Draw and interpret the displayed formula of a molecule to show all the atoms and all the bonds

Organic Chemistry – Formulae, Functional Groups & Terminology (IGCSE 0620)

Learning Objectives

• Draw and interpret the displayed (structural) formula of any organic molecule, showing every atom and every covalent bond.
• Use a displayed formula to count atoms, write the molecular and empirical formulae, and identify functional groups.
• Write the correct IUPAC name (and common name where relevant) for alkanes, alkenes, alcohols and carboxylic acids containing up to four carbon atoms.
• Recognise the general formulae and repeat units of the four core homologous series studied in the syllabus.

1. What is a Displayed (Structural) Formula?

• Each atom is represented by its chemical symbol (C, H, O, N, …).
• Each covalent bond is shown as a line:
  • single bond – one line (‑)
  • double bond – two parallel lines (=)
  • triple bond – three parallel lines (≡)
• Hydrogen atoms attached to carbon are usually omitted for brevity; they are understood to be present to satisfy carbon’s valency of four.
• All other atoms (O, N, halogens, etc.) are drawn explicitly, together with any hydrogen attached to them (e.g. the H in –OH).
• Key terminology:
  • Saturated – only single C–C bonds (alkanes).
  • Unsaturated – contains one or more C=C or C≡C bonds (alkenes, alkynes).
  • Molecular formula – the actual number of each type of atom in a molecule.
  • Empirical formula – the simplest whole‑number ratio of the atoms.

2. Quick‑Check Checklist for Interpreting a Displayed Formula

1. Identify every atom symbol (C, H, O, N, halogen…).
2. Count the lines attached to each atom to determine the number of bonds.
3. Apply valency rules (C = 4, H = 1, O = 2, N = 3, halogen = 1) to work out how many hydrogens are *implied* but not drawn.
4. Write the molecular formula by adding up all explicit atoms and the implied hydrogens.
5. Derive the empirical formula by reducing the molecular formula to the smallest whole‑number ratio.
6. Spot functional groups (‑OH, ‑C(=O)‑, ‑COOH, ‑NH₂, etc.).
7. Name the compound using the functional‑group hierarchy (see Section 4).

3. The Four Core Homologous Series

4. Functional‑Group Hierarchy (IGCSE 0620)

When more than one functional group is present, the group highest in the hierarchy determines the suffix of the IUPAC name.

1. Carboxylic acid (‑COOH) → suffix “‑oic acid”
2. Ester (‑COOR) → suffix “‑oate”
3. Alcohol (‑OH) → suffix “‑ol”
4. Aldehyde (‑CHO) → suffix “‑al”
5. Ketone (‑C=O‑) → suffix “‑one”
6. Amine (‑NH₂, ‑NHR, ‑NR₂) → suffix “‑amine”
7. Alkene (C=C) → suffix “‑ene”
8. Alkyne (C≡C) → suffix “‑yne”
9. Alkane (no higher‑priority group) → suffix “‑ane”

5. Drawing & Interpreting Displayed Formulae

5.1 Drawing a Displayed Formula from a Name

1. Identify the longest carbon chain – this is the “parent”.
2. Number the chain from the end that gives the lowest possible numbers to the principal functional group.
3. Insert the appropriate functional‑group symbols at the indicated positions.
4. Draw single bonds as straight lines; double bonds as “=”, triple bonds as “≡”.
5. Omit H atoms on carbon unless they are needed to show a functional group (e.g. the H in –OH).

5.2 Interpreting a Displayed Formula (Deriving the Molecular Formula)

1. Count every explicit atom (C, O, N, halogen, etc.).
2. For each carbon, count the number of bonds shown; add enough H atoms to give a total of four bonds.
3. For hetero‑atoms, add H atoms to satisfy their normal valency (O = 2, N = 3, halogen = 1).
4. Sum all atoms – this is the molecular formula.
5. Reduce the molecular formula to the smallest whole‑number ratio – this is the empirical formula.

6. Worked Examples

Example 1 – Ethanol

Displayed formula (exam style): CH₃–CH₂–OH

1. Explicit atoms: C = 2, O = 1, H (shown) = 3 + 2 + 1 = 6.
2. Carbon valency check – both carbons already have four bonds; no hidden H.
3. Oxygen has two bonds (to C and H) – satisfied.
4. Molecular formula: C₂H₆O.
5. Empirical formula: C₂H₆O (already simplest).
6. Functional group: –OH → alcohol.
7. IUPAC name: ethanol (common name: ethyl alcohol).

Example 2 – Propan‑1‑ol

Displayed formula: CH₃–CH₂–CH₂–OH

1. Explicit atoms: C = 3, O = 1, H (shown) = 3 + 2 + 2 + 1 = 8.
2. All carbons satisfy valency; no hidden H.
3. Molecular formula: C₃H₈O.
4. Empirical formula: C₃H₈O.
5. Functional group: –OH → alcohol.
6. IUPAC name: propan‑1‑ol (common name: 1‑propanol).

Example 3 – Acetic Acid

Displayed formula: CH₃–C(=O)–OH

1. Explicit atoms: C = 2, O = 2, H (shown) = 3 + 1 = 4.
2. Carbonyl carbon has four bonds (C, O double, O single) – satisfied.
3. Carboxyl‑OH oxygen already has its H shown.
4. Molecular formula: C₂H₄O₂.
5. Empirical formula: C₁H₂O₁ (or CH₂O).
6. Functional group: –COOH → carboxylic acid.
7. IUPAC name: ethanoic acid (common name: acetic acid).

Example 4 – But‑2‑ene

Displayed formula: CH₃–CH=CH–CH₃

1. Explicit atoms: C = 4, H (shown) = 3 + 2 + 2 + 3 = 10.
2. Each carbon has four bonds (including the double bond); no hidden H.
3. Molecular formula: C₄H₈.
4. Empirical formula: C₂H₄.
5. Functional group: C=C double bond → alkene.
6. IUPAC name: but‑2‑ene (the “‑2‑” indicates the position of the double bond).

7. Naming Displayed Formulae (Alkanes, Alkenes, Alcohols, Carboxylic Acids ≤ 4 C)

8. Practice Questions (with Answers)

1. Draw the displayed formula for butan‑2‑ol and write its molecular formula.
   Answer: CH₃–CH(OH)–CH₂–CH₃ → C₄H₁₀O.
2. Identify the functional group(s) in the displayed formula CH₃–C(=O)–OH and give the IUPAC name.
   Answer: Functional group – carboxylic acid (‑COOH). IUPAC name – ethanoic acid.
3. For the displayed formula CH₂=CH–CH₃, determine the number of double bonds and name the compound.
   Answer: One C=C double bond; IUPAC name – prop‑1‑ene (common name – propene).
4. Convert the IUPAC name “propan‑2‑ol” into a displayed formula and give the molecular formula.
   Answer: Displayed formula – CH₃–CH(OH)–CH₃. Molecular formula – C₃H₈O.
5. Write the displayed formula for the carboxylic acid “butanoic acid” and state its molecular formula.
   Answer: Displayed formula – CH₃–CH₂–CH₂–COOH. Molecular formula – C₄H₈O₂.
6. Given the displayed formula CH₃–CH₂–CH=CH₂, (a) state whether the compound is saturated or unsaturated, (b) write its molecular formula, and (c) give the IUPAC name.
   Answer: (a) Unsaturated (contains one C=C). (b) C₄H₈. (c) but‑1‑ene.

9. Quick‑Scan of Syllabus Alignment (Section 11 – Organic Chemistry)