90 total
By the end of these notes, you should be able to:
A functional group is a specific atom or group of atoms within an organic molecule that is responsible for the chemical reactions that molecule undergoes. Different functional groups react in different, predictable ways.
When a molecule contains more than one functional group, it can undergo more than one type of reaction — one for each functional group present.
The table below summarises the key functional groups, their identifying tests or reactions, and what you observe:
| Functional Group | Example | Identifying Reaction / Test | Observation |
|---|---|---|---|
| Alkene (C=C) | Ethene | Add bromine water (Br₂ in water) | Orange/brown bromine water is decolourised |
| Halogenoalkane (C–X, where X = Cl, Br, I) | Chloroethane | Add aqueous silver nitrate (AgNO₃) after heating with NaOH(aq) | White/cream/yellow precipitate of silver halide |
| Alcohol (–OH) | Ethanol | Add acidified potassium dichromate(VI) (K₂Cr₂O₇) | Orange solution turns green (if oxidation occurs); primary and secondary alcohols are oxidised, tertiary are not |
| Aldehyde (–CHO) | Ethanal | Add Tollens' reagent (ammoniacal silver nitrate) OR Fehling's solution | Tollens': silver mirror forms; Fehling's: brick-red precipitate |
| Ketone (C=O in chain) | Propanone | Tollens' or Fehling's — no reaction; use 2,4-DNPH | Orange/yellow precipitate with 2,4-DNPH confirms carbonyl (C=O) group |
| Carboxylic acid (–COOH) | Ethanoic acid | Add Na₂CO₃ or NaHCO₃ | Effervescence (bubbles of CO₂ gas) |
| Amine (–NH₂) | Ethylamine | Smell (fishy odour); turns damp red litmus blue; reacts with acid | Alkaline; forms a salt with acids |
| Amide (–CONH₂) | Ethanamide | Hydrolyse with hot aqueous acid or alkali | Produces carboxylic acid/carboxylate + ammonia or amine |
| Ester (–COO–) | Ethyl ethanoate | Distinctive fruity smell; hydrolyse with acid or alkali | Produces alcohol + carboxylic acid (or carboxylate salt) |
| Phenol (–OH attached to benzene ring) | Phenol | React with FeCl₃ solution | Violet/purple colour formed |
| Benzene ring (arene) | Benzene, phenol | React with Br₂ in presence of a halogen carrier catalyst (e.g. AlBr₃) | Electrophilic substitution; HBr gas produced |
Remember: When a molecule has multiple functional groups, each group reacts independently. You must consider all groups present and predict all the reactions that could occur.
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