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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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Presentation Transcript
slide1

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
arrhenius acids and bases

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

br nsted lowry definitions
Brønsted-Lowry Definitions
  • Acid: a proton donor
  • Base: a proton acceptor
conjugate acids bases
Conjugate Acids & Bases
  • Brønsted-Lowry does not require water only H+ transfer
conjugate acids bases1
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

conjugate acids bases2
Conjugate Acids & Bases
  • curved arrows show the flow of electrons in an acid-base reaction

recall resonance - also uses curved arrows

conjugate acids bases3
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?

conjugate acids bases4
Conjugate Acids & Bases
  • resonance

octets greater contribution even with plus charge on O

conjugate acids bases5

H+ on the carbonyl

Conjugate Acids & Bases
  • H+ on the hydroxyl

can “write” contributing structures create & separate charge

pi electrons as basic sites
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

lewis acids and bases
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

lewis acids and bases1
Lewis Acids and Bases
  • examples

carbocations very strong Lewis acid

slide13

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
acids base strengths
Acids & Base Strengths
  • acid strength - expressed by equilibrium constant

dissociation (ionization) of acetic acid

pKa = -log Ka

acid base equilibria

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
acid base equilibria1

CO2

+

H2O

Acid-Base Equilibria
  • acetic acid + sodium bicarbonate (omit Na+ )

pKa= 4.76

6.36

molecular structure and acidity
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

molecular structure and acidity1

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

molecular structure and acidity2
Molecular Structure and Acidity
  • Size of A:(-) Within a column -
  • the larger the atom bearing the (-), the greater its stability
molecular structure and acidity3
Molecular Structure and Acidity
  • Resonance delocalized of charge in A-
    • Compare alcohol and carboxylic acid acidity

resonance stabilization

molecular structure and acidity4
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

molecular structure and acidity5
Molecular Structure and Acidity
  • inductive effect: butanoic and chlorobutanoic acids
molecular structure and acidity6
Molecular Structure and Acidity
  • Hybridization
    • greater the % s character with (-) the more stable the anion
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