State that a saturated compound has molecules in which all carbon-carbon bonds are single bonds

Displaying Formulas (Bond‑line / Skeletal Diagrams)

In the exam you will be asked to draw or interpret bond‑line (displayed) formulas. Remember:

• Each vertex (or line end) = a carbon atom.
• Hydrogen atoms attached to carbon are **implicit** – you do not write them.
• Double bonds are shown as “=”, triple bonds as “≡”. (Only double bonds are required for the core.)
• Functional groups (‑OH, ‑COOH, etc.) are drawn explicitly.

Examples

   H   H   H   H
   |   |   |   |
H–C–C–C–C–H
   |   |   |
   H   H   H

   H   H   H   H
   |   |   |   |
H–C–C = C–C–H
   |       |
   H       H

   H   H
   |   |
H–C–C–O–H
   |   |
   H   H

   H   H
   |   |
H–C–C–C–O–H
       ||
       O

Placement of Double Bonds

When a molecule contains more than one possible double‑bond location (e.g., but‑1‑ene vs. but‑2‑ene) you must show the exact position in the bond‑line diagram. This is a common exam pitfall.

Naming Unbranched Compounds (core requirement)

CH₃‑CH₃

CH₃‑CH₂‑CH₃

CH₃‑CH₂‑CH₂‑CH₃

CH₃‑CH=CH‑CH₃

CH₂=CH‑CH₂‑CH₃

CH₃‑CH₂‑OH

CH₃‑CH₂‑COOH

Structural Isomers (supplementary topic)

Only C₄H₁₀ isomers are required for the supplement. You should be able to:

• Identify the two isomers (n‑butane and isobutane).
• Draw each structure from its name.

Examples

• n‑Butane: CH₃‑CH₂‑CH₂‑CH₃ (straight chain)
• Isobutane (2‑methylpropane): CH₃‑CH(CH₃)‑CH₃ (branched)

Alkynes – why they are not in the core

Alkynes contain C≡C triple bonds and are not listed in the 0620 syllabus. They are omitted to keep the core focused on alkanes, alkenes, alcohols and carboxylic acids.

State Symbols (required for symbol equations)

• (s) – solid
• (l) – liquid
• (g) – gas
• (aq) – aqueous solution

Example symbol equation (combustion of ethane):

C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l)

Common Misconceptions

• “All organic compounds are saturated.” – False. Alkenes have C=C double bonds and are unsaturated.
• “A compound with a functional group cannot be saturated.” – False. Saturation refers only to the carbon skeleton. Ethanol (CH₃CH₂OH) is saturated because its C‑C bonds are all single.
• “Alkynes are part of the IGCSE core.” – False. They are excluded from the 0620 syllabus.
• “Hydrogen can be added to any organic molecule to make it saturated.” – False. Hydrogen can only be added if the carbon skeleton already contains only single C‑C bonds; otherwise a bond must be broken first.

Quick‑Check Questions

1. Write the molecular formula for the saturated hydrocarbon containing five carbon atoms.
2. Is cyclohexane (C₆H₁₂) saturated or unsaturated? Explain using the definition of saturation.
3. Identify the functional group in CH₃CH₂COOH and name the compound.
4. Draw the bond‑line (displayed) formula for prop‑1‑ene (C₃H₆).
5. Give the IUPAC name for the displayed formula

   CH₃‑CH(CH₃)‑CH₃

   .
6. Write a balanced symbol equation (including state symbols) for the combustion of ethane.

Answers to Quick‑Check

1. C₅H₁₂ (using CₙH₂ₙ₊₂ for alkanes).
2. Cyclohexane is saturated. Although it is a ring, each carbon is bonded to two other carbons and two hydrogens – all C‑C bonds are single, satisfying the definition.
3. Functional group: **carboxyl** (‑COOH). The compound is **propanoic acid**.
4. Bond‑line for prop‑1‑ene:
   

      H   H
      |   |
   H–C = C–C–H
      |   |
      H   H
   

5. The name is **2‑methylpropane** (commonly called isobutane).
6. Combustion of ethane:
   

    C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l)