Acid-Base-Strengths - acids and base strength practice PDF

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acids and base strength practice...

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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

Acidity Increases H

H H C C

C C H

H

H H

Basicity Increases

H C C

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. Δ G° (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 H3 C

O

+

H

Cl

H H3 C

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...