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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S3.2 - Functional groups - Classification of organic compounds

3.2.1 Representing Organic Compounds 3.2.2 Functional Group 3.2.3 Homologous Series 3.2.4 Trends and Properties of Homologous Series 3.2.5 Nomenclature 3.2.6 Structural Isomerism 3.2.7 Stereoisomerism and Chirality (AHL) 3.2.8 Mass Spectrometry (MS) of Organic Compounds (AHL) 3.2.9 Infrared (IR) Spectroscopy and Greenhouse Gases (AHL) 3.2.10 Proton NMR Spectroscopy (AHL) 3.2.11 Splitting Patterns in Proton NMR Spectroscopy (AHL) 3.2.12 Interpreting Spectra (AHL)

Structural Isomerism

Specification Reference 3.2.6

Quick Notes

  • Structural isomers have the same molecular formula, but different connectivity of atoms.
  • Types to recognise:
    • Chain isomers – straight vs. branched carbon chains
    • Position isomers – same functional group in different locations
    • Functional group isomers – different functional groups with the same atoms
  • Classification of primary (1°), secondary (2°), tertiary (3°) is used for alcohols, halogenoalkanes and amines.

Full Notes

What Are Structural Isomers?

Structural isomers are molecules that have the same molecular formula but different arrangements of atoms.

There are three types of structural isomerism you need to know – chain isomerism, position isomerism and functional group isomerism.

Chain Isomerism

Carbon chain is arranged differently.

Example C4H10 (butane, methylpropane)

IB Chemistry diagram showing chain isomers butane and methylpropane, illustrating straight vs branched chains.

Butane: Straight chain.
2-Methylpropane: One branch.

Position Isomerism

Same functional group, different position on the chain.

Example C5H11Br (Bromopentane isomers)

IB Chemistry diagram showing position isomers of bromopentane with Br atom on carbons 1, 2, and 3.

1-Bromopentane: Br on carbon 1.
2-Bromopentane: Br on carbon 2.
3-Bromopentane: Br on carbon 3.

Functional Group Isomerism

Different functional groups, same molecular formula.

Example C3H6O (Aldehyde and Ketone Isomers)

IB Chemistry diagram showing functional group isomers propanal (aldehyde) and propanone (ketone).

Propanal (Aldehyde, –CHO)
Propanone (Ketone, –CO– group)

Classification of Alcohols, Halogenoalkanes, and Amines

Compounds can be classified as primary, secondary, or tertiary based on the number of carbon atoms directly bonded to the carbon (or nitrogen) atom that carries the functional group.

This classification helps determine their reactivity and is especially important in understanding reaction mechanisms.

Alcohols

IB Chemistry diagram showing classification of alcohols into primary, secondary, and tertiary based on carbon attached to –OH group.

Amines

IB Chemistry diagram showing classification of amines into primary, secondary, and tertiary depending on number of alkyl groups attached to nitrogen.

Halogenoalkanes

IB Chemistry diagram showing classification of halogenoalkanes into primary, secondary, and tertiary based on carbon bonded to halogen atom.

Linked Course Question

AHL Structure 2.2 – Linked Course Question

How does the fact that there are only 3 isomers of dibromobenzene support the current model of benzene’s structure?

If benzene had alternating single and double bonds (like in Kekulé's model), more than 3 isomers of dibromobenzene would be possible. The fact that only three isomers (1,2- / 1,3- / 1,4-dibromobenzene) exist supports the idea that all C–C bonds in benzene are identical, consistent with the delocalised electron model. This means the ring is symmetrical, and all positions are equivalent in pairs.

Summary