Presentation on theme: "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."— Presentation transcript:
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- 3 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- 4 Brønsted-Lowry Definitions Acid: Acid: a proton donor Base: Base: a proton acceptor
4- 5 Conjugate Acids & Bases Brønsted-Lowry does not require water only H + transfer
4- 6 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- 7 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- 8 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- 9 Lewis Acids and Bases examples carbocations very strong Lewis acid
4- 10 Acids & Base Strengths By equilibrium constant e.g.: dissociation (ionization) of acetic acid pKa = -log Ka How is acid [base] strength expressed/compared?
4- 12 Acid-Base Equilibria Strong acid - completely to product(s) Weak - incomplete, gives an Equilibrium direction favors reaction of stronger acid-base pair pKa 4.76 pKa 9.24 What favors the direction of acid - base reactions?
4- 13 Acid-Base Equilibria acetic acid + sodium bicarbonate (omit Na + ) 6.36 pK a = 4.76 CO 2 H2OH2O +
4- 15 How does Structure effect 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- 16 within a period - the greater the electronegativity of A: (-) the more A: (-) is stablized the stronger the acid Conjugate base Acidelectronegativity pK a 38 pK a 16 pK a 51 incerasing acidity
4- 17 Size of A: (-) Within a column - the larger the atom bearing the (-), the greater its stability
4- 18 inductive effect, electron-withdrawing -covalent bonds transmit electronegativity (polarizing) effects -push or pull shared e (-) s of adjacent atoms decreases w/ distance
4- 19 inductive effect: butanoic and chlorobutanoic acids
4- 20 Hybridization greater the % s character with (-) the more stable the anion
4- 21 1. On your quiz write which of these 2 compounds is the stronger acid. 2. Write and complete this acid-base reaction. Include electronic arrows that account for the reaction. Lecture quiz Name ____________, seat ______, TA or section______ Open notes, 3 minutes, hand in promptly
4- 22 Conjugate Acids & Bases curved arrows show the flow of electrons in an acid-base reaction recall resonance - also uses curved arrows
4- 23 Resonance delocalized of charge on A: (-) Compare alcohol and carboxylic acid acidity resonance stabilization (of A: - )
4- 24 Use of resonance theory 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- 25 resonance octets greater contribution even with plus charge on O Use of resonance theory
4- 26 H + on the hydroxyl can “write” contributing structures create & separate charge H + on the carbonyl Use of resonance theory