Identification of Functional Groups by Test-Tube Reactions

• Alkenes ($C=C$): The presence of a carbon-carbon double bond is confirmed by shaking the sample with bromine water. The orange-brown solution will decolorize (turn colorless) as the bromine adds across the double bond in an electrophilic addition reaction.

• Halogenoalkanes ($R-X$): These are identified by first warming the sample with aqueous sodium hydroxide ($NaOH$) to release halide ions through nucleophilic substitution. The mixture is then acidified with dilute nitric acid ($HNO_3$) before adding silver nitrate ($AgNO_3$).

• Halide Observations: The color of the resulting silver halide precipitate identifies the specific halogen: Silver Chloride ($AgCl$) is a white precipitate, Silver Bromide ($AgBr$) is cream, and Silver Iodide ($AgI$) is yellow.

• Alcohols ($R-OH$): Primary and secondary alcohols are identified using acidified potassium dichromate ($K_2Cr_2O_7$). The solution changes from orange to green as the chromium(VI) ions are reduced to chromium(III) ions during the oxidation of the alcohol.

• Aldehydes ($R-CHO$): These are distinguished from ketones using mild oxidizing agents like Tollen's reagent or Fehling's solution. Aldehydes are easily oxidized to carboxylic acids, while ketones are not.

• Tollen's Reagent: Often called the 'silver mirror test,' a positive result for an aldehyde produces a coating of metallic silver on the inside of the test tube.

• Fehling's Solution: A positive result involves the blue copper(II) solution forming a brick-red precipitate of copper(I) oxide ($Cu_2O$) upon warming with an aldehyde.

Carboxylic acids ($R-COOH$) are identified by their reaction with carbonates or hydrogen carbonates, such as sodium hydrogen carbonate ($NaHCO_3$).

The reaction produces a salt, water, and carbon dioxide gas ($CO_2$). The observation of immediate effervescence (bubbling) is the definitive positive result for an acid.

To confirm the gas is $CO_2$, it can be bubbled through limewater, which will turn cloudy as calcium carbonate forms.

• Tertiary Alcohols: It is critical to note that tertiary alcohols do not react with acidified potassium dichromate because they lack a hydrogen atom on the carbon bonded to the $-OH$ group, preventing oxidation.

• Aldehydes vs. Ketones: While both contain a carbonyl group ($C=O$), only aldehydes react with Tollen's or Fehling's reagents. This is a primary method for distinguishing between these two isomers.