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Br ø nsted-Lowry and Lewis Acids/Bases

Chapter 4 Acid - Base Chemistry. Br ø nsted-Lowry and Lewis Acids/Bases Acid Dissociation Constants, pKa, the Relative strength of Acids and Bases. [electron pushing, arrows, electronic view rxs.] Equilibrium in Acid-Base Reactions Molecular structure and acidity

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Br ø nsted-Lowry and Lewis Acids/Bases

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  1. Chapter 4 Acid - Base Chemistry • Brønsted-Lowry and Lewis Acids/Bases • Acid Dissociation Constants, pKa, the Relative strength of Acids and Bases. • [electron pushing, arrows, • electronic view rxs.] • Equilibrium in Acid-Base Reactions • Molecular structure and acidity • [periodicity: electronegativity & size of atom • hybridization, inductive effects, resonance] • See the Chapter Summary

  2. H3O+(aq) H+(aq) + H2O(I) H2O KOH K+ + -OH(aq) Arrhenius Acids and Bases • acid: substance that produces H3O+ ions aqueous solution hydronium ion base: substance that produces OH- ions in aqueous solution

  3. Brønsted-Lowry Definitions • Acid: a proton donor • Base: a proton acceptor

  4. Conjugate Acids & Bases • Brønsted-Lowry does not require water only H+ transfer

  5. Conjugate Acids & Bases • acid-base reaction:aproton (H+)transfer reaction • conjugate acid-base pair:a pair of molecules (ions) that can be interconverted byH+transfer conjugate base acid

  6. Conjugate Acids & Bases • curved arrows show the flow of electrons in an acid-base reaction recall resonance - also uses curved arrows

  7. Conjugate Acids & Bases • molecules may have 2 or more sites that can accept a H+ e.g. carboxylic acids, esters, and amides • protonation favored where the charge is more delocalized which oxygen is protonated?

  8. Conjugate Acids & Bases • resonance octets greater contribution even with plus charge on O

  9. H+ on the carbonyl Conjugate Acids & Bases • H+ on the hydroxyl can “write” contributing structures create & separate charge

  10. Pi Electrons As Basic Sites • pi electrons - base - donate a pair of e-s Result: formation of a carbocation, C has 6-e-s,+1charge

  11. Lewis Acids and Bases • Lewis acid:molecule/ion that accepts a pair of electrons • Lewis base:molecule/ion that donates a pair of electrons forms a new covalent bond

  12. Lewis Acids and Bases • examples carbocations very strong Lewis acid

  13. Chapter 4 Acid - Base Chemistry • Brønsted-Lowry and Lewis Acids/Bases • Acid Dissociation Constants, pKa, the Relative strength of Acids and Bases. • [electron pushing, arrows, • electronic view rxs.] • Equilibrium in Acid-Base Reactions • Molecular structure and acidity • [periodicity: electronegativity & size of atom • hybridization, inductive effects, resonance] • See the Chapter Summary

  14. Acids & Base Strengths • acid strength - expressed by equilibrium constant dissociation (ionization) of acetic acid pKa = -log Ka

  15. 3.18 HF

  16. pKa 4.76 pKa 9.24 Acid-Base Equilibria • Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

  17. CO2 + H2O Acid-Base Equilibria • acetic acid + sodium bicarbonate (omit Na+ ) pKa= 4.76 6.36

  18. Molecular Structure and Acidity • relative acidities the more A:(-) stable, greater the acidity of H-A Ways to stabilize A:(-) the negative charge ON a more electronegative atom ON larger atom RESONANCE delocalized STABILIZED by inductive effect IN an orbital with more s character

  19. C o n j u g a t e b a s e A c i d Molecular Structure and Acidity • within a period - the greater the electronegativity of A:(-) • the more A:(-) isstablized •  the stronger the acid pKa16 pKa38 pKa51

  20. Molecular Structure and Acidity • Size of A:(-) Within a column - • the larger the atom bearing the (-), the greater its stability

  21. Molecular Structure and Acidity • Resonance delocalized of charge in A- • Compare alcohol and carboxylic acid acidity resonance stabilization

  22. Molecular Structure and Acidity • inductive effect, electron-withdrawing • covalent bonds transmit electronegativity (polarizing) effects • push or pull shared e(-)s of adjacent atoms decreases w/ distance

  23. Molecular Structure and Acidity • inductive effect: butanoic and chlorobutanoic acids

  24. Molecular Structure and Acidity • Hybridization • greater the % s character with (-) the more stable the anion

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