Introduction to Organic Chemistry & IUPAC Nomenclature I
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.
Practise this lesson
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.
Know
- The general formulas for alkanes (CnH2n+2), alkenes (CnH2n), alkynes (CnH2n-2)
- The molecular shape and bond angle around carbon for single, double, and triple bonds (tetrahedral/109.5°, trigonal planar/120°, linear/180°)
- The eight chain-length IUPAC prefixes: meth– to oct–
- The seven major functional group classes and their suffixes
- What a homologous series is and how it differs from isomers
Understand
- Why carbon's tetravalency creates the structural diversity of organic chemistry
- Why the number of bonds between two carbons determines the shape around them, and how to predict bond angles from bond type alone
- Why members of a homologous series share chemical behaviour but differ in physical properties
- When each type of structural formula (full, condensed, skeletal) is appropriate
Can Do
- Name any straight-chain or branched alkane up to C8
- Name any alkene or alkyne with correct double/triple bond locant
- Draw full structural, condensed, and skeletal formulas and convert between them
- Verify a molecular formula using the relevant general formula
- Identify the functional group class from a structural formula
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.
- Every atom and bond explicitly
- Atoms grouped by carbon; branches in brackets
- Zigzag; C and H implied; heteroatoms shown
- Short molecules ≤ C4; "draw the structural formula"
- Medium chains; reaction equations
- Longer molecules (C5+); complex organic reactions
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:
- Find the LONGEST continuous carbon chain this is the parent chain and gives the base name.
- Number the chain from the end CLOSEST to the first branch giving substituents the lowest possible locants.
- Name each substituent as a prefix with its locant, listed alphabetically (ignoring di–, tri– multiplying prefixes when alphabetising).
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.
- Find the longest continuous carbon chain
- Number from the end nearest the first branch
- Name substituents alphabetically (not by chain length)
- Counting from a branch rather than tracing the longest path
- Numbering from the wrong end, giving unnecessarily high locants
- Listing ethyl after methyl because methyl appears shorter
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:
- Find the longest chain that includes the C=C double bond this is the parent chain.
- Change the suffix from –ane to –ene.
- Add a locant immediately before –ene showing the lower-numbered carbon of the C=C pair.
- Number the chain from the end closer to the double bond (the double bond takes priority over branches in numbering).
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.
- Ethene
- Prop-1-ene
- But-2-ene
- Ethyne
- Pent-1-yne
- Pent-2-yne
- 2C, one C=C, no locant needed
- 3C, C=C at C1 from nearer end
- 4C, C=C at C2 (same from either end)
- 2C, triple bond, no locant needed
- 5C, C≡C starts at C1
- 5C, C≡C starts at C2 from nearer end
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.
- Single bonds only (as in alkanes): the carbon is bonded to four atoms, which spread out as far apart as possible, pointing to the corners of a tetrahedron, with bond angles of 109.5°. Methane (CH₄) is the classic example.
- A double bond (as in alkenes): the two double-bonded carbons and the atoms attached to them all lie in a flat plane, with bond angles of about 120°. Ethene (CH₂=CH₂) is flat.
- A triple bond (as in alkynes): the triple-bonded carbons and their attached atoms lie in a straight line, with a bond angle of 180°. Ethyne (HC≡CH) is linear.
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.
Bond angle: 109.5°
Example: Methane (CH₄)
Bond angle: ~120°
Example: Ethene (CH₂=CH₂)
Bond angle: 180°
Example: Ethyne (HC≡CH)
Name the compound CH₃CH₂CH(CH₃)CH₂CH₃ and draw its full structural formula.
Write the IUPAC name for: CH₃CH₂CH(CH₃)CH=CH₂
A student draws CH₃–C(CH₃)₂–CH₂–CH₂–CH₃ for "2,2-dimethylpentane" and writes molecular formula C₆H₁₄. Assess whether both are correct.
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
Show All Answers
Activity 1, Naming Practice
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 ✓.
Activity 2, Structure from Name
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).
Multiple Choice
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.
Short Answer Model Answers
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:
- In "hex-1-ene": were you right about the number of carbons? ("hex" = 6 carbons)
- What does the "1" actually mean, and was your prediction correct? (locant of the double bond, the C=C starts at C1)
- What does "–ene" tell you about the compound? (contains a C=C double bond; trigonal planar geometry, ~120° bond angles at the C=C carbons; formula C₆H₁₂)
- Write the condensed structural formula for hex-1-ene from memory and verify it against the alkene general formula.
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?