Qualitative tests for identifying carboxylic acids, amides, and esters involve distinct reactions that capitalize on the unique reactivity of each functional group.
Below, we delve into the tests for each group, explaining the chemistry behind them and how they're conducted.
Carboxylic Acids
Litmus Test
Principle: Carboxylic acids are acidic and can donate a proton (H⁺) to blue litmus, turning it red.
Procedure: Apply a drop of the test substance on blue litmus paper. A red coloration indicates the presence of an acidic compound, such as a carboxylic acid.
Sodium Bicarbonate Test
Principle: Carboxylic acids react with sodium bicarbonate (NaHCO₃) to produce carbon dioxide (CO₂), water (H₂O), and a sodium salt of the carboxylic acid. The evolution of CO₂ gas is an observable and distinct indication of a carboxylic acid.
Procedure: Add a small amount of the test substance to an aqueous solution of NaHCO₃. Effervescence due to CO₂ evolution indicates a positive test.
Reaction: RCOOH + NaHCO₃ ⟶ RCOO⁻Na⁺ + H₂O + CO₂↑
Amides
Hydrolysis Test
Principle: Amides can be hydrolyzed to carboxylic acids and amines (or their salts) upon heating with dilute acids or alkalis. The resulting carboxylic acid can then be identified using carboxylic acid tests.
Acidic Hydrolysis Procedure: Heat the amide with dilute HCl. Test the reaction mixture for the presence of carboxylic acid using the litmus test or the sodium bicarbonate test.
Basic Hydrolysis Procedure: Heat the amide with NaOH solution. Test the alkaline mixture for the presence of an amine (by its smell or with nitrous acid) and for the carboxylate ion with the sodium bicarbonate test indirectly.
Reactions: Acidic: RCONH₂ + H₂O + H⁺ ⟶ RCOOH + NH₄⁺ Basic: RCONH₂ + OH⁻ ⟶ RCOO⁻ + NH₃
Hofmann Degradation (Ammonia Release Test)
Principle: Primary amides react with bromine in an alkaline solution to produce a primary amine with one less carbon atom, releasing ammonia in the process. The presence of ammonia turns moist red litmus paper blue.
Procedure: Treat the amide with bromine water and NaOH. Place red litmus paper above the reaction container. A change to blue indicates ammonia formation.
Reaction: RCONH₂ + 4Br₂ + 4NaOH ⟶ RNH₂ + 2Na₂CO₃ + 4NaBr + 6H₂O
Esters
Hydrolysis Test
Principle: Esters react with water in the presence of an acid or base to yield carboxylic acids and alcohols. The carboxylic acid formed can be identified using the tests for carboxylic acids.
Acidic Hydrolysis Procedure: Heat the ester with dilute HCl. Test for the presence of a carboxylic acid as described above.
Basic Hydrolysis (Saponification) Procedure: Heat the ester with NaOH solution. The resulting carboxylate can be tested with the addition of dilute acid to precipitate the carboxylic acid.
Reactions: Acidic: RCOOR' + H₂O + H⁺ ⟶ RCOOH + R'OH Basic: RCOOR' + OH⁻ ⟶ RCOO⁻ + R'OH
Smell Test
Principle: Many esters have characteristic fruity Odors. While not definitive, this property can serve as an initial indicator of their presence.
Procedure: Simply smell the test substance cautiously. Note that this is subjective and should be followed by more concrete tests.