Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.4 Racemic mixtures-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.4 Racemic mixtures- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.4 Racemic mixtures- 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
15.4 Racemic Mixture
A racemic mixture contains equal amounts of two enantiomers. Although each enantiomer is individually optically active, the mixture as a whole is optically inactive because the effects cancel each other.
Definition
A racemic mixture is a mixture containing equal amounts of two enantiomers.
Why It Is Optically Inactive
- One enantiomer rotates plane-polarised light clockwise.
- The other rotates it anticlockwise by the same amount.
- Equal and opposite rotations cancel out.
Therefore:
No overall rotation occurs
Key Characteristics
- Contains 50% of each enantiomer.
- Optically inactive overall.
- Still contains chiral molecules.
How Racemic Mixtures Form
Racemic mixtures are commonly produced when:
- Reactions occur in non-chiral conditions.
- A planar intermediate allows attack from either side equally.
Example
Formation of 2-bromobutane from but-2-ene can produce a racemic mixture because both enantiomers form in equal amounts.
Important Distinction
- Optically active substance: contains mainly one enantiomer.
- Racemic mixture: contains equal amounts of both enantiomers.
Key Features
- Equal mixture of two enantiomers.
- No overall optical activity.
- Rotations cancel due to equal and opposite effects.
Example 1:
Define the term racemic mixture.
▶️ Answer/Explanation
A racemic mixture contains equal amounts of two enantiomers.
It is optically inactive because the rotations cancel out.
Example 2:
Explain why a racemic mixture is optically inactive even though it contains optically active molecules.
▶️ Answer/Explanation
Each enantiomer rotates plane-polarised light in opposite directions.
Since equal amounts are present, the rotations cancel exactly.
Therefore, there is no net rotation of light.