Acids and Bases;

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Chapter 7 Acids and Bases Intermolecular
Attractions
2
Problems for Chapter 7
Skip section 7.4, 7.10, and 7.14 Read (no
lecture) Section 7.11, 7.12 and 7.13 In Text 1
a, b, g, h, 2b, 6b, 8a, 11b, c 14, 15, 18
through 22 24, 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
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Section 7.1 Bronsted-Lowry Theory
conjugate base
conjugate acid
base
acid
HA B- A- BH
-H
H
acid proton donor
base proton acceptor
Conjugate Acid Base Proton Conjugate Base
Acid - Proton
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Section 7.2 pKa
pKa - log Ka Strong acid large Ka
small pKa Weak acid small Ka large pKa
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Sect. 7.2 Relative Acid and Base Strengths
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Acid Strengths
Strong Acid Conjugate base is weak pKa is
small Weak Acid Conjugate base is strong
pKa is large
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Section 7.3 Lewis Acid-Base theory
more general than Bronsted theory
BASE
B
electron-pair donor
A
ACID
electron-pair acceptor
Fe3 BF3 H3O
Some Lewis Acids
NH3 H2O
Some Lewis Bases
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Section 7.5 Electronegativity and Size Effects
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Sect. 7.5 Electronegativity and Size
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Effect of Atomic Size on Acidity
increasing atom size
pKa Values
3.5
16
5
-7
7
-9
4
-10
3
1.36 A
1.81 A
1.95 A
2.16 A
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Effect of Electronegativity on Acidity
increasing electronegativity
pKa Values
gt45
45
20
34
35
15
16
18
5
3.5
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Section 7.6 Resonance Effects
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Take Away LessionResonance strengthens
acidResonance weakens base
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Resonance in the Acetate Ion
equivalent structures charge on oxygens
O
C
H
C
3
O

O
-H
_
C
H
C
O
H
3
base
_
acetic acid
O
C
H
C
3
O
acetate ion
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Phenolate ion Resonance
_
-
non-equivalent structures charge on carbon and
oxygen
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Nitrophenols
Placing a nitro group on the benzene ring of a
phenol increases its acidity.
The effect is largest when the nitro group is
placed in an ortho or a para position on the
ring, and considerably smaller for the meta
position.
Multiple nitro groups at the ortho and
para positions can increase the pKa of a
phenol to the point that it becomes a very strong
acid.
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pKa Values of Nitrophenols
7.2
10
0.4
9.3
4.0
7.3
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Resonance in p-Nitrophenol
_
When in an ortho or para position a nitro group
can participate in resonance.

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Section 7.7 Alpha Hydrogen compounds
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(No Transcript)
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Take Away LessionResonance strengthens
acidResonance weakens base
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Section 7.8 Inductive Effects
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Resonance operates through the p bonding system.
It doesnt drop off with distance. Inductive
effect operates through the sigma bonding system.
It drops off with increasing distance.
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Types of Inductive Effects
ELECTRON WITHDRAWING GROUPS
ELECTRON DONATING GROUPS
d-
d
CH3
C
R, CH3
F, Cl, Br, NO2 , NR3
electronegative elements take electron
density from cabon. This strenthens the acidity.
alkyl groups donate electron density to carbon.
This weakens the acidity.
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Inductive Effects on Haloacids
O
d
d-
C
Cl
O
O
d-
d
d-
d
d-
d
Cl
C
C
C
O
The effect diminishes with distance - it
carries for about 3 bonds.
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Inductive Effects
increasing electronegativity
multiple substituents
pKa Values
3.13
4.75
2.81
2.87
2.81
1.29
2.66
0.65
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Inductive Effects
pKa Values
4.8
3.75
4.5
4.75
4.0
4.87
4.81
2.9
distance
5.02
When the chlorine atom is moved further away
from the carboxyl group, acidity decreases
Alkyl groups release electrons. This decreases
acidity
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Section 7.9 Hybridization Effects
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Effect of Hybridization
pKa values
sp3
ca. 50
sp orbitals are more electron withdrawing than
sp3 orbitals (sp orbitals have more s
character). sp gt sp2 gt sp3
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sp2
sp
25
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Section 7.11 Solvent Effects
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Solvation Effects
4.75
C
O
O
H
C
H
3
5.02
4.87
C
O
O
H
C
H
C
H
3
2
4.81
C
O
O
H
C
H
C
H
C
H
3
2
2
Notice that these are all similar
.but this one has a larger pKa
This is probably a solvation effect. Solvation
lowers the energy of the ion.
steric hindrance
Stronger Acid
The bulky t -butyl group is not as well solvated.
Weaker Acid
unbranched
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• Sect. 7.12 Intermolecular Attractions
• hydrogen bonding
• dipole-dipole attractions
• London forces or van der Waals attractions
• Sect. 7.13 Solubility

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SUMMARY
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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
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Classification of Weak and Strong Acids
by Functional Group
weak acids
strong acids
40 20 10
5 0
pKa
di- and tri-
nitrophenols
inorganic acids oxyacids
carboxylic acids nitrophenols
alkanes
phenols
alcohols ketones amides alkynes
alkenes amines
b-diketones
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Electron-Withdrawing Effects Strengthen Acids
(-)
(-)
-
-
-
-
-
Any effect that bleeds electron density away
from the negatively-charged end of the conjugate
base will stabilize (lower the energy) of the
conjugate base and make it a weaker base. The
parent acid will be a stronger acid.
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Conversely ..
Electron-Donating Effects Weaken Acids
-
(-)
(-)
-
-
-
-
-
Any effect that pushes extra electron density
toward the negatively-charged end of the
conjugate base will destabilize (increase the
energy) of the conjugate base and make the base
weaker. The parent acid will be stronger.