The apparent pKa values of the positively charged, hydrophilic indicators 2‐hydroxy‐5‐bromo‐trimethylanilinium chloride (HBTC) and 2‐hydroxy‐5‐nitro‐trimethylanilinium chloride (HNTC) in the presence of water‐in‐oil microemulsion of the surfactants benzylcetyldimethyl ammonium chloride (BCDC) in benzene and polyoxyethylene (4) dodecyl ether (Brij‐30) in heptane were determined spectrophotometrically. The experimental variables were the type of buffer (benzimidazole, phosphate, and tris) and the ratios [water]/[surfactant] and [buffer]/[surfactant]. It is shown that: a) the indicators are confined within the second hydration shell of the nanodroplet, i.e. are not adsorbed at the water/oil interface; b) the use of the starting pH values of the buffer solutions, i.e., those measured outside the micellar domain, to calculate the micellar pKa values of the indicators is inadequate; c) buffer‐independent micellar pKa values can be obtained if the starting pHs of the buffer solutions were corrected for ion exchange with the counter‐ion of the positively charged surfactant, and for the lower polarity of the micelle‐solubilized water (relative to bulk water); d) the micellar ΔpKa (=pKa in the aggregate −pKa in water) ranged from ‐0.45 to ‐0.20 unit for HBTC/BCDC, from ‐0.44 to ‐0.13 unit for HNTC/BCDC, from 0.22 to 0.64 unit for HBTC/Brij‐30, and from 0.27 to 0.59 unit for HNTC/Brij‐30; e) these micelle‐induced pKa shifts can be analyzed in terms of the lower polarity of the micelle‐solubilized water (relative to bulk water) and perturbation of the acid‐base equilibria by the electrostatic effect of the positively charged W/O interface (BCDC); f) the magnitude of latter effect, at the average solubilization site of the indicator was calculated to range from 28 to 67 mV.
Imidazole (Imz)‐catalysed hydrolysis of benzoate esters proceeds via the intermediate formation of N‐benzoylimidazoles. This paper considers the second step of this reaction, viz., Imz‐catalysed hydrolysis of N‐(4‐X‐benzoyl)imidazoles, X = CH3, H, Cl, CN and NO2, and N‐(disubstituted benzoyl)imidazoles, 2‐chloro‐4‐nitro, 2, 4‐dinitro and 3,5‐dinitro, in water–acetonitrile mixtures (10% or 14%, v/v, in organic solvent). On the basis of catalytic rate constants and the kinetic solvent isotope effect, it is shown that catalysis by Imz is of the general‐base type. Unexpectedly, the hydrolysis of N‐(2,4‐dinitrobenzoyl)imidazole was found to be slower than that of N‐(4‐nitrobenzoyl)imidazole. It is shown that this reactivity order is due to a combination of a steric effect and stabilization of the reactant state due to a donor–acceptor interaction between the Imz moiety and the 2,4‐dinitrophenyl ring.
The kinetics of methanolysis (solvolysis) in 97.4% MeOH-dioxan of a series of 4'-substituted 4-benzyloxybenzyl chlorides, and of 4-anisy1, 4-phenoxybenzy1, and benzyl chlorides have been studied and discussed, including comparisons with the data for the corresponding series of benzoyl chlorides, previously reported by us. The 4'substituted precursor alcohols, chlorides, and product methyl ethers are all new compounds. 4-Anisyl chloride and the series of benzyloxybenzyl chlorides react by the S, 1 mechanism, whereas benzyl chloride reacts by the S,2 mechanism. 4-Phenoxybenzyl chloride shows intermediate behaviour. A similar pattern was observed with the corresponding benzoyl compounds. In both series the reactivity order is CH,O > 4'-CH,C,H4CH,0 (-0.76) >C,H5CH,0 (-0.74) 7 4'-CIC,H4CH20 (-0.69) > 4'-N02C,H4CH,0 (-0.60) > C6H50 > H (values in parentheses are new IS+ values). At 25" the overall range of rates is 4 290 in the benzyl series, compared with only 2.42 in the benzyl series. The Arrhenius parameters in the two series demonstrate, however, an underlying simil
The experimental results for the 2-propanolysis of benzoyl, benzyl, benzene sulphenyl and benzene sulphonyl chlorides obtained by conductimetric technique were compared with estimates for chlorobenzene which is extremely unreactive as an electrophile. We thus obtained the following reactivity sequence: PhSCl>PhCOCl>PhSO 2 Cl>PhCH 2 Cl>PhCl with rate-coefficiente ratios (in the same order): 9.5 x 10 4 : 1: 7.14 x 10 -2 : 4.7 x 10 -3 : about 10 -26 . We have discussed these results in specific terms and with the aid of general conclusions which stem from our own classification of electrophiles.Keywords: 2-propanolysis; electrophilic carbon and sulphur centres; solvolysis mechanism. ARTIGO INTRODUÇÃONo campo da substituição nucleofílica, as pesquisas relativas às reatividades de substratos que têm como centro eletrofílico o carbono saturado 1,2 , o carbono carbonílico 3,4 e o carbono aromá-tico 5 têm recebido muita atenção; entretanto, estudos comparativos entre si são raros. Algumas comparações entre as reatividades dos carbonos: aromático e saturado, carbonílico e saturado, aromático e carbonílico, aromático, heteroaromático e sistema tropílico, carbonílico e imidoílico, e de arenos coordenados e não-coordenados são descritas na literatura [6][7][8][9][10][11][12][13][14][15] . Todavia, muito pouco é conhecido acerca da reatividade do enxofre como centro eletrofílico, embora haja uma quantidade razoável de trabalhos sobre centros sulfonílicos 16 e pouco sobre centros sulfenílicos.Estudos comparativos das reatividades relativas de compostos com carbono e enxofre como centros eletrofílicos, em solvólise nucleofílica, são, ainda, desconhecidos. Assim, no presente trabalho dispensou-se atenção aos cloretos de benzenossulfenila em reações de solvólise com 2-propanol e, para comparação, foram relacionadas reações de substituição nucleofílica nos grupos carbonila e sulfonila e nos carbonos saturado e aromático.A escolha do 2-propanol para o estudo da solvólise nucleofí-lica foi baseada na facilidade para seguir as reações usando uma técnica convencional (condutimetria), no nível conveniente de velocidade de reação, na escassez de dados de sua solvólise nucleofílica e no desejo de comparar os diferentes sistemas, sem as complicações introduzidas se algum deles reagisse por um mecanismo unimolecular (S N 1). Julgou-se que isso não ocorre em 2-propanol anidro para os compostos não substituídos e muito menos para os que possuem substituintes receptores de elé-trons no anel benzênico. Julgou-se, também, que entre as classes de compostos estudados, aquelas que oferecem maiores chances de operar por um mecanismo unimolecular são os sistemas benzílico e benzoílico. Entretanto, é muito relevante o fato de ter-se constatado que, mesmo na metanólise, nem cloreto de benzila e nem de benzoíla reagiu por um mecanismo S N 1 11,17 . Escolheram-se sistemas arílicos por conveniência e porque planejou-se estender os estudos cinéticos para incluir arenos eletrofí-licos. A escolha do cloreto como nucleófugo se deveu ao fato de ele se consti...
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