Solvent nucleophilicity scales based upon

A comparison of the rates of solvolysis of the tert-butyldimethylsulfonium ion and the 1-adamantyldimethylsulfonium ion presents strong evidence that the solvent dependence of the tert-butyldimethylsulfonium ion solvolysis rates is governed primarily by solvent nucleophilicity effects. Leaving-group contributions based upon 1-adamantyldimethylsulfonium ion solvolyses are better incorporated into the establishment of the solvent nucleophilicity scale based upon triethyloxonium ion solvolysis. Alternative solvent nucleophilicity scales based upon the solvolysis of S-methylbenzo-thiophenium ions are discussed. Analyses of the extent of nucleophilic participation by the solvent in the solvolyses of methyldiphenyl-sulfonium and benzhydryldimethylsulfonium ion will be presented. The relative nucleophilicities of various anionic and neutral nucleophiles toward the triethyloxonium ion in ethanol have been determined. 

Solvent Nucleophilicity Scales Based upon R-X+ Substrates... 

For situations where solvent nucleophilicity may be a factor, Kevill (8) favors the use of the extended Grunwald-Winstein equation (equation 1). Scales of NOTs and OTs values based upon the use of methyl tosylate and 2-adamantyl tosylate as model SN2- and SNl-reacting substrates have been developed (15, 16). Also Y scales have been developed for other anionic leaving groups using 1-adamantyl or 2-adamantyl derivatives (17-19), where Sn2 reaction is impossible or severely hindered. 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

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