Testing for Organic Functional Groups

The Reagent: Bromine water ($Br_2(aq)$) is an orange-yellow aqueous solution used to detect carbon-carbon double bonds ($C=C$).

The Reaction: When added to an alkene, an electrophilic addition reaction occurs where the bromine atoms add across the double bond to form a colourless dibromoalkane.

Positive Result: A positive test is indicated by the rapid decolourisation of the orange solution upon shaking with the unknown sample.

Distinction: Saturated compounds like alkanes do not react with bromine water under standard conditions, meaning the solution will remain orange.

The Reagent: Acidified potassium dichromate ($K_2Cr_2O_7$ acidified with $H_2SO_4$) acts as a strong oxidising agent.

Primary and Secondary Alcohols: These alcohols can be oxidised, causing the $Cr_2O_7^{2-}$ ions (orange) to be reduced to $Cr^{3+}$ ions (green).

Tertiary Alcohols: These lack a hydrogen atom on the carbon bonded to the hydroxyl group, making them resistant to oxidation; thus, the solution remains orange.

Methodology: The reaction requires gentle heating, typically in a water bath at approximately $60$ degrees Celsius to ensure safety with flammable organic liquids.

Aldehydes vs. Ketones: Aldehydes can be easily oxidised to carboxylic acids, whereas ketones cannot, providing a basis for differentiation.

Fehling's Solution: This alkaline solution contains $Cu^{2+}$ ions (blue); when warmed with an aldehyde, the $Cu^{2+}$ is reduced to $Cu^+$, forming a brick-red precipitate of copper(I) oxide ($Cu_2O$).

Tollens' Reagent: Also known as ammoniacal silver nitrate, it contains $[Ag(NH_3)_2]^+$ ions. When warmed with an aldehyde, silver ions are reduced to metallic silver ($Ag$), coating the inside of the test tube in a 'silver mirror'.

Ketone Result: Ketones yield a negative result for both tests, meaning Fehling's remains blue and Tollens' remains a clear solution.

The Principle: Carboxylic acids are weak acids (typically pH 3) that react with carbonates to produce a salt, water, and carbon dioxide gas.

The Reagent: Sodium carbonate ($Na_2CO_3$) or sodium hydrogencarbonate ($NaHCO_3$) is added to the organic sample.

Positive Result: The observation of effervescence (bubbling) indicates the release of $CO_2$ gas. This gas can be confirmed by bubbling it through limewater, which will turn cloudy.

Specificity: This test distinguishes carboxylic acids from other organic compounds like alcohols or phenols, which are not acidic enough to react with carbonates.