Unsaturation Test (Br₂ water, KMnO₄ test)

The bromine water test works through electrophilic addition. Bromine molecule (Br₂) is polarized when it approaches the electron-rich double bond. The π electrons attack Br₂, forming a bromonium ion intermediate. Bromide ion (Br⁻) then attacks from the opposite side, resulting in anti-addition product. For alkenes: R-CH=CH-R + Br₂ → R-CHBr-CHBr-R. For alkynes: R-C≡C-R + 2Br₂ → R-CBr₂-CBr₂-R (two molecules of Br₂ add). The red-brown color of bromine disappears, and the solution becomes colorless. Reaction occurs rapidly at room temperature without catalyst. Saturated compounds show no color change as they don't react with bromine under these conditions.

Baeyer's test uses cold, dilute alkaline KMnO₄ solution (Bayer's reagent). Unsaturated compounds are oxidized to glycols (1,2-diols) while KMnO₄ is reduced from Mn⁷⁺ (purple) to Mn⁴⁺ (brown MnO₂ precipitate). Reaction: 3R₂C=CR₂ + 2KMnO₄ + 4H₂O → 3R₂C(OH)-C(OH)R₂ + 2MnO₂ + 2KOH. The purple color disappears and brown precipitate forms. For terminal alkynes: R-C≡CH + 2KMnO₄ → R-COOH + CO₂ + 2MnO₂. The test must be performed cold to prevent further oxidation. Saturated compounds show no reaction. This test is more specific for unsaturation than bromine test as some compounds (phenols, anilines) also decolorize bromine water.

Bromine Water Test Procedure: Take 2 mL of organic compound (or dissolve in CCl₄), add bromine water dropwise with shaking. Positive test: Rapid decolorization of bromine. Negative test: Color persists. Baeyer's Test Procedure: Take 2 mL of compound, add 2% KMnO₄ solution dropwise with shaking. Positive test: Purple color disappears, brown precipitate forms. Negative test: Purple color persists. Both tests should be performed in well-ventilated areas. Bromine is toxic and corrosive - handle with care. Results should be observed immediately as some reactions are slow.

Alkenes: Both tests positive. Ethene: C₂H₄ + Br₂ → C₂H₄Br₂ (colorless). Alkynes: Both tests positive. Acetylene: C₂H₂ + 2Br₂ → C₂H₂Br₄ (colorless). Alkanes: Both tests negative. No reaction with bromine water or KMnO₄. Aromatic compounds: Benzene shows negative results with both tests under normal conditions due to resonance stability. However, with substitution (alkyl benzenes) or under UV light, bromination may occur. Cycloalkanes: Cyclohexane (saturated) - negative, Cyclohexene (unsaturated) - positive. Compounds with both types: Show positive tests due to unsaturated portions.

Bromine test limitations: Compounds that readily undergo substitution (phenols, anilines, enols) also decolorize bromine. Aldehydes and ketones may react slowly. Some unsaturated compounds with electron-withdrawing groups react slowly. Baeyer's test limitations: Compounds easily oxidized (aldehydes, formic acid, SO₂) give false positives. Aromatic aldehydes may react. The test doesn't work well in acidic medium. Combined interpretation: When both tests are positive - likely alkene/alkyne. When bromine test positive but Baeyer's negative - likely aromatic or compound undergoing substitution. When both negative - likely saturated compound. Always confirm with other tests when in doubt.

Unsaturation tests are crucial for: Organic compound identification - distinguishing between saturated and unsaturated hydrocarbons, Quality control - checking purity of commercial alkenes, Reaction monitoring - following hydrogenation or hydration reactions, Educational purposes - teaching organic chemistry principles, Industrial applications - quality assessment of petroleum fractions and polymers. These simple tests provide quick, inexpensive methods for preliminary analysis before more sophisticated techniques like IR spectroscopy or GC-MS. Understanding these tests is fundamental to organic chemistry laboratory work and helps build intuition about molecular structure and reactivity relationships.