Unlock the language of organic chemistry — learn how carbon's tetravalency and IUPAC naming rules let you encode and decode the structure of any organic molecule up to C8.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Before you read on, write down what you think each part of the name "hex-1-ene" is telling you. How many carbons do you think it has? What do you think the "1" means? What does the "-ene" ending suggest?
Hold your answer — you will return to test and revise it at the end of the lesson.
Tetravalency · carbon's structural diversity
Carbon's ability to form four stable covalent bonds — with itself and with H, O, N, and halogens — creates a structural diversity no other element comes close to matching.
Organic chemistry is the study of carbon compounds (with conventional exceptions such as carbonates, oxides and carbides). Carbon sits in Group 14 and has four valence electrons, so it forms four covalent bonds to complete its octet. This tetravalency means carbon can bond to itself in straight chains, branched chains, and rings of essentially unlimited length while simultaneously bonding to other elements. The result is millions of known organic compounds and new ones synthesised every year.
Full structural · Condensed · Skeletal · Converting between them
Organic structures can be drawn at three levels of detail, and knowing which one a question is asking for — and how to read each — is a practical exam skill that affects marks in almost every Module 7 question.
A full structural formula (displayed formula) shows every atom and every bond explicitly as individual lines. For propane: each C–H and C–C bond is drawn separately. This is required when a question says "draw the structural formula" without further specification, especially for short molecules (≤ C4).
A condensed structural formula groups hydrogen atoms with their carbon: propane = CH₃CH₂CH₃; but-1-ene = CH₂=CHCH₂CH₃. Branches are shown in brackets — e.g. 2-methylpropane = CH₃CH(CH₃)CH₃.
A skeletal (line) formula represents the carbon chain as a zigzag, where each vertex and each endpoint is an implied carbon atom, and all H atoms on carbon are implied. Heteroatoms (O, N, Cl, Br) and their attached H atoms must be shown explicitly. Skeletal formulas are fully accepted in NSW HSC responses.
Families of compounds · –CH₂– increment · predictable properties
A functional group is the part of a molecule that reacts; a homologous series is a family of molecules that share a functional group and differ only in chain length — once you know the family rules, you know how any member will behave.
A functional group is a specific atom or group of atoms responsible for the characteristic reactions of a molecule. Two molecules with the same functional group will undergo the same types of reactions regardless of chain length. The seven major classes:
| Class | Functional group | Suffix | Simple example |
|---|---|---|---|
| Alkane | C–C only (no functional group) | –ane | Ethane |
| Alkene | C=C double bond | –ene | Ethene |
| Alkyne | C≡C triple bond | –yne | Ethyne |
| Alcohol | –OH (hydroxyl) | –ol | Ethanol |
| Aldehyde | –CHO (at chain end) | –al | Ethanal |
| Ketone | C=O (in chain, not terminal) | –one | Propanone |
| Carboxylic acid | –COOH (at chain end) | –oic acid | Ethanoic acid |
A homologous series is a sequence of compounds sharing the same functional group and general formula, differing by one –CH₂– unit between consecutive members. Physical properties shift gradually and predictably: boiling point, viscosity, and melting point all increase because dispersion forces strengthen with molecular size. Chemical properties remain similar because the functional group is unchanged.
Longest chain · Lowest locants · Alphabetical substituents
IUPAC names are built from a small set of rules applied in a fixed order — learn the rules, and you can name or decode any straight-chain or branched alkane you encounter.
An IUPAC name for an alkane is built in three steps:
Chain-length prefixes to memorise: meth– (1C), eth– (2C), prop– (3C), but– (4C), pent– (5C), hex– (6C), hept– (7C), oct– (8C). All alkane names end in –ane.
Common substituents: methyl (–CH₃), ethyl (–C₂H₅). If the same substituent appears more than once, add di–, tri–, or tetra– and list all locants separated by commas — e.g. 2,3-dimethylbutane.
Double/triple bond locant · Priority numbering · Suffix –ene / –yne
Alkenes and alkynes follow the same chain-length and branch-naming rules as alkanes, with two additions: the suffix changes and a locant is required to show where the double or triple bond begins.
For alkenes:
For alkynes: Same rules; suffix = –yne.
When a molecule has both a branch and a double bond: the double bond takes priority — give the double bond the lowest possible locant, even if this means a branch receives a higher locant.
Shape from bond type · Predict angles · Observable pattern
The number of other atoms a carbon is bonded to determines the shape around it — and you can predict bond angles directly from the type of bond present.
The pattern: more bonds between two carbons pulls the molecule into a flatter, then straighter shape. You can state the shape and bond angle for any carbon just by spotting whether it sits at a single, double, or triple bond.
Why organics need specific safety and disposal procedures
Many organic substances are flammable, volatile and/or toxic, so they must be handled and disposed of using procedures that control those specific hazards.
Handling: many organics (e.g. alcohols, alkanes) are highly flammable, so keep them away from naked flames and heat sources and warm them with a water bath, not a Bunsen flame. Volatile and toxic organics must be used in a fume cupboard, with gloves and safety glasses, because vapours can be inhaled or absorbed through the skin.
Disposal: organic liquids must NOT be poured down the sink — many are immiscible with water, persistent and toxic to aquatic life. They are collected in labelled organic-waste containers for safe disposal (e.g. by licensed processing or incineration). Always consult the Safety Data Sheet (SDS) and a risk assessment before use.
Copy the highlighted handling and disposal points into your book.
Waste organic liquids should not be poured down the sink; instead they are collected in labelled organic-_____ containers for safe disposal.
C₅H₁₂ is an alkane (CₙH₂ₙ₊₂ with n=5) — suffix is '-ane'. There are three structural isomers: pentane (unbranched), 2-methylbutane (one methyl branch on C2), and 2,2-dimethylpropane (two methyl branches on C2). Counting isomers requires systematically shortening the parent chain and moving branches.
No numerical calculation formulas this lesson — nomenclature is conceptual. Memorise general formulas and verify any molecular formula by substituting n.
Complete the Learn phase to unlock Practice.
For each compound below, write the correct IUPAC name. Show your working (identify the parent chain length, numbering direction, and substituents/locants). Verify each molecular formula using the appropriate general formula.
| # | Condensed structural formula | Your IUPAC name | Molecular formula check |
|---|---|---|---|
| 1 | CH₃CH₂CH₂CH₂CH₃ | ||
| 2 | CH₃CH(CH₃)CH₂CH₃ | ||
| 3 | CH₂=CHCH₂CH₂CH₃ | ||
| 4 | CH₃CH₂C≡CCH₂CH₃ | ||
| 5 | CH₃CH₂CH(CH₃)CH(CH₃)CH₃ |
For each IUPAC name below: (A) Draw the condensed structural formula. (B) State the molecular shape and bond angle around each carbon in the chain. (C) State the molecular formula and verify it against the appropriate general formula.
1. 3-methylhexane
2. 2,3-dimethylbutane
3. 4-methylpent-2-ene
Click an option to check your answer.
1. What is the correct IUPAC name for CH₃CH₂CH(CH₃)CH₂CH₂CH₃?
2. A carbon atom in an organic molecule has bond angles of approximately 120° to its neighbours. What molecular geometry and bond type does this indicate?
3. What is the correct IUPAC name for HC≡CCH₂CH₂CH₃?
4. A compound has the molecular formula C₅H₁₀. Which homologous series does it most likely belong to?
5. Which condensed structural formula correctly represents 3-methylpent-1-ene?
6. (a) State the molecular shape and bond angle around each carbon in propene (CH₃CH=CH₂). (b) Identify the bond type responsible for the shape at C1 and C2, and the bond type at C3. (c) Explain, in terms of the bonds present, why C1 and C2 have different geometry to C3. 4 MARKS
7. (a) Write the IUPAC name for the compound with condensed formula CH₃CH₂C(CH₃)₂CH₂CH₃. (b) State its molecular formula and verify it using the appropriate general formula. (c) State the molecular shape and bond angle around all carbon atoms in this compound and explain what structural feature leads to this. 4 MARKS
8. A student claims that propane (C₃H₈), butane (C₄H₁₀), and 2-methylpropane (C₄H₁₀) are all members of the same homologous series. (a) Identify which pair consists of isomers and explain how you know. (b) Identify which pair are consecutive members of a homologous series and explain. (c) Predict, with reasoning, whether propane and 2-methylpropane will have similar chemical reactivity. 5 MARKS
1. CH₃CH₂CH₂CH₂CH₃: Longest chain = 5C → pentane. No branches. Name: pentane. Formula: C₅H₁₂ ✓ (CₙH₂ₙ₊₂, n=5 → 12H).
2. CH₃CH(CH₃)CH₂CH₃: Main chain: CH₃–CH–CH₂–CH₃ = 4C → butane. Branch: –CH₃ at C2. Name: 2-methylbutane. Formula: C₅H₁₂ ✓.
3. CH₂=CHCH₂CH₂CH₃: Functional group = C=C (alkene). Chain including C=C = 5C. Double bond at C1. Name: pent-1-ene. Formula: C₅H₁₀ ✓ (CₙH₂ₙ, n=5 → 10H).
4. CH₃CH₂C≡CCH₂CH₃: Functional group = C≡C (alkyne). Chain = 6C. Triple bond at C3 (from either end). Name: hex-3-yne. Formula: C₆H₁₀ ✓ (CₙH₂ₙ₋₂, n=6 → 10H).
5. CH₃CH₂CH(CH₃)CH(CH₃)CH₃: Longest chain: CH₃–CH₂–CH–CH–CH₃ = 5C. Two methyl branches at C2 and C3 from the CH(CH₃)CH₃ end. Name: 2,3-dimethylpentane. Formula: C₇H₁₆ ✓.
1. 3-methylhexane: Condensed: CH₃CH₂CH(CH₃)CH₂CH₂CH₃. All C: tetrahedral geometry, 109.5° (single bonds only). Formula: parent 6C + branch 1C = 7C total → C₇H₁₆ ✓ (n=7 → 16H).
2. 2,3-dimethylbutane: Condensed: CH₃CH(CH₃)CH(CH₃)CH₃. All C: tetrahedral geometry, 109.5° (single bonds only). Formula: parent 4C + 2 methyl branches = 6C → C₆H₁₄ ✓ (n=6 → 14H).
3. 4-methylpent-2-ene: Pent-2-ene = 5C chain, C=C at C2–C3. Methyl at C4: CH₃–CH=CH–CH(CH₃)–CH₃. C2 and C3: trigonal planar, ~120° (C=C present); C1, C4, C5: tetrahedral, 109.5°. Formula: 5C parent + 1C branch = 6C → C₆H₁₂ ✓ (CₙH₂ₙ, n=6 → 12H).
1. C — 3-methylhexane. Main chain = 6C (hexane). Methyl branch at C3. Options A and B have 7C in the main chain (heptane) — wrong.
2. B — Trigonal planar, ~120°, double bond present. Bond angles of ~120° → trigonal planar geometry → C=C double bond present. Tetrahedral (109.5°) indicates single bonds only; linear (180°) indicates a triple bond.
3. A — Pent-1-yne. HC≡C–CH₂–CH₂–CH₃: longest chain including C≡C = 5C → pent. Suffix = –yne. Numbered from the HC≡C end gives locant 1. Modern IUPAC: pent-1-yne.
4. B — Alkene (CₙH₂ₙ). C₅H₁₀: test n=5. Alkene: 2(5)=10 → C₅H₁₀ ✓. Alkane: 2(5)+2=12 → C₅H₁₂ ✗.
5. A — CH₂=CHCH(CH₃)CH₂CH₃. 3-methylpent-1-ene: pent-1-ene = 5C with C=C at C1. Methyl at C3. Option B has double bond at C3. Option C has methyl at C4. Option D has methyl at C2.
Q6 (4 marks): (a) C1 (CH₂=): trigonal planar, ~120°; C2 (=CH–): trigonal planar, ~120°; C3 (–CH₃): tetrahedral, 109.5° [2 — 1 mark for C1/C2 set, 1 mark for C3]. (b) C1 and C2 are part of the C=C double bond; C3 has only single bonds [1]. (c) C1 and C2 are each bonded to three groups via the C=C arrangement — three groups spread into a flat plane at ~120°. C3 is bonded to four groups via single bonds only — four groups spread to tetrahedral positions at 109.5° [1].
Q7 (4 marks): (a) CH₃–CH₂–C(CH₃)₂–CH₂–CH₃: main chain = 5C (pentane). Two methyl branches at C3. Name: 3,3-dimethylpentane [1]. (b) Total C: 5+2 = 7C. CₙH₂ₙ₊₂, n=7: H=16. Formula = C₇H₁₆ ✓ [1]. (c) All 7 carbons have tetrahedral geometry, 109.5° bond angles [1] because this compound has only single bonds — no C=C or C≡C present, so all carbons bond to four groups and adopt tetrahedral positions [1].
Q8 (5 marks): (a) Isomers = butane and 2-methylpropane (both C₄H₁₀) [1] — same molecular formula but different structural arrangements [1]. (b) Homologous series pair = propane and butane [1] — they are consecutive members of the alkane series, differing by exactly one –CH₂– unit, sharing general formula CₙH₂ₙ₊₂ [1]. (c) Propane and 2-methylpropane will have similar chemical reactivity [1] — they share the alkane functional group class (C–C and C–H single bonds only). Chemical reactivity in a homologous series depends on the functional group, which is identical for both.
Back at the start you predicted what "hex-1-ene" meant. Now you know: "hex-" = 6 carbons, "-ene" = a carbon–carbon double bond, and the "1" tells you the double bond starts at carbon 1. You can now decode any name like this — and that's the same skill that let you read "hexyl acetate" off the shampoo bottle.
Go back to your Think First response. Now that you've studied IUPAC nomenclature:
What is the general formula for alkanes, and what does it mean in terms of structure?
For each bond type (single, double, triple), what shape forms around the carbon and what bond angle results?
What are the three IUPAC naming steps for a branched alkane?
What is the difference between a homologous series and structural isomers?
In IUPAC naming, what takes priority when numbering a chain — a branch or a double bond?
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