Carboxyl groups estimation

Figure 3. Dependence of product yields from Mylar on dose (2). Carboxyl groups estimated by titration (PohFs method) (13)...

While the surface of diamonds from volcanic chimneys is hydrophobic, that from secondary deposits is hydrophilic due to the formation of oxides (Boehm et al. 1964). Powdered diamonds with oxides on their surface show a higher wetting heat than those without. Treating diamonds oxidised on their surface with water forms carboxyl groups. Estimation of active hydrogen after Zerewitinoff or by deuterium exchange revealed that nearly all hydrogen atoms were carboxylic (Sappok and Boehm, quoted from Boehm 1966). Diamonds cleared of oxides at 10" to 10" torr in vacuo at 800 °C do not show an essential resonance absorption (11 pval free radicals/g diamond). 

In this study we examined the influence of concentration conditions, acidity of solutions, and electrolytes inclusions on the liophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 at the cloud point temperature. The liophilic properties of micellar phases formed under different conditions were determined by the estimation of effective hydration values and solvatation free energy of methylene and carboxyl groups at cloud-point extraction of aliphatic acids. It was demonstrated that micellar phases formed from the low concentrated aqueous solutions of the surfactant have more hydrophobic properties than the phases resulting from highly concentrated solutions. The influence of media acidity on the liophilic properties of the surfactant phases was also exposed. 

For quantitative work, it is necessary to estimate the concentration of 5-amino-l-(P-D-ribofuranosyl)imidazole in aqueous solution. It seems that the only available method is the Bratton-Marshall assay, which was originally developed for the estimation of arylamines in biological fluids. The principle of the method is the spectrometric estimation of a salmon-pink colored dyestuff obtained by diazotation in situ, followed by coupling with /V-( 1 -naphthyl)ethyl-enediamine.65 The only remaining problem then is to know the molar extinction of this dye because pure samples of AIRs are not available. A value of 16800 at 520 nM was obtained for the dyes prepared from a model compound, 5-amino-l-cyclohexylimidazole-4-carboxylic acid (54), which is crystalline. A comparable molar extinction can be expected for the dye prepared from imidazole 55, if the carboxyl group does not exert too much influence on the chromophore. Actually, its influence is perceptible even with the naked eye, the dyestuff prepared from 53 having a somewhat different, wine-red color, with max>520 nM. The molar extinction for 55 is 17400 at 500 nM. When the decarboxylation of 54 was conducted under mild acidic conditions (pH 4.8, 50°C, 1 hour), estimation of 5-aminoimidazole 55 by the Bratton-Marshall method led to the conclusion that the reaction was almost quantitative.66 Similar conditions for the final decarboxylation were adopted in the preparation of samples of AIRs labeled with stable isotopes.58... 

The pH-dependence of the inhibition also indicated that unprotonated castanospermine is a better inhibitor than the protonated form. However, as essential carboxyl groups of the enzyme ionize in the same range of pH as castanospermine (pK, 6.09), it was not possible to estimate the inhibitory potency of protonated castanospermine. 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(=328 K) of the monolayer [31]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform-infrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-nr1. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. 

Steffens et al., 1975. The rate of the same reference reaction (note J) was extrapolated from 60° to 75° using an activation energy of 22 kcal mol-1 (Kirby and Younas, 1970), and the same correction for leaving group applied as in j. The pK, of the substrate carboxyl group was estimated to be 4.4... 

In another estimate (Kirby and Percy, 1989), the carboxyl group in l-methoxymethoxy-8-naphthoic acid and the dimethylammonium group in the l-methoxymethoxy-8-A, A -dimethylnaphthylammonium ion are estimated to lead to rate increases by intramolecular catalysis of < ca. 900 and 1.9 X 10 compared to the value of ca. 1 x 10 calculated for the intramolecular catalytic effect of the carboxyl group in 2-methoxymethoxybenzoic acid. The salicylate ion remains the most efficient leaving group thus far discovered that can take part in hydrogen-bond catalysis of the hydrolysis of acetals. 

Cytidine 5 -phospho-A-acetylneuraminate (298), the coenzyme of sialyltransferases, is a sugar-nucleotide in which the leaving group is a nucleotidyl monophosphate that contains a carboxylate group directly attached to the anomeric centre. Studies of its hydrolysis reveal that at pH 5 specific acid catalysis occurs (Scheme 34) to yield a glycosyl carbocation as a tight ion pair, the lifetime of which was estimated... 

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