Qualitative Tests for Functional Groups

Study Notes

Saturated hydrocarbons (alkanes) contain only single bonds, while unsaturated hydrocarbons (alkenes and alkynes) have double or triple carbon-carbon bonds.
Two qualitative tests for unsaturation:
- Bromine Test: Orange-red color of bromine solution disappears in the presence of unsaturated compounds.
- Baeyer’s Test (Alkaline KMnO4 Test): Pink color of KMnO4 fades when reacting with unsaturated hydrocarbons, possibly forming brown MnO2 precipitate.

Alcohols contain a hydroxyl group (-OH) attached to an aliphatic carbon; categorized as mono (one -OH), di (two -OH), or trihydric (three -OH).
Alcohols can be primary (1°), secondary (2°), or tertiary (3°) based on the carbon to which the -OH is bonded.
Sodium Metal Test: Alcohols react with sodium, releasing hydrogen gas (effervescence).
Ester Test: Alcohols react with carboxylic acids to form esters, with sulfuric acid as a catalyst.
Ceric Ammonium Nitrate Test: Alcohols react with ceric ammonium nitrate, yielding red alkoxy cerium (IV) compound.
Acetyl Chloride Test: Alcohols form esters with acetyl chloride and release HCl gas, producing white ammonium chloride fumes with ammonium hydroxide.
Iodoform Test: Positive for acetaldehyde, methyl ketones, and alcohols with CH3-CH-OH group, indicated by yellow precipitate of iodoform (CHI3).

Phenols contain -OH group attached to an aromatic ring, often colorless but can turn colored upon oxidation.
Examples include o-cresol, m-cresol, p-cresol, quinol, and catechol.
Litmus Test: Phenol, being a weak acid, turns blue litmus paper red.
Ferric Chloride Test: Reaction with ferric ions produces a violet complex.
Liebermann’s Test: Treatment with sodium nitrite and concentrated sulfuric acid yields deep blue/green, switching to red or brown with water; blue/green can regenerate with strong alkali.
Phthalein Dye Test: Heating phenol with phthalic anhydride and sulfuric acid forms colorless phenolphthalein, which turns pink in alkali.

Aldehydes and ketones are characterized by a carbonyl group (C=O).
Aldehydes have the carbonyl carbon bonded to at least one hydrogen; formaldehyde has two hydrogens. Ketones have the carbonyl carbon bonded to two carbon groups.
2,4-Dinitrophenyl Hydrazine Test (2,4-DNP Test): Detects carbonyl groups; a positive result yields yellow/orange-red precipitate.
Sodium Bisulfite Test: Aldehydes and ketones form white crystalline addition products with sodium bisulfite.
Differentiating tests for aldehydes:
- Schiff’s Test: Produces pink or magenta with Schiff’s reagent.
- Tollen’s Test: Aldehydes reduce Tollen’s reagent to silver mirror effect in reaction vessel.
- Fehling’s Test: Cu(II) ions in Fehling’s solution are reduced to red cuprous oxide precipitate by aldehydes.
Differentiating tests for ketones:
- m-Dinitrobenzene Test: Ketones show violet coloration.
- Sodium Nitroprusside Test: Ketonate anions form red colored complex with nitroprusside.

Carboxylic acids contain a carboxyl functional group, categorized as aliphatic (water-soluble) or aromatic (sparingly soluble).
Formic and acetic acids are simplest aliphatic acids; benzoic acid is the simplest aromatic acid.
Litmus Test: Carboxylic acid turns blue litmus red due to higher acidity than alcohols.
Sodium Hydrogen Carbonate Test: Brisk effervescence indicates carbon dioxide production when reacting with sodium hydrogen carbonate.
Ester Test: Carboxylic acid and alcohol react (with sulfuric acid catalyst) to form fruity-smelling esters.

Amines are ammonia derivatives with one or more hydrogen atoms replaced by alkyl or aryl groups.
A primary amine (RNH2) is formed when one hydrogen is replaced by an alkyl or aryl group.