4-4-11

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

Chapter 4 Acid - Base Chemistry

OC O

H3C

HO H

OC O

H3CO H

H

4-4-22road trip started

Br

OR

Br

O

OH

O OH

H

OH

OH

OHOH

OR

OH

OH

OR

R

O

H

OH

NH2

O

O

OH

Nuc(Anionic)

OH

R

Br

Li

CuLi

HO

Br

O

OHOH

OR OR

OH

Br

Br

Br

Cl

2

1 other product

2 other product

a

b

c

e

f

g

h

i

jk

l

m

no

ct

wu

vx

u

y

a2

c3c4

a8

u

b7

b5

b6

b4

b4

b1b1

r

z

b

a7

b3

b2O

O

ab

4-4-33Arrhenius Acids and BasesArrhenius Acids and Bases

acid:acid: substance that produces H3O+ ions aqueous solution

H+(aq) + H2O(I) H3O+(aq)

hydronium ion

base:base: substance that produces OH- ions in aqueous solution

KOH K+ + -OH(aq) H2O

4-4-44Brønsted-Lowry DefinitionsBrønsted-Lowry Definitions

Acid:Acid: a proton donor

Base:Base: a proton acceptor

+O H

H

H

H

H

H + N+O

H

N H

H

H

+H H

+ -O H

H

OH

H

O HH + +O H H

4-4-55Conjugate Acids & BasesConjugate Acids & Bases

Brønsted-Lowry does not require water only H+ transfer

NH3NH3+ +

conjugate acid-base pair

conjugate acid-base pair

CH3 CO

O HCH3 C

O

O

H

4-4-77Pi Electrons As Basic SitesPi Electrons As Basic Sites

pi electrons - base - donate a pair of e-s

Result: formation of a carbocationcarbocation, C has 6-e-s, +1charge

H3CC

CH3C H

H

H Br+

H3CC

CH3C H

H

Br+H

4-4-88Lewis Acids and BasesLewis 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

A + :B A Bforms a new covalent bond

4-4-99

H3CC

CH3C

H

HH

Br

Lewis Acids and BasesLewis Acids and Bases

examplesH3C

C

CH3C H

H

Br+H

C

OC

C

C

HH

HH

H

HH

HH

H B

F F

F

+

C

OC

C

C

HH

HH

H

HH

HH

H B

F F

F

carbocations very strong Lewis acid

4-4-1010Acids & Base StrengthsAcids & Base Strengths

By equilibrium constant

e.g.: dissociation (ionization) of acetic acid

[H3O+][A-]

[HA][H2O]Keq =

Keq[H2O]Ka[H3O+][A-]

[HA]= =

pKa = -log Ka

How is acid [base] strength expressed/compared?

+ OH

H

H3C CO

O

H+ OH

H

H3C CO

O H

4-4-1111

HI I-

HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6H5COOH C6H5COO-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6H5OH C6H5O-

HCO3-

CO32-

CH3NH2CH3NH3+

H2O HO-

CH3CH2OH CH3CH2O-

HC CH HC C-H2 H-NH3 NH2

-CH2=CH2 CH2=CH

-CH3CH3 CH3CH2

-Acid Formula pKa Conjugate BaseEthane

Ammonia

EthanolWater

Bicarbonate ionPhenol

Ammonium ion

Carbonic acidAcetic acid

3525

Benzoic acidPhosphoric acid

Sulfuric acidHydrogen chlorideHydrogen bromideHydrogen iodide

51

38

10.33

15.715.9

4.766.36

9.24

9.95

-5.2-7

-9-8

4.19

2.1-1.74Hydronium ion

Strongerconjugate

base

Weakerconjugate

base

Weaker acid

Stronger acid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

AcetyleneHydrogen

Ethylene 44

3.18 HF

4-4-1212Acid-Base EquilibriaAcid-Base Equilibria

Strong acid - completely to product(s)

Weak - incomplete, gives an Equilibrium

direction favors reaction of stronger acid-base pair

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

ammoniumacetate ionpKa 4.76 pKa 9.24

What favors the direction of acid - base reactions?

4-4-1313Acid-Base EquilibriaAcid-Base Equilibria

acetic acid + sodium bicarbonate (omit Na+ )

6.36pKa= 4.76

CO2

H2O++CH3 C

O

O

C O-H

O

O

H

+CH3 CO

O H

C O-H

O

O

4-4-1414Acid-Base Theories

Brønsted-Lowry: Brønsted-Lowry: acidacid-proton donor, -proton donor, basebase-proton acceptor -proton acceptor

Lewis: Lewis: acidacid-accept pair electrons, -accept pair electrons, basebase-donate pair electrons-donate pair electrons

H3CC

CH3C H

H

H Br+

H3CC

CH3C H

H

Br+H

H3CC

CH3C

H

HH

Br

H3CC

CH3C H

H

Br+H

4-4-1515How does Structure effect Acidity?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 atomON larger atomRESONANCE delocalized STABILIZED by inductive effectIN an orbital with more s character

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

A:-A-H :B H-B+

4-4-1616

within a period - the greater the electronegativity of A:(-)

the more A:(-) is stablized

the stronger the acid

Conjugate baseAcid

electronegativityelectronegativity

pKa 38

pKa 16

pKa 51

incerasing acidity

CH3 OH3C O H

CH3 NH3C N HH H

CH3 CH3C C HH H

H H

4-4-1717

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

CH3 O

pKa 7.0 pKa 16

+ +H3C S H H3C O HCH3 S

4-4-1818

:B

C C

F

F

F

H

H

O

inductive effect, electron-withdrawing-covalent bonds transmit electronegativity (polarizing) effects -push or pull shared e(-)s of adjacent atoms

pKa: 15.9

C C

H

H

H

H

H

OH

C C

F

F

F

H

H

OH

12.4

C C

H

C

H

H

H

OH

FF

F

14.6

C C

H

C

H

H

H

OH

HC

H

F

FF

15.4

:B

C C

H

H

H

H

H

O

decreases w/ distance

4-4-1919

inductive effect: butanoic and chlorobutanoic acids

pKa 4.82 pKa 4.52

OH

O

OH

OCl

pKa 3.98

OH

OCl

pKa 2.83

OH

O

Cl

4-4-2020

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

HO-H HO–

Weak Acid

water

51

15.7

ConjugateBase pKa

HC C

H H

HH H

HC C

H HH H

(s-character)

(25%)

44H

C CH H

HC C

H H

H(33%)

25H C C H H C C (50%)

4-4-2222Conjugate Acids & BasesConjugate Acids & Bases

curved arrows show the flow of electrons in an acid-base reaction

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

ammoniumacetate ion

recall resonance - also uses curved arrows

4-4-2323

Resonance delocalized of charge on A:(-)

Compare alcohol and carboxylic acid acidity

resonance stabilization (of A:-)

+ pKa = 15.9C OC

H H

HH

HH

OH

H

+C OC

H H

HH

H OH

HH

+ pKa = 4.76OH

H

CH3 CO

OH

CH3 CO

O+ O

H

HH

CH3 CO

O

4-4-2424

+HSO4-C

OH3C

O

H

HORC

OH3C

OH

H

C

O+H3C

OH

H OSO3H

Use of resonance theoryUse of resonance theory

molecules may have 2 or more sites that can accept a H+

e.g. carboxylic acids, esters, and amidesprotonation favored where the charge is more delocalized

which oxygen is protonated?

4-4-2525

resonance

octets greater contribution even with plus charge on O

C

O+ H2SO4 +HSO4

-H3C

OH

C

O

H3C

OH

H

C

O

H3C

O

H

HOR

C

OH3C

OH

H

C

OH3C

OH

H

Use of resonance theoryUse of resonance theory

4-4-2626

C

O+ H2SO4 +HSO4

-H3C

OH

C

O

H3C

OH

H

C

O

H3C

O

H

HOR

C

OH3C

OH

H

C

OH3C

OH

H H+ on the hydroxyl

C

OH3C

O

H

H

can “write” contributing structures create & separate charge

H+ on the carbonyl

Use of resonance theoryUse of resonance theory