Journal of the Chemical Society. Perkin transactions II p. 46 - 51 (1980)
Update date:2022-08-11
Topics:
Landini, Dario
Maia, Angelamaria
Montanari, Fernando
Pirisi, Filippo M.
Anion-promoted nucleophilic substitutions carried out in aqueous-organic two-phase systems in the presence of catalytic amounts of perhydrodibenzo-18- crown-6 follow the classic mechanism of phase-transfer catalysis. The observed pseudo-first-order rate constants are linearly related to the concentration of complexed crown ether in the organic phase. The narrow reactivity range and the sequence found in the reaction between n-octyl methane-sulphonate and a homogeneous series of anions in the PhCl-H2O two-phase system (N 3- > I- ～ Br- > CN - > Cl- > SCN-) remain largely unaltered in anhydrous PhCl. From this point of view complexed crown ethers differ substantially from lipophilic quaternary salts and cryptates. Indeed, removal of the hydration sphere of the anions in going from two-phase to anhydrous conditions is balanced by a larger cation-anion interaction, resulting in a very small variation of anion reactivity. This indicates that, unlike cryptates, complexed crown ethers can hardly be considered as a source of 'naked anions.' A comparison is also reported among lipophilic crown ethers, quaternary salts, and cryptands as phase-transfer catalysts.
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