Basal cell carcinoma is the most common malignant neoplasm of the skin, whereas the localization at the nail unit is very rare. We report the case of a 58-year-old patient with a periungual basal cell carcinoma at the thumb. The specific feature of the reported case is the frequent exposure to fishing baits that the patient had stained with an unknown colorant. The use of chromatography, mass spectrometry and infrared spectroscopy revealed the colorant as the azo pigment Solvent Red 8. Solvent Red 8 is a widespread synthetic azo pigment that is applied to stain consumer products. Compounds such as Solvent Red 8 can be cleaved to carcinogenic amines under, for example, light exposure, in particular after incorporation into the human body. As a result of the frequent skin contact to this azo pigment, this hazard compound might have induced the basal cell carcinoma in our patient.
Eight generalizations are given arising from world-wide studies of proton transfer reactions in aqueous media carried out over the past twentyfive years. Future directions of research on pro ton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, acetonitrile, and benzonitrile) are reviewed.Inorganic solution chemistry often involves proton trans fers to and from solvated metal ions as well as to and from the acids and bases that complex metal ions. Eight generalizations are presented below that attempt to summarize the insights re garding proton transfer reactions that have emerged in the past quarter century.The masterful reviews by Eigen (1_) and Bell (2) provide much more extensive analysis of most of these points.
Eight Generalizations
1.For thermodynamically favorable reactions (K»l) of the typeinvolving oxygen or nitrogen acids with BLO, HLO + , or OH in 10 -1 -1 aqueous solution at room temperature, k^~10 M s and the rate constant is smaller by a factor of 10^^ in the reverse (unfavorable) direction (1,2).2. Eigen 1 s mechanism for proton transfer reactions be tween acids (AH) and bases (B) proceeds through a neutral hydro-
Proton transfer reactions of the five substituted phenols 3‐Cl−4‐nitrophenol CNP, 4‐nitrophenol NP, 3‐methyl−4‐nitrophenol MNP, 4‐Cl−3,5‐dimethylphenol CMP, and 3,5‐dimethylphenol DMP with acridine orange, and the formation of hydrogen bonded complexes AHA− of the anions A− of CNP, NP, and MNP with the respective phenols AH in benzonitrile solution were investigated by means of absorption spectroscopy. Thermodynamic data for the formation of the complexes AHA− and of the ionic reactions (1) and (2) of CNP, NP, and MNP with the acridine orange base B
and for the reactions (2) and (3) of CMP and DMP with B were determined. The reactions (1) ‐ (3) are exothermic. The absolute values of ΔH1 increase with the acidity of the phenols. The formation enthalpies of the hydrogen bonded anions AHA‐ are much larger than ΔH1, so that ΔH2 is mainly determined by this association. On the other hand there is no significant difference between ΔH2 and ΔH3; thus the hydrogen bond formed between a phenol and a complexed anion AHA‐ is very weak. Substituent effects on the acidity of phenols are about twice as large in benzonitrile as in aqueous solution.
The kinetics of proton transfer between five substituted phenols (AH) and the acridine orange base (B) are studied in benzonitrile solution using the temperature jump method. In the overall reactions (1)–(3)
the formation and dissociation of an intermediate ion pair A‐.. HB+ is rate determining. The rate constants of the bimolecular recombination of the ions to the ion pair are kb1, kb2, and kb3. Their values are in the range (0.18‐ 1.7) · 109 M‐1 s‐1 (25 °C, I = 0.1 M). kb2 and kb3 do not depend on the pK of the phenol involved, although these rates are not diffusion controlled. The rate of proton transfer from the protonated acridine orange (BH+) to the free anions (A‐) does depend to some degree on the acidity of the corresponding phenol. In the case of the 3‐methyl‐ 4‐nitrophenolate ion this rate is diffusion controlled, as shown by the dependence of kb1 on the temperature and the ionic strength. Lower limits for the formation constants of the ion pairs are estimated from the rate data. The equilibria and the kinetics are discussed in terms of an extended Eigen scheme of proton transfer, and the separate steps of anion solvation by hydrogen bonding to phenol molecules shown to depend in different ways on the basicity of the anion.
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