200 research outputs found
1,3-Dipolar Cycloaddition of Phenyl Azide to Norbornene in Aqueous Solutions
No full textSecond-order rate constants for the cycloaddition of phenyl azide to norbornene were determined in aqueous solutions. In organic solvents this d o n shows a very small solvent effect. In highly aqueous media, however, remarkable accelerations are observed. The solvent depedence of the rate constants on solvent composition in mixed aqueous solutions resembles those observed for Diels-Alder reactions, most likely because both types of cycloadditions are mechanistically related.
Cycloadditions in mixed aqueous solvents: the role of the water concentration
No full textWe examined the kinetics of a series of cycloaddition reactions in mixtures of water with methanol, acetonitrile and poly(ethylene glycol) (MW 1000). The reactions include the Diels–Alder (DA) reaction between cyclopentadiene and N-n-butylmaleimide or acridizinium bromide, the retro-Diels-Alder (RDA) reaction of 1,4,4a,9a-tetrahydro-4a-methyl-(1α,4α,4aα,9aα)-1,4-methaneanthracene-9,10-dione and the 1,3-dipolar cycloaddition of benzonitrile oxide with N-n-butylmaleimide. Plots of logk vs the molar concentration or volume fraction of water are approximately linear, but with a characteristic break around 40 M water. This break, absent for the RDA reaction, is ascribed to hydrophobic effects. Comparison with aqueous mixtures of the more hydrophobic 1-propanol shows that these mixtures induce qualitatively similar effects on the rate, but that preferential solvation effects cause the mixtures of 1-propanol to exhibit a more complex behavior of logk on composition. The results are analyzed using the Abraham–Kamlett–Taft model. The solvent effects in aqueous mixtures are not satisfactorily described by this model. For some cycloadditions, small maxima in rate are observed in highly aqueous mixtures of alcohols. The origin of these maxima and the aforementioned breaks is most likely the same.
Diels-Alder reactions in water: Enforced hydrophobic interaction and hydrogen bonding
No full textSecond-order rate constants have been measured for the Diels-Alder (DA)
reactions of cyclopentadiene with dienophiles of varying hydrophobicity and
hydrogen-bond acceptor capacity in water, in a series of organic solvents and in alcohol-water mixtures. The intramolecular DA reaction of N-furfuryl-Nalkylacrylamides was also investigated in the same reaction media.
The often huge rate accelerations in water appear to be inherent in
the activation process of the cycloaddition and do not originate from
diene-dienophile association. A pseudothermodynamic analysis of the rate acceleration in water relative to 1- propanol and 1-propanol-water mixtures provides evidence for two factors dominating the rate enhancement in water: hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process.
Surface Potentials of Hydrophobic Microdomains in Aqueous Solutions of Poly(dimethyldiallylammonium-co-methyl-n-dodecyldiallylammonium salts) with Different Counterions. Evidence for the Existence of a Critical Aggregation Concentration
No full textThe solvatochromic acid-base indicator 1-hexadecyl-4-[(oxocyclohexadienylidene)ethylene]-1,4-dihydropyridine (HOED) was used to determine surface potentials (Y) of micelles formed from methyldiallyl-ndodecylammonium salts containing different counterions (MDDAX, X- = Cl, Br, I, benzoate, salicylate). Y decreased in the order Cl > Br > I > benzoate, salicylate consistent with conductometrically determined values for the counterion binding. HOED was also used to probe the microdomains formed in aqueous solutions of poly(dimethyldiallylammonium-co-methyl-n-dodecyldiallylammonium salts) containing different counterions (Copol C1-12 90/10 X). A similar dependence of Y on the nature of the counterion was found. Raising the temperature led to higher values of Y, while increasing the polysoap concentration appeared to lower the surface potentials for all polysoaps. With the exception of Copol C1-12 90/10 I and Sal, an additional peak in the UV-vis absorption spectrum of the probe was observed which was attributed to the aggregation of probe molecules on nondomain forming strains of the polysoap. Support for this conclusion was obtained from the observation that the intensity of the additional peak greatly decreased upon increasing the polysoap concentration. This behavior can be reconciled with the occurrence of a critical aggregation concentration. The sudden rise of the aggregation numbers of Copol C1-12 90/10 Br and benzoate at ca. 10 mM, leading to a constant value at higher concentrations, provides additional evidence for this conclusion
Diels-Alder reactions in water:Enforced hydrophobic interaction and hydrogen bonding
No full textSecond-order rate constants have been measured for the Diels-Alder (DA) reactions of cyclopentadiene with dienophiles of varying hydrophobicity and hydrogen-bond acceptor capacity in water, in a series of organic solvents and in alcohol-water mixtures. The intramolecular DA reaction of N-furfuryl-Nalkylacrylamides was also investigated in the same reaction media. The often huge rate accelerations in water appear to be inherent in the activation process of the cycloaddition and do not originate from diene-dienophile association. A pseudothermodynamic analysis of the rate acceleration in water relative to 1- propanol and 1-propanol-water mixtures provides evidence for two factors dominating the rate enhancement in water: hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process
Stimuli response of polysoap hydrogels in aqueous solution and DC electric fields
No full textNovel types of polysoap hydrogels based on hydrophobically-modified polyelectrolytes crosslinked with N,N-methylenebisacrylamide have been prepared by free radical polymerization at 70–80°C in aqueous solution with ammonium persulfate as initiator. Poly(diallylamine-co-N,N-dodecylmethyldiallylammonium bromide) (PDA-C12), poly(N-methyldiallyl-co-N,N-dodecylmethyldiallylammonium bromide) (PMDA-C12) both contain hydrophobic side chains with 12 carbon atoms. The swelling behavior of these polysoap hydrogels was studied by immersion of the gels in buffered solutions at various pHs and ionic strengths. It was found that the structure of the polysoap backbone influenced the pH-dependent swelling and deswelling. The swelling process is reversible after repeating cycles of swelling and deswelling induced by a change of pH in appropriate buffer solutions. SEM micrographs of polysoap gels indicate that the network structures are characterized by the presence of large open pores or small closed pores. The stimuli response of the polysoap gels in electric fields was also investigated. In a contact electric field, deswelling was observed at the anode side of the gels. In a non-contact electric field, the gels bend towards the anode. The gels can turn back to the original shape and bend toward the cathode with time when higher electric potentials are applied. These properties of the gels are related to both the change of osmotic pressure caused by mobile ions and by hydrophobic interactions.
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