OCR A Level Chemistry A: Basic concepts of organic chemistry (H432) – Revision Notes

These revision notes condense section 4.1.1, Basic concepts of organic chemistry, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover every learning outcome, 4.1.1(a) to (i), at the start of Module 4, Core organic chemistry. H432 is the full A level. Each idea is explained in full, with longer worked examples, in the Basic concepts of organic chemistry study guide.

Once these notes feel secure, try the practice questions. All H432 topics are on the OCR chemistry hub, and the printable checklist lists every outcome. For a short check across the course, try the free diagnostics.

4.1.1(a): Naming

Stems (learn all ten): meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for 1 to 10 carbons. Swap -ane for -yl to name the alkyl group: methyl, ethyl, propyl and so on up to decyl.

Naming is limited to the functional groups in the specification. Give a locant wherever the position of a group could vary.

Feature	Name part	Example
C=C	-ene	pent-2-ene
–OH	-ol, or hydroxy-	butan-2-ol
–F, –Cl, –Br, –I	fluoro-, chloro-, bromo-, iodo-	2-iodobutane
alkyl branch	methyl-, ethyl- and so on	3-ethylhexane
–CHO	-al (no locant; always C1)	propanal
C=O in a chain	-one	pentan-3-one
–COOH	-oic acid (always C1)	butanoic acid
ring of carbons	cyclo-	cyclohexane

Method in steps

1. Pick the longest chain containing the main group.
2. Number from the end nearest the suffix group (or C=C).
3. If still tied, give substituents the lowest set of numbers.
4. If still tied, the substituent first in the alphabet gets the lower number.
5. Write prefixes alphabetically; di-, tri- and tetra- do not count for order.
6. Commas between numbers, hyphens between numbers and letters.

Reminder: CH₃CH(OH)CH₂CHClCH₃ is numbered from the OH end. OH on C2, Cl on C4: 4-chloropentan-2-ol.

4.1.1(b): Formula types

Type	What it shows	Example for propan-1-ol
Molecular	number of each atom	C₃H₈O
General	algebraic form for a series	CₙH₂ₙ₊₁OH
Structural	arrangement, minimum detail	CH₃CH₂CH₂OH
Displayed	every atom and every bond	all C–H, C–C, C–O and O–H bonds drawn
Skeletal	carbon skeleton and functional groups, no H on chains	two lines for the carbon chain, then a line to OH

• Definitions of these types are not required. Using them correctly is.
• In structural formulae, write the carboxyl group as COOH and the ester group as COO.
• Cyclohexane is a plain hexagon. Benzene is a hexagon with a circle inside.
• In a skeletal formula, every line end and every corner is a carbon atom unless a different atom is written there.

4.1.1(c): Key terms

Homologous series (definition required): a series of organic compounds having the same functional group, with each successive member differing by CH₂.

Term	Meaning
Functional group	the group of atoms responsible for a compound’s characteristic reactions
Alkyl group	CₙH₂ₙ₊₁, often written R
Aliphatic	C and H in straight chains, branched chains or non-aromatic rings
Alicyclic	aliphatic, in non-aromatic rings, with or without side chains
Aromatic	contains a benzene ring
Saturated	C–C single bonds only
Unsaturated	at least one multiple C–C bond: C=C, C≡C or an aromatic ring

R can also stand for any fragment of a molecule that is not involved in the reaction being shown.

Must-know distinctions

• Alicyclic vs aromatic. Cyclohexene is a ring and unsaturated, but it is alicyclic, not aromatic. Only a benzene ring makes a compound aromatic.
• Alicyclic vs aliphatic. Every alicyclic compound is also aliphatic.
• Saturated vs “degree of saturation”. The specification keeps saturated and unsaturated for C–C multiple bonds. Degree of saturation is the wider idea that also counts other multiple bonds and rings.

4.1.1(d): General formulae

Series	General formula	Mᵣ in terms of n (C = 12.0, H = 1.0, O = 16.0)
Alkanes	CₙH₂ₙ₊₂	14n + 2
Alkenes (one C=C)	CₙH₂ₙ	14n
Cycloalkanes (one ring)	CₙH₂ₙ	14n
Alcohols	CₙH₂ₙ₊₁OH	14n + 18
Haloalkanes (one X)	CₙH₂ₙ₊₁X	14n + 1 + Aᵣ(X)

Method: write Mᵣ in terms of n, set it equal to the value given, solve for n, then write the formula.

Small reminder: a haloalkane CₙH₂ₙ₊₁Br with Mᵣ 108.9, using Br = 79.9: 14n + 1 + 79.9 = 108.9, so 14n = 28 and n = 2. The compound is C₂H₅Br.

4.1.1(e): Structural isomers

Structural isomers: same molecular formula, different structural formula.

Method in steps

1. Decide what the formula allows. CₙH₂ₙ₊₂ type means no ring and no C=C. CₙH₂ₙ means one C=C or one ring.
2. Draw the longest chain first and move each group to every distinct position.
3. Shorten the chain by one carbon and repeat. Then try two.
4. Name every structure. A repeated name is a repeated structure, so cross it out.

The fastest check on a list of isomers is to name them all. A structure that bends round a corner is still the same chain, so look for the longest continuous run of carbons each time.

4.1.1(f) and (g): Fission and radicals

	Homolytic fission	Heterolytic fission
Electrons go	one to each bonding atom	both to one bonding atom
Products	two radicals	a cation and an anion
Example	Br–Br → Br• + Br•	H–Cl → H⁺ + Cl⁻
Shown with	equations using dots	a curly arrow from the bond

• Radical: a species with an unpaired electron.
• Put a dot on every species with a single unpaired electron: Cl•, CH₃•, CH₃CH₂CH₂•.
• Dots are not required for diradicals, such as an oxygen atom.
• Radical mechanisms are written as a sequence of equations. Half curly arrows are not required (4.1.2 f).

4.1.1(h) and (i): Curly arrows

A curly arrow stands for an electron pair on the move. It shows either heterolytic fission or the formation of a covalent bond.

Arrow tail can start at	Arrow head goes to
a covalent bond	the atom that takes the pair (heterolytic fission), or an atom it now bonds to (as when a C=C bond attacks)
a lone pair	an atom, forming a new bond
a negative charge	an atom, forming a new bond

• Never start a tail from an atom with no lone pair shown, or from a positive charge.
• Add the relevant dipoles, such as Cδ+–Clδ−, on polar bonds before you draw arrows to them.
• When one bond forms and another breaks in one step, draw both arrows.

Quick self-test

1. Name CH₃CH₂CH(CH₃)CH₂CH₂CH₃.
2. Name CH₃CH=CHCH₂OH.
3. Give the molecular formula of the alkene with eight carbon atoms.
4. Give the general formula of an alkyl group.
5. Is methylcyclopentane aliphatic, alicyclic or aromatic? Is it saturated?
6. Define homologous series.
7. How many structural isomers have the formula C₂H₄BrCl? Name them.
8. An alkane has Mᵣ = 142.0. Give its molecular formula and name.
9. A skeletal formula is a zigzag of five lines with nothing else drawn. Name the compound.
10. Which type of bond fission forms radicals? Write the equation for this fission of Br₂.
11. State the three places a curly arrow may start.
12. Is benzene saturated or unsaturated?

Answers

1. 3-methylhexane (six-carbon chain; numbering from the left gives 3, from the right 4).
2. But-2-en-1-ol (the OH takes C1; the C=C then starts at C2).
3. C₈H₁₆.
4. CₙH₂ₙ₊₁.
5. Alicyclic (so also aliphatic), and saturated. Formula C₆H₁₂.
6. A series of organic compounds with the same functional group, each successive member differing by CH₂.
7. Two: 1-bromo-1-chloroethane (CH₃CHBrCl) and 1-bromo-2-chloroethane (BrCH₂CH₂Cl).
8. 14n + 2 = 142.0, so n = 10: C₁₀H₂₂, decane.
9. Six carbons (five lines have six ends and corners): hexane.
10. Homolytic fission: Br–Br → Br• + Br•.
11. A covalent bond, a lone pair, or a negative charge.
12. Unsaturated. Aromatic rings count as multiple carbon–carbon bonding.

Where marks are usually lost

• Choosing a chain that is not the longest, especially when the chain turns a corner in the drawing.
• Numbering so the substituents get low numbers before checking the suffix group.
• Using the same number twice without di-, as in “2-chloro-2-chloropropane” for 2,2-dichloropropane.
• Writing “C–OH” in a displayed formula with no O–H bond.
• Drawing hydrogens on carbon in a skeletal formula, or leaving off the H of an OH group.
• Giving the definition of homologous series without “same functional group” or without “differ by CH₂”.
• Listing one isomer twice with different numbering, such as 1-chloro and 3-chloro on a propane chain.
• Missing the dot on a radical, or writing a radical as an ion.
• Arrows that start from an atom or a δ+ end, or that point the wrong way along a bond.
• Leaving out the dipole on the bond being attacked.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 4.1.1 Basic concepts of organic chemistry, learning outcomes (a) to (i). Published by OCR.