A functional group is a small cluster of atoms attached to a carbon backbone, and it, not the backbone, decides how the molecule reacts. Six groups cover most of what a rebuilding community handles: hydroxyl (alcohols and sugars), carboxyl (vinegar and fatty acids), amine (proteins and many dyes), ester (fats, waxes and fragrances), carbonyl (acetone and aldehydes) and ether (solvents). Identify the group and you can predict whether the compound is acidic, basic, volatile or reactive.
Part of Organic Chemistry
The reactive atom clusters on carbon chains that determine acidity, basicity, solubility and reactivity of organic compounds.
Carbon Chains Are Inert; The Group Does The Work
The carrier’s table lists six groups with a representative substance and one property each. The table below keeps the carrier’s entries and adds the practical consequence for a workshop.
| Group | Formula | Where it appears | What it means in practice |
|---|---|---|---|
| Hydroxyl, -OH | R-OH | Alcohols, sugars | Dissolves in water; can be oxidised toward an acid, which is how ethanol becomes vinegar (see Vinegar Production) |
| Carboxyl, -COOH | R-COOH | Vinegar, fatty acids | Acidic; reacts with bases to make salts, which is the chemistry of soap |
| Amino, -NH2 | R-NH2 | Proteins, dyes | Basic; reacts with acids, the property used in alkaloid chemistry |
| Ester, -COO- | R-COO-R’ | Fats, fragrances, waxes | Often fragrant; made from an acid plus an alcohol |
| Carbonyl, C=O | R-CO-R’ | Acetone, aldehydes | Reactive and often volatile |
| Ether, -O- | R-O-R’ | Diethyl ether, anisole | Good solvents, relatively unreactive, and in the case of diethyl ether highly flammable |
Reading A Molecule By Its Group
Look for the group first and the chain second. The same chain with a different group is a different material: a long chain with a carboxyl group is a fatty acid, and the same chain joined as an ester to glycerol is a fat. Key Reactions is largely a list of what happens when two groups meet: carboxyl plus hydroxyl gives an ester (Esterification), and an ester plus a strong alkali gives a soap salt and glycerol, the reaction covered in Saponification.
Method: Naming The Group In An Unknown Property List
Step 1: Test acidity. With a cabbage-juice indicator, a liquid turning red points to a carboxyl group; one turning green or blue points to an amine or an alkali.
Step 2: Test water solubility. A small molecule with a hydroxyl or carboxyl group usually mixes with water; a long bare chain does not.
Step 3: Smell with care. A sweet, fruity smell suggests an ester. Waft; do not inhale from the container.
Step 4: Record it in condensed notation (see Structural Formulas).
Worked Example: Reading Aspirin’s Parent, Salicylic Acid
Salicylic acid, the compound behind Aspirin from Willow, carries two groups on a ring: a hydroxyl and a carboxyl. The carboxyl makes it acidic and the hydroxyl adds polarity, so it dissolves in alcohol. That is the prediction available from the groups alone, with no measurement.
Common Mistakes
- Calling every -OH compound “alcohol”. Sugars and acids can contain hydroxyl groups too. The group name describes the group, not the whole molecule.
- Treating the carrier’s amine row as “all plant medicines”. Amine groups are common in alkaloids but not universal in medicines; salicin, which the carrier lists with the alkaloids, is not one.
Safety
The group predicts the hazard class, not the dose
Ethers are the lightest solvents and the most flammable; carboxylic acids burn tissue when concentrated; amines can be caustic and toxic. None of this tells you a safe quantity. For any substance handled as a drug see Dosage and Safety.
What’s Next
- Learn next: Naming Conventions — how the group is encoded in a compound’s name
- Combine with: Esterification — the group reaction you can perform with vinegar and alcohol