Elucidation of Salt Effects on the Indicator Acidity in Acetonitrile

Abstract

The salt effects on the proton-transfer from CF3COOH to an acid-base indicator, ‘Methyl Yellow’ [p-(dimethylamino)-azobenzene], were examined in acetonitrile by means of spectrophotometry. The addition of LiClO4 to the indicator (Ind) and CF3COOF mixture (both 2.0 × 10−4 mol dm−3) caused an increase and a successive decrease in the absorbance (λmax = 514 nm) of IndH+. The promoted increase of the IndH+ concentration with increasing concentration of LiClO4 was explained on a quantitative basis concerning the formation of a triple cation, CF3COO−(Li+)2, as well as an ion pair, CF3COO−Li+: Ind–CF3COOH + 2 Li+ IndH+ + CF3COO−(Li+)2. The successive decrease in the absorbance at 514 nm with a large excess of LiClO4 was attributed to an ion-exchange reaction: IndH+ + Li+ IndLi+ + H+. The addition of NaClO4 to Ind–CF3COOH caused only an increase in the IndH+ concentration, and no deprotonation from IndH+. Alkaline-earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) promoted protonation by the formation of complex species, CF3COO−M2+. Although distinct deprotonation from IndH+ was observed with Mg(ClO4)2, no deprotonation occurred with Ba(ClO4)2. The effects of the chloride ion (Et4NCl) on deprotonation from IndH+ (In + CF3SO3H) were explained by the formation of homoconjugated species (HCl2−), and not by the mere formation of HCl. The apparent changes in pH or the Hammett acidity function with the presence of various salts were elucidated by the ‘complex’ formation in acetonitrile.

UV−Visible and 1H or 13C NMR Spectroscopic Studies on the Specific Interaction between Lithium Ions and the Anion from Tropolone or 4-Isopropyltropolone (Hinokitiol) and on the Formation of Protonated Tropolones in Acetonitrile or Other Solvents

“…24 We have proposed that the properties of bulk water are altered into those of "dihydrogen ether" R[H]-O-[H]R when the huge network of bulk water is distorted by added organic solvents and concentrated salts. 24,25 We have also explained the large salt effects on the indicator acidity (or Hammett acidity function) 20 and on the proton transfer from tropolone 21 or nitrophenols 22 to amine bases in acetonitrile solution in terms of the chemical interaction between the anions and M + or M 2+ . We have reported the direct effects of M + or M 2+ on the equilibria of acid-base and metal indicators.…”

“…12 Fuoss and Kraus 13 proposed the concept of "symmetrical" triple ions, that is, anionic and cationic triple ions (to the same extent), based on mere electrostatic interaction between (spherical) ions in an electric field of low permittivity. By means of voltammetry, 14 conductometry, 15,16 and UVvisible and NMR spectroscopy, [17][18][19][20][21][22] we have certainly demonstrated that alkali metal (M + ) and alkaline earth metal ions (M 2+ ) have "weak" but unexpectedly distinct chemical (i.e., covalent bonding or coordinating) as well as Coulombic interactions with many simple anions, not only in solvents of low permittivity ( r < 10) 16 but also in higher permittivity media (20 < r < 65) with poor solvating ability. 14,15 The coordination chemistry of the lithium ion with simple anions as well as cyclic ligands has been reviewed by Olsher et al 23 Stable carbocations can be produced from trityl halides by addition of perchlorates of Li + , Na + , Mg 2+ , and Ba 2+ in acetonitrile; [17][18][19] the carbocations have been detected by UV-visible, 17 1 H, and 13 C NMR spectroscopy.…”

Observation by UV−Visible and NMR Spectroscopy and Theoretical Confirmation of 4-Isopropyltropolonate Ion, 4-Isopropyltropolone (Hinokitiol), and Protonated 4-Isopropyltropolone in Acetonitrile

“…and so on) not only through Coulombic forces but also by coordination provided the solute ions are not subject to strong solvation. 2 We have explained the large salt effects on the indicator acidity (or Hammett acidity function) 3 using 4-(dimethylamino)azobenzene 4 and on the proton transfer from tropolone 5 or nitrophenols 6 toward amine bases in acetonitrile solution in terms of the direct chemical interaction between the anions and M + or M 2+ and not merely as ion-exchange reactions or the alternation between contact ion pairs (CIP) and solvent-separated ion pairs (SSIP). 6 At last, we obtained decisive 1 H NMR evidence of the formation of "reverse-coordinated" 2,5 species, (Li + ) 2 L -, for the anions (L -) of tropolone and hinokitiol (4isopropyltropolone) in acetonitrile-d 3 but not in DMF-d 6 (dime-thylformamide-d 6 ).…”

“…Reichardt et al 9 described, using the solvation model of Frank and Wen, 10 that "at high salt concentrations (c > 5 mol/L), region C can be abolished and only regions A and B survive, resulting in an aqueous solvent called 'dihydrogen ether'". [1][2][3][4][5][6][7][8][9][10][11][12][13]…”

Concentrated salt effects on solvolysis rates in 1,4-dioxane–H2O mixed solvent and the expansion of the use of the Hammett equation for salt effects

“…Our studies have used the techniques of voltammetry, 11 conductometry, 12 and UV-visible 13 and NMR spectroscopy. 14 We have explained the large salt effects on the indicator acidity (or Hammett acidity function) using p-(dimethylamino)azobenzene 15 and on the proton transfer from tropolone 16 or nitrophenols 17 to amine bases in acetonitrile in terms of the chemical interaction between the anions and M ϩ or M 2ϩ , and not merely as ion-exchange reactions or the alternation between the contact ion pair (CIP) and the solvent-separated ion pair (SSIP). The formation of trityl (Ph 3 C ϩ ) and 4-methoxy-substituted trityl cations from the corresponding halide compounds in acetonitrile solution has been demonstrated, and this formation is attributed to the chemical interaction between M ϩ or M 2ϩ and the halide ions (X Ϫ = Cl Ϫ and Br Ϫ ): 19 In addition, the quantitative formation of the 4-methoxy-substituted trityl cations brought about by the addition of Ba(ClO 4 ) 2 to a solution of (4-MeOC 6 H 4 )Ph 2 CCl in acetonitrile was found to be mainly due to the precipitation of a mixed-anion salt, BaCl(ClO 4 ).…”