4- 1 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 [periodicity: electronegativity & size of atom hybridization, inductive effects, resonance] See the Chapter Summary Chapter 4 Acid - Base Chemistry

4- 2 Arrhenius Acids and Bases acid: acid: substance that produces H 3 O + ions aqueous solution H + (aq) + H 2 O (I) H 3 O + (aq) hydronium ion base: base: substance that produces OH - ions in aqueous solution KOH K OH (aq) H2OH2O

4- 3 Brønsted-Lowry Definitions Acid: Acid: a proton donor Base: Base: a proton acceptor

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

4- 5 Conjugate Acids & Bases acid-base reaction: acid-base reaction: a proton (H + ) transfer reaction conjugate acid-base pair: conjugate acid-base pair: a pair of molecules (ions) that can be interconverted by H + transfer acid conjugate base

4- 6 Conjugate Acids & Bases curved arrows show the flow of electrons in an acid-base reaction recall resonance - also uses curved arrows

4- 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?

4- 8 Conjugate Acids & Bases resonance octets greater contribution even with plus charge on O

4- 9 Conjugate Acids & Bases H + on the hydroxyl can “write” contributing structures create & separate charge  H + on the carbonyl

4- 10 Conjugate Acids & Bases Problem 4.3 Problem 4.3 Does proton transfer to an amide group occur preferentially on the amide oxygen or the amide nitrogen?

4- 11 Pi Electrons As Basic Sites pi electrons - base - donate a pair of e - s carbocation Result: formation of a carbocation, C has 6-e - s,  +1charge

4- 12 Lewis Acids and Bases Lewis acid: Lewis acid: molecule/ion that accepts a pair of electrons Lewis base: Lewis base: molecule/ion that donates a pair of electrons forms a new covalent bond

4- 13 Lewis Acids and Bases examples carbocations very strong Lewis acid

4- 14 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 [periodicity: electronegativity & size of atom hybridization, inductive effects, resonance] See the Chapter Summary Chapter 4 Acid - Base Chemistry

4- 15 Acids & Base Strengths acid strength - expressed by equilibrium constant dissociation (ionization) of acetic acid pKa = -log Ka

HF

4- 17 Acid-Base Equilibria Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base pKa 4.76 pKa 9.24

4- 18 Acid-Base Equilibria acetic acid + sodium bicarbonate (omit Na + ) 6.36 pK a = 4.76 CO 2 H2OH2O +

4- 19 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

4- 20 within a period - the greater the electronegativity of A: (-) the more A: (-) is stablized  the stronger the acid Conjugate base Acid Molecular Structure and Acidity pK a 38 pK a 16 pK a 51

4- 21 Size of A: (-) Within a column - the larger the atom bearing the (-), the greater its stability Molecular Structure and Acidity

4- 22 Resonance delocalized of charge in A - Compare alcohol and carboxylic acid acidity Molecular Structure and Acidity resonance stabilization

4- 23 inductive effect, electron-withdrawing covalent bonds transmit electronegativity (polarizing) effects push or pull shared e (-) s of adjacent atoms Molecular Structure and Acidity decreases w/ distance

4- 24 inductive effect: butanoic and chlorobutanoic acids Molecular Structure and Acidity

4- 25 Hybridization greater the % s character with (-) the more stable the anion Molecular Structure and Acidity