Representations and Nomenclature (A Level Organic)

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-.

Key Ideas (What Earns Marks)

• Every carbon must have 4 bonds (count double bond as 2, triple as 3).
• Identify the longest carbon chain that includes the functional group.
• Number the chain so the functional group gets the lowest possible number.
• Use suffixes for the highest-priority functional group (e.g. -ol, -al, -one, -oic acid) and prefixes for substituents (e.g. chloro-, methyl-).

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)

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

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

PropaneEthane

Carbon bearing chlorine

Carbon 2Carbon 1

Check my attempt

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

FourThree

Groups bonded to central carbon

Three CH₃ groups and HTwo CH₃ groups and two H

Check my attempt

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.

Next step

Try naming the five-carbon skeletal alcohol independently.

Common Mistakes

• Miscounting carbons in skeletal structures (each vertex counts).
• Forgetting that heteroatoms must be shown explicitly (you can’t “assume an oxygen”).
• Numbering from the wrong end, giving higher locants than necessary.
• Treating a displayed formula like a skeletal formula (displayed must show every atom and bond).

Use the Organic Chemistry topic check to practise and check your understanding.

Exam Tips

• If the question shows a skeletal structure, rewrite it mentally as a displayed formula before deciding the name.
• When asked to “draw a displayed formula”, show every bond and every atom label.
• If a name includes a locant (e.g. butan-2-ol), the position number is a mark: do not omit it.

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.