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Acidity of Superacids

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Acidity of Superacids OH O2N NO2 NO Superacid derivatives 2 in your pocket? Ivo Leito F C F F F [email protected] F F F F F F University of Tartu F Estonia F University of Shanghai for Science and Technology, March 2011 Overview • Acidity, basicity, acids, bases • In solutions • In the gas phase • Measurement of acid strength • Superacids • Measurement of their strength • Design of superacids 2 1 “Acid is a sour substance” • Acidshavebeeninusesincelongtime • Food preservation • Making paints and dyes • … • Obtaining: • Sour fruits (citric acid) HO H H • Fermentation (acetic acid) O H • Heating minerals (sulfuric acid, …) 3 “Alkali is caustic substance” • Alkalies (inorganic bases) have also been in use for a long time • Washing (NaOH, one of the starting materials of soap) • Constrauction (Lime - CaO, Ca(OH)2 – lime mortar) • Obtained: • Ashes • Limestone 4 2 Ancient landmarks Soap-making, Babylon, 2800 BC Lime processing, Since antiquity Vineager, Since antiquity Preparation of H2SO4 and HCl Jābir ibn Hayyān, 8. saj In 18. Century all common acids and alkalies were known and in use Images: Wikipedia 5 The essence of acidity? 18. century: The role of acids is large But the origin of acidity was still largely unclear 6 3 The Origin of Acidity? Antoine Lavoisier, 1776: acids are compounds that contain oxygen (oxygen: producer of acid) Humphrey Davy, 1810: HCl, HBr, H2S etc do not contain oxygen Justus von Liebig, 1838: acids are compounds, in which H atom can be substituted by a metal atom Images: Wikipedia 7 Modern Understanding • Acids dissociate in solution giving H+ ion Svante Arrhenius (1859-1927) Nobel prize 1903 • Different acids dissociate to a different extent Wilhelm Ostwald (1853-1932) Nobel prize 1909 • Acidity of a solution: + pH = -log[H ] Søren Sørensen (1868-1939) • Acid: proton donor, conjugate acids and bases Images: Wikipedia Johannes Nicolaus Brønsted (1879-1947) 8 4 Acidity of molecules and acidity of media • Brønsted acidity of a molecule refers to its ability to donate proton to other molecules • Usually defined in terms of equilibrium constants (Ka, pKa) or deprotonation energies (GA or ΔGacid) • Brønsted acidity of a medium refers to the ability to donate proton to molecules in the medium • In aqueous solution: pH • Strongly acidic solutions: H0 9 Acidity of molecules in solution • Acidity (acid strength) of molecules in solution is usually defined in the framework of the Brønsted theory via the pKa values Ka HA + S A- + SH+ Acid Conjugate acid Base Conjugate base of acid HA of base S a(A− )⋅a(SH+ ) pKa = −log Ka = −log a(HA) 10 5 Acidity and molecular structure OH O + + S ' + SH H2O pKa =10 OH O- O2N NO2 O2N NO2 + + S ' + SH NO2 NO2 H2O 11 pKa = ~ 0 Acid-base reaction in a non-aqueous solvent K1 K2 K3 ¯ + ¯ + (AH)S + (:B)S ' (AH⋅⋅⋅:B)S ' (A :⋅⋅⋅HB )S or (A :HB )S ' HB complex HB complex contact ion pair K3 K4 ¯ + ¯ + ' (A : // HB )S ' (A :)S + (HB )S solvent-separated ion pair free ions 12 6 Acidity of molecules in the gas phase • Acidity of molecules in the gas phase is expressed via deprotnation Gibbs' free energy Ka HA A- + H+ 0 GA = ΔGacid = −RT ln Ka • ΔGºacid is called gas-phase acidity of HA • "º" is often omitted 13 Example of acidity differences in different media Acid pKa pKa ΔGa (GP) pKa (water) (MeCN) kcal/mol (GP) HBr ca -9 5.5 318.3 233.4 2,4-Dinitro- 3.96 16.7 308.6 226.2 phenol • In water HBr is 1013 times stronger than 2,4- DNP • In the gas phase HBr is 107 times weaker than 2,4-DNP 14 7 Gas phase Dissociation in the gas phase – Gas-phase acidiy ΔG°a(HA) >>> 0 HA H+ + A- + −ΔG°a(SH ) << 0 S desolva- tion + - SH ΔG°s(A )<< 0 ΔG°ds(HA) > 0 + ΔG°s(SH ) << 0 Solution Dissociation S S S S S S S in solution S S S + S S HA + - ΔG°a(HA,s) S SH S A S S S S S S S S S 15 Acidity of solutions: pH • Simplified: pH = -log[H+] • Expresses the ability of te medium to donate proton 16 8 Unified acidty scale + μabs(H ) –800 –900 –1000–1100 –1200 –1300 –1400 –1500 in kJ mol–1 superacidic medium acidic medium H SO 14 H O 2 4 3.1 2 HF 12.6 34 DMSO HSO3F 7 39 MeCN Et2O 86* CH2Cl2 86* C6H6 120* SO2 1 bar 10–20 bar gaseous HCl (for comparison) pHabs 140 160 180180 200200 220220 240 260 *approximate value, calculated autoprotolysis 17 Himmel, Goll, Leito, Krossing Chem.Eur.J.2011, 17, 5808 What is a superacid? • Superacidic medium: A Brønsted superacid is a medium, in which the chemical potential of the proton is higher than in pure sulfuric acid 18 Himmel, Goll, Leito, Krossing Chem.Eur.J.2011, 17, 5808 9 What is a superacid? • Superacidic molecule: Superacidic molecule in a given medium is one that is more acidic than H2SO4 in that medium 19 Why do we need to measure acidity? • Acidity is a core parameter of an acidic molecule • Rationalization and prediction of mechanisms of chemical and industrial processes • Design of novel acids and bases • Development of theoretical calculation methods 20 10 How to measure solution acidities of highly acidic molecules? • H0 scale • The classical approach • Different H0 refer to different media • H0 is rather a characteristic of a medium than of a molecule • X-H vibrational frequencies • Indirect: characterizes Stoyanov, et al JACS, 2006, 128, 8500 hydrogen bond rather than acidity • Equilibrium measurement (pKa) in a low basicity solvent • Obvious, butalmostunusedappraoch! 21 Solvent • As low basicity as possible • As high ε as possible • As high pKauto as possible • Easy to purify, reasonably inert, electrochemically stable, transparent in UV, readily available, widely used • There is no ideal solvent • Wth increasing ε as a rule basicity also increases • Water cannot be used 22 11 Non-aqueous pKa values: not trivial • Processes are often more complex than simple ionic dissociation • Ion-pairing, homoconjugation, … • Measurement of a(SH+) is not trivial • Traces of moisture can significantly affect results K. Kaupmees et al, J. Phys. Chem. A 2010, 114, 11788 • Working in an inert gas atmosphere (glovebox) is necessary 23 Approach: Relative measurement • Measurement of pKa differences • ΔpKa values • No need to measure a(SH+) in solution • Many of the error sources cancel out, either partially or fully • Traces of moisture • Impurities in compounds • Baseline shifts and drifts • Technique: UV-Vis spectrometry 24 12 Compound 1 UV-Vis ΔpKa measurement Compound 2 2 2 1.8 1.8 OH CN CF SO SO CF 1.6 1.6 anion 2 2 2 3 anion 1.4 NC CN 1.4 1.2 1.2 SO2CF3 1 1 0.8 0.8 Absorbance (AU) Absorbance (AU) 0.6 0.6 0.4 0.4 0.2 0.2 neutral 0 0 neutral 190 240 290 340 390 190 240 290 340 390 wavelength (nm) wavelength (nm) Mixture 2 OH 1.8 CF SO SO CF 1.6 2 2 2 3 ΔpK = pK (HA ) − pK (HA ) 1.4 a a 2 a 1 anion 1.2 CN SO2CF3 - 1 [A ]⋅[HA ] NC CN 1 2 0.8 = −log K = log - Absorbance (AU) Absorbance 0.6 [HA1]⋅[A2 ] 0.4 0.2 neutral = 0.87 0 190 240 290 340 390 25 wavelength (nm) 1,2-Dichloroethane • Advantages of 1,2-DCE: • Very low basicity (B' = 40), low anion-solvating ability → strong superacids are measurable • pKauto unmeasurably high • Reasonably inert and stable, readily available, widely used, dissolves also many ionic compounds well • Transparent in UV down to 230 nm • Limitations: • ~Low polarity (ε = 10) • Ion-pair acidities 26 13 a No Acid pK a(DCE) Directly measured ΔpK ip values in DCE pK 1,2-DCE acidity 1 Picric acid 0.0 2 HCl -0.4 0.73 0.71 1.48 0.36 3 2,3,4,6-(CF3)5-C6H-CH(CN)2 -0.7 1.09 0.77 0.74 c 2.08 1.00 scale 4 4-NO2-C6H4SO2NHTos -1.5 0.28 1.78 5 HNO3 -1.7 1.01 1.13 6 4-NO2-C6H4SO2NHSO2C6H4-4-Cl -2.4 0.88 0.08 • The most acidic 0.24 7 H2SO4 -2.5 0.12 1.26 8 C6(CF3)5CH(CN)2 -2.6 equilibrium acidity 1.02 1.02 1.05 1.48 9 (4-NO2-C6H4-SO2)2NH -3.7 0.47 0.80 10 3-NO2-4-Cl-C6H3SO2NHSO2C6H4-4-NO2 -4.1 scale in a constant- 0.36 1.35 11 (3-NO2-4-Cl-C6H3SO2)2NH -4.5 0.39 0.93 composition 12 HBr -4.9 0.62 0.19 1.03 13 4-NO2-C6H4SO2CH(CN)2 -5.1 0.80 medium 1.33 14 2,4,6-(SO2F)3-Phenol -5.9 d 0.64 15 2,4,6-Tf3-Phenol -6.4 0.94 16 CH(CN)3 -6.5 0.42 1.14 1.12 17 4-Cl-C H SO(=NTf)NHTos -6.8 0.33 0.65 6 4 0.51 0.05 18 NH -TCNP e -6.8 0.24 • Relative acidities 2 0.20 19 2,3,5-tricyanocyclopentadiene -7.0 20 Pentacyanophenol -7.6 0.67 0.84 1.77 21 4-Cl-C6H4SO(=NTf)NHSO2C6H4-4-Cl -7.6 1.56 22 HI -7.7 1.64 1.00 • Acidity increases 0.98 1.13 1.10 0.93 23 4-NO2-C6H4SO2NHTf -7.8 1.04 1.01 0.81 24 Me-TCNP -8.6 0.96 0.90 downwards 0.09 0.13 1.42 25 3,4-(MeO)2-C6H3-TCNP -8.7 -0.02 0.12 26 4-MeO-C6H4-TCNP -8.7 0.12 0.46 0.24 27 C(CN)2=C(CN)OH -8.8 0.28 1.61 0.22 0.59 28 4-Cl-C6H4SO(=NTf)NHSO2C6H4-NO2 -8.9 0.74 0.67 0.06 29 2,4-(NO2)2-C6H3SO2OH -8.9 0.59 0.60 30 C6F5CH(Tf)2 -9.0 0.52 31 HB(CN)(CF3)3 -9.3 0.47 0.44 A.
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