Naming organic compounds and structural isomers

Homologous series

A homologous series has the same functional group and general formula; successive members differ by CH2, and they show a trend in physical properties and similar chemical properties. A functional group is the atom or group responsible for the characteristic reactions. Alkanes are CnH2n+2 and are saturated (only single C-C bonds); cycloalkanes are CnH2n.

IUPAC names

Stem from the longest carbon chain: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). Suffix or prefix from the functional group: -ane, -ene, -ol, chloro-, bromo-. Number the chain to give the lowest numbers; name branches as methyl, ethyl, in alphabetical order, with di-, tri- for repeats.

Example: CH3CH(CH3)CH2CH3 is 2-methylbutane.

Types of formula

• Molecular: C4H10
• Structural: CH3CH2CH2CH3
• Displayed: every atom and bond drawn
• Skeletal: lines for the carbon chain, with H on carbon left out; each line end or corner is a carbon
• General: CnH2n+2

Structural isomers

Structural isomers have the same molecular formula but a different structural formula. They can differ in chain (branching), in the position of a functional group, or in the functional group itself. C4H10 has two: butane and 2-methylpropane. C5H12 has three; C6H14 has five.

Worked example

Name CH3CH2CH(CH3)CH2CH2CH3.

1. Longest chain: 6 carbons, hexane.
2. Number from the end nearer the branch: the methyl is on carbon 3.

Answer: 3-methylhexane

Check yourself

1. What is the name of CH3CH(CH3)CH3?
   • Butane
   • 2-methylpropane
   • 1-methylpropane
   • 2-methylbutane
   Show answer

   2-methylpropane. The longest chain has 3 carbons with a methyl on carbon 2.

2. How many structural isomers have the formula C5H12?
   Show answer

   3. Pentane, 2-methylbutane and 2,2-dimethylpropane.

3. How many structural isomers have the formula C6H14?
   Show answer

   5. Hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane.

4. Which is the molecular formula of a cycloalkane?
   • C6H14
   • C6H6
   • C6H12
   • C6H10
   Show answer

   C6H12. Cycloalkanes are CnH2n.

5. Successive members of a homologous series differ by
   • CH2
   • CH3
   • C2H4
   • H2
   Show answer

   CH2. Each member has one more CH2 unit.

6. What is the correct name of CH3CH2C(CH3)2CH3?
   • 3,3-dimethylbutane
   • 2-ethyl-2-methylpropane
   • 2,2-dimethylbutane
   • Hexane
   Show answer

   2,2-dimethylbutane. Longest chain is 4 C; numbering from the nearer end puts both methyls on C2.

7. What does saturated mean for a hydrocarbon?
   • It contains only single carbon-carbon bonds
   • It cannot dissolve any more solute
   • It contains a C=C double bond
   • It contains oxygen
   Show answer

   It contains only single carbon-carbon bonds. Saturated compounds have no C=C or C≡C bonds.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.