Effect of Substituents & Qualitative Tests

Effect of Substituents & Qualitative Tests


• 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


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


• Structure of carboxylate ion – resonance hybrid

• Electron withdrawing substituents – increases acidity of carboxylic acid

• Electron releasing substituents – decreases acidity of carboxylic acid

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