The full, readable lecture — why carbon is unique, the functional groups, the homologous series, structural and stereo isomerism, and how organic elements are detected. As you scroll, the panel on the right tells each idea through an everyday picture: LEGO bricks, a family queue, twins, your own two hands.
1 — Why carbon is unique
Organic chemistry is the study of carbon compounds (except a few like CO, CO₂, carbonates and cyanides). Carbon forms an enormous number of compounds — over ten million — because of a unique set of properties.
- Tetravalency — carbon has 4 valence electrons (2s²2p²) and forms four strong covalent bonds, like a LEGO brick with four studs ready to click.
- Catenation — carbon's ability to bond to other carbon atoms, building long straight chains, branched chains and rings. C–C bonds are strong (~347 kJ/mol), so the skeletons are stable.
- Multiple bonding — carbon forms single (C–C), double (C=C) and triple (C≡C) bonds; sp³, sp² and sp hybridisation give 109.5°, 120° and 180° shapes.
- Small size — carbon is a small atom, so its bonds are short and strong and it can form stable π-bonds (unlike heavier Group-14 elements).
On the right, watch four-studded bricks snap together — chains, a branch, then a ring. That single clicking talent is why life and almost every material around you is built from carbon.
2 — Functional groups
A functional group is an atom or group of atoms that gives a molecule its characteristic chemical properties. Compounds with the same group react in similar ways — it is the key that opens one particular lock, so the group decides the family, the prefix and the suffix.
| Class | Functional group | Suffix | Example |
|---|
| Alkene | >C=C< | -ene | ethene |
| Alcohol | –OH | -ol | ethanol |
| Aldehyde | –CHO | -al | ethanal |
| Ketone | >C=O | -one | propanone |
| Carboxylic acid | –COOH | -oic acid | ethanoic acid |
| Amine | –NH₂ | -amine | methanamine |
When several groups are present, one is the principal group (named by suffix) and the rest become prefixes. Priority (high→low): –COOH > –CHO > >C=O > –OH > –NH₂.
3 — Homologous series
- Homologous series — a family of compounds with the same functional group and general formula, in which each member differs from the next by a single –CH₂– unit.
Characteristics
- Members share the same general formula and functional group.
- Consecutive members differ by CH₂ (14 mass units) — like each cousin in the queue standing one head taller.
- Physical properties change gradually (b.p. rises with chain length); chemistry stays similar.
| Series | General formula | First members |
|---|
| Alkanes | CₙH₂ₙ₊₂ | CH₄, C₂H₆, C₃H₈ |
| Alkenes | CₙH₂ₙ | C₂H₄, C₃H₆ |
| Alcohols | CₙH₂ₙ₊₁OH | CH₃OH, C₂H₅OH |
4 — Isomerism: structural
- Isomers — compounds with the same molecular formula but a different arrangement of atoms, and therefore different properties — twins built from one identical kit of parts.
Structural (constitutional) isomerism — the atoms are connected in a different order. The main kinds:
| Type | Differs in… | Example (same formula) |
|---|
| Chain | branching of the skeleton | n-butane vs isobutane (C₄H₁₀) |
| Position | position of the group | propan-1-ol vs propan-2-ol (C₃H₈O) |
| Functional | the functional group itself | ethanol vs dimethyl ether (C₂H₆O) |
🌍 Real-world: glucose and fructose share C₆H₁₂O₆ but glucose has a –CHO and fructose a >C=O — functional isomers your body tastes and uses differently.
5 — Isomerism: optical (chirality)
Stereoisomers have the same connectivity but a different arrangement in space. The deepest example is chirality — handedness.
- Chiral carbon — a carbon bonded to four different groups. Its molecule is non-superimposable on its mirror image, exactly like your left and right hands.
- Enantiomers — the two mirror-image forms. They rotate plane-polarised light in opposite directions (one +/d, one −/l) and are otherwise chemically identical.
🌍 Why drug chirality matters: a chiral drug's two enantiomers can act completely differently — one heals while its mirror twin is inactive or harmful. Modern medicines are often sold as a single hand.
6 — Isomerism: geometric (cis–trans)
Because a C=C double bond cannot rotate (its π-bond locks it like a road divider you cannot cross), groups on the two carbons stay fixed on the same side or on opposite sides.
- cis-isomer — the two like groups sit on the same side of the double bond (e.g. cis-but-2-ene).
- trans-isomer — the two like groups sit on opposite sides (e.g. trans-but-2-ene).
Requirement: each doubly-bonded carbon must carry two different groups. cis and trans have different melting points, boiling points and shapes.
7 — Detection of elements
To know which elements an organic compound contains, we convert them into recognisable inorganic ions and test for them.
| Element | Method | Positive test |
|---|
| C & H | heat with dry CuO | CO₂ turns limewater milky; H₂O blues white CuSO₄ |
| Nitrogen | Lassaigne → NaCN | Prussian-blue with FeSO₄/Fe³⁺ |
| Sulphur | Lassaigne → Na₂S | black PbS; violet with nitroprusside |
| Halogens | Lassaigne + HNO₃ + AgNO₃ | white (Cl), pale-yellow (Br), yellow (I) ppt |
Lassaigne's test: the compound is fused with hot sodium metal, which converts N, S and halogens into soluble Na salts (NaCN, Na₂S, NaX) that are then detected.
8 — Exam recap
- Carbon is unique: tetravalency, catenation, multiple bonds; sp³/sp²/sp give 109.5°/120°/180°.
- Functional groups decide the family — know the prefix/suffix table.
- Homologous series: same general formula, differ by CH₂, gradual property change.
- Structural isomerism: chain, position, functional, metamerism.
- Stereoisomerism: cis–trans (geometric) and optical (chiral / enantiomers).
- Detection (Lassaigne's) of N, S, halogens; C/H by limewater & CuSO₄; purification methods.