AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
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)

Stereoisomers and Chirality HL Only

Specification Reference S3.2.7

Quick Notes

  • Stereoisomers have the same structural formula (same atoms and connectivity) but different spatial arrangements.
  • Two key types:
    • Cis-trans isomerism (geometric isomers)
    • Optical isomerism (enantiomers)
  • Cis-trans:
    • Occurs in non-cyclic alkenes and small cycloalkanes (C3, C4).
    • Cis = groups on same side; trans = groups on opposite sides.
  • Chiral carbon: bonded to four different groups.
    • Produces enantiomers (non-superimposable mirror images).
  • Enantiomers rotate plane-polarised light in opposite directions (optical activity).
  • A racemic mixture contains equal amounts of both enantiomers and shows no optical activity.

Full Notes

Stereoisomerism

Stereoisomers have the same structural formula, but atoms are arranged differently in 3D space.

Note – Cis and Trans and E and Z isomerism has been covered in more detail here.
This page is just what you need to know for AHL IB chemistry :)

Geometrical Isomerism (Cis/Trans)

Cis-Trans isomerism occurs in alkenes due to restricted rotation around the C=C double bond, as pi-bonds are unable to rotate without breaking.

To show cis/trans isomerism:

Example But-2-ene

IB Chemistry diagram showing cis and trans isomers of but-2-ene with CH3 groups on same or opposite sides of C=C.

Cis-but-2-ene: CH3 groups on same side of C=C.
Trans-but-2-ene: CH3 groups on opposite sides of C=C.

This type of isomerism is also common in cyclic compounds when rotation is restricted.

Example 1,2-dimethylcyclopropane

IB Chemistry diagram showing cis and trans isomers of 1,2-dimethylcyclopropane.

The ring prevents free rotation, so the two methyl groups can be locked in position on the same side (cis) or opposite sides (trans) of the ring plane.

Optical Isomerism

If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule).

There are two possible ways the atoms or groups can be arranged giving two possible stereoisomers.

The isomers are non-superimposable mirror images of one another and are called optical isomers (enantiomers).

IB Chemistry diagram showing mirror image enantiomers of a chiral molecule.

Example 2-hydroxypropanoic acid (Lactic Acid)

IB Chemistry diagram showing the two enantiomers of lactic acid, a chiral molecule with four different groups on central carbon.

Chiral carbon: CH3CH(OH)COOH
Two enantiomers that exist as mirror images.

These isomers rotate plane-polarised light in opposite directions and have identical physical properties, except how they interact with light and other chiral substances.

A mixture that contains equal amounts of two enantiomers is called a racemic mixture and has no overall optical activity (doesn't rotate plane-polarised light).

Some molecules can have more than one chiral centre in them, especially biological molecules such as amino acids.

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Matt’s exam tip

The terms used for describing optical isomerism can be confusing. Make sure you are comfortable with the following:

  • Chiral: A carbon is chiral if it is bonded to four unique atoms or groups.
  • Optical activity: The ability of a chiral compound to rotate the plane of polarised light.
  • Enantiomer: One of two non-superimposable mirror image forms of a chiral molecule.
  • Racemic mixture: A 50:50 mixture of two enantiomers that shows no overall optical activity.

Identifying Chiral Centres and Isomer Types

From a structural formula, you should be able to:

IB Chemistry diagram showing example of chiral carbon in hydroxynitrile molecule. IB Chemistry diagram showing cis/trans pairs possible in alkenes.

Summary