Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.1–15.2 Optical isomerism and enantiomers-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.1–15.2 Optical isomerism and enantiomers- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.1–15.2 Optical isomerism and enantiomers- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.

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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics

15.1 Optical Isomerism and Chirality

Optical isomerism is a type of stereoisomerism that occurs when molecules are chiral. A chiral molecule exists as two non-superimposable mirror images called enantiomers.

Definition of Optical Isomerism

Optical isomerism occurs when compounds have the same structural formula but differ in the spatial arrangement of atoms, forming non-superimposable mirror images.

Chirality

A molecule is chiral if it cannot be superimposed on its mirror image.

Chirality usually arises from the presence of a:

Chiral centre (asymmetric carbon atom)

Chiral Centre

A chiral centre is a carbon atom bonded to four different atoms or groups.

General Form

\( \mathrm{C^*(WXYZ)} \)

where all four groups are different.

Enantiomers

  • Pair of optical isomers.
  • Non-superimposable mirror images.
  • Identical physical properties except interaction with plane-polarised light and chiral substances.

Optical Activity

  • One enantiomer rotates plane-polarised light clockwise.
  • The other rotates it anticlockwise by the same amount.

Hence the term optical isomerism.

Conditions for Optical Isomerism

  • Presence of a chiral centre.
  • Four different groups attached to carbon.
  • Molecule lacks internal symmetry.

Examples of Chiral Molecules

  • \( \mathrm{CHBrClF} \)
  • 2-butanol
  • Lactic acid

Non-Chiral Example

\( \mathrm{CH_3CH_2OH} \) is not chiral because no carbon has four different groups attached.

Key Features

  • Optical isomerism results from chirality.
  • Usually caused by a carbon bonded to four different groups.
  • Produces enantiomers (mirror-image isomers).
  • Enantiomers rotate plane-polarised light in opposite directions.

Example 1:

Explain why 2-butanol shows optical isomerism.

▶️ Answer/Explanation

The second carbon atom is bonded to four different groups:

  • \( \mathrm{H} \)
  • \( \mathrm{OH} \)
  • \( \mathrm{CH_3} \)
  • \( \mathrm{CH_2CH_3} \)

Therefore, it contains a chiral centre and exists as enantiomers.

Example 2:

Explain why propane does not exhibit optical isomerism.

▶️ Answer/Explanation

No carbon atom in propane is attached to four different groups.

Therefore, propane has no chiral centre and is not optically active.

15.2 Optical Isomerism, Chiral Centres and 3D Enantiomers

Optical isomerism occurs because of the presence of one or more chiral centres in a molecule. These give rise to a pair of stereoisomers called enantiomers, which are non-superimposable mirror images of each other.

Chiral Centre (Asymmetric Carbon Atom)

A chiral centre is usually a carbon atom bonded to:

Four different atoms or groups

Such a carbon is called an asymmetric carbon atom.

Formation of Optical Isomers

Because the groups around the chiral carbon can be arranged in two different spatial forms, two stereoisomers are produced:

  • Mirror images of each other
  • Cannot be superimposed

These are called: Enantiomers

Non-Superimposable Mirror Images

Enantiomers behave like left and right hands:

  • They are mirror images.
  • They cannot be placed exactly on top of each other.

3D Representation of Optical Isomers

Optical isomers are drawn using wedge-and-dash notation.

  • Straight line: bond in plane of page
  • Solid wedge: bond coming out of page
  • Dashed wedge: bond going behind page

Example: 2-Butanol

Chiral carbon attached to:

  • \( \mathrm{OH} \)
  • \( \mathrm{H} \)
  • \( \mathrm{CH_3} \)
  • \( \mathrm{CH_2CH_3} \)

Two mirror-image forms exist.

Simple 3D Sketch Principle

If one enantiomer has:

  • \( \mathrm{OH} \) on wedge
  • \( \mathrm{H} \) on dash

Then the other has:

  • \( \mathrm{OH} \) on dash
  • \( \mathrm{H} \) on wedge

Properties of Enantiomers

  • Same structural formula
  • Same melting/boiling points
  • Rotate plane-polarised light in opposite directions
  • Interact differently with chiral molecules

Important Exam Skills

  • Identify asymmetric carbon atoms.
  • Draw mirror-image structures correctly.
  • Use wedge-and-dash notation accurately.
  • Recognise non-superimposable mirror images.

Key Features

  • Optical isomerism arises from chiral centres.
  • Chiral carbon bonded to four different groups.
  • Enantiomers are mirror images that cannot superimpose.
  • 3D diagrams use wedge-and-dash notation.

Example 1:

Explain why 2-chlorobutan-1-ol exhibits optical isomerism.

▶️ Answer/Explanation

One carbon atom is bonded to four different groups.

  • \( \mathrm{H} \)
  • \( \mathrm{Cl} \)
  • \( \mathrm{CH_2OH} \)
  • \( \mathrm{CH_2CH_3} \)

Therefore, the molecule is chiral and exists as enantiomers.

Example 2:

State the relationship between two optical isomers of a chiral molecule.

▶️ Answer/Explanation

They are non-superimposable mirror images of each other.

They are called enantiomers.

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