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PKeh

Common pKa Values. Evans pKa TableAlkane~ 50 Alcohol~ 15Alkene~ 45Malonate~ 12Allyl~ 43Phenol~ 10Phenyl ~ 43Nitro~ 10Amine~ 35Carboxylic acid~ 5 Alkyne~ 25Ester~ 25Ketone~ 20. Definition of pKa. pKa ? How acidic a proton isAH(aq) H2O(l) H3O (aq) A

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PKeh

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    1. pKeh? Neil Stevenson 21/07/04

    2. Common pKa Values Evans pKa Table Alkane ~ 50 Alcohol ~ 15 Alkene ~ 45 Malonate ~ 12 Allyl ~ 43 Phenol ~ 10 Phenyl ~ 43 Nitro ~ 10 Amine ~ 35 Carboxylic acid ~ 5 Alkyne ~ 25 Ester ~ 25 Ketone ~ 20

    3. Definition of pKa pKa – How acidic a proton is AH(aq) + H2O(l) H3O+(aq) + A-(aq) Keq = [H3O+][A-] Ka = [H3O+][A-] [AH][H2O] [AH] pKa = - logKa Higher Ka => Lower pKa \ Stronger acid has lower pKa

    4. pH and pKa pKa = pH when acid is exactly half dissociated i.e. [A-] = [AH] Ka = [H3O+][A-] [AH] pH > pKa Exists as A- pH < pKa Exists as AH

    5. Solvent and pKa pKa dependant on solvent AH + solvent A- + solvent-H+ More basic solvent => equilibrium lies to right Stabilisation of conjugate acid via H-bonding => lower pKa

    6. Solvent and pKa pKa (H2O) = 15.74 pKa (H3O+) = -1.74 \ pKa range of reactions that can be performed in water is –1.74 to 15.74 Strongest base in water is OH- Strongest acid in water is H3O+ Deprotonation of ethyne: HCºCH + NH2- HCºC- + NH3 pKa (NH3) = 38 > pKa (HCºCH) = 24

    7. Acid Strength Strong acid/base – complete ionisation Weak acid/base – incomplete ionisation AH + B A- + BH+ Acid Base Conjugate Base Conjugate Acid pKa depends on stability of conjugate base CH3CO2H CH3OH CH4 4.76 15.54 48 Higher pKa Less stable conjugate base CH3CO2- CH3O- CH3-

    8. Stability of Conjugate Base Electronegativity CH4 NH3 H2O HF 48 33 16 3 Increased electronegativity => more stable anion => lower pKa A-H bond strength HF HCl HBr HI 3 -8 -9 -10 Weaker A-H bond => proton more easily dissociated => lower pKa

    9. Stability of Conjugate Base Delocalisation of negative charge HClO HClO2 HClO3 HClO4 7.5 2 -1 -10 Increased delocalisation => more stable anion =>lower pKa

    10. Stability of Conjugate Base Delocalisation of negative charge 22.8 18.5 11 Increased delocalisation => lower pKa Used in Fmoc protecting group:

    11. Stability of Conjugate Base Aromaticity 15 20 62 Conjugate bases: 6pe- 10pe- 4pe- Aromatic Aromatic Anti-Aromatic Aromaticity formed in conjugate base => lower pKa

    12. Stability of Conjugate Base Electron withdrawing groups 4.76 3.6 2.4 1.7 More electron withdrawing => lower pKa Electron donating groups 15.5 16.0 17.1 19.2 More electron donating => higher pKa

    13. Stability of Conjugate Base Hybridisation 50 44 26 sp3 sp2 sp Increased s character in anion => lower pKa Remote hybridisation 4.9 4.2 1.9 Increased s character => less e- donating => lower pKa

    14. Base Strength Stronger base => Conjugate acid has higher pKa i.e. HCºC- is a stronger base than HO- since pKa(HCCH) = 25 > pKa (H2O) = 15.74 More accessible electrons => stronger base Neutral bases – need to know pKaH pKaH = pKa of conjugate acid pKaH (NH3) = pKa (NH4+) = 9.2 pKa (NH3) = 38 Strength of neutral base determined by: - availability of lone pair - stabilisation of positive charge

    15. Neutral Nitrogen Bases Electron donating groups 9.2 10.6 10.8 9.8 Alkyl groups donate e- => stabilise +ve charge => stronger base => higher pKa However => less stabilisation in solvent by H-bonding Aromaticity Lone pair involved in aromatic ring => lower pKaH 5.2 -4

    16. Neutral Nitrogen Bases Lone pair delocalisation Amides weak bases (pKaH ~ -0.5) => N lone pair delocalised => less basic more favoured less favoured Amidines strong bases (pKaH ~ 12) e.g. DBU Protonate at sp2 centre

    17. Neutral Nitrogen Bases Hybridisation 10.7 9.5 8.2 Increased s character => less e- donating => lower pKa Proton sponges Steric effects can affect pKa Significant release of steric strain upon protonation => stronger base => higher pKa 12.0 5.1

    18. Questions Estimate pKa’s: ? ? 15 Estimate pKa: Place in order of pKa:

    19. Questions Assign pKa’s: 4, 7, 9, 10, 11 Place in order of pKa:

    20. Questions Assign pKa’s to appropriate functional groups and draw structure at pH 1, 5, 9, & 12: pKa’s: 1.8, 8.3, 10.8 Estimate pKa’s: Are conjugate bases aromatic? Predict products of following reactions:

    21. Questions Explain the difference in pKa: 2.98 4.58 Place in order of pKa:

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