Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
Can we predict the relative basicity of two bases ?
Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
Can we predict the direction of equilibrium of the following reactions?
Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
The relationship between structure (size) and acidity
FHO-
Cl-
s
e
s
a
re
c
In
y
it
il
b
ta
S
Br-
B
a
HS-
I-
HSe-
The extent of electron delocalization (overall size) of the
corresponding conjugate bases may correlate with the acidity.
Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
The relationship between electronegativity and acidity
Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
The relationship between structure / electronegativity and acidity
Predicting the Strength of Acid / Base
The stronger the acid, the weaker will be its conjugate base.
(The larger the pKa of the conjugate acid, the stronger the base)
The relationship between structure / hybridization and acidity
*The higher the s-character, the higher is the carbon’s electronegativity.
H
H
C C
H C C H
H
H
H
H
H
H
C C
H
H
H
H
Acidity Increases
H
H C C
C C
H
H
H
Basicity Increases
C C
H
H
The Strength of Acid / Base: pKa
The Strength of Acid / Base: Inductive Effect
Inductive Effect: propagation of bond polarity through σ-bond
network (decrease with the distance)
The Strength of Acid / Base: Resonance Effect
requires energy
for the change
endothermic
Resonance Effect: stabilization of the system by having resonance
structures (increase with the # of equivalent resonance structures)
pKa vs. ΔG° (Standard Free-Energy Change)
requires energy
for the change
endothermic
ΔG° = −RT ln Keq
pKa = −log Ka
ΔG° = 2.303RT (−log Ka)
= 2.303 (8.314x10-3 kj/mol°K)
x (273+25)°K (−log Ka)
= (5.7 kj/mol) x 4.75
= 27.1 kj/mol
Resonance Effect: stabilization of the system by having resonance
structures (increase with the # of equivalent resonance structures)
Organic Compounds as Bases
carboxylic and and alcohol as acids
carboxylic as a base
H
O
O
H
H3C
O
+
H
Cl
H
H3C
O
+
Cl-
The Strength of Acid / Base: pKa
Mechanism for Organic Reactions
Why acid–base?
Step 1
Many reactions in organic chemistry are:
either acid–base reaction themselves
or involves acid–base reaction at some stage
Mechanism for Organic Reactions
Why acid–base?
Many reactions in organic chemistry are:
either acid–base reaction themselves
or involves acid–base reaction at some stage
Step 1
Step 2
Step 3
Acid / Base in Nonaqueous Solution
Leveling Effect: No stronger base than OH- can exist in water.
Synthesis of Tritium-labeled Compound