( iv) Substitution reactions in the hydrocarbon part α-hydrogen atoms in carboxylic acids are acidic in nature and can be easily replaced by halogen atoms in HVZ reaction
The reaction is known as Hell-Volhard-Zelinsky reaction.
( v) Arndt-Eistert reaction It is method of converting lower carboxylic acids to their higher homologues
( vi) Reducing property Among carboxylic acids, formic acid is the only acid that acts as reducing agent. It reduces, acidified KMnO4 to MnSO4,
HgCl2 to Hg, Tollen ‟ s reagent to silver mirror and Fehling ‟ s solution to red ppt. and itself gets oxidised to CO2 and H2O.
HCOOH + HgCl2→ Hg + 2HCI + CO2
( vii) Electrophilic substitution reactions of aromatic acids-COOH group shows-R effect, therefore, acts as a deactivating and meta-directing group. Carboxylic acids do not undergo Friedel-Craft ‟ s reaction because the carboxylic group IS deactivating and the catalyst
AlCl3( anhy.) gets bonded to the carboxyl group.