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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.

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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:

1 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

2 4- 2 road trip started

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

5 4- 5 Conjugate Acids & Bases Brønsted-Lowry does not require water only H + transfer

6 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

7 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

8 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

9 4- 9 Lewis Acids and Bases examples carbocations very strong Lewis acid

10 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?

11 HF

12 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?

13 4- 13 Acid-Base Equilibria acetic acid + sodium bicarbonate (omit Na + ) 6.36 pK a = 4.76 CO 2 H2OH2O +

14 4- 14 Acid-Base Theories Brønsted-Lowry: acid-proton donor, base-proton acceptor Lewis: acid-accept pair electrons, base-donate pair electrons

15 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

16 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

17 4- 17 Size of A: (-) Within a column - the larger the atom bearing the (-), the greater its stability

18 4- 18 inductive effect, electron-withdrawing -covalent bonds transmit electronegativity (polarizing) effects -push or pull shared e (-) s of adjacent atoms decreases w/ distance

19 4- 19 inductive effect: butanoic and chlorobutanoic acids

20 4- 20 Hybridization greater the % s character with (-) the more stable the anion

21 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

22 4- 22 Conjugate Acids & Bases curved arrows show the flow of electrons in an acid-base reaction recall resonance - also uses curved arrows

23 4- 23 Resonance delocalized of charge on A: (-) Compare alcohol and carboxylic acid acidity resonance stabilization (of A: - )

24 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?

25 4- 25 resonance octets greater contribution even with plus charge on O Use of resonance theory

26 4- 26 H + on the hydroxyl can “write” contributing structures create & separate charge  H + on the carbonyl Use of resonance theory


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