Weakly nucleophilic media, reactivity

Among the reactive groups in wool the most important are amino, imino, and hydroxyl groups. Reactions occur in a weakly acidic medium (pH 3-5) and include nucleophilic substitution of leaving groups (usually Cl, F, and, rarely, sulfonate or ammonium groups) or addition reactions to polar aliphatic double bonds. 

With 50% aqueous acetone, there is a weak nucleophile (H20) and a highly polar reaction medium favoring ionization, or the SN1 mechanism. In this mechanism, the reactivity order of alkyl halides is tertiary > secondary > primary. Therefore,... 

The retrogradation of the dissociation equilibrium is brought about by addition to the reaction medium of common ion salts whose cation is inert toward the polymerization system. As for the nature of the counterions, those that bring about a certain covalency of the active centers are preferred. Addition of a weak nucleophile is also an efficient means to curb the reactivity of carbocations. The following systems (monomer, initiating system, solvent) qualify, more or less perfectly, for the category of living polymerizations ... 

These data suggest that o-QM reactivity is heavily affected by general acid catalysis in the gas phase or in low polar medium. The general acid catalysis can be provided by a water molecule for nucleophiles bearing weak acid hydrogens (such as those in ammonia). 

Almost all the CDs are modified through chemical reactions on the hydroxyl groups. The reactivity of the three kinds of hydroxyls follows the order C-6>C-2 >C-3. The acidities sequence is C-2>C-3jJ>C-6. The C-6 hydroxyl is most easily replaced via the reaction of CDs with reagent in a strong alkaline environment. However, in the weak alkahne environment, the C-2 hydroxyl is easier to be activated and replaced. Because of the steric hindrance, the large molecular size reagent is more convenient to attack the C-6 hydroxyl. The C-3 hydroxyl can be replaced only if the C-2 and C-6 hydroxyls are shielded. When the CDs are used to encapsulate the guest, the dimension of the cavity plays the key role in the stability of the complexes. But the main factor of unstable complexes is nucleophilicity. In addition, the reaction medium and cavity size both had effect on the structure of the reaction product. 

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

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