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Brønsted–Lowry Acids and BasesAn acid is a proton donorA base is a proton acceptor
acid base
H2O H3OHBr Br+ +
NH3 H2O+ NH4 + OHacidbase
Note that water can act as an acid or a base
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Brønsted–Lowry Acids and Bases
acid base
H2O H3OHBr Br+ +
The remaining species after the proton has been donated is the conjugate base.
conjugate acid
conjugate base
Every acid–base reaction involving proton transfer
has two conjugate acid–base pairs.
HBr Br+ +OH H
O
HHH
The resulting species after the proton has been accepted is the conjugate acid.
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Copyright © 2010 Pearson Education, Inc.
pH
The concentration of hydrogen ions is used as a measure of acidity
This concentration is expressed as pH
pH = – log[H3O+]
The higher the concentration, the more acidic the solution and the lower the pH
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pH
Neutral water: [H3O+] = 1.0 × 10–7 M
pH = – log[H3O+] = 7
pH < 7.00 Acidic solution
pH = 7.00 Neutral solution
pH > 7.00 Basic solution
H2O + H2O OH H3O+
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The Acidity Constant, Ka
The strength of an acid is represented by its ionization constant (acidity constant), Ka
Ka=product of concentrations of ionized species
concentration of intact acid
HA + H2O A H3O+
Ka =A H3O
HA
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The Acidity Constant, Ka
The Ka implies the concentrations of the acid and the ions
Ka > 1 Ionized products greater than intact acid.
Ka < 1 Ionized products less than intact acid.
Ka >> 1 Ionization goes to completion (strong acid).(e.g., > 103)
Ka << 1 Ionization does not occur to an appreciable amount. (e.g., < 10–3)
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pKa = – log (Ka)
The Acidity Constant, Ka
Since the Ka values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use
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Compare pKa and Ka Values
pKa 14121086420
strong acids weak acids
Ka10-1410-1010-610-2
The smaller the value of the pKa
the stronger the acid.
-2
102
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Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Acid Strength
HA + H2O H3O+ + A-
HA
A-
A-
Has a strong conj. base(higher energy)
WEAK ACID
STRONG ACID
ENERGY
ionizationeasier
The difference between a strong acid and a weak acid can be described by the stability of the conjugate base.
Has a weak conj. base(lower energy)
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Acid StrengthA more stable conjugate base means a
stronger acid.
HA
stabilization
ENERGY
A-
A-
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Acid StrengthFactors that influence stability of the
conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects
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Acid StrengthFactors that influence stability of the
conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects
Copyright © 2010 Pearson Education, Inc.
Acid StrengthFactors that influence stability of the
conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects
Copyright © 2010 Pearson Education, Inc.
Acid StrengthFactors that influence stability of the
conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects
Copyright © 2010 Pearson Education, Inc.
Acid StrengthFactors that influence stability of the
conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects
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Resonance Effects
More or better resonance structures of the conjugate base lead to a stronger acid.
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Resonance Effects
18
10
5
45
30
25
20
9
28
25
15
pKa Valuesincreasing qualityof 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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Resonance EffectsThe Acetate Ion
acetate ion
acetic acid
CH3 C OH
O-H+
Base
CH3 C O
O
CH3 C O
O
Resonance Stabilized Equivalent structures (charges on oxygens)
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Resonance Effects
O
-
More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen)
O O O O O
The Phenolate Ion
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Electronegativity
Placing the negative charge on a more electronegative element (from the same period) in the conjugate base leads to a stronger acid.
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ElectronegativitypKa Values
increasing electronegativity
C
O
CH3R 20
15
5
C
O
NH2R
C
O
OHR
CH4
NH3
H2O
HF
>45
34
16
3.5
RCH3
RNH2
ROH
45
35
18
Consider the conjugate bases
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ElectronegativitypKa Values
Consider the conjugate bases
increasing electronegativity
C
O
CH3R 20
15
5
C
O
NH2R
C
O
OHR
C
H
H
H
NH
H
OH
F
CH4
NH3
H2O
HF
> 45
34
16
3.5
RCH3
RNH2
ROH
45
35
18
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Atomic Size
Placing the negative charge on a larger atom (from the same group) in the conjugate base leads to a stronger acid.
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Atomic SizepKa Values
increasing size
H2O
H2S
H2Se
H2Te
16
7
4
3
Consider the ionic radii
C
O
OHR
C
O
SHR
C
S
SHR
HF
HCl
HBr
HI
3.5
– 7
– 9
– 10
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F–
Cl–
I–
Br–
ElectronegativitypKa Values
Consider the ionic radii
increasingsize
HF
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 Å
1.81 Å
1.95 Å
2.16 Å
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Hybridization
• More s character in the orbital bearing the negative charge in the conjugate base leads to a stronger acid.
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Hybridization
sp3
sp2
sp
> 45
35
25
As electrons in hybrid orbitals become closer to the nucleus, they are lower in energy
-1.74
-7
:
:
:pKa
pKa
C
H
H
H
H
C CH
H
H
H
C CH H
C
C
C
O
H
HH
CR
RO H
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Inductive EffectsElectron-withdrawing effects due to
differences in electronegativity pull electron density away from the negatively charged end of the conjugate base, lowering the energy and stabilizing the conjugate base, making the acid stronger.
R CO
OC
O
OO SR
O
O
O
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Inductive EffectsElectron-donating effects due to
differences in electronegativity push electron density toward the negatively charged end of the conjugate base, increasing the energy and destabilizing the conjugate base, making the acid weaker.
R CO
OC
O
OO SR
O
O
O
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Inductive EffectsElectron-withdrawing Groups
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
Remember, the electron-withdrawing and -donating groups work throughthe bond system, while resonance groups work through the system.
Electron-donating Groups
Cl C
CH3 C
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Inductive EffectsChlorine helps to stabilize – CO2
– by withdrawing electrons
Cl CO
O
This effect diminishes with distance—it extends for about 3 bonds
C CO
CCl O
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Inductive Effects
3.13
2.87
2.81
2.66
4.75
2.81
1.29
0.65
pKa Values
increasing electronegativity
increasing substitution
I CH2COOH
Br CH2COOH
Cl CH2COOH
F CH2COOH
CH3 COOH
CH2 COOHCl
CH COOHCl
Cl
C COOHCl
Cl
Cl
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Inductive Effects
2.814.75pKa:
Increasing substitution
C
O
OHCH3 C
O
OHCH2Cl C
O
OHCCl
Cl
Cl 0.65