Strc Effects& Acid Base Prop Part 2

Acids and Bases

Lewis Acids and Bases H

+

Lewis acid Electron deficient

H

O

O

H

Lewis base Electron rich

H

H

H

Adduct New covalent bond

Acids and Bases

Lewis Acids and Bases Examples of Lewis Acids: Fe3+ AlCl3 H3O+ Examples of Lewis Bases: NH3 H2O

Cl

−

Acids and Bases

• pH = – log[H3O+]

A + H3O

HA + H2O

Ka =

Acids and Bases

A

H3O

HA

pKa = – log (Ka)

Acid Strength HA + H2O

H3O+ + -

E N E R G Y

HA

Acids and Bases A-

WEAK ACID A Has a strong conj. base (∴higher energy) - STRONG ACID A Has a weak conj. ionization base easier (∴lower energy)

Acids and Bases

Acid Strength

Acids and Bases

• Factors that influence stability of the conjugate base include: – – – – –

Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Acids and Bases

Resonance Effects

increasing quality of resonance

R OH

pKa Values

18

OH

5

45

CH3

30

CH3O C CH3 O

25

10

O R C OH

R CH3

R NH2 NH2

O

R C CH3 O

25

O 20

O

R C CH2 C R

28

9

R C NH2

15

• The Acetate Ion

O

O CH3

Acids and Bases

C OH

-H+

CH3

C O

Base

acetic acid

O CH3

C O

acetate ion Resonance Stabilized Equivalent structures (charges on oxygens)

Acids and Bases

• The Phenolate Ion O

O

O

O

O

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

O

Acids and Bases

Electronegativity

increasing electronegativity

pKa Values

CH4

>45

RCH3

45

NH3

34

RNH2

35

H2O

16

ROH

18

HF

3.5

O R C CH3 O R C NH2 O R C OH

20

15

5

increasing size

Acids and ElectronegativityBases pKa Values

HF

3.5

HCl

–7

F–

1.36 Å

H2O

16

1.81 Å

H2S

7

H2Se

4

Cl– HBr

1.95 Å

–9

H2Te

Br– HI

2.16 Å

–10 I–

3

O R C OH O R C SH S R C SH

Acids and Bases

Inductive Effects Electron-withdrawing Groups

δ− Cl

δ+ C

Electron-donating Groups

δ+ CH3

δ− C

F, Cl, Br, O, N

R, CH3, B, Si

electronegative elements pull electron density away from carbon

alkyl groups and elements less electronegative than carbon push electron density toward carbon

Acids and Bases

Inductive Effects Chlorine helps to stabilize – CO2– by withdrawing electrons

δ− Cl

δ+O C O

This effect diminishes with distance—it extends for about 3 bonds

δ − δ + δ−δ+ δ− δ+ Cl C C C

O O

Acids and Bases

Inductive Effects

increasing electronegativity

I Br Cl F

CH2COOH CH2COOH CH2COOH CH2COOH

pKa Values 3.13

2.87

2.81

CH3 Cl

COOH

CH2

COOH

Cl Cl

CH

COOH

increasin g substitut ion 4.75

2.81

1.29

Cl 2.66

Cl

C Cl

COOH

0.65

Electrophiles

 e- pair acceptor

R

 electron poor  may possess formal positive charge

R

 incomplete octets  greatly reduced e- density  those that can yield e- poor species +

X2 in protic solvent X

R

O C

O

OH

SO3 +

SO3H

δ+ CH3

R C+

AlCl3 δ−

AlCl4

R.

X.

: CH2

NO2+

CO2

R C+ O

H2O

H+

BF3 FeBr3

Nucleophiles

 e- pair donor  excess negative charges

OH-

 fractional negative charges

RO-

 may have a lone pair e-

R R C: R

NH2-

O R

C

-

O

CN-

X-

N3O-