Solutes in sulfuric acid. Part IX. Effect of para‐substitution on the ionization of the oxonium ions of phenol and anisole

Abstract

Abstract. The ionization of the oxonium ions of phenol, anisole, 4-chloro-and 4-methyl-phenol and 4-bromo-, 4-fluoro-and 4-methyl-anisole in concentrated aqueous sulfuric acid has been investigated using UV spectroscopy at 25°C. The sulfuric acid concentrations at half-ionization are within the range 75.2 to 81.8% H,S04, corresponding with pK,'s between -5.87 and -6.52 based on the Hoa acidity function. The substituent effects on the ionization of PhO+H2 and PhO'HMe are small, the Hummet p values being 0.5 kO.1 and 0.9kO.1, respectively.

Requirements for Terpene Cyclizations inside the Supramolecular Resorcinarene Capsule: Bound Water and Its Protonation Determine the Catalytic Activity

“…This indicates that assemblies II and V are not protonated by the external acid, at least on the 1 H NMR (500 MHz) time scale . A protonation of the phenolic groups of II and V in the presence of unbound water seems unlikely considering the p K a values of H 3 O + (p K a = 0) and protonated phenols (p K a ≈ −6) . The integrity of the assemblies I – VI in the presence of 1.0 equiv of HCl was confirmed by DOSY-NMR experiments (see Supporting Information Chapter 10 for details).…”

“…17 A protonation of the phenolic groups of II and V in the presence of unbound water seems unlikely considering the pK a values of H 3 O + (pK a = 0) 18 and protonated phenols (pK a ≈ −6). 19 The integrity of the assemblies I−VI in the presence of 1.0 equiv of HCl was confirmed by DOSY-NMR experiments (see Supporting Information Chapter 10 for details). Importantly, the observed protonation of the host correlates well with the observed catalytic activity of the respective assemblies in the cyclization of monoterpenes.…”

“…As reported by us, 10 no reaction was observed with assembly II (Figure 2e), while assembly I (Figure 2a) catalyzed the formation of αterpinene (8) from acetate 7 in 30−35% yield. The reaction proceeds in good selectivity with other common cyclization products (Figure 2g) such as eucalyptol (15), terpinolene (16), limonene (17), γ-terpinene (18), and isoterpinolene (19) being only formed in minor quantities (<10% yield). Assembly III (Figure 2b), formed from monomer 3, featuring one additional hydroxyl group compared to resorcinarene 1, also displayed catalytic activity and likewise yielded α-terpinene (8) as the main product, albeit in a lower yield.…”

“…While I readily encapsulates ammonium salts (e.g., tetrabutylammonium bromide (TBAB) 14) as ion pairs, the encapsulation of anions in assembly II has been shown to be energetically unfavorable. 10 The binding of anions can be easily investigated with 19 F NMR when utilizing fluorine-containing anions (e.g., triflate). Figure 3 shows the 19 F NMR spectra of the assemblies I−VI in the presence of 1.0 equiv of tetrabutylammonium triflate (27).…”

“…10 The binding of anions can be easily investigated with 19 F NMR when utilizing fluorine-containing anions (e.g., triflate). Figure 3 shows the 19 F NMR spectra of the assemblies I−VI in the presence of 1.0 equiv of tetrabutylammonium triflate (27). In all cases, the presence of the assembly causes a shift and broadening of the signal corresponding to the triflate anion, with the sole exception being assembly II, where no effect on the triflate signal was observed.…”