Year 12 Chemistry Module 7 — Organic Chemistry ⏱ ~45 min Lesson 1 of 23 IQ1

Introduction to Organic Chemistry & IUPAC Nomenclature I

Every synthetic fabric, fuel, medicine and flavouring on Earth is built from variations on the same carbon-chain framework — and the naming system you learn today is the universal key for reading and writing the language of all of them.

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Think First

Look at the back of a shampoo bottle, a bag of chips, or a sunscreen tube. You will find names like "hexyl acetate," "octanoic acid," and "2-methylpropanol." These are not brand names — they are systematic chemical names that encode the exact structure of the molecule.

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.

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Key Formulas & Relationships — This Lesson

Alkane general formula: C_nH_2n+2

Saturated — single C–C bonds only e.g. n=4: C₄H₁₀ (butane) | n=1: CH₄ (methane)

Alkene general formula: C_nH_2n

One C=C double bond (one degree of unsaturation) e.g. n=4: C₄H₈ (but-1-ene or but-2-ene)

Alkyne general formula: C_nH_2n−2

One C≡C triple bond (two degrees of unsaturation) e.g. n=5: C₅H₈ (pent-1-yne)

Hybridisation → Geometry → Bond angle

sp³ → tetrahedral → 109.5° (alkanes) sp² → trigonal planar → ~120° (alkenes) sp → linear → 180° (alkynes)

No numerical calculation formulas this lesson — nomenclature and hybridisation are conceptual. Memorise general formulas and verify any molecular formula by substituting n.

📖 Know

The general formulas for alkanes (C_nH_2n+2), alkenes (C_nH_2n), alkynes (C_nH_2n−2)
The three hybridisation states of carbon: sp³, sp², sp
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 hybridisation state, geometry, and bond angle always travel as a linked set
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

📚 Core Content

Key Terms — scan these before reading

HydrocarbonAn organic compound containing only carbon and hydrogen atoms.

Functional groupA specific atom arrangement that determines characteristic chemical reactions.

Homologous seriesA family of compounds with the same functional group, differing by CH₂.

IsomerCompounds with the same molecular formula but different structural arrangements.

Addition reactionA reaction where atoms add across a carbon-carbon multiple bond.

Substitution reactionA reaction where one atom or group replaces another in a molecule.

Misconceptions to Fix

✗

Wrong: Organic chemistry only studies molecules produced by living organisms.

✗

Right: Organic chemistry is the study of carbon-containing compounds, regardless of origin. Many organic compounds are synthesised industrially (plastics, pharmaceuticals, synthetic dyes). The "organic" label refers to carbon-based structure, not biological source.

Why Carbon is the Backbone of Organic Chemistry

Tetravalency · sp³/sp²/sp hybridisation · geometry and bond angles

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, and the three hybridisation states of carbon are what determine the geometry and reactivity of each part of a molecule.

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.

The three hybridisation states control geometry and reactivity:

sp³: Four equivalent hybrid orbitals point to the corners of a tetrahedron — bond angles 109.5°, only single bonds. All carbons in an alkane are sp³.
sp²: Three hybrid orbitals in a plane at ~120°, one unhybridised p-orbital forms a π bond sideways with an adjacent p-orbital → creates the C=C double bond. All alkene C=C carbons are sp².
sp: Two hybrid orbitals pointing in opposite directions (180°), two p-orbitals form two π bonds → creates C≡C triple bond. All alkyne C≡C carbons are sp.

You do not need to explain the quantum mechanics of why hybridisation occurs. You do need to link each state to its geometry, bond angle, and example compound.

sp³

Bond type: Single bonds only

Geometry: Tetrahedral

Bond angle: 109.5°

Example: Ethane (CH₃CH₃)

sp²

Bond type: Includes C=C

Geometry: Trigonal planar

Bond angle: ~120°

Example: Ethene (CH₂=CH₂)

sp

Bond type: Includes C≡C

Geometry: Linear

Bond angle: 180°

Example: Ethyne (HC≡CH)

HSC MethodIn any question about the shape or bond angles around a specific carbon: identify the hybridisation FIRST (from the bond type present), then state the geometry, then state the bond angle. These three always travel as a linked set — giving only the bond angle without naming the geometry and hybridisation earns partial marks at best.

Common ErrorStudents write "sp² carbon has a bond angle of 120° with its four bonds." This is misleading — sp² carbon does form four bonds total (3σ + 1π), but the THREE sigma bonds determine the 120° planar geometry. The π bond sits above and below the plane and does not add a "fourth direction." Never describe the π bond as contributing to the shape.

InsightHybridisation directly predicts reactivity. Alkenes and alkynes have π bonds — high-electron-density regions accessible to electrophilic reagents, making addition reactions possible. Alkanes have only σ bonds, which are stronger and harder to break — this is why alkanes are the least reactive organic class and require UV light or very high temperatures to react.

Homologous Series and Functional Groups

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 you need to recognise by their structural signature:

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. Because only chain length changes, 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 — the reactive part — is unchanged.

MethodLearn to identify a functional group from a structural formula before applying any naming rule. Functional group identification is the first move in every naming, property, and reaction question in Module 7. If you cannot identify the group, you cannot name, predict properties, or predict reactions correctly.

Common Error — CriticalStudents confuse "homologous series" with "isomers." Members of a homologous series have different molecular formulae (C₂H₆, C₃H₈, C₄H₁₀ …) — they differ by CH₂ each time. Isomers have the same molecular formula arranged differently. These are opposite concepts and using one when you mean the other will always cost marks.

Types of Structural Formulae

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 (enough H to give each C four bonds). Heteroatoms (O, N, Cl, Br) and their attached H atoms must be shown explicitly. Skeletal formulas are fully accepted in NSW HSC responses and are the fastest to draw for longer molecules.

What is shown

Every atom and bond explicitly

Atoms grouped by carbon; branches in brackets

Zigzag; C and H implied; heteroatoms shown

When to use

Short molecules ≤ C4; "draw the structural formula"

Medium chains; reaction equations

Longer molecules (C5+); complex organic reactions

HSC TipWhen an HSC question says "draw the structural formula," draw the FULL structural formula unless the question specifies "condensed" or "skeletal." For molecules longer than C5, a labelled skeletal formula is acceptable — but write "skeletal formula" beside your drawing so the marker knows it is intentional.

Common ErrorIn skeletal formulas, students miscount implied hydrogens. A terminal C (end of zigzag) has ONE chain bond → needs THREE implied H atoms (–CH₃). A vertex C has TWO chain bonds → needs TWO implied H (–CH₂–). A vertex with a branch has THREE chain bonds → needs ONE implied H (–CH–). Miscounting changes the molecular formula and will be penalised.

IUPAC Naming Rules for Alkanes

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.

Action

Find the longest continuous carbon chain

Number from the end nearest the first branch

Name substituents alphabetically (not by chain length)

Common failure

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

Method — Step 1 is CriticalThe FIRST step before writing any name is to find the longest continuous chain. Redraw the molecule as a straight chain with branches if the original drawing is confusing. Students who start naming from the visible branches consistently get the parent chain wrong — always find the longest path first.

Common Error"3-methylbutane" — students number from the wrong end and give the methyl branch locant 3. The correct name is 2-methylbutane because numbering from the nearer end gives the branch at C2. Always use the lowest possible locants. If two numbering directions give the same first locant, compare the second — choose the direction with the lower second locant.

InsightIUPAC names are fully reversible — from a name you reconstruct a unique structure; from a structure you derive a unique name. Exam questions test both directions equally. Practise drawing a structure FROM a name (not just naming drawn structures) — students who only practise in one direction are caught off-guard when the question runs in reverse.

IUPAC Naming Rules for Alkenes and Alkynes

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.

IUPAC name

Ethene

Prop-1-ene

But-2-ene

Ethyne

Pent-1-yne

Pent-2-yne

Reasoning

2C, one C=C — no locant needed (only one possible position)

3C, C=C at C1 from nearer end

4C, C=C at C2 (same from either end — use C2)

2C, triple bond — no locant needed

5C, C≡C starts at C1

5C, C≡C starts at C2 from nearer end

IUPAC FormatThe locant in a modern IUPAC alkene name must appear immediately before the suffix, hyphenated: "but-1-ene" is correct; "1-butene" is the older format still seen in some textbooks and accepted in NSW HSC, but use the hyphenated form in written responses to demonstrate current IUPAC fluency.

Common ErrorStudents forget to include the double bond carbon in the parent chain. If the C=C does not sit in the longest chain, you have chosen the wrong parent chain — at HSC level, questions are written so the C=C is always in the longest chain. If you find yourself with an alkene named from a chain not including the C=C, go back to step 1.

📓 Copy Into Your Books

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General Formulas

Alkane: C_nH_2n+2 (saturated, sp³ C only)
Alkene: C_nH_2n (one C=C, sp² C at double bond)
Alkyne: C_nH_2n−2 (one C≡C, sp C at triple bond)
Verify: substitute n → check H count matches formula

Hybridisation — Three-Part Rule

sp³ → tetrahedral → 109.5° → alkanes (single bonds only)
sp² → trigonal planar → ~120° → alkenes (C=C present)
sp → linear → 180° → alkynes (C≡C present)
Always state all three: hybridisation + geometry + angle

IUPAC Naming — 3 Steps

1. Longest continuous chain → parent name (meth–oct–)
2. Number from end nearest first branch/double bond
3. Name substituents alphabetically with locants
Alkene/alkyne: suffix –ene/–yne; locant before suffix (but-1-ene)

Homologous Series vs Isomers

Homologous series: DIFFERENT molecular formula (differ by CH₂)
Isomers: SAME molecular formula, different arrangement
Same homologous series → similar chemical properties
Longer chain → higher boiling point (stronger dispersion forces)

🧪 Activities

🔬 Activity 1 — Naming Practice

From Structure to IUPAC Name

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

✏️ Activity 2 — Structure from Name

From IUPAC Name to Structural Formula

For each IUPAC name below: (A) Draw the condensed structural formula. (B) State the hybridisation of 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

✏️ Worked Examples

Worked Example 1 — Naming a branched alkane (straightforward)

Name the compound CH₃CH₂CH(CH₃)CH₂CH₃ and draw its full structural formula.

Find the longest continuous carbon chain. Trace: CH₃–CH₂–CH–CH₂–CH₃ = 5 carbons in the main chain. The –CH₃ branch hangs off the middle carbon. Parent chain = pentane (5C).

Number from the end closer to the branch. From left: C1–C2–C3(branch)–C4–C5 → branch at C3. From right: C1–C2–C3(branch)–C4–C5 → branch also at C3. Both directions give C3, so either direction is valid.

Name the substituent. –CH₃ = methyl, locant = 3. Full IUPAC name: 3-methylpentane.

Verify molecular formula. Parent chain = 5C; methyl branch = 1C; total = 6C. Using C_nH_2n+2: n = 6 → H = 2(6)+2 = 14. Molecular formula = C₆H₁₄ ✓.

Worked Example 2 — Naming an alkene with a branch (intermediate)

Write the IUPAC name for: CH₃CH₂CH(CH₃)CH=CH₂

Identify the functional group. C=C double bond → alkene. Find the longest chain that includes the C=C: CH₂=CH–CH(CH₃)–CH₂–CH₃ = 5 carbons including both C=C carbons. Parent chain = pentene.

Number from the end closer to the double bond. From the CH₂= end: C1=C2–C3–C4–C5. Double bond at C1. From the other end: C1–C2–C3=C4–C5 → double bond at C3. Choose C1 (lower locant for the double bond). ✓

Locate the branch. With C1 at CH₂=, the CH₃ branch is at C3. Substituent = 3-methyl.

Assemble the name. 5C chain + alkene + double bond at C1 + methyl at C3 = 3-methylpent-1-ene. Verify: 6C total, one C=C → C₆H₁₂ (C_nH_2n, n=6 → 12H ✓).

Worked Example 3 — Evaluating a student's structure and molecular formula (hard)

A student draws CH₃–C(CH₃)₂–CH₂–CH₂–CH₃ for "2,2-dimethylpentane" and writes molecular formula C₆H₁₄. Assess whether both are correct.

Decode the IUPAC name. "2,2-dimethylpentane" = parent chain 5C (pentane); two methyl groups, both at C2.

Check the drawn structure. CH₃–C(CH₃)₂–CH₂–CH₂–CH₃: C1(CH₃)–C2(C(CH₃)₂)–C3(CH₂)–C4(CH₂)–C5(CH₃). C2 has bonds to C1, C3, and two CH₃ branches (four bonds total) — this matches 2,2-dimethylpentane. Structure is correct ✓

Check the molecular formula. Count all carbons: parent chain 5C + two methyl branches 2C = 7C total. Using C_nH_2n+2: n=7 → H = 2(7)+2 = 16. Correct formula = C₇H₁₆.

Diagnose the error. The student wrote C₆H₁₄ — this is the formula for hexane (6 carbons, no branches). The student counted 5 (parent) + 1 (one methyl) = 6, forgetting to add the second methyl branch. Molecular formula is wrong ✗ — should be C₇H₁₆.

✅ Check Your Understanding

Multiple Choice — 5 Questions

Click an option to check your answer. Aim for all 5 before looking at the answers accordion.

ApplyBand 3

1. What is the correct IUPAC name for CH₃CH₂CH(CH₃)CH₂CH₂CH₃?

3-methylheptane

4-methylheptane

3-methylhexane

5-methylhexane

ApplyBand 3

What is NOT the correct IUPAC name for CH₃CH₂CH(CH₃)CH₂CH₂CH₃?

3-methylheptane

4-methylheptane

3-methylhexane

5-methylhexane

UnderstandBand 3

2. A carbon atom in an organic molecule has bond angles of approximately 120° to its neighbours. Which hybridisation state and bond type does this indicate?

sp³ — single bonds only

sp² — double bond present

sp — triple bond present

sp² — triple bond present

sp² — double bond present

sp — triple bond present

sp² — triple bond present

ApplyBand 3

3. What is the correct IUPAC name for HC≡CCH₂CH₂CH₃?

Pent-1-yne

Pent-1-ene

1-pentyne

Pent-2-yne

ApplyBand 3

What is NOT the correct IUPAC name for HC≡CCH₂CH₂CH₃?

Pent-1-yne

Pent-1-ene

1-pentyne

Pent-2-yne

UnderstandBand 3

4. A compound has the molecular formula C₅H₁₀. Which homologous series does it most likely belong to?

Alkane — general formula C_nH_2n+2

Alkene — general formula C_nH_2n

Alkyne — general formula C_nH_2n−2

Alcohol — general formula C_nH_2n+2O

ApplyBand 4

5. Which condensed structural formula correctly represents 3-methylpent-1-ene?

CH₂=CHCH(CH₃)CH₂CH₃

CH₃CH(CH₃)CH=CHCH₃

CH₂=CHCH₂CH(CH₃)CH₃

CH₂=C(CH₃)CH₂CH₂CH₃

CH₃CH(CH₃)CH=CHCH₃

CH₂=CHCH₂CH(CH₃)CH₃

CH₂=C(CH₃)CH₂CH₂CH₃

✅ Comprehensive Answers

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🔬 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 (from the end nearer the branch). Name: 2-methylbutane. Formula: C₅H₁₂ ✓.

3. CH₂=CHCH₂CH₂CH₃: Functional group = C=C (alkene). Chain including C=C = 5C. Double bond at C1 (lower locant). 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 — same locant). 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 C3 and C4 from the CH₃CH₂ end (giving lower locants than C2 and C3 from the other end — check: from left C3,C4 vs from right C2,C3 → choose C2,C3 direction). Name: 2,3-dimethylpentane. Formula: C₇H₁₆ ✓.

✏️ Activity 2 — Structure from Name

1. 3-methylhexane: Condensed: CH₃CH₂CH(CH₃)CH₂CH₂CH₃. All C = sp³. Formula: parent 6C + branch 1C = 7C total → C₇H₁₆ ✓ (n=7 → 16H).

2. 2,3-dimethylbutane: Condensed: CH₃CH(CH₃)CH(CH₃)CH₃. All C = sp³. 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 = sp²; C1, C4, C5 = sp³. Formula: 5C parent + 1C branch = 6C → C₆H₁₂ ✓ (CnH2n, n=6 → 12H).

❓ Multiple Choice

1. C — 3-methylhexane. Trace: CH₃–CH₂–CH(CH₃)–CH₂–CH₂–CH₃. Main chain = 6C (hexane). Methyl branch at C3 (from either end — symmetric in this case, but count: C1–C2–C3 from the left gives the branch at C3; from the right it is also C4 — lower locant = C3). Name: 3-methylhexane. Option A (3-methylheptane) has 7C in main chain — wrong. Option B would require a 7C chain with branch at C4 — wrong.

2. B — sp², double bond present. 120° bond angles → sp² hybridisation → trigonal planar geometry. sp³ gives 109.5°; sp gives 180°. Option D incorrectly pairs sp² with a triple bond.

3. A — Pent-1-yne. HC≡C–CH₂–CH₂–CH₃: longest chain including C≡C = 5C → pent. Suffix = –yne (triple bond). Numbered from the HC≡C end gives locant 1 (vs locant 4 from the other end). Modern IUPAC: pent-1-yne. Option C (1-pentyne) is the older format — still accepted in HSC but less precise.

4. B — Alkene (CnH2n). C₅H₁₀: test n=5 in each formula. Alkane: 2(5)+2=12 → C₅H₁₂ ✗. Alkene: 2(5)=10 → C₅H₁₀ ✓. Alkyne: 2(5)−2=8 → C₅H₈ ✗. Note: C₅H₁₀ could also be a cycloalkane — but at HSC level the expected answer is alkene.

5. A — CH₂=CHCH(CH₃)CH₂CH₃. 3-methylpent-1-ene: pent-1-ene = 5C with C=C at C1 (CH₂=CH–). Methyl at C3. Condensed: CH₂=CH–CH(CH₃)–CH₂–CH₃ ✓. Option B has the double bond at C3 (not C1). Option C has methyl at C4 → 4-methylpent-1-ene. Option D has methyl at C2 → 2-methylpent-1-ene.

📝 Short Answer Model Answers

Q6 (4 marks): (a) C1 (CH₂=): sp² [1]; C2 (=CH–): sp² [implied by same answer]; C3 (–CH₃): sp³ [1]. (b) C1 and C2: trigonal planar geometry, ~120° bond angles. C3: tetrahedral geometry, 109.5° bond angles [1]. (c) C1 and C2 are sp²-hybridised because they are part of the C=C double bond — each has three sigma bonds (determining planar geometry) plus one p-orbital contributing to the pi bond. C3 has only single bonds (sp³), so all four hybrid orbitals point to tetrahedral positions [1].

Q7 (4 marks): (a) CH₃CH₂C(CH₃)₂CH₂CH₃: main chain = CH₃–CH₂–C–CH₂–CH₃ = 5C (pentane). Two methyl branches at C3. Name: 3,3-dimethylpentane [1]. (b) Total C: 5 (parent) + 2 (methyls) = 7C. Using CnH2n+2, n=7: H = 2(7)+2=16. Formula = C₇H₁₆ ✓ [1]. (c) All 7 carbons are sp³-hybridised [1] because this compound has only single (C–C and C–H) bonds — no double or triple bonds are present, so all carbons adopt tetrahedral geometry [1].

Q8 (5 marks): (a) Isomers: butane and 2-methylpropane (C₄H₁₀) [1] — they have the same molecular formula (C₄H₁₀) but different structural arrangements [1]. (b) Homologous series pair: propane (C₃H₈) and butane (C₄H₁₀) [1] — they are consecutive members of the alkane series, differing by exactly one –CH₂– unit, and share the same general formula CnH2n+2 [1]. (c) Similar chemical reactivity [1] — propane and 2-methylpropane 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. Physical properties (boiling point) would differ slightly due to the branched structure of 2-methylpropane reducing surface area and thus dispersion forces.

Introduction to Organic Chemistry & IUPAC Nomenclature I

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Key Formulas & Relationships — This Lesson

📖 Know

💡 Understand

✅ Can Do

Misconceptions to Fix

Why Carbon is the Backbone of Organic Chemistry

sp³

sp²

sp

Homologous Series and Functional Groups

Types of Structural Formulae

IUPAC Naming Rules for Alkanes

IUPAC Naming Rules for Alkenes and Alkynes

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General Formulas

Hybridisation — Three-Part Rule

IUPAC Naming — 3 Steps

Homologous Series vs Isomers

From Structure to IUPAC Name

From IUPAC Name to Structural Formula

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.

Multiple Choice — 5 Questions

Extended Questions

✅ Comprehensive Answers

🔬 Activity 1 — Naming Practice

✏️ Activity 2 — Structure from Name

❓ Multiple Choice

📝 Short Answer Model Answers

Revisit Your Thinking

IUPAC Nomenclature & Functional Groups

Mark lesson as complete