Structure, functional derivatives and acidity of carboxylic group

Carboxylic Acids

Contents

• Carboxyl group

• Functional derivatives of carboxylic acids

• Acidity of carboxylic acid

Learning Objectives

At the end of this lecture, student will be able to

• Identify structure of carboxyl group

• Explain functional derivatives of carboxylic acid

• Explain acidity of carboxylic acids

Carboxylic Acids

• Carboxyl group → carbonyl group + hydroxyl group

• Functional derivatives of carboxylic acids

 

• Functional derivatives of carboxylic acids

Amide

Acid anhydrides

Ionisation of Carboxylic Acids

• Carboxylic acids in solution dissociate into carboxylate ion

• K_a - acidity constant

• Larger the ka → greater the ionization → stronger the acid

Acidity of Carboxylic Acids

• Carboxylic acids – more acidic than corresponding alcohols

• Example

• Resonance effect

Resonance stabilised acetate ion

• Equivalent resonance structures for acetate ion

• Greater resonance stabilization

• Reduces energy for ionisation

No resonance stabilization for alcohol and alkoxide ion

• Carboxylic acid yields resonance-stabilized anion, stronger acid than alcohol

Inductive effect

• Inductive effect of carbonyl group – responsible for acidity of carboxylic acids

• In both compounds - O—H bond - highly polarized by greater electronegativity of the oxygen atom

• Carbonyl group – more powerful electron attracting inductive effect than CH2 group

• Large resonance stabilization of carboxylate ion

• Acidity of a carboxylic acid - due to powerful resonance stabilization of its anion

• Stabilization and the resulting acidity possible because of the presence of the carbonyl group

Summary

• Carboxyl group – carbonyl and hydroxyl group

• Functional derivatives of carboxylic acid – acid halide, ester, amide, acid anhydride

• Acidity of carboxylic acid - acidity constant Ka