Crown ethers K+ complex

When 18-crown-6 is added to the reaction of CH3CH2Br with KCN, for example, the crown ether forms a tight complex with that has nonpolar C-H bonds on the outside, making the complex soluble in nonpolar solvents like benzene (CeHg) or hexane. When the crown ether/K" complex dissolves in the nonpolar solvent, it carries the "CN along with it to maintain electrical neutrality. The result is a solution of tightly complexed cation and relatively unsolvated anion (nucleophile). The anion, therefore, is extremely nucleophilic because it is not hidden from the substrate by solvent molecules. 

The rate constants for the substitution reaction of 4-bromonitrobenzene with potassium phenoxide in dioxan in the presence of crown ethers are linearly correlated with the stability constants of the crown ether-K+ complex in methanol. ... 

Since it is well known that stability of a complex between a crown ether and an alkali metal cation greatly depends on the size of the inner cavity of the former and of the latter and that of all the alkali metal ions dibenzo-18-crown-6 forms the most stable complex with K" ",27 it was interesting to see whether these facts in a parallel way influence the present polymerization reactions. We conducted a series of pol3nnerizations under the same condition as described in "Synthetic Reactions", using various alkali hydroxides (LiOH, NaOH, KOH, and CsOH) the results are tabulated in Table 2. The results strongly imply that phase transfer of the nucleophiles, bisphenolate anions, from the aqueous to the nitrobenzene phase was most efficient when the metal ion was K. This, in turn, indicates that the crown ether-K" " complex is the most stable among the complexes expected to be formed with the alkali metals examined, as has been amply demonstrated by others. For example, Pedersen ... 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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