and skin. In general, the toxicology of the alkan olamines is typical of alkaline materials, ie, the greater the base strength, the greater the effect. Neutralized alkan olamines are much less toxic their stearate soaps, for instance, have been found to be nonhazardous. 

Equation 20 is the rate-controlling step. The reaction rate of the hydrophobes decreases in the order primary alcohols > phenols > carboxylic acids (84). With alkylphenols and carboxylates, buildup of polyadducts begins after the starting material has been completely converted to the monoadduct, reflecting the increased acid strengths of these hydrophobes over the alcohols. Polymerization continues until all ethylene oxide has reacted. Beyond formation of the monoadduct, reactivity is essentially independent of chain length. The effectiveness of ethoxylation catalysts increases with base strength. In practice, ratios of 0.005—0.05 1 mol of NaOH, KOH, or NaOCH to alcohol are frequendy used. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

For discussions of field effects on acid and base strength and on reactivity, see Chapters 8 and 9, respectively. 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

The tendency for N-nitrosamides to undergo hydrolysis by a nucleophilic catalysed pathway has been confirmed by studies of N-alkylnitroso acetamides (22) Results summarised in Table I for N -n-butyl-JJ -nitroso acetamide show that its decomposition is also subject to steric constraints (2,6-lutidinestrong nucleophiles (eg. imidazole, thiols) irrespective of their base strength (pK ). Further, the second order dependence on [Imidazole] is more clearly defined for the decomposit-... 

In summary, the effect of pH on the dissolution rate of a drug from an oral dosage form depends on (a) the pH of the GI fluids, a patient variable (b) the acid or base strength of the drug, a pharmaceutical variable as well as (c) the physicochemical properties of the dosage form, another pharmaceutical variable. Furthermore, by intentionally designing the dosage form such that it buffers the diffusion layer, we can control a patient variable by a pharmaceutical variable. 

A base is a nucleophile Electronic effects which shift electron density to the atom with the lone-pair increases base-strength. 

Concerning the details of the initiation, we do know from the polarographic studies of Kabir-ud-Din and Plesch (1978) that for CF3S03H in CH2C12 at ca. 20 °C, at any rate in the presence of the base electrolyte needed for polarography (0.1 mold"1 ( -Bu)4NTJF4 ), the K2 for (57) is 7.2 x 10 3, and that there is no formation of HA2 in the absence of the base-electrolyte, K2 is likely to be smaller because of the ionic-strength effect. Therefore, since... 

Phenol red immobilized PVA membrane for an optical pH sensor is developed based on the same approach, since the molecular structure of phenol red is similar to that of phenolphthalein. Phenol red was first reacted with the formaldehyde to produce hydroxymethyl groups, and then it was attached to PVA membrane via the hydroxymethyl groups. The changes of spectra characteristics after immobilization, the ionic strength effects, response time, reproducibility and long-term stability of the sensor membrane are discussed by Z. Liu et al. [170],... 

W Williams, GT Young. Amino acids and peptides. XXXV. Effect of solvent on the rates of racemization and coupling of acylamino acid p-nitrophenyl esters. Base strengths of amines in organic solvents, and related investigations. J Chem Soc Perkin Trans 1 1194, 1972. 

All these data show that the ionic strength can induce a change in pKa up to 0.5-0.6 unit. Because the ionic strength effect causes spectral changes indistinguishable from those caused by pH, a correction method is desirable. A method based on a double pH indicator system, i.e. pyranine and 4-methylumbellifer-one, was proposed by Opitz and Lubbers 1. 

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