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