Solvent Effect on a Model of S_NAr Reaction in Conventional and Non-Conventional Solvents

In this chapter some theoretical and experimental reports in order to elucidate solvent effects (preferential solvation and iso-solvation effects, respectively) over nucleophilic aromatic substitution reactions as reaction model were examined. Solvent effects phenomena are introduced to predict their mechanism highlighting the hydrogen bond role mainly in ionic liquids, a new generation of solvents that can be designed in order to improve the reactivities of the reacting pair and intermediate species through of the potential energy surface (PES). Then, the preferential solvent effect may be defined as the difference between local and bulk compositions of the solute with respect to the various components of the solvent; usually mixtures of solvents and iso-solvation effect indicate the composition of a mixture in which the probe under consideration is solvated by approximately an equal number of cosolvent molecules in the solvent mixture

Activation of Electrophile/Nucleophile Pair by a Nucleophilic and Electrophilic Solvation in a SNAr Reaction

Nucleophilic aromatic substitution reactions of 4-chloroquinazoline toward aniline and hydrazine were used as a model system to experimentally show that a substrate bearing heteroatoms on the aromatic ring as substituent is able to establish intramolecular hydrogen bond which may be activated by the reaction media and/or the nature of the nucleophile

Coupling planktonic and benthic shifts during a bloom of Alexandrium catenella in southern Chile:Implications for bloom dynamics and recurrence

Cell abundances and distributions of Alexandrium catenella resting cysts in recent sediments were studied along time at two locations in the Chilean Inland Sea exposed to different oceanographic conditions: Low Bay, which is much more open to the ocean than the more interior and protected Ovalada Island. The bloom began in interior areas but maximum cyst concentrations were recorded in locations more open to the ocean, at the end of the Moraleda channel. Our results showed a time lapse of around 3 months from the bloom peak (planktonic population) until the number of resting cysts in the sediments reached a maximum. Three months later, less than 10% of the A. catenella cysts remained in the sediments. Maximum cyst numbers in the water column occurred one month after the planktonic peak, when no cells were present. The dinoflagellate assemblage at both study sites was dominated by heterotrophic cysts, except during the A. catenella bloom. CCA analyses of species composition and environmental factors indicated that the frequency of A. catenella blooms was associated with low temperatures, but not with salinity, chlorophyll a concentration, and predator presence (measured as clam biomass). However, resting cyst distribution was only related to cell abundance and location. The occurrence of A. catenella cysts was also associated with that of cysts from the toxic species Protoceratium reticulatum. By shedding light on the ecological requirements of A. catenella blooms, our observations support the relevance of encystment as a mechanism of bloom termination and show a very fast depletion of cysts from the sediments (<3 months), which suggest a small role for resting cyst deposits in the recurrence of A. catenella blooms in this area.Postprin

Surfactant-mediated enzymatic superactivity in water/ionic liquid mixtures, evaluated on a model hydrolytic reaction catalyzed by α-chymotrypsin

In this work, the influence of the C 12 -, C 14 -, and C 16 -alkyl chain derivatives of 1-alkyl-3-methyl-imidazole tetrafluoroborate over the hydrolisis of p-nitrophenyl trimethylacetate catalyzed by α-chymotrypsin was studied in water and water - ionic liquids mixtures. The ionic liquid used is 1-butyl-3-methyl-imidazole tetrafluoroborate (BMIMBF 4 ). 1-alkyl-3-methylimidazole tetrafluoroborate derived surfactants can increase the catalytic efficiency of α-chymotrypsin on the hydrolysis of p-nitrophenyl trimethylacetate. This effect is negatively affected by the decrease on the surfactant's critical micelle concentration (CMC) and the partition of the substrate between the micelles and the external media observed above the critical miscellar concentration (CMC). In water/BMIMBF 4 mixtures, the presence of the ionic liquid (IL), render the C 14 and C 16 surfactants insoluble, and overrides the effect elicited by the C 12 alkyl chain surfactant on the activity of α-chymotrypsin. The loss of the surfactants influence on the enzymatic activity due to the presence of BMIMBF 4 seems to be a complex process, controlled by a decreased affinity between α-chymotrypsin and the surfactant molecules and, to a larger extent, the appearance of an “acquired resistance” of the enzyme to the influence of the surfactants, related to a more compact and resilient conformation of the protein in the presence of BMIMBF 4 . The influence of BMIMBF 4 is not only limited to the surfactant/enzyme interaction, but also it does modifies the partition of the substrate between the aqueous media and the micellar environment

Are Electrophilicity and Electrofugality Related Concepts? A Density Functional Theory Study

Artículo de publicación ISIIt is proposed that the electrofugality of a fragment within a molecule is determined by its group nucleophilicity. The variation of electrofugality should be tightly related to the electron releasing ability of the substituent attached to the electrofuge moiety. This contribution closes the set of relationships between philicity and fugality quantities: while nucleofugality appears related to the group electrophilicity of the leaving group, electrofugality is related to the group nucleophilicity of the permanent group.Fondecyt 1070715 110049