P£b value

When using relation (12c) it was found that the determination of Pa is subject to large errors, as a result of which the p B values naturally became very inacciu-ate (Gold and Tye, 1952b). Plattner and collaborators (1949) also drew attention to this source of error when evaluating partition equilibria of azulene derivatives. 

As mentioned at the very beginning, it is impossible to give the p a value of and the p b value of OH on a strictly thermodynamic basis [9]. However, using the concentration of water in pure water Oog = 1 -24), one can construct the diagram... 

In aqueous solution, N2H4 usually forms [N2H5] (hydra-zinium) salts, but some salts of [N2H6] have been isolated, e.g. [N2H6][S04). The p b values for hydrazine are given in equations 15.36 and 15.37, and the first step shows N2H4 to be a weaker base than NH3 (equation 15.22). 

Figure 4.11 Left Certain unsaturated heterocycles, such as pyridine, also have the lone pair in a sp -hybrid orbital and thus have p/(b values equivalent to those of imines. Right Attachment of electron-withdrawing groups will raise the p/Cb of imines, but to an unpredictable degree. It is proper to estimate the p/fb as being >10.

The compounds have limited value as dye-stufTs intermediates, nitrobenzenef CeHjNOj, PhN02-Colourless, highly refractive liquid with characteristic smell m.p. b.p. 21 TC. 

When, in a column headed M.p., a value is given in parenthesis, it indicates that the compound is liquid at room temperature and that the value given is consequently the boiling-point. Conversely in a column headed B.p., values given in parenthesis are those of the melting-point. A blank space indicates that the compound has not apparently been recorded. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The parameters and Ca are associated with the Lewis acid, and Eg and Cb with the base. a and b are interpreted as measures of electrostatic interaction, and Ca and Cb as measures of covalent interaction. Drago has criticized the DN approach as being based upon a single model process, and this objection applies also to the — A/y fBFs) model. Drago s criticism is correct, yet we should be careful not to reject a simple concept provided its limits are appreciated. Indeed, many very useful chemical quantities are subject to this criticism for example, p o values are measures of acid strength with reference to the base water. 

For the theory of LFER s, two consequences follow (a) The reaction constant p should depend on T according to eq. (18), and (b) values of AH and AS should obey the pertinent relationship as well as log k. The experimental evidence seems to be in accord with the first postulate (9, 17). Sometimes the simple proportionality to T" was claimed (124), corresponding to the isoentropic series but this is only a particular case, of course. 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

C18-0146. The figure beiow shows the titration curves for 50.0-mL samples of weak bases A, B, and C. The titrant was 0.10 M HCl. (a) Which is the strongest base (b) Which base has the largest p (c) What are the initial concentrations of the three bases (d) What are the approximate p K- values of the conjugate acids of the three bases (e) Which of the bases can be titrated quantitatively using indicators Explain your answers, and identify an appropriate indicator for each base that can be titrated successfully. 

It should be noted that relative intensity measurements suffice for the determination of z. The absolute value of ie/Ie required for calculation of the molecular weight usually is determined at 90° and the appropriate correction factor 1/P(90°), obtained from the dissymmetry as described above, is applied to obtain Pgo°. The factor P B) is strictly applicable only at infinite dilution, and it is therefore impor-... 

Carrupt, P. A., El Tayar, N., Karlen, A., Testa, B. Value and limits of molecular electrostatic potentials for characterizing drug-biosystem interactions. Methods Enzymol. 1991, 203, 638-677. 

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