Organic Chemistry: Formulae and Naming — Revision Notes

Condensed for the final weeks. For the full explanation, use the Organic Chemistry: Formulae and Naming study guide.

Tier note: items marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus: 11.1 outcomes 7–9 (structural formulae, structural isomers, the full list of homologous-series characteristics), 11.2 outcomes 3–4 (naming unbranched compounds up to four carbons, including locants, and naming esters), plus empirical formula (3.1 outcome 5) and the reactions from 11.4, 11.5 and 11.7 that are Supplement (substitution and addition as reaction types, UV light, oxidising ethanol, making an ester). 0620 Core names only methane, ethane, ethene, ethanol and ethanoic acid (and the products of the Core reactions), and recognises a compound’s type from its name ending or formula (11.2 outcomes 1–2). 5070 has no tiers, so O Level candidates need all of it.

The homologous series

0620 Core: a family of similar compounds with similar chemical properties because they have the same functional group (11.1 outcome 4).

(0620 Extended, 5070 required) The full description: the same functional group, the same general formula, differing by CH₂, showing a gradual trend in physical properties and similar chemical properties (11.1 outcome 9).

The functional group is the specific atom or group of atoms responsible for a series’ characteristic reactions — the –OH in alcohols, the –COOH in carboxylic acids, the C=C in alkenes. It’s the functional group, not the length of the carbon chain, that determines how a compound reacts.

Series	Functional group	General formula	Suffix
Alkanes	C–C single	CₙH₂ₙ₊₂	-ane
Alkenes	C=C double	CₙH₂ₙ	-ene
Alcohols	–OH	CₙH₂ₙ₊₁OH	-ol
Carboxylic acids	–COOH	CₙH₂ₙ₊₁COOH	-oic acid
Esters (0620 Extended, 5070 required)	–COO–	—	-oate

Stem names — count the carbons

1 meth-   2 eth-   3 prop-   4 but-   5 pent-   6 hex-

(Background — beyond both syllabuses.) pent- and hex-: 0620 and 5070 both limit naming to compounds with up to four carbon atoms per molecule.

So: C₂H₅OH = ethanol · CH₃COOH = ethanoic acid (0620 Core). (0620 Extended, 5070 required) C₃H₈ = propane · C₄H₈ = but-1-ene or but-2-ene (the unbranched alkenes): naming other unbranched compounds up to four carbons is 11.2 outcome 3.

(0620 Extended, 5070 required) Locant numbers show where a feature sits on a longer or branched chain: propan-1-ol has the –OH on the end carbon, propan-2-ol has it on the middle carbon — same molecular formula, different compound, different physical properties.

(0620 Extended, 5070 required) Esters are named from the alcohol and acid that formed them: an alcohol ending in -yl, followed by the acid’s -oate. Ethanol + ethanoic acid gives ethyl ethanoate.

Types of formula

Type	Ethanol example
Empirical (0620 Extended, 5070 required)	C₂H₆O (simplest ratio)
Molecular	C₂H₆O
Structural (0620 Extended, 5070 required)	CH₃CH₂OH
Displayed	Every atom and bond drawn out

A displayed formula is what examiners expect when a question asks you to “draw the structure” — it is the most detailed of the four representations above.

In a displayed formula, show every C–H bond. Omitting them is a common lost mark.

Worked example. Use the general formula to find the molecular formula of the alkane with 5 carbon atoms.

CnH2n+2, with n = 5
C5H(2x5)+2 = C5H12  (pentane)

Using the general formula is 0620 Core (11.1 outcome 2). The name pentane is background, beyond the four-carbon naming limit.

Isomers (0620 Extended, 5070 required)

Structural isomers — same molecular formula, different arrangement of atoms (a different structural formula).

C4H10:   butane        CH3CH2CH2CH3      (straight chain)
         methylpropane CH3CH(CH3)CH3     (branched)

Same formula, different structure, therefore different physical properties (branched isomers have lower boiling points — less surface contact between molecules). The molecular formula alone never tells you which isomer you’re looking at — that’s exactly why a question distinguishing between isomers always expects a structural or displayed formula, not just C₄H₁₀.

Saturated vs unsaturated

• Saturated — only single C–C bonds (alkanes).
• Unsaturated — contains at least one carbon-carbon bond that isn’t single, most commonly C=C (alkenes).

Test: add bromine water. Orange-brown → colourless indicates unsaturation (alkene). No change (bromine water stays orange-brown) means the compound is saturated.

This is the single most examined organic test. (0620 Extended, 5070 required) It also underpins the difference in typical reaction type between the two series, since alkanes mostly undergo substitution while alkenes mostly undergo addition.

Key reactions to recall

(0620 Extended, 5070 required) Alkanes react by substitution (one atom replacing another), while alkenes react by addition (atoms adding across the double bond) — the saturated/unsaturated distinction above is exactly why.

alkane + halogen  --UV light-->   substitution
alkene + bromine  ---->           ADDITION (decolourises)
alkene + steam    --catalyst-->   alcohol
ethanol  --oxidation (acidified KMnO4, or bacteria)-->  ethanoic acid
alcohol + acid    ---->           ESTER + water

0620 Core: alkanes are generally unreactive except in combustion and substitution by chlorine (11.4 outcome 2); the bromine water test (11.5 outcome 4); ethanol made by adding steam to ethene (11.6 outcome 1). (0620 Extended, 5070 required) The meaning of substitution and addition, UV light for substitution (11.4 outcomes 3–4, 11.5 outcomes 5–6), oxidising ethanol to ethanoic acid (11.7 outcome 2) and making an ester (11.7 outcome 3).

Exam traps

• Alkanes are saturated and relatively unreactive; alkenes are unsaturated and reactive.
• Bromine water goes colourless, not “clear” — clear is not a colour.
• Displayed formulae must show all bonds including C–H.
• Alkene general formula is CₙH₂ₙ — no +2.
• (0620 Extended, 5070 required) Isomers must have the same molecular formula; different formulae are just different compounds.
• (0620 Extended, 5070 required) Confusing propan-1-ol with propan-2-ol — the locant number is part of the name, not an optional extra.
• (0620 Extended, 5070 required) Naming an ester the wrong way round — it is always alcohol (-yl) + acid (-oate), in that order.

Self-test

1. Give the general formula of the alkenes.
2. (0620 Extended, 5070 required) Name C₃H₇OH and C₃H₇COOH.
3. How would you distinguish hexane from hexene?
4. (0620 Extended, 5070 required) Draw the two structural isomers of C₄H₁₀ (describe them).
5. What defines a homologous series?
6. Use the general formula to find the molecular formula of the alkane with 6 carbon atoms.
7. (0620 Extended, 5070 required) Name the ester formed from methanol and ethanoic acid.
8. (0620 Extended, 5070 required) What is the difference between propan-1-ol and propan-2-ol?

Answers: 1. CₙH₂ₙ. 2. Propan-1-ol or propan-2-ol (the formula C₃H₇OH fits both; it does not show where the –OH is) and butanoic acid (the carboxyl carbon counts, so C₃H₇COOH has four carbons). 3. Add bromine water: hexene decolourises it (orange-brown → colourless); hexane produces no change. 4. Butane — an unbranched chain of four carbons; methylpropane — a three-carbon chain with a CH₃ branch on the middle carbon. 5. Compounds sharing the same functional group and general formula, each differing from the next by CH₂, with a gradual trend in physical properties and similar chemical properties. (0620 Core needs only: a family of similar compounds with similar chemical properties due to the same functional group.) 6. C6H(2x6)+2 = C₆H₁₄ (hexane; the name is background, beyond the four-carbon naming limit). 7. Methyl ethanoate. 8. Both are C₃H₇OH, but the –OH is on the end carbon in propan-1-ol and on the middle carbon in propan-2-ol — same molecular formula, different structure.

For the full worked explanation with additional detail, see the Organic Chemistry: Formulae and Naming study guide; for exam-style questions with full mark schemes, see the Organic Chemistry practice questions.