Chapter 4 Acid - Base Chemistry
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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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Br nsted lowry and lewis acids bases

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


Br nsted lowry and lewis acids bases

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


Br nsted lowry and lewis acids bases

3.18 HF


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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