J. R. Leis

Novel catanionic vesicles from calixarene and single-chain surfactant

The mixed system between p-sulfonatocalix[4]arene and tetradecyltrimethylammonium bromide forms unilamellar vesicles after sonication of the aqueous dispersion. Furthermore these vesicles can be stored, without use of lyoprotectants, by lyophilization and then rehydration without change in size or shape.

Evidence for complexes of different stoichiometries between organic solvents and cyclodextrins

The influence of the organic solvent on the acid and basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the presence of alpha- and beta-cyclodextrins has been studied. The observed rate constant was found to decrease through the formation of an unreactive complex between MNTS and the cyclodextrins. In the presence of dioxane, acetonitrile or DMSO, the inhibitory effect of beta-CD decreased on increasing the proportion of organic cosolvent as a result of a competitive reaction involving the formation of an inclusion complex between beta-CD and the cosolvent. The disparate size of the organic solvent molecules resulted in stoichiometric differences between the complexes; the beta-CD-dioxane and beta-CD-DMSO complexes were 1 : 1 whereas the beta-CD-acetonitrile complex was 1 : 2. The basic and acid hydrolysis of MNTS in the presence of alpha-CD showed a different behavior; thus, the reaction gave both 1 : 1 and 2 : 1 alpha-CD-MNTS complexes, of which only the former was reactive. This result was due to the smaller cavity size of alpha-CD and the consequent decreased penetration of MNTS into the cavity in comparison to beta-CD. The acid hydrolysis of MNTS in the presence of alpha-CD also revealed decreased penetration of MNTS into the cyclodextrin cavity, as evidenced by the bound substrate undergoing acid hydrolysis. In addition, the acid hydrolysis of MNTS in the presence of acetonitrile containing alpha-CD gave 1 : 1 alpha-CD-acetonitrile inclusion complexes, which is consistent with a both a reduced cavity size and previously reported data.

Cyclodextrin effect on solvolysis of substituted benzoyl chlorides

A kinetic study was carried out on the solvolysis of substituted benzoyl chlorides in the presence of alpha-, beta- and gamma-CD. Combination of the substituent dependent mechanism for solvolysis of benzoyl chlorides and the complexation ability of the cyclodextrin yields the following experimental behavior: (i) catalysis by beta- and gamma-CD for solvolysis of electron-attracting substituted benzoyl chlorides due to the reaction with its hydroxyl group C(6); (ii) absence of alpha-CD influence on solvolysis of benzoyl chlorides with electron withdrawing substituents; (iii) inhibition of solvolysis of benzoyl chlorides with electron-donating groups. This behavior is observed for solvolysis of meta/para substituted substrates in the presence of beta-CD, solvolysis of meta-substituted benzoyl chlorides in the presence of alpha-CD and solvolysis of para-substituted benzoyl chlorides in the presence of gamma-CD. This decrease in the rate constant is a consequence of the complexation of the substrate in the cyclodextrin cavity and its low solvation ability, causing the rate of solvolysis in its interior to be negligible. (iv) The solvolysis of meta-substituted benzoyl chlorides in the presence of gamma-CD yields a new behavior where the reaction of the complexed substrate is not negligible in the interior of the cyclodextrin cavity, which has been interpreted as a consequence of incomplete expulsion of hydration water from its cavity when the complexation takes place. (v) The experimental results obtained in the presence of alpha-CD show that meta-substituted benzoyl chlorides give rise to host : guest complexes with 1 : 1 stoichiometries, whereas those which are para-substituted cause a 2 : 1 stoichiometry to be formed. This difference in behavior has been interpreted taking into account the size of the different benzoyl chlorides and their accommodation in the alpha-CD cavity.

Modification of crystal violet : sulfite ion equilibrium induced by SDS micelles

The presence of sodium dodecylsulfate micelles was found to displace the equilibrium between crystal violet and sulfite ion in the direction of the former and lead to complete recovery of the dye. The underlying mechanism for this anomalous behaviour was elucidated and the rate of the reversal process determined.

Reactivity of sulfur nucleophiles with N-methyl-N-nitroso-p-toluenesulfonamide

The transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to cysteine (CYS) and 2-aminoethanethiol (AET) has been studied in a pH range between pH = 7 and pH = 13. Kinetic results clearly indicate that both nucleophiles react through the corresponding thiolate to give the corresponding nitrosothiol. The existence of two (AET) or three (CYS) macroscopic acidity constants has been kinetically evidenced and the nitrosation rates of the corresponding bases have been identified. Nitrosation rate constants of the different species present in the reaction medium have been determined and a Bronsted-type plot has been established giving a beta(nuc) value approximately equal to 0.08 clearly different from the values of beta(nuc) approximately equal to 0.7 obtained in the nitrosation of primary and secondary amines by MNTS. The low beta(nuc) value has been attributed to the need for previous desolvation of the nucleophile.

Reactivity in w/o microemulsions. Activation parameters for solvolysis in AOT/isooctane/water systems

A kinetic study was carried out on the solvolysis of different benzoyl halides in microemulsions of AOT/isooctane/water: 4-methoxybenzoyl chloride (4-CH3O), 3-methoxybenzoyl chloride (3-CH3O), benzoyl fluoride (BzF) and 4-trifluoromethylbenzoyl chloride (4-CF3). The application of the formalism of the pseudophase allows us to obtain the true rate constants for the solvolysis in the interface, ki, which decreases 350 times on going from W=50 to W=2 (W=[H2O]/[AOT]) for 4-CH3O; 35 times for 3-CH3O and 3 times for BzF. In the solvolysis of 4-CF3ki decreases 3.5 times on going from W=2 to W=50. This behavior has been interpreted as a consequence of a decrease of the electrophilic character of water when W decreases, whereas the nucleophilic character increases. As a consequence the rate constant for the dissociative processes decreases as W decreases, due to a reduction in the water’s ability to solvate the leaving group, and the rate of the associative processes increases due to the greater nucleophilicity of the water. The obtained results are compatible with the activation parameters determined in the microemulsion: for the solvolysis of BzF and 4-CF3, ΔS≠ takes values which are close to ΔS≠≈−180 J mol−1 K−1 and ΔS≠≈−150 J mol−1 K−1 respectively. These values are lower than those observed in pure water and compatible with a greater degree of structuring of the interfacial water than that of bulk water. For the solvolysis of 4-CH3O it is observed that ΔS≠ decreases from ΔS≠≈−40 J mol−1 K−1 for W=50 to ΔS≠≈−150 J mol−1 K−1 for W=2. This behavior is consistent with a change from a dissociative mechanism to an associative one which also is shown when studying a Hammett correlation using Taft σ values.

Microemulsion-promoted changes of reaction mechanisms: solvolysis of substituted benzoyl chlorides

When the hydrolysis of substituted benzoyl chlorides is carried out in a water/oil (w/o) microemulsion, the point in the substituent ς+/lg k plot at which the mechanism changes from associative to dissociative depends on the water content of the microemulsion.