What this chapter is about
This chapter introduces you to the foundations of organic chemistry, the branch that studies carbon-containing compounds. Carbon forms more compounds than any other element because of its unique ability to bond with itself and other atoms in chains, rings and complex three-dimensional shapes. You will learn why carbon is special, how organic compounds are classified, and the rules for naming them systematically using IUPAC nomenclature.
The chapter also covers practical laboratory techniques used to purify and identify organic compounds, such as distillation, crystallisation, sublimation and chromatography. These skills let chemists isolate pure substances from natural sources or reaction mixtures. Understanding purification is essential before you can study reactions in later chapters.
By the end, you should be able to write structural formulas, assign IUPAC names to simple organic molecules, recognise functional groups, and describe how to separate a mixture of organic compounds in the laboratory.
Key ideas
- Tetravalency of carbon: Carbon has four valence electrons and forms four covalent bonds, allowing it to create stable chains, branched structures and rings.
- Catenation: Carbon atoms bond strongly with each other, forming long chains and closed rings; this property explains the vast number of organic compounds.
- Structural representation: Organic molecules can be shown as complete structural formulas, condensed formulas (CH₃–CH₂–OH) or bond-line formulas where each vertex represents a carbon atom.
- Functional groups: Atoms or groups of atoms that give characteristic chemical properties, such as –OH (alcohol), –CHO (aldehyde), –COOH (carboxylic acid), C=C (alkene) and –NH₂ (amine).
- Homologous series: A family of compounds with the same functional group and general formula, differing by a –CH₂– unit, showing gradual change in physical properties.
- IUPAC nomenclature: A systematic method for naming compounds using a root word (number of carbons), a suffix (functional group) and prefixes (substituents with their positions).
- Isomerism: Compounds with the same molecular formula but different structural arrangements; chain isomers differ in carbon skeleton, position isomers differ in the location of a functional group.
- Purification techniques: Distillation separates liquids by boiling-point differences; crystallisation purifies solids by dissolving and cooling; chromatography separates components based on differential adsorption.
Formulas and facts to remember
1. General formula of alkanes: CₙH₂ₙ₊₂ (saturated hydrocarbons with only single bonds).
2. General formula of alkenes: CₙH₂ₙ (one carbon–carbon double bond).
3. General formula of alkynes: CₙH₂ₙ₋₂ (one carbon–carbon triple bond).
4. Degree of unsaturation (DBE): (2C + 2 + N − H − X) / 2, where C = carbons, N = nitrogens, H = hydrogens, X = halogens; counts rings plus multiple bonds.
5. Rf value in chromatography: Rf = (distance moved by substance) / (distance moved by solvent front); a characteristic ratio used to identify compounds.
6. Boiling-point trend: In a homologous series, boiling point rises by roughly 20–30 °C for each added –CH₂– unit because of increasing molecular mass and surface area.
7. Electronegativity order affecting polarity: F > O > N > C; bonds to more electronegative atoms are polar and influence solubility and reactivity.
8. Inductive effect: Electron-withdrawing groups (–NO₂, –Cl) pull electron density through sigma bonds; electron-donating groups (–CH₃) push electron density; both affect acidity and basicity.
Worked examples
### Example 1 – Writing the IUPAC name
Problem: Name the compound CH₃–CH(CH₃)–CH₂–CH₂–OH.
Solution: 1. Identify the longest carbon chain containing the functional group –OH. The chain has five carbons: C1 bears –OH, then C2, C3, C4, C5. 2. Number from the end nearest the –OH group. Counting from the –OH end: position 1 has –OH, position 4 has a –CH₃ branch. 3. Root word for five carbons is pent-; suffix for alcohol is -ol. 4. Name: 4-methylpentan-1-ol.
### Example 2 – Calculating degree of unsaturation
Problem: A compound has molecular formula C₆H₁₀. Find its degree of unsaturation and suggest possible structures.
Solution: DBE = (2 × 6 + 2 − 10) / 2 = (12 + 2 − 10) / 2 = 4 / 2 = 2.
The molecule has two degrees of unsaturation. Possible structures:
- Two double bonds: CH₂=CH–CH=CH–CH₂–CH₃ (hexa-1,3-diene, rearranged).
- One triple bond: CH≡C–CH₂–CH₂–CH₂–CH₃ (hex-1-yne).
- One double bond plus one ring: cyclohexene (six-membered ring with one C=C).
### Example 3 – Choosing a purification method
Problem: How would you purify a small amount of naphthalene contaminated with common salt?
Solution: Naphthalene is a white solid that sublimes on gentle heating; common salt does not sublime. 1. Place the impure mixture in a china dish and cover with an inverted glass funnel. 2. Heat gently; naphthalene vapour rises and condenses on the cooler funnel surface. 3. Collect the pure naphthalene crystals from the funnel; salt remains in the dish.
This technique, sublimation, works because the two substances differ greatly in volatility.
Common mistakes
- Writing –OH at any position and forgetting to number from the end nearest the principal functional group → always number the chain so the principal group gets the lowest locant.
- Confusing molecular formula with structural formula and assuming one molecular formula gives one compound → remember that isomers share the same molecular formula but have different structures.
- Using common names such as acetone or acetic acid in IUPAC answers → use systematic names (propan-2-one, ethanoic acid).
- Thinking chromatography always uses paper → chromatography includes thin-layer (TLC), column and gas chromatography, each suited to different mixtures.
- Ignoring the inductive effect when comparing acidities → electron-withdrawing groups stabilise the conjugate base and increase acidity; electron-donating groups do the opposite.
Quick revision
1. Carbon's tetravalency and catenation explain the enormous variety of organic compounds.
2. IUPAC names follow the pattern: locant + prefix + root + suffix (e.g., 2-methylpropan-1-ol).
3. Functional groups determine chemical behaviour; the carbon skeleton determines physical properties.
4. Degree of unsaturation tells you how many rings or multiple bonds to look for.
5. Distillation separates by boiling point; crystallisation purifies solids; sublimation separates volatile solids; chromatography separates by differential adsorption.
6. Rf value in chromatography is a ratio of distances and helps identify substances.