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8/14/2019 Structural Effects and Acid-Base Property (ORG CHEM)
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Roldan M. de GuiaDepartment of BiochemistryFaculty of Pharmacy
University of
Santo Tomas
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electron delocalization
(Resonance)
Structural Effects
CH2 CH Cl..
..:
..
..:Cl
CO
NH2
NH2
..
..
..
..
..
..CH3 C
O
O H
: :
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electron delocalization (Resonance)Structural Effects
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electron delocalization (Resonance)Structural Effects
http://en.wikipedia.org/wiki/Image:Benz4.png8/14/2019 Structural Effects and Acid-Base Property (ORG CHEM)
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electron delocalization (Resonance)Structural Effects
OH
: :
::
C
O
H
NH2..
N
O O
+
-
: : :....
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HyperconjugationStructural Effects
C
H
H
H
CH CH2
CHH
CH CH2
..- -..
CH
CH CH2
H-..
CH CH CH2
HH
+
H
+
H+
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Hyperconjugation
Structural Effects
Dienes
conjugated
cumulative
isolated
CH2 CH CH CH2
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Hyperconjugation
Structural Effects
C
H
H H C
H
HH+
..-
C
H
HH+
-..
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Hyperconjugation
Structural Effects
C
H
H
H
C
O
H
CH
HH
CO
-H+
OO
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Inductive EffectStructural Effects
Electron attracting inductiveeffectwith excess positive charges
N+
RR
R
+NH3
+NO2
those with electronegative atoms
NH2 OH OCH3
F
Cl
Brgroups exhibiting orbital electronegativity
C N N N
those with easily polarizable valence electrons
I-
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Inductive EffectStructural Effects
Electron attracting inductiveeffect
H3C Cl
+
+
CH3 CH2 Cl
++
+
CH3 CH2 OH +
N+
H3C CH3
CH3
--
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Inductive EffectStructural Effects
Electron repelling inductiveeffectalkyl groups
CH3
negative groups
COO- S- O-
H3C C
O
O
--
H3C S-
CH
H3C
H3C
C N
R C
O
O
C
O
O
O SR
O
O
O
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Steric EffectStructural Effects
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Hydrogen BondingStructural Effects
+F H
O H
N H
+
+
F
O
N
H
H
H
+
+
+
OH
H
HO
H
HO
R
O
H
R
R C
O
O H
H O
C
O
R
C O
H
O H
O
F
H
OHOH
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Acids and Bases
BrnstedLowry Acids and Bases
An acid is a proton donor
A base is a proton acceptor
acid base
H2O H3OHBr Br + +
NH3 H2O+ NH4 + OHacidbase
Note that water can act as an acidora base
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Acids and Bases
BrnstedLowry Acids and Bases
acid base
H2O H3OHBr Br + +
conjugate
acid
conjugate
base
Every acidbase reaction involving proton transfer
has two conjugate acidbase pairs.
HBr Br+ +O
H H
O
H
HH
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Acids and Bases
Lewis Acids and Bases
A Lewis acid is an electron pairacceptor.(think empty orbital)
(think filled orbital)
A Lewis base is an electron pairdonor.
The result of a Lewis acidbase reaction is often
called an adduct.
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Acids and Bases
Lewis Acids and Bases
+
O
H HH
O
HH
H
Lewis acid Lewis base Adduct
Electrondeficient
Electronrich
New covalentbond
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Acids and Bases
Lewis Acids and Bases
Examples of Lewis Acids:
Fe3+ AlCl3 H3O+Examples ofLewis Bases:
NH3
H2
O Cl
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Acids and Bases
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Acids and Bases
pH = log[H3O+]
HA + H2O A H3O+
Ka = AH
3O
HA
pKa = log (Ka)
id d
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Acids and Bases
Acid Strength
HA + H2O H3O+ + A-
HA
A-
A-
Has a strong conj. base( higher energy)
WEAK ACID
STRONG ACID
E
N
E
R
GY
ionization
easier
Has a weak conj. base
( lower energy)
A id d
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Acids and Bases
A id d B
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Acids and Bases
Acid StrengthFactors that influence stability of the
conjugate base include:
Resonance
Electronegativity
Atomic Size
Hybridization
Inductive Effects
A id d B
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Acids and Bases
Resonance Effects
18
10
5
45
30
25
20
9
28
25
15
pKa Valuesincreasing quality
of 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
A id d B
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Acids and BasesThe 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)
A id d B
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Acids and Bases
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
A id d B
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Acids and Bases
ElectronegativitypKaValuesincreasingelectronegativity
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
A id d B
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Acids and Bases
F
Cl
I
Br
ElectronegativitypKaValues
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
1.81
1.95
2.16
A id d B
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Acids and Bases
Inductive Effects
Electron-withdrawing Groups
F, Cl, Br, O, N R, CH3, B, Si
electronegative elementspull electron density
away from carbon
alkyl groups and elementsless electronegative than carbon
push electron density toward carbon
Electron-donating Groups
Cl C
+CH3 C
+
A id d B
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Acids and Bases
Inductive Effects
Chlorine helps to stabilizeCO2
by withdrawing
electronsCl C
O
O
+
This effect diminishes with distanceit extends for about 3
bonds
C C
O
CCl
+++ O
A id d B
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Acids and Bases
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
Electrophiles
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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+BF
3
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
-
RCO
O-
CN-
NH2-
N3-
O-
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