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Representations and Nomenclature (A Level Organic)

Learn and apply Representations and Nomenclature (A Level Organic) in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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H2 Organic Representations and Nomenclature: Orientation

Organic marks are often lost before the chemistry starts: misreading skeletal formulae, miscounting carbons, and naming/numbering errors. This lesson makes those rules automatic so you can decode structures quickly and write exam-safe names.

This topic gets cleaner if you cross-check Organic Mechanisms: Curly Arrows, Electrophiles, Nucleophiles while navigating from the Organic Chemistry hub.

Definitions (Must Know)

A. Structural (displayed) formula

A displayed formula shows all atoms and all bonds explicitly.

B. Skeletal formula

A skeletal formula is a line-angle drawing where:

  • each line end / vertex represents a carbon atom,
  • hydrogen atoms on carbon are omitted, and
  • heteroatoms (e.g. O, N, halogens) are shown explicitly with their attached hydrogens.

C. Functional group

A functional group is the atom/group of atoms responsible for a molecule’s characteristic reactions (e.g. -OH in alcohols).

D. Homologous series

A homologous series is a family of organic compounds with the same functional group and general formula, where successive members differ by -CH₂-.

Detailed Explanations

A. Common representations (what the question might show)

  • Empirical formula: simplest whole-number ratio of atoms.
  • Molecular formula: actual number of each type of atom (e.g. C₄H₁₀). No connectivity is shown.
  • Structural formula: shows the order in which atoms are joined, usually in a compact form such as CH₃CH₂OH.
  • Displayed formula: all bonds shown.
  • Skeletal formula: fastest for larger molecules; you must infer omitted hydrogens correctly.
  • Three-dimensional formula: a solid wedge shows a bond coming out of the plane; a dashed wedge shows one going behind it.

B. Workflow: reading a skeletal structure correctly

  1. Count the carbon atoms (each vertex/line end is one carbon).
  2. Identify any heteroatoms (they are drawn explicitly).
  3. For each carbon, count the bonds already shown; add H atoms until that carbon has 4 bonds.

Because carbon forms 4 covalent bonds, therefore any “missing bonds” in a skeletal structure are completed by hydrogens.

Mini example:

  • A 4-vertex zig-zag chain with no heteroatoms is a 4-carbon alkane → C₄H₁₀ (butane).

C. Functional groups (core set)

Functional groupExampleNotes
AlkaneCH₃CH₃no functional group; only single C–C bonds
AlkeneCH₂ = CH₂contains C=C
HalogenoalkaneCH₃CH₂ClC–X where X = F/Cl/Br/I
AlcoholCH₃CH₂OHcontains –OH
AldehydeCH₃CHOterminal -CHO
KetoneCH₃COCH₃internal C = O
Carboxylic acidCH₃COOHcontains -COOH
AmineCH₃NH₂contains –NH2 (or substituted)
AreneC₆H₆contains a benzene ring
PhenolC₆H₅OH–OH attached directly to a benzene ring
EsterCH₃COOCH₃contains -COO⁻
Acyl chlorideCH₃COClcontains -COCl
AmideCH₃CONH₂contains -CONH₂ or a substituted amide group
NitrileCH₃CNcontains -C#N
Amino acidH₂NCH₂COOHcontains both amino and carboxyl groups

D. Workflow: naming (IUPAC basics)

  1. Choose the longest chain containing the functional group.
  2. Number to give the functional group the lowest locant.
  3. Name substituents with locants, list alphabetically (ignore di-/tri- for ordering).
  4. Apply the suffix and functional group locant (where needed).

Mini example:

  • CH₃CH(Cl)CH₃ has a 3-carbon chain (propane) with Cl on carbon 2 → 2-chloropropane.

E. Local shape, hybridisation and bonds

Carbon centreHybridisationLocal shapeApproximate angleBonding feature
four electron regionssp³tetrahedral109.5°four σ bonds
carbon in C=C or C=Osp²trigonal planar120°three σ regions and one π bond
carbon in C≡C or C≡Nsplinear180°two σ regions and two π bonds

A single bond is one σ bond. A double bond is one σ plus one π bond; a triple bond is one σ plus two π bonds. Use the local carbon centre, not the shape of the whole molecule, when stating hybridisation and bond angle.

Worked Examples

Modelled example 1

Name a Straight-Chain Alcohol

Core

Problem

Name CH₃CH₂CH₂OH.
Study the worked solution

  1. Choose the parent

    Method

    Count three carbons in the longest chain containing the alcohol group.

    Reason

    The principal functional group must belong to the parent chain.

    Working

    Three-carbon parent: propane.

  2. Number from the correct end

    Method

    Number from the end nearest OH, placing it on carbon 1.

    Reason

    The principal functional group receives the lowest possible locant.

    Working

    OH locant: 1.

  3. Build the name

    Method

    Replace the final “e” of propane with the alcohol suffix and retain the locant.

    Reason

    The suffix identifies the functional group and the locant distinguishes its position.

    Working

    Name: propan-1-ol.

Common misconception 2

Locate Chlorine on the Parent Chain

Find and correct the mistake

Learner claim

A learner names CH₃CH(Cl)CH₃ as 1-chloropropane because they started numbering at the left-hand carbon. Correct the name and explain the locant.

Try this before viewing the solution

Parent chain
Carbon bearing chlorine

View solution step by step

  1. Trace the carbon chain

    Method

    Label the three connected carbons 1–2–3.

    Reason

    Locants describe connectivity, not where a symbol happens to appear on the page.

    Working

    CH₃-CH(Cl)-CH₃: Cl is attached to the middle carbon.

  2. State the corrected name

    Method

    Name the compound 2-chloropropane.

    Reason

    Numbering from either end places chlorine on carbon 2.

    Working

    Correct name: 2-chloropropane.

Challenge 3

Draw 2-Methylpropane from Its Name

Minimal support

Representation transfer

Draw an unambiguous structural formula of 2-methylpropane and use carbon valency to check the central carbon.

Try this before viewing the solution

Total carbon atoms
Groups bonded to central carbon

Hints

Hint 1: build the parent
Draw a three-carbon propane chain and mark carbon 2.
Hint 2: add the substituent
Attach one methyl group to carbon 2, then complete that carbon’s four bonds.
View solution step by step

  1. Construct the named skeleton

    Method

    Attach a methyl group to carbon 2 of propane.

    Reason

    The parent and substituent parts of the name account for all four carbons.

    Working

    CH₃-CH(CH₃)-CH₃.

  2. Audit the central carbon

    Method

    Confirm that the central carbon has four single bonds: three to carbon and one to hydrogen.

    Reason

    Carbon valency distinguishes the intended branched structure from an incomplete drawing.

    Working

    Unambiguous structural formula: (CH₃)₃CH.

Mind Stretchers

Mind stretcher 1Extension

A skeletal structure has 5 vertices in a straight chain, with an -OH shown on the second vertex from one end. What is the IUPAC name?

Show Hint

Count the carbon vertices first, then number from the end nearest the principal group.

Show Answer

Mark scheme:

  • 5-carbon chain = pentane.
  • Number from the end that gives -OH the lowest locant → -OH on carbon 2.
  • Name: pentan-2-ol.

Mind stretcher 2: Resolving an ambiguous condensed formulaExtension

Question. A student writes C₃H₆O and calls it propanone. Explain why the molecular formula alone does not prove that name, and give one alternative structure.

Show Hint

A molecular formula gives atom counts but not connectivity or functional-group position.

Show Answer

C₃H₆O does not show how the atoms are connected, so it can represent more than one constitutional isomer. Propanal, CH₃CH₂CHO, is one alternative to propanone, CH₃COCH₃.