pKa values and gas-phase acidities of superacid molecul

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UV-Vis ΔpK a measurement Compound 1 Compound 2 Absorbance (AU) Absorbance (AU) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 anion neutral 190 240 290 340 390 wavelength (nm) Mixture 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 anion NC NC CN CN CN OH CF 2 SO 2 CN SO 2 CF 3 0.2 neutral 0 190 240 290 340 390 wavelength (nm) SO 2 CF 3 Absorbance (AU) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 neutral 190 240 290 340 390 wavelength (nm) 0 ΔpK a = 0.87 = pK a anion (HA ) − pK (HA ) - [A1] ⋅[HA2] = − log K = log - [HA ] ⋅[A ] 2 CF 2 SO 2 SO 2 CF 3 1 a OH 2 SO 2 CF 3 1 15 Acetonitrile (MeCN) • Advantages of MeCN: • Low basicity (B' = 160), low anion-solvating ability → quite strong acids measurable • High polarity (ε = 36) • High pK auto (≥ 33) • Transparent in UV, easy to purify, reasonably inert, electrochemically stable, readily available, widely used • Limitations: • Very strong bases decompose MeCN • Already Et-P2(pyrr) slowly decomposes MeCN • Impossible to study the strongest superacids 16 8
Self-consistent acidity scale in MeCN Acid pK a (AN) a Directly measured ΔpK a b 1 9-C 6F 5-Fluorene 28.11 2 (4-Me-C 6F 4)(C 6H 5)CHCN 26.96 3 (4-NC 5F 4)(C 6H 5)NH 26.34 4 (C 6H 5)(C 6F 5)CHCN 26.14 5 (4-Me 2N-C 6F 4)(C 6F 5)NH 25.12 6 (4-Me-C 6F 4)(C 6F 5)NH 24.94 7 Octafluorofluorene 24.49 8 Fluoradene 23.90 9 9-COOMe-Fluorene 23.53 10 Acetic acid 23.51 11 (C 6F 5)CH(COOEt) 2 22.85 12 2-NO 2-Phenol 22.85 13 (4-Me-C 6F 4) 2CHCN 22.80 14 (4-Me-C 6F 4)(C 6F 5)CHCN 21.94 15 Benzoic acid 21.51 16 9-CN-Fluorene 21.36 17 (4-H-C 6F 4)(C 6F 5)CHCN 21.11 18 (C 6F 5) 2CHCN 21.10 19 (CF 3) 3COH 20.55 20 (4-Cl-C 6F 4)(C 6F 5)CHCN 20.36 21 2,4,6-Br 3-Phenol 20.35 22 (2,4,6-Cl 3-C 6F 2)(C 6F 5)CHCN 20.13 23 2,3,5,6-F 4-Phenol 20.12 24 2,3,4,5,6-F 5-Phenol 20.11 25 (2-C 10F 7)(C 6F 5)CHCN 20.08 26 1-C 10F 7OH 19.72 27 2,4,6-(SO 2F) 3-Aniline 19.66 28 (2-C 10F 7) 2CHCN 19.32 29 9-C 6F 5-Octafluorofluorene 18.88 30 2-C 10F 7OH 18.50 31 (4-CF 3-C 6F 4)(C 6F 5)CHCN 18.14 32 4-C 6F 5-2,3,5,6-F 4-Phenol 18.11 33 (4-H-C 6F 4)CH(CN)COOEt 18.08 34 2,3,4,5,6-Cl 5-Phenol 18.02 35 2,3,4,5,6-Br 5-Phenol 17.83 36 (C 6F 5)CH(CN)COOEt 17.75 37 4-Me-C 6H 4CH(CN) 2 17.59 38 (2-C 10F 7)CH(CN)COOEt 17.50 39 (4-Cl-C 6F 4)CH(CN)COOEt 17.39 40 2,4-(NO 2) 2-Phenol 16.66 41 4-CF 3-2,3,5,6-F 4-Phenol 16.62 42 (4-NC 5F 4)(C 6F 5)CHCN 16.40 43 (4-CF 3-C 6F 4) 2CHCN 16.13 44 (4-CF 3-C 6F 4)CH(CN)COOEt 16.08 45 (4-NC 5F 4)(2-C 10F 7)CHCN 16.02 46 4-NC 5F 4-OH 15.40 47 (4-NC 5F 4)CH(CN)COOEt 14.90 1.15 0.63 0.20 1.03 0.64 0.96 0.67 0.90 0.60 1.01 0.21 0.42 0.40 0.44 0.38 0.36 0.06 0.59 1.49 1.32 1.96 1.20 0.45 0.58 1.05 0.92 0.42 0.39 1.02 0.01 0.43 0.34 1.19 0.75 0.77 0.22 0.32 0.06 1.12 019 1.78 0.17 -0.01 1.49 0.25 0.76 0.81 0.79 0.46 0.19 0.08 1.10 0.60 1.35 0.82 0.68 0.04 0.26 0.53 0.26 0.03 0.03 0.85 0.73 0.04 1.15 0.54 1.43 0.73 0.75 0.71 0.48 1.02 0.41 0.92 1.92 1.59 0.10 0.27 0.27 0.05 0.70 0.69 1.75 1.70 1.51 1.21 0.63 0.88 0.37 -0.01 0.82 1.46 0.72 1.03 0.26 1.02 0.28 0.29 0.25 0.00 0.80 0.06 1.32 47 (4-NC 5F 4)CH(CN)COOEt 14.90 0.69 0.19 48 3-CF 3-C 6H 4CH(CN) 2 14.72 0.15 49 Saccharin 14.57 0.84 0.71 1.89 50 4-Me-C 6F 4CH(CN) 2 13.87 0.40 51 (4-NC 5F 4) 2CHCN 13.46 0.87 0.45 0.89 52 C 6F 5CH(CN) 2 13.01 0.48 0.03 0.04 53 4-H-C 6F 4CH(CN) 2 12.98 0.79 0.74 54 2-C 10F 7CH(CN) 2 12.23 1.38 0.26 55 Tos 2NH c 11.97 0.62 56 4-NO 2-C 6H 4CH(CN) 2 11.61 0.03 57 4-MeO-C 6H 4C(=O)NHTf d 11.60 0.28 0.19 0.14 58 4-Me-C 6H 4C(=O)NHTf 11.46 1.21 0.98 59 (C 6H 5SO 2) 2NH 11.34 0.60 0.60 0.57 60 4-Cl-C 6H 4SO 2NHTos 11.10 0.49 0.36 61 C 6H 5C(=O)NHTf 11.06 1.43 0.10 0.07 62 Picric acid 11.00 63 4-F-C 6H 4C(=O)NHTf 10.65 0.91 0.79 0.87 64 4-Cl-C 6H 4C(=O)NHTf 10.36 0.82 0.54 65 (4-Cl-C 6H 4SO 2) 2NH 10.20 0.15 0.56 -0.01 66 4-CF 3-C 6F 4CH(CN) 2 10.19 0.13 1.15 0.14 67 4-NO 2-C 6H 4SO 2NHTos 10.04 0.52 1.20 1.06 0.73 68 4-Cl-3-NO 2-C 6H 3SO 2NHTos 9.71 1.05 0.46 69 4-NO 2-C 6H 4C(=O)NHTf 9.49 0.53 0.23 70 4-NO 2-C 6H 4SO 2NHSO 2C 6H 4-4-Cl 9.17 2.3 0.56 1.73 71 TosOH 8.6 1.21 0.23 72 (4-NO 2-C 6H 4SO 2) 2NH 8.32 1.3 0.25 73 1-C 10H 7SO 3H 8.02 0.50 0.19 1.04 74 C 6H 5CHTf 2 7.85 0.54 1.25 75 4-Cl-C 6H 4SO 3H 7.3 0.53 76 3-NO 2-C 6H 4SO 3H 6.78 0.51 77 4-NO 2-C 6H 4SO 3H 6.73 78 4-MeO-C 6H 4C(=NTf)NHTf 6.54 0.51 0.44 1.28 0.23 79 4-Me-C 6H 4C(=NTf)NHTf 6.32 0.75 0.23 0.05 0.62 80 TosNHTf 6.30 0.5 0.16 0.35 81 C 6H 5C(=NTf)NHTf 6.17 0.36 1.11 0.47 82 C 6H 5SO 2NHTf 6.02 0.59 0.26 83 4-F-C 6H 4C(=NTf)NHTf 5.79 0.83 1.64 84 4-Cl-C 6H 4C(=NTf)NHTf 5.69 0.77 1.37 85 2,4,6-(SO 2F) 3-Phenol 5.66 1.25 0.64 86 4-Cl-C 6H 4SO 2NHTf 5.47 0.42 0.59 0.71 87 4-Cl-C 6H 4SO(=NTf)NHTos 5.27 0.53 88 4-NO 2-C 6H 4C(=NTf)NHTf 5.13 0.38 1.20 89 2,4,6-Tf 3-Phenol 4.93 0.41 1.1 90 4-NO 2-C 6H 4SO 2NHTf 4.52 0.05 0.87 91 4-Cl-C 6H 4SO(=NTf)NHSO 2C 6H 4-4-Cl 4.47 1.10 1.15 0.31 92 2,3,5-tricyanocyclopentadiene 4.16 0.74 0.50 93 4-Cl-C 6H 4SO(=NTf)NHSO 2C 6H 4-4-NO 2 3.75 A. Kütt et al, J. Org. Chem. 2006, 71, 2829 17 A. Kütt et al, J. Org. Chem. 2011, 76, 391 Properties of the MeCN acidity scale • Consistency, checked by circular validation measurements: • 93 acids, 203 relative acidity measurements • Consistency standard deviation: 0.03 pK a units • Anchored to Picric acid (pK a = 11.0) • pK a range: 3.7 .. 28.1 • Useful tool for further studies • Acids in the lower part are superacids • pK a (H 2 SO 4 ) ≈ 8 in MeCN 18 9
Page 4 and 5: What is a superacid? • Superacidi
Page 6 and 7: Solvent • As low basicity as poss
Page 10 and 11: 67 4NO 2 C 6 H 4 SO 2 NHTos 10.04 6
Page 12 and 13: Important aspects for superacids
Page 14 and 15: Fluorine • Highly electronegative
Page 16 and 17: 250 256 255.5 HSbF 6 250.8 HAuF 6 2

pKa values and gas-phase acidities of superacid molecul

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