The Reagent: Bromine water () is an orange-yellow aqueous solution used to detect carbon-carbon double bonds ().
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 ( acidified with ) acts as a strong oxidising agent.
Primary and Secondary Alcohols: These alcohols can be oxidised, causing the ions (orange) to be reduced to 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 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 ions (blue); when warmed with an aldehyde, the is reduced to , forming a brick-red precipitate of copper(I) oxide ().
Tollens' Reagent: Also known as ammoniacal silver nitrate, it contains ions. When warmed with an aldehyde, silver ions are reduced to metallic silver (), 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 () or sodium hydrogencarbonate () is added to the organic sample.
Positive Result: The observation of effervescence (bubbling) indicates the release of 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.
| Functional Group | Reagent | Positive Observation | Negative Observation |
|---|---|---|---|
| Alkene | Bromine Water | Decolourises (Orange to Colourless) | Remains Orange |
| Alcohol (1°/2°) | Acidified | Orange to Green | Remains Orange (3° Alcohol) |
| Aldehyde | Tollens' Reagent | Silver Mirror | Remains Clear (Ketone) |
| Carboxylic Acid | Sodium Carbonate | Effervescence () | No Bubbling |