Organic Naming Rules

Organic naming is the gateway to every organic chemistry question. If you cannot name the compound, you cannot write its formula, and if you cannot write its formula, you cannot write the equation. The 0620 syllabus keeps naming simple: straight chains, up to four carbons, five homologous series.

The prefix: counting carbons

The prefix tells you how many carbon atoms are in the longest continuous chain:

Number of carbons	Prefix	Example alkane
1	Meth-	Methane (CH4)
2	Eth-	Ethane (C2H6)
3	Prop-	Propane (C3H8)
4	But-	Butane (C4H10)

These four prefixes are the only ones needed for 0620. Memorise them.

The suffix: identifying the homologous series

The suffix tells you which functional group is present:

Homologous series	Functional group	Suffix	General formula
Alkanes	C-C single bonds only	-ane	CnH2n+2
Alkenes	C=C double bond	-ene	CnH2n
Alcohols	-OH	-ol	CnH2n+1OH
Carboxylic acids	-COOH	-oic acid	CnH2n+1COOH
Esters	-COO-	-yl -anoate	varies

Putting prefix and suffix together

Compound	Carbons	Series	Name
CH4	1	Alkane	Methane
C2H4	2	Alkene	Ethene
C3H7OH	3	Alcohol	Propanol
CH3COOH	2 (including COOH carbon)	Carboxylic acid	Ethanoic acid
C4H9OH	4	Alcohol	Butanol

The five homologous series in detail

Alkanes (covered in alkanes)

• General formula: CnH2n+2
• Functional group: none (only C-C and C-H single bonds)
• Saturated hydrocarbons (all single bonds)
• Examples: methane CH4, ethane C2H6, propane C3H8, butane C4H10

Alkenes (covered in alkenes)

• General formula: CnH2n
• Functional group: C=C double bond
• Unsaturated hydrocarbons (contain a double bond)
• Examples: ethene C2H4, propene C3H6, butene C4H8
• No methene exists (you need at least 2 carbons for a C=C bond)

Alcohols (covered in alcohols)

• General formula: CnH2n+1OH
• Functional group: hydroxyl group -OH
• Examples: methanol CH3OH, ethanol C2H5OH, propanol C3H7OH, butanol C4H9OH
• The -OH is written at the end of the molecular formula

Carboxylic acids (covered in carboxylic acids)

• General formula: CnH2n+1COOH
• Functional group: carboxyl group -COOH
• Examples: methanoic acid HCOOH, ethanoic acid CH3COOH, propanoic acid C2H5COOH
• The carbon in COOH counts toward the total carbon count

Esters (covered in esters)

• Formed from alcohol + carboxylic acid
• Named as: [alcohol part]-yl [acid part]-anoate
• Example: ethanol + ethanoic acid produces ethyl ethanoate
• Esters have fruity smells and are used as flavourings and solvents

Naming esters

Ester naming follows a specific pattern:

1. The alcohol part gives the first word: remove -ol, add -yl
2. The acid part gives the second word: remove -oic acid, add -anoate

Alcohol	Acid	Ester name	Formula
Methanol	Ethanoic acid	Methyl ethanoate	CH3COOCH3
Ethanol	Ethanoic acid	Ethyl ethanoate	CH3COOC2H5
Ethanol	Propanoic acid	Ethyl propanoate	C2H5COOC2H5
Propanol	Methanoic acid	Propyl methanoate	HCOOC3H7

The ester linkage is -COO-, where the C=O comes from the acid and the O comes from the alcohol.

Drawing structural formulae

For IGCSE, you need to draw displayed (full structural) formulae showing every bond. Rules:

• Carbon always forms 4 bonds
• Hydrogen always forms 1 bond
• Oxygen always forms 2 bonds
• Nitrogen always forms 3 bonds

Common errors in structural formulae

• Drawing 5 bonds to carbon
• Forgetting the double bond in alkenes (C=C)
• Placing the -OH group in the middle of an alcohol chain instead of on a carbon
• Drawing the -COOH group incorrectly

Isomerism (Supplement)

Isomers are compounds with the same molecular formula but different structural formulae, covered in isomerism. For example:

• Butanol (C4H9OH) has isomers: butan-1-ol and butan-2-ol (the OH on different carbons)
• Butane and methylpropane are isomers of C4H10

For 0620, you need to identify that isomers exist and draw them. Full IUPAC naming with locants is not required.

Properties trends within a homologous series

As chain length increases (more carbon atoms):

Property	Trend	Reason
Boiling point	Increases	Larger molecules have stronger intermolecular forces
Viscosity	Increases	Larger molecules tangle more
Flammability	Decreases	Higher proportion of carbon, harder to ignite

These trends apply within each homologous series and are common MCQ topics.

Common exam mistakes

1. Calling ethanoic acid “acetic acid”: Use IUPAC names, not common names, in exams.
2. Writing C2H4 as an alkane: C2H4 is ethene (alkene). Ethane is C2H6.
3. Forgetting that the COOH carbon counts: Ethanoic acid CH3COOH has 2 carbons total, not 1.
4. Naming esters backwards: It is ethyl ethanoate, not ethanoate ethyl. Alcohol part first as -yl, acid part second as -anoate.

Worked exam questions

Name the compound with the structural formula CH3CH2CH2OH. State which homologous series it belongs to. [2 marks]

• Propanol / propan-1-ol [1]
• Alcohols [1]

Reasoning: 3 carbon chain (prop-) with -OH group (-ol) = propanol, belonging to the alcohols.

An ester is formed from methanol and propanoic acid. Name the ester and write the word equation for its formation. [3 marks]

• Methyl propanoate [1]
• Methanol + propanoic acid -> methyl propanoate + water [1]
• Catalyst: concentrated sulfuric acid [1]

The boiling points of four alkanes are: methane -162 C, ethane -89 C, propane -42 C, butane -1 C. Explain the trend. [2 marks]

• Boiling point increases as the chain length / number of carbon atoms increases [1]
• Larger molecules have stronger intermolecular forces, requiring more energy to overcome [1]