Organic Chemistry – Formulae, Functional Groups & Terminology
What You’ll Learn
By the end of this lesson you’ll be able to draw any organic molecule from its formula, identify all atoms and bonds, and recognise common functional groups.
Think of a molecule as a city map – every atom is a building, every bond is a road, and functional groups are the city’s landmarks.
Key Terms
- Atom – the basic unit of a chemical element.
- Bond – the connection between two atoms (single, double, triple).
- Skeletal (Line‑Angle) Formula – a simplified way of drawing organic molecules where carbon atoms are at the vertices or ends of lines.
- Functional Group – a specific arrangement of atoms that determines a molecule’s chemical behaviour.
- Isomer – compounds with the same formula but different arrangements of atoms.
Drawing Skeletal Formulae
- Draw a backbone of carbon atoms using straight lines.
- Each line end or vertex represents a carbon atom unless otherwise specified.
- Attach heteroatoms (O, N, S, halogens) directly to the backbone.
- Show hydrogen atoms explicitly only when they are attached to heteroatoms or when the molecule is small.
- Use double/triple bonds by drawing two or three parallel lines.
⚗️ Analogy: Imagine drawing a road network – each intersection is a carbon, roads are bonds, and special buildings (like hospitals) are heteroatoms.
Common Functional Groups
| Functional Group | Representative Formula | Typical Bonding |
|---|
| Alcohol | \$R{-}OH\$ | C–O single bond, O–H |
| Ketone | \$R{-}C(=O)-R'\$ | C=O double bond |
| Aldehyde | \$R{-}C(=O)-H\$ | C=O double bond, terminal H |
| Carboxylic Acid | \$R{-}C(=O)-OH\$ | C=O double bond, C–O single bond |
| Amine | \$R{-}NH_2\$ | C–N single bond, N–H bonds |
| Ether | \$R{-}O{-}R'\$ | C–O single bond, O–C single bond |
Example: Ethanol (\$C2H5OH\$)
Step‑by‑step drawing:
- Start with a straight line of two carbon atoms: C–C.
- Add the hydroxyl group to the second carbon: C–C–OH.
- Show the hydrogen atoms on the first carbon (three H) and the second carbon (two H) if you wish to be explicit.
Resulting skeletal formula: CH₃–CH₂–OH (hydrogens omitted for carbon atoms).
Exam Tip Box
🔍 Always check:
- All heteroatoms are shown.
- Each carbon has a total of four bonds (including hydrogens).
- Double/triple bonds are clearly indicated.
- Functional groups are identified correctly – they’re the “landmarks” of the molecule.
💡 Tip: If you’re unsure, draw a quick Lewis structure first to confirm valence.
Practice Questions
- Draw the skeletal formula for 2‑butanone (\$CH3COCH2CH_3\$) and label the functional group.
- Identify the functional group in the following skeletal formula: CH₃–CH₂–CH₂–OH.
- Convert the following skeletal formula to a full structural formula, showing all hydrogens: CH₃–CH₂–CH₂–CH₃.
- Write the formula for an ether with two methyl groups and a central oxygen.
Quick Reference Cheat Sheet
| Group | Bond Pattern | Common Example |
|---|
| Alcohol | C–O–H | Ethanol, Methanol |
| Aldehyde | C=O–H | Formaldehyde, Acetaldehyde |
| Ketone | C=O–C | Acetone, Butanone |
| Carboxylic Acid | C=O–OH | Acetic acid, Benzoic acid |
| Amine | C–N–H | Methylamine, Ethylamine |
| Ether | C–O–C | Dimethyl ether, Diethyl ether |
🧪 Remember: Practice drawing a variety of molecules. The more you practise, the faster you’ll recognise functional groups and draw accurate structures.