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Acids andBases

Lewis Acids and Bases

+

O

H HH

O

HH

H

Lewisacid

Lewisbase

Adduct

Electrondeficient

Electronrich

Newcovalent

bond

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Acids andBases

Lewis Acids and Bases

Examples of Lewis Acids:

Fe3+ AlCl3H3O+Examples ofLewis Bases:

NH3 H2O Cl

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Acids andBases

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Acids andBases

pH =

log[H3O+

]HA + H2O A H3O+

Ka = A H3O

HA

pKa = log (Ka)

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Acids andBases

Acid Strength

HA + H2O H3O+ + A-

HA

A-

A-

Has a strongconj. base(higher energy)

WEAK ACID

STRONG ACID

E

N

E

R

GY

ionization

easier

Has a weak conj.

base(lower energy)

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Acids andBases

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Acids andBases

Acid Strength Factors that influence stability

of the conjugate base include:

Resonance

Electronegativity

Atomic Size

Hybridization

Inductive Effects

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Acids andBasesResonance Effects

18

10

5

45

30

25

20

9

28

25

15

pKa Valuesncreasing qualityf resonance

R OH

OH

R C

O

OH

R CH3

CH3

CH3O C

O

CH3

R C

O

CH3

R C

O

CH2 C R

O

R NH2

NH2

R C

O

NH2

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Acids andBases The Acetate Ion

acetate ion

acetic acid

CH3 C OH

O

-H+

Base

CH3 C O

O

CH3 C O

O

Resonance StabilizedEquivalent structures (charges on oxygens)

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Acids andBases

O

-

More resonance structures, but not more stable than acetateNonequivalent structures (note charges on carbon and oxygen)

O O O O O

The Phenolate Ion

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Acids andBases

ElectronegativitypKaValuesincreasingelectronegativity

C

O

CH3R20

15

5

C

O

NH2R

C

O

OHR

CH4

NH3

H2O

HF

>45

3416

3.5

RCH3

RNH2

ROH

45

35

18

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Acids andBases

F

Cl

I

Br

ElectronegativitypKaValues

increasing

sizeHF

HCl

HBr

HI

3.5

7

9

10

H2O

H2S

H2Se

H2Te

16

7

4

3

C

O

OHR

C

O

SHR

C

S

SHR

1.36

1.81

1.95

2.16

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Acids andBasesInductive Effects

Electron-withdrawing Groups

F, Cl, Br, O, N R, CH3, B, Si

electronegative elementspull electron density awayfrom carbon

alkyl groups and elementsless electronegative than carbonpush electron density toward carbon

Electron-donating Groups

Cl C

+CH3 C

+

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Acids andBasesInductive Effects

Chlorine helps to stabilize CO2

by withdrawing

electrons Cl C

O

O

+

This effect diminishes with distanceit extends for about 3 bonds

C C

O

CCl +++

O

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Acids andBases

Inductive Effects

3.13

2.87

2.81

2.66

4.75

2.81

1.29

0.65

pKa Values

increasing

electronegativity

increasin

gsubstitution

I CH2COOH

Br CH2COOH

Cl CH2COOH

F CH2COOH

CH3 COOH

CH2 COOHCl

CH COOHCl

Cl

C COOHCl

Cl

Cl

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Electrophiles

e- pair acceptor

electron poor

may possess formal positive charge

incomplete octets

greatly reduced e- density

those that can yield e- poor species

R

C+

R

R

R.

NO2+CH2:

R C+

O

CO2

H2O

H+ BF3

FeBr3

AlCl3X2 in protic solvent

X.

R C

O

O OH

SO3

SO3H+

X+

CH3 AlCl4

+

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Nucleophiles

e- pair donor

excess negative charges

fractional negative charges

may have a lone pair e-

R

CR

R

: -

OH-

RO-

X-

RC

O

O-

CN-

NH2

-

N3-

O-

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