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Effects on the Reactivity by Changing the Electrophilic Center from CO to CS: Contrasting Reactivity of Hydroxide, p ‐Chlorophenoxide, and Butan‐2,3‐dione Monoximate in DMSO/H 2 O Mixtures
Author(s) -
Um IkHwan,
Han JeongYoon,
Buncel Erwin
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801534
Subject(s) - reactivity (psychology) , nucleophile , chemistry , electrophile , solvent , solvent effects , medicinal chemistry , hydroxide , reaction rate constant , stereochemistry , kinetics , organic chemistry , catalysis , medicine , physics , alternative medicine , pathology , quantum mechanics
Second‐order rate constants have been measured spectrophotometrically for the reactions of O ‐ p ‐nitrophenyl thionobenzoate ( 1 , PNPTB) with HO − , butan‐2,3‐dione monoximate (Ox − , α‐nucleophile), and p ‐chlorophenoxide ( p ‐ClPhO − , normal nucleophile) in DMSO/H 2 O of varying mixtures at (25.0±0.1) °C. Reactivity of these nucleophiles significantly increases with increasing DMSO content. HO − is less reactive than p ‐ClPhO − toward 1 up to 70 mol % DMSO although HO − is over six p K a units more basic in these media. Ox − is more reactive than p ‐ClPhO − in all media studied, indicating that the α‐effect is in effect. The magnitude of the α‐effect (i.e., k Ox −/ k p ‐ClPhO −) increases with the DMSO content up to 50 mol % DMSO and decreases beyond that point. However, the dependency of the α‐effect profile on the solvent for reactions of 1 contrasts to that reported previously for the corresponding reactions of p ‐nitrophenyl benzoate ( 2 , PNPB); reactions of 1 result in much smaller α‐effects than those of 2 . Breakdown of the α‐effect into ground‐state (GS) and transition‐state (TS) effects shows that the GS effect is not responsible for the α‐effect across the solvent mixtures. The role of the solvent has been discussed on the basis of the bell‐shaped α‐effect profiles found in the current study as well as in our previous studies, that is, a GS effect in the H 2 O‐rich region through H‐bonding interactions and a TS effect in the DMSO‐rich media through mutual polarizability interactions.