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.

The homologous series

A family of compounds with the same functional group, the same general formula, differing by CH₂, showing a gradual trend in physical properties and similar chemical properties.

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	–COO–	—	-oate

Stem names — count the carbons

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

So: C₃H₈ = propane · C₄H₈ = butene · C₂H₅OH = ethanol · CH₃COOH = ethanoic acid.

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.

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	C₂H₆O (simplest ratio)
Molecular	C₂H₆O
Structural	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)

Isomers

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 — 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

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
alcohol + oxygen  --oxidation-->  carboxylic acid
alcohol + acid    ---->           ESTER + water

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.
• Isomers must have the same molecular formula; different formulae are just different compounds.
• Confusing propan-1-ol with propan-2-ol — the locant number is part of the name, not an optional extra.
• 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. Name C₃H₇OH and C₃H₇COOH.
3. How would you distinguish hexane from hexene?
4. 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. Name the ester formed from methanol and ethanoic acid.
8. What is the difference between propan-1-ol and propan-2-ol?

Answers: 1. CₙH₂ₙ. 2. Propanol and butanoic acid (note 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. 6. C6H(2x6)+2 = C₆H₁₄ (hexane). 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.