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We report on the ethanolysis of the P=O and P=S compounds ethyl and methyl paraoxon (
1a and
1b)and ethyl and methyl parathion (
2a and
2b). Plots of spectrophotometrically measured rate constants,
kobsd versus [MOEt], the alkali ethoxide concentration, show distinct upward and downward curvatures,pointing to the importance of ion-pairing phenomena and a differential reactivity of free ions and ionpairs. Three types of reactivity and selectivity patterns have been discerned: (1) For the P=O compounds
1a and
1b, LiOEt > NaOEt > KOEt > EtO
-; (2) for the P=S compound
2a, KOEt > EtO
- > NaOEt> LiOEt; (3) for P=S,
2b, 18C6-crown-complexed KOEt > KOEt = EtO
- > NaOEt > LiOEt. Theseselectivity patterns are characteristic of both catalysis and inhibition by alkali-metal cations dependingon the nature of the electrophilic center, P=O vs P=S, and the metal cation. Ground-state (GS) vstransition-state (TS) stabilization energies shed light on the catalytic and inhibitory tendencies. Theunprecedented catalytic behavior of crowned-K
+ for the reaction of
2b is noteworthy. Modeling revealsan extreme steric interaction for the reaction of
2a with crowned-K
+, which is responsible for the absenceof catalysis in this system. Overall, P=O exhibits greater reactivity than P=S, increasing from 50- to60-fold with free EtO
- and up to 2000-fold with LiOEt, reflecting an intrinsic P=O vs P=S reactivitydifference (thio effect). The origin of reactivity and selectivity differences in these systems is discussedon the basis of competing electrostatic effects and solvational requirements as function of anionic electricfield strength and cation size (Eisenman's theory).
