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9701 · 13.1

Formulas, functional groups and the naming of organic compounds — practice questions

Practice and worked examples for 9701 Formulas, functional groups and the naming of organic compounds. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

An organic compound has the structural formula CH3CH(CH3)CH2CH2OHCH_3CH(CH_3)CH_2CH_2OH. Give its IUPAC name.

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  1. Identify the principal functional group: The -OH group indicates it is an alcohol, so the suffix will be '-ol'.
  2. Find the longest carbon chain containing the functional group: The longest chain has 4 carbon atoms. The stem is 'but-'. So, it is a butanol.
  3. Number the chain: We must give the -OH group the lowest possible number. Numbering from right to left gives the -OH group position 1. CH3−CH(CH3)−CH2−CH2OHCH_3-CH(CH_3)-CH_2-CH_2OH becomes C4−C3(CH3)−C2−C1OHC_4-C_3(CH_3)-C_2-C_1OH.
  4. Identify substituents: There is a methyl group (CH3CH_3) on carbon 3.
  5. Assemble the name: The substituent is '3-methyl'. The parent alcohol is 'butan-1-ol'. Combining these gives the full IUPAC name: 3-methylbutan-1-ol.

Worked example 2

Draw the skeletal formula for 2,2-dichloro-3-methylbutane and state its molecular formula.

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  1. Analyse the name:
    • 'butane' means a 4-carbon main chain.
    • '3-methyl' means a CH3CH_3 group on carbon 3.
    • '2,2-dichloro' means two chlorine atoms, both on carbon 2.
  2. Draw the chain and number it: Draw a 4-carbon zig-zag chain. Let's number from left to right for convenience.
  3. Add substituents:
    • At position 2, add two bonds to 'Cl'.
    • At position 3, add a single line to represent the methyl group.
  4. Final Skeletal Formula: A 3-segment zig-zag line. The second vertex has two lines pointing to 'Cl'. The third vertex has one extra line (the methyl group).
  5. Determine Molecular Formula:
    • Main chain: 4 carbons.
    • Methyl group: 1 carbon.
    • Total Carbons = 5 (C5C_5).
    • Hydrogens: C1 has 3 H's. C2 has 0 H's (bonded to 2xCl, C1, C3). C3 has 1 H (bonded to C2, C4, and methyl C). C4 has 3 H's. The methyl group has 3 H's. Total H = 3+0+1+3+3 = 10 (H10H_{10}).
    • Halogens: 2 chlorine atoms (Cl2Cl_2).
    • Molecular Formula: C5H10Cl2C_5H_{10}Cl_2.
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