Introduction to Organic Chemistry
CollegeThe chemistry of carbon
Organic chemistry is the chemistry of carbon compounds. Carbon's power comes from four features: it forms four covalent bonds, bonds strongly to itself (catenation) into chains and rings, forms single, double and triple bonds, and combines with H, O, N, and halogens — yielding millions of stable compounds.
The simplest are hydrocarbons (C and H only):
| Family | Bonding | General formula | Example |
|---|---|---|---|
| Alkanes | All single (saturated) | CnH2n+2 | ethane C₂H₆ |
| Alkenes | One C=C double bond | CnH2n | ethene C₂H₄ |
| Alkynes | One C≡C triple bond | CnH2n−2 | ethyne C₂H₂ |
A functional group is an atom or group that gives a molecule characteristic reactions — e.g. hydroxyl (–OH, alcohols), carboxyl (–COOH, carboxylic acids), amino (–NH₂, amines). Molecules with the same molecular formula but different arrangements are isomers.
Families, homologous series and reactions
Organic chemistry is manageable because compounds fall into homologous series — families that share a functional group and a general formula, with each member differing from the next by a CH₂ unit. Members of a series show a smooth gradation in physical properties (boiling point rises with chain length) and, crucially, the same chemical reactions, because reactivity is dictated by the functional group, not the whole molecule. Learn the group, and you know the chemistry of the entire family.
Two reaction patterns are foundational. Saturated alkanes are relatively unreactive and undergo substitution (e.g. with halogens under UV light). Unsaturated alkenes, with their reactive C=C double bond, undergo addition — the double bond opens to take on a new atom or group across it. The bromine-water test exploits exactly this: alkenes decolourise orange bromine water by addition, alkanes do not, giving a simple lab test for unsaturation.
Worked example — combustion and an addition reaction
(a) Complete combustion of ethane. Balance C₂H₆ + O₂ → CO₂ + H₂O.
- Balance carbon: 2 C → 2 CO₂.
- Balance hydrogen: 6 H → 3 H₂O.
- Count oxygen on the right: 2(2) + 3(1) = 7 O atoms = 3.5 O₂.
- Clear the fraction by doubling every coefficient: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
(b) Addition of bromine to ethene. The C=C bond opens and a Br adds to each carbon:
- Identify the reactive site: the electron-rich C=C double bond of ethene.
- The double bond breaks open, freeing two bonding positions.
- One bromine atom bonds to each carbon; no atoms are lost — this is addition, not substitution.
- Orange bromine water is decolourised, confirming an alkene (a positive unsaturation test).
Draw and name the two structural isomers of C₄H₁₀.
Both have formula C₄H₁₀ but differ in carbon skeleton: butane (a straight chain, CH₃–CH₂–CH₂–CH₃) and 2-methylpropane / isobutane (a branched chain, a central CH with three CH₃ groups and one H). Same formula, different structure and slightly different properties.
How can you chemically distinguish ethane from ethene?
Add bromine water. Ethene (an alkene, C=C) decolourises the orange bromine water rapidly by addition. Ethane (a saturated alkane) does not react and the colour stays. Decolourisation is the classic test for unsaturation.
Why do alkanes undergo substitution but alkenes undergo addition?
Alkanes are saturated — every carbon already has four single bonds, so a new atom can only enter by replacing an existing atom (substitution). Alkenes have a reactive C=C double bond that can open up, letting atoms add across it without anything leaving (addition).
What functional group makes ethanol an alcohol, and what does it become on oxidation?
The hydroxyl group, –OH, defines alcohols. On mild oxidation ethanol (CH₃CH₂OH) is oxidised first to the aldehyde ethanal (CH₃CHO) and, on further oxidation, to the carboxylic acid ethanoic acid (CH₃COOH) — the acid in vinegar.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Write the balanced combustion of methane, CH4.
- Products of complete combustion are CO2 and H2O.
- Balance carbon and hydrogen, then oxygen.
- This gives CH4 + 2O2 -> CO2 + 2H2O.
Example 2What forms when ethene reacts with bromine?
- Ethene, C2H4, has a C=C double bond that opens up.
- Bromine (Br2) adds across the double bond in an addition reaction.
- Each carbon gains a Br, forming 1,2-dibromoethane.
Now you try
Work each one out first, then tap to reveal the worked answer.