Effect of Substituents & Qualitative Tests

Effect of Substituents & Qualitative Tests

Contents

• Resonance stabilized acetate ion

• Effect of substituents

Learning Objectives

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

• Identify structure of carboxyl group

• Explain effect of substituent on acidity

Structure of Carboxylate Ions

• Carboxylate ion – resonance hybrid of 2 structures

• Evidence – bond length measurements

• Example – formic acid and sodium formate

• Carboxylate ion. – Orbital picture

• Overlap of p orbitals    in both directions - delocalization of π electrons and dispersal of charge

Effect of Substituents on Acidity

• Factors that increase the stability of carboxylate ion increase the acidic strength of acid.

• Electron withdrawing substituents like halo groups – increase acidity

• Electron releasing substituents like alkyl groups– decrease acidity

• Electronegativity and electron withdrawing power of the halogen substituent and strength of the acid decreases in the order

FCH₂COOH   >   ClCH₂COOH   >   Br   CH₂COOH   >   ICH₂COOH   > CH₃COOH

• Number of electron withdrawing chloro group and acid strength decreases in the order

Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH

• Inductive effect & acid strength decreases rapidly with increasing distance from the carboxylic group

Summary

• Structure of carboxylate ion – resonance hybrid

• Electron withdrawing substituents – increases acidity of carboxylic acid

• Electron releasing substituents – decreases acidity of carboxylic acid