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Predicting the Strength of Acid / Base

Can we predict the relative basicity of two bases ?

The stronger the acid, the weaker will be its conjugate base.(The larger the pKaof the conjugate acid, the stronger the base)

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Predicting the Strength of Acid / Base

Can we predict the direction of equilibrium of the following reactions?

The stronger the acid, the weaker will be its conjugate base.(The larger the pKaof the conjugate acid, the stronger the base)

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Predicting the Strength of Acid / Base

The relationship between structure (size) and acidity

The stronger the acid, the weaker will be its conjugate base.

(The larger the pKaof the conjugate acid, the stronger the base)

The extent of electron delocalization (overall size) of the

corresponding conjugate bases may correlate with the acidity.

Cl-

Br-

F-

Stability Increases

I-

HO-

HS-

HSe-B

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Predicting the Strength of Acid / Base

The relationship between electronegativity and acidity

The stronger the acid, the weaker will be its conjugate base.

(The larger the pKaof the conjugate acid, the stronger the base)

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Predicting the Strength of Acid / Base

The relationship between structure / electronegativity and acidity

The stronger the acid, the weaker will be its conjugate base.

(The larger the pKaof the conjugate acid, the stronger the base)

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Predicting the Strength of Acid / Base

The relationship between structure / hybridization and acidity

The stronger the acid, the weaker will be its conjugate base.

(The larger the pKaof the conjugate acid, the stronger the base)

*The higher the s-character, the higher is the carbons electronegativity.

Acidity Increases

C C HH C C

H

H H

H

C C

H

HHH

HH

C CH C C

H

H

H

C C

H

HH

HH

Basicity Increases

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The Strength of Acid / Base: pKa

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The Strength of Acid / Base: Inductive Effect

Inductive Effect: propagation of bond polarity through -bondnetwork (decrease with the distance)

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The Strength of Acid / Base: Resonance Effect

endothermic

requires energyfor the change

Resonance Effect: stabilization of the system by having resonance

structures (increase with the # of equivalent resonance structures)

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pKavs . G

(Standard Free-Energy Change)

endothermic

requires energyfor the change

G = 2.303RT(log Ka)

= 2.303 (8.314x10-3 kj/molK)

x (273+25)K (log Ka)

= (5.7 kj/mol) x 4.75= 27.1 kj/mol

G= RTln Keq pKa= log Ka

Resonance Effect: stabilization of the system by having resonance

structures (increase with the # of equivalent resonance structures)

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Organic Compounds as Bases

O

H3C OH H Cl+

O

H3C OH

H

Cl-+

carboxylic and and alcohol as acids

carboxylic as a base

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The Strength of Acid / Base: pKa

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Mechanism for Organic Reactions

Step 1

Why acidbase? Many reactions in organic chemistry are:

either acidbase reaction themselvesor involves acidbase reaction at some stage

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Mechanism for Organic Reactions

Step 1

Step 2 Step 3

Why acidbase? Many reactions in organic chemistry are:

either acidbase reaction themselvesor involves acidbase reaction at some stage

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Acid / Base in Nonaqueous Solution

Leveling Effect: No stronger base than OH- can exist in water.

Synthesis of Tritium-labeled Compound