Salt Effects on Proton Transfer from Nitrophenols to Amine or Pyridine Bases in Acetonitrile

Article abstract of DOI:10.1021/j100045a020

The addition of MClO4 (M⁺ = Li⁺ and Na⁺) to 1.0 x 10^-1 mol dm^-3 2,4-dinitrophenol and the equivalent concentration of γ-picoline (4-methylpyridine) in acetonitrile caused a promoted proton transfer from the phenol to the base to the extent of 62 and 23 percent for 0.1 mol dm^-3 Li⁺ and Na⁺, respectively.Alkaline-earth metal perchclorates (M(ClO4)2) had larger effects even at low concentrations of M²⁺, e. g., the proton-transfer ratio was 90 percent for 1.0 x 10^-2 mol dm^-3 Mg(ClO4)2.The effects of M²⁺ decreased with increasing size of the cation: Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺.With the addition of MClO4 or M(ClO4)2, a higher proton transfer ratio was observed for 2,5-dinitrophenol + γ-collidine than for 2,4-dinitrophenol + γ-picoline.The salt effects of MClO4 or M(ClO4)2 on the acid-base reactions were accounted for by the formation of triple cations or complex species of M⁺ or M²⁺ with the phenolate ions (B represents base): (NO2)2 PhOH-B + 2M⁺ ->/<- (NO2)2PhO^-(M⁺)2 + BH⁺; (NO2)2PhOH-B + M²⁺ ->/<- (NO2)2PhO^-M²⁺ + BH⁺.The stronger the base, the greater the proton transfer (based on the addition of salts) from the phenols.The effects of LiClO4 on 2,4,6-trinitrophenol (picric acid) + 2-chloropyridine were very small; however, the effects on the proton transfer from 2- and 4-nitrophenol to triethylamine were very large.The extent of the proton transfer (based on the addition of LiClO4) from nitrophenols in the presence of appropriate bases was coincident with the order of the formation constants of ion aggregates (ion pairs and triple ions) for lithium nitrophenolates in acetonitrile: 2- >> 4-, 2,5- > 2,4- >> 2,4,6-.The "free" phenolate ions were produced by the addition of tetraalkylammonium halides (R4N⁺X^-: R = Et, n-Bu; X^- = Cl^-, Br^-, I^-) to the nitrophenols in the presence of bases inacetonitrile: (NO2)nPhOH-B + 2X^- ->?<- (NO2)nPhO^- + BH⁺(X^-)2.The presence of a base was not essential for the deprotonation of picric acid with R4NX.