AQA A-Level Chemistry: Introduction to organic chemistry (7405) – Revision Notes

These notes condense section 3.3.1 Introduction to organic chemistry of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.1.1 Nomenclature, 3.3.1.2 Reaction mechanisms and 3.3.1.3 Isomerism. It is all AS content (nothing is A-level only) and there is no required practical in this section. It is examined on Paper 2 at AS and A-level, and A-level Paper 3 can test any content.

For full explanations and worked examples, use the introduction to organic chemistry study guide. Then try the practice questions. The course hub and printable checklist cover the whole specification, and the free 10-minute diagnostics show where your gaps are.

Key definitions

Term	Definition
Empirical formula	Simplest whole-number ratio of atoms of each element in a compound
Molecular formula	Actual number of atoms of each element in a molecule
General formula	Algebraic formula for all members of a homologous series, e.g. CₙH₂ₙ₊₂
Structural formula	Arrangement of atoms carbon by carbon, e.g. CH₃CH(OH)CH₃
Displayed formula	Shows every atom and every bond
Skeletal formula	Lines for the carbon skeleton, H on carbon omitted, functional groups drawn
Homologous series	A series of compounds containing the same functional group
Functional group	Atom or group of atoms responsible for the typical reactions of a compound
Radical	Species with an unpaired electron, shown by a dot
Structural isomers	Same molecular formula, different structural formula
Stereoisomers	Same structural formula, different arrangement of atoms in space

3.3.1.1 Formulas and homologous series

Features of a homologous series

• same general formula
• successive members differ by CH₂
• same functional group, so similar chemical properties
• gradual trend in physical properties (e.g. boiling point rises with chain length).

General formulas to know

Family	General formula
Alkanes	CₙH₂ₙ₊₂
Alkenes (one C=C) and cycloalkanes	CₙH₂ₙ
Alcohols	CₙH₂ₙ₊₁OH

Skeletal formula checklist

• Every corner and every line end is a carbon.
• Count H on each carbon as 4 minus the bonds drawn.
• Draw –OH, =O, –Cl and similar groups in full, including H on O.

3.3.1.1 IUPAC naming

Stems: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). You need chains and rings of up to six carbons each.

Group	Suffix or prefix	Carbon 1 fixed?
C=C	-ene	No — give the lowest number
–OH	-ol	No — give the lowest number
C=O in chain	-one	No — give the lowest number
–CHO	-al	Yes
–COOH	-oic acid	Yes
–C≡N	-nitrile (CN carbon is counted)	Yes
–F, –Cl, –Br, –I	fluoro-, chloro-, bromo-, iodo-	Prefix
–CH₃, –C₂H₅	methyl-, ethyl-	Prefix
Ring	cyclo-	—

Method in steps

1. Longest chain containing the functional group → stem.
2. Number from the end that gives the main functional group the lowest number.
3. Prefixes in alphabetical order (chloro before methyl), each with its locant.
4. di-, tri- for repeats: 2,3-dichloro…
5. Commas between numbers, hyphens between numbers and letters.

Small reminders

• CH₃CH(Cl)CH₂CH₃ → 2-chlorobutane (not 3-chlorobutane).
• CH₃CH₂CH₂CHO → butanal (no number).
• CH₃COCH₂CH₂CH₃ → pentan-2-one.
• CH₂=CHCH(OH)CH₃ → but-3-en-2-ol: –OH takes the suffix and the lower number.

3.3.1.2 Mechanism conventions

	Free-radical mechanisms	Other mechanisms
What you draw	Balanced equation for each step	Structures plus curly arrows
Symbol	Dot for the unpaired electron (Br•, C₂H₅•)	Curly arrow = movement of an electron pair
Curly arrows?	Not required	Required

Curly-arrow rules

• Bond formed: arrow starts at a lone pair or at a covalent bond, and ends where the new bond forms.
• Bond broken: arrow starts at the bond, and ends at the atom that keeps the pair.
• Show lone pairs and δ+/δ− where they explain the arrows.

Free-radical step check

• Atoms balance on both sides.
• A step with one radical in has one radical out; two radicals combining give no radical.
• The names initiation, propagation and termination are taught with the chlorination of methane in 3.3.2.4.

3.3.1.3 Isomerism

Type	Same	Different	Example
Chain	Molecular formula, functional group	Carbon skeleton	butane / methylpropane
Position	Molecular formula, skeleton, functional group	Position of the group	1-chloropropane / 2-chloropropane
Functional group	Molecular formula	Functional group	hex-1-ene / cyclohexane
E–Z (stereo)	Structural formula	Arrangement in space about C=C	(E)- and (Z)-pent-2-ene

E–Z: method in steps

1. Find the C=C.
2. Check that each carbon of the C=C has two different groups. If either has two identical groups, there are no E–Z isomers.
3. On each carbon, rank the two atoms directly attached by atomic number. Higher atomic number = higher priority.
4. If they tie, compare the atoms one bond further out; stop at the first difference.
5. Higher-priority groups on the same side = Z; on opposite sides = E.

Why E–Z isomers exist: restricted rotation about the planar C=C double bond.

Worked reminder: in BrCH=CHCl, one carbon has Br and H, the other Cl and H. Br beats H and Cl beats H. If Br and Cl are on the same side of the C=C, the isomer is (Z)-1-bromo-2-chloroethene; if they are on opposite sides, it is the E isomer.

    Br      Cl              Br      H
      \    /                  \    /
       C=C                     C=C
      /    \                  /    \
     H      H                H      Cl
       (Z)                     (E)

Common priority comparisons

Higher	Lower	Reason
–Br (35)	–Cl (17)	Atomic number
–Cl (17)	–CH₃ (C = 6)	Atomic number
–CH₂CH₃	–CH₃	Next atoms (C,H,H) beat (H,H,H)
–OH (8)	–CH₃ (6)	Atomic number

Must-know distinctions

• Empirical vs molecular formula: ethene’s molecular formula is C₂H₄; its empirical formula is CH₂.
• Structural vs displayed formula: CH₃CH₂OH is structural; showing every C–H, C–C, C–O and O–H line is displayed.
• Structural isomers vs stereoisomers: structural isomers differ in which atoms are joined to which; stereoisomers have the same bonds but a different arrangement in space.
• E/Z vs cis/trans: the specification uses E and Z with CIP priorities. Z is not always the “both H on one side” form — apply the rules each time.
• Lone pair vs negative charge: a curly arrow starts at a lone pair, not at the minus sign.

Quick self-test

1. Give the empirical formula of hexane.
2. State the general formula of the alkanes and give the molecular formula of the alkane with 8 carbons.
3. How many carbon atoms are in pentanenitrile? Give its structural formula.
4. Name CH₃CH₂CH₂OH.
5. Name (CH₃)₂C=CHCH₃ and state whether it has E–Z isomers.
6. Give the name of a functional group isomer of butanal.
7. 1-chloropropane and 2-chloropropane are isomers. What type?
8. Which has higher CIP priority: –CH₂OH or –CH₂CH₃?
9. What does the dot in Br• represent?
10. State the two places a curly arrow that forms a new bond can start.
11. Define the term stereoisomers.
12. Explain why the C=C double bond gives rise to stereoisomers.

Answers

1. C₃H₇ (C₆H₁₄ divided by 2).
2. CₙH₂ₙ₊₂; C₈H₁₈.
3. Five; CH₃CH₂CH₂CH₂CN (the CN carbon is counted).
4. Propan-1-ol.
5. 2-methylbut-2-ene. No E–Z isomers: carbon 2 carries two identical CH₃ groups.
6. Butanone (C₄H₈O).
7. Position isomers.
8. –CH₂OH: the next atoms are (O, H, H) against (C, H, H), and O has the higher atomic number.
9. An unpaired electron.
10. From a lone pair of electrons, or from a covalent bond.
11. Compounds with the same structural formula but a different arrangement of atoms in space.
12. There is restricted rotation about the planar C=C, so groups on each carbon are fixed on one side.

Where marks are usually lost

• Writing the molecular formula when asked for the empirical formula, or not simplifying (C₂H₄ for ethene’s empirical formula).
• Numbering from the wrong end, so the functional group gets a higher locant than it should.
• Prefixes not in alphabetical order (methylchloro… instead of chloro…methyl…).
• Forgetting that the CN carbon counts in a nitrile name.
• Skeletal formulas with H drawn on carbon, or with the end carbons missed in the count.
• Curly arrows in a free-radical mechanism, or a radical with no dot.
• A curly arrow starting from an atom, or from a charge, instead of from a lone pair or bond.
• E–Z assigned from “H on the same side” rather than from CIP priorities.
• A definition of stereoisomers that says “same molecular formula” only — the structural formula must be the same.
• Missing “restricted rotation” when explaining why E–Z isomers exist.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.1 Introduction to organic chemistry (3.3.1.1 to 3.3.1.3).