Chapter 7Chapter 7Acids and Bases; Acids and Bases; Intermolecular AttractionsIntermolecular Attractions

Skip section 7.4, 7.10, and 7.14Read (no lecture) Section 7.11, 7.12 and 7.13In Text: 1 a, b, g, h, 2b, 6b, 8a, 11b, c

14, 15, 18 through 2224, 25, 27, 28, 31, 32, 33, 34

End of Chap: 36, 38, 42, 43, 45 a, b, e, 48 through 52 54, 55, 56, 58, 59,

62, 63

Problems for Chapter 7

HA + B- A- + BHacid base

conjugate base

conjugate acid

Section 7.1Section 7.1Bronsted-Lowry TheoryBronsted-Lowry Theory

+H+

-H+

acid = proton donorbase = proton acceptorConjugate Acid = Base + ProtonConjugate Base = Acid - Proton

pKa = - log Kaa

Strong acid = large Ka = small pKaa

Weak acid = Weak acid = small Ka = large pKaa

Section 7.2 pKa

HClO4 ClO4

_stronger

weaker

ACIDSTRENGTH

weaker

stronger

BASESTRENGTH

ACID CONJ. BASE Ka pKa

10 -1010

10 -5

10-10

10 50-50

5

-

10

10 16-16

OH O

CH3 C OH

O

CH3 C O

O

CH3 CH3 CH3 CH2

_

CH3CH2O-H CH3CH2O

Sect. 7.2 Relative Acid and Base Strengths

Strong Acid Conjugate base is weak pKa is small

Weak Acid Conjugate base is strong pKa is large

Acid Strengths

Section 7.3: Lewis Acid-Base theory

B: electron-pair donor

BASE

A ACIDelectron-pair acceptor

Some Lewis Acids: Fe3+ BF3 H3O+

Some Lewis Bases: NH3 H2O

more general than Bronsted theory

Section 7.5Section 7.5Electronegativity Electronegativity and Size Effects and Size Effects

C N O F

P S Cl

Br

I

Se

Si

electronegativity increases acidityincreased

increased

size

increasesacidity

Sect. 7.5: Electronegativity and Size

Effect of Atomic Size on AcidityEffect of Atomic Size on Acidity

R C

O

OH

R C SH

O

R C SH

S

HOH

HSH

HSeH

HTeH

HF

HCl

HBr

HI

increasingatom size pKa Values

3.5

-7

-9

-10

16

7

4

3

5

F- Cl- Br- I-

1.36 A 1.81 A 1.95 A 2.16 A

CH4

NH3

H2O

HF

RCH3

RNH2

ROH

R C

O

CH3

R C NH2

O

R C OH

O

>45

34

16

3.5

45

35

18

20

15

5

Effect of Electronegativity on AcidityEffect of Electronegativity on Acidityincreasing electronegativity pKa Values

CH

H

H

H N

H

H O F_ _ _ _

Section 7.6 Section 7.6 Resonance EffectsResonance Effects

Take Away Lession:

Resonance strengthens acidResonance weakens base

CH3 C O

O

H-H+

CH3 C

O

O

CH3 CO

O

base

_

_

acetate ion

Resonance in the Acetate Resonance in the Acetate IonIon

acetic acid

equivalent structurescharge on oxygens

Phenolate ion ResonancePhenolate ion Resonance

O

-

_

_

_

_

_

O O

OOO

non-equivalent structurescharge on carbon and oxygen

NitrophenolsNitrophenols

Placing a nitro group on the benzene ringof a phenol increases its acidity.

The effect is largest when the nitro groupis placed in an ortho or a para position onthe ring, and considerably smaller for themeta position.

Multiple nitro groups at the ortho and parapositions can increase the pKa of a phenolto the point that it becomes a very strong acid.

OH

O2N

NO2OH

NO2

OH

NO2

OH

NO2

OH OH

NO2O2N

NO2

10

7.2

7.3

0.4

pKa Values of NitrophenolspKa Values of Nitrophenols

9.3 4.0

_Resonance in Resonance in pp--NitrophenolNitrophenol

_

_

_

_

O

NO2

O

NO2

O

NO O

O

NO2

O

NO2

O

NO O

_

_ _+

extrastructure

When in an orthoor para positiona nitro group canparticipate inresonance.

+

Section 7.7Section 7.7Alpha Hydrogen Alpha Hydrogen compoundscompounds

Acid pKa

CH3CH2CH2CH2-H

H

H

H

H

R H

O

OCH2CH3H

O

CH3H

Acid pKa

O

OCH2CH3CH3CH2O

O

H

~50

44

25

24

20

13

O

CH3H3C

O

H

9

10H-CH2NO2CNH 25

CNNC11

H

NO2O2N4

H

Take Away Lession:

Resonance strengthens acidResonance weakens base

Section 7.8Section 7.8Inductive EffectsInductive Effects

Resonance operates through the bonding system. It doesn’t drop off with distance.

Inductive effect operates through the sigma bonding system. It drops off with increasing distance.

Cl-

C CH3

-

C

ELECTRONWITHDRAWINGGROUPS

ELECTRONDONATINGGROUPS

F, Cl, Br, NO2 , NR3+ R, CH3

Types of Inductive Types of Inductive EffectsEffects

electronegative elements take electron densityfrom cabon. This strenthens the acidity.

alkyl groups donate electron density to carbon. This weakensthe acidity.

Cl C C C-

- + - +

O

O

Inductive Effects on Inductive Effects on HaloacidsHaloacids

The effect diminishes with distance - it carries for about 3 bonds.

Cl C

O-Chlorine helps

to stabilize -CO2-

by withdrawingelectrons

O

Inductive Effects Inductive Effects

I CH2COOH

Br CH2COOH

Cl CH2COOH

F CH2COOH

CH3 COOH

Cl CH2 COOH

Cl CH COOH

Cl

Cl C COOH

Cl

Cl

3.13

2.87

2.81

2.66

4.75

2.81

1.29

0.65

pKa Valuesincreasingelectronegativity

multiplesubstituents

CH3CH2CH2 COOH

COOHCH2CH

Cl

CH3

COOHCH

Cl

CH3CH2

COOHCH2CH2CH2

Cl

COOHH

COOHCH3

COOHCH3CH2

COOHCH3CH2CH2

COOHCCH3

CH3

CH3

Inductive EffectsInductive Effects

pKa Values

3.75

4.75

4.87

4.81

5.02

4.8

4.5

4.0

2.9

distance

Alkyl groups release electrons.This decreases acidity

When the chlorine atom is moved further away from the carboxyl group,acidity decreases

Section 7.9Section 7.9Hybridization EffectsHybridization Effects

Effect of HybridizationEffect of Hybridization

C C HH

C C HH

H H

C HH

H

H

sp3

sp2

sp

ca. 50

35

25

sp orbitals are more electron withdrawing than sp3 orbitals (sp orbitals have more s character). sp > sp2 > sp3

pKa values

Section 7.11Section 7.11Solvent Effects Solvent Effects

CO

O-

H

OH

H

OH

H

OHH

O H

H

O H

Solvation EffectsSolvation Effects

4.75

4.87

4.81

Notice that these are all similar

COOHCH3

COOHCH3CH2

COOHCH3CH2CH2

COOHCCH3

CH3

CH3

5.02

….but this one has a larger pKa

This is probably a solvation effect. Solvation lowers the energy of the ion.

The bulkyt -butyl groupis not as wellsolvated.

StrongerAcid

WeakerAcid

unbranched

steric hindrance

CO

O

-H

OH

H

OH

H

OHO

H

H

H

O H

Sect. 7.12: Intermolecular Attractions•hydrogen bonding•dipole-dipole attractions•London forces or van der Waals attractions

Sect. 7.13: Solubility

SUMMARYSUMMARY

Electronegativity of atom bearing acidic hydrogen; more electronegative = more acidic

Size of atom bearing acidic hydrogen; larger = more acidic.

Resonance: greater charge delocalization in conjugate base = more acidic.

alpha-hydrogens: carbonyls, nitro, cyano, etc. Greater charge delocalization in conjugate base = more acidic.

Inductive effect: electronegative atoms withdraw electrons making the acid more acidic.

Hybridization; more s-character = more acidic

40 20 10 5 0

HCl

HBr

HIR CH3 C C

H

H

H

R

R NH2

H2SO4

HClO4

HNO3

weak acids strong acids

Classification of Weak and Strong Acids Classification of Weak and Strong Acids by Functional Groupby Functional Group

pKa

inorganic acidsoxyacids

carboxylic acidsnitrophenols

phenols-diketones

alcoholsketonesamidesalkynes

alkenesamines

alkanes

C C HR

OH

R

O

CH2 R

OR C

O

OH

nitrophenolsdi- and tri-R C CH3

O

R C NH2

OR OH

Electron-Withdrawing Effects Electron-Withdrawing Effects Strengthen AcidsStrengthen Acids

R C

O

O

C

O

O

O R S

O

O

O

Any effect that “bleeds” electron density away from the negatively-charged end of the conjugate base willstabilize (lower the energy) of the conjugate base and make it a weaker base.The parent acid will be a stronger acid.

- -

- -

-

(-) (-)

Electron-Donating Effects Electron-Donating Effects Weaken AcidsWeaken Acids

R C

O

O

C

O

O

O R S

O

O

O

Any effect that “pushes” extra electron density toward the negatively-charged end of the conjugate base willdestabilize (increase the energy) of the conjugate baseand make the base weaker. The parent acid will be stronger.

- -

- --

-

(-) (-)

Conversely …..