Acid-Base Strength Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/29/2015 Acid-Base Strength The strength of an acid-base interaction depends on the nature of the reacting species and solvent effects. Solvent effects can be eliminated by considering gas phase reactions. The strength of an acid is a measure of its ability to dissociate and protonate another compound. Non-aqueous solvents can be used to distinguish between strong acids.

6/29/2015 Gas Phase Basicity Proton Affinity An energy cycle can be used to determine the strength of a base in the absence of solvent effects. B(g) + H(g) -  H A,BH BH(g) +e - -  H IE,H -e -  H IE,BH B(g) + H + (g)  H = PA BH + (g)

6/29/2015 Gas Phase Basicity Substituted Amines N 3- NH 2- N(CH 3 ) basicity 3084 kJ·mol NH 2 CH NH NF NH(CH 3 ) proton affinity values

Solution Phase Basicity Substituted Amines NR 3 The basicity of amines depends on inductive effects of substituents steric effects pK b solvent effects NF 3 no basic character 604 N(CH 3 ) NH 2 CH NH NH(CH 3 ) PA

6/29/2015 Gas Phase Acidity Proton Affinity The conjugate base of stronger acids have lower proton affinities. HF(g)  H = PA H + (g) + F - (g) CH 3 COO kJ·mol -1 CF 3 COO kJ·mol -1

6/29/2015 Gas Phase Acidity Proton Affinity I-I- Br - Cl - F-F- CH 3 COO - OH Conjugate Acid Acidity 1315 kJ·mol HI HBr HCl HF CH 3 COOH H2OH2O

Solution Phase Acidity Acid Strength in Solution In water, HCl, HBr and HI are all considered strong acids. Why are they all considered equally strong in water? How can the strongest acid be identified?

Solution Phase Acidity Acid Strength in Solution In which solvent would HCl ionize less? CH 3 COOH H2OH2O NH 3

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions Comparison of many acid-base reactions indicates that both ionic and covalent factors contribute to the enthalpy of reaction. Each acid and base can be assigned an electrostatic (E) factor and a covalent (C) factor. -ΔH rx = E A E B + C A C B

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions O H H S H H K+K+ I - I Which acid-base pair reactions are more exothermic? E B = 2.28 C B = 0.10 E B = 0.04 C B = 1.56 E A = 0.50 C A = 2.00 E A = 3.78 C A = kcal/mol

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions -ΔH rx = E A E B + C A C B These factors provide ∙ an estimate of the enthalpy of reaction ∙ insight into the bonding nature of each species I - I E A = 0.50 C A = 2.00 K+K+ E A = 3.78 C A = 0.10

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions -ΔH rx = E A E B + C A C B + R A T B More precise estimates can include a correction factor for the transference of charge from the base to the receptor acid. K+K+ E A = 3.78 C A = 0.10 R A = O H H E B = 2.28 C B = 0.10 T B = 0.43 (more important for ions)

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions O H H K+K+ E B = 2.28 C B = 0.10 T B = 0.43 E A = 3.78 C A = 0.10 R A = kcal/mol -ΔH rx = E A E B + C A C B + R A T B

6/29/2015 Acid-Base Reactions Ionic and Covalent Contributions E, C, R & T values represent gas phase reactions. F - has stronger ionic and covalent terms than Cl -, Br - or I - F Cl Br I

6/29/2015 Acid-Base Reactions Bond Energies Three contributing factors to bond energy covalent energy Madelung (Coulombic) energy electronegativity energy -ΔH rx = E A E B + C A C B + R A T B

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Hard and Soft Acids and Bases Hard non-polarizable high charge density Soft polarizable large, low charge cations anions H +, Groups I & II, Ti 4+, Cr 3+, Fe 3+, Co 3+ Cu +, Ag +, Hg 2 2+, Hg 2+, Pd 2+, Pt 2+ (class a)(class b) N>>P>As>Sb O>>S>Se>Te F>Cl>Br>I N As>Sb O<<S<Se  Te F<Cl<Br<I

6/29/2015 Hard and Soft Acids and Bases Dithiooxalate ion can chelate on two sides. C C O - O S - S C C - O O S S - C C O O S S -- prefers harder cations prefers softer cations

6/29/2015 Hard and Soft Acids and Bases Dithiooxalate ion can chelate on two sides. C C O O S S - - prefers harder cations prefers softer cations C C O O S S - - Fe 3+ Cu +

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Capable of protonating compounds that don’t want to be protonated. Causing isomerization in the most unlikely candidates. More acidic than concentrated sulfuric acid.

6/29/2015 FF HH + (CH 3 ) 3 COH(CH 3 ) 3 C + + H 3 O + I2I2 I 2 + and/or I 3 +

6/29/2015 Acids which are stronger than pure sulfuric acid. Super acids are measured with the Hammett acidity function (H o ) H o = pK BH + - log [BH + ] [B] B is an indicator base: e.g., nitroaniline, benzene, picramide

6/29/2015 Superacid Acidity 100% sulfuric acidH 2 SO hydrofluoric acidHF-11.0 perchloric acidHClO fluorosulfuric acidHSO 3 F-15.6 triflic acidHSO 3 CF trifluoromethanesulfonic acid magic acid TM HSO 3 F-SbF to -25 fluoroantimonic acidHF-SbF to -28 H o non-aqueous systems

6/29/2015 (C 6 H 5 ) 3 COH + superacid → (C 6 H 5 ) 3 C + + H 3 O + trivalent carbocation (carbenium ion) alkyl-Cl + superacid → alkyl + + HCl hydrocarbon + superacid → hydrocarbon-H + pentacoordinate carbocation (carbonium ion) George Olah 1994 Nobel Prize

6/29/2015 Formed by mixing fluorine containing Bronsted acid or metal oxide fluorinated Lewis acid HF, HSO 3 F, HSO 3 CF 3 BF 3, SbF 5, TaF 5 TiO 2, SiO 2

6/29/ HF + 2 SbF 5 H 2 F + + F F F F F F Sb F F F F F - → HSO 3 F + SbF 5 fluorosulfonic acid antimony pentafluoride complexing agent F F F F F H-O-S—F O O Sb → Why are superacids so acidic? magic acid TM fluoroantimonic acid

6/29/2015 Hammett acidity is effectively the extension of pH below zero. H o = pK BH + - log [BH + ] [B] B is an indicator base: e.g., nitroaniline, benzene, picramide BH +  B + H + pK BH +

6/29/2015 Hammett acidity is determined spectrophotometrically by determining [BH + ] and [B] H o = pK BH + - log [BH + ] [B] B is an indicator base: e.g., nitroaniline, benzene, picramide

6/29/2015 Applications Isomerization/polymeriztion R + + CC CC+C+ R CC R CC+C+ CC Heterogeneous catalysts solid superacids ZrO 2 -SO 4, TiO 2 -SO 4 for cracking & isomerizing alkanes

6/29/2015 Applications Hydrocracking heavy oil bitumen H2H2 lower boiling distillate also, coal liquifaction

6/29/2015 Redox Reactions F-S-O-O-S-F O O O O peroxydisulfuryldifluoride + I 2 I FO 2 SO OSO 2 F I2+I2+ I3+I3+ + SO 3 F -

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HF + SO 3  HSO 3 F

6/29/2015 Non-aqueous Solvents

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Acid-Base Reactions Ionic and Covalent Contributions O H H S H H K+K+ I - I Which acid-base pair reactions are more exothermic? E B = 2.28 C B = 0.10 E B = 0.04 C B = 1.56 E A = 0.50 C A = 2.00 E A = 3.78 C A = kcal/mol

6/29/2015 Superacids

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