Chemistry · Ch 12 — Carbonyl Compounds and Carboxylic Acids
Chemical Properties of Carboxylic Acids
Chemical Properties of Carboxylic Acids
Because the carboxyl group's resonance delocalisation makes it a poor electrophile for simple nucleophilic addition (Section 12.9), a carboxylic acid does NOT give the characteristic addition reactions of an ordinary carbonyl group the way aldehydes and ketones do. Instead, its reactions are organised into four distinct classes, based on WHICH bond of the -C(=O)-O-H system is broken: (A) reactions involving cleavage of the O-H bond (acting as a Bronsted acid); (B) reactions involving cleavage of the C-OH bond (the hydroxyl behaving like that of an alcohol, replaceable by another nucleophile); (C) reactions involving the -COOH group as a whole (reduction, decarboxylation, and amide/anhydride formation); and (D) substitutio …
Reducing Action of Formic Acid and Tests for the Carboxylic Acid Group
Formic acid (HCOOH) is structurally unique among carboxylic acids: it contains BOTH an aldehyde-type C-H directly on the carboxyl carbon AND the acid group itself, so, like any true aldehyde, it is easily oxidised and consequently acts as a genuine REDUCING agent -- a property no other carboxylic acid shares, since every other acid's carboxyl carbon has no such C-H to donate. (i) Formic acid reduces Tollens' reagent (ammoniacal AgNO3) to metallic silver: HCOO- + 2 Ag+ + 3 OH- gives 2 Ag (the mirror) + CO3(2-) + 2 H2O. (ii) Formic acid reduces Fehling's solution, turning the blue Cu2+ solution to a red precipitate of cuprous oxide: HCOO- + 2 Cu2+ + 5 OH- gives Cu2O (red) + CO3(2-) + 3 H2O -- exactly the answer to Question 12, distinguishing formic acid from acetic acid (which, lacking that aldehydic C-H, gives neither test) or benzophenone (a ketone, which cannot reduce Tollens' reagent under any circumstance). Separately, three simple qualitative TESTS confirm the -COOH group generally, in any carboxylic acid: (i) in aqueous solution, a carboxylic acid turns blue litmus red; (ii) a carboxylic acid gives brisk effervescence with sodium bicarbonate, f …