Compare the properties of enantiomers
Describe the relationship between mirror-image enantiomers and distinguish shared properties from chirality-dependent effects.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 29. Follow the formula, naming and optical-isomerism limits stated in the syllabus. Benzene-ring displayed formulas are not expected; use the supplied convention for aromatic skeletal formulas. Do not extend the optical-isomerism scope to naming diastereoisomers or meso compounds. Written practice is self-checked
Before you begin
- Use covalent formula conventions, functional groups, reaction terminology, and the AS organic chemistry foundations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What relationship defines enantiomers?
What you will learn
- Recognise a pair of non-superimposable mirror-image molecules
- State which physical and chemical properties are the same
- Identify optical rotation and possible biological activity as differences
Enantiomers are mirror images that cannot be superimposed
Enantiomers are a pair of optical isomers that are mirror images but cannot be placed on top of one another so that all corresponding atoms and bonds match. A tetrahedral carbon attached to four different groups is a common source of this chirality.
The syllabus notes that compounds can contain more than one chiral centre, but naming diastereoisomers and meso compounds is not required. Keep this topic focused on the stated enantiomer outcomes.
Most ordinary properties match; chiral interactions may differ
A pure pair of enantiomers has identical physical and chemical properties in an achiral environment, apart from its effect on plane-polarised light and its potential biological activity.
Biological molecules and receptors are themselves chiral, so two enantiomers may interact differently with them. The word potential matters: different biological effects can occur, but they should not be assumed for every pair without evidence.
Pause and explain
Which property can differ between pure enantiomers in an achiral environment?
Worked example
Compare two pure enantiomers in an achiral environment and in a biological receptor site.
Show the worked solution
- In an achiral environment their ordinary physical and chemical properties are identical.
- They rotate plane-polarised light in opposite directions.
- A chiral biological site can interact differently with each, so activity may differ.
Answer Enantiomers have the same ordinary physical and chemical properties in an achiral environment, rotate plane-polarised light oppositely, and may have different biological activity because a receptor is chiral.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State two pieces of evidence needed to identify a proposed pair as enantiomers.
Give me a hint
Check both mirror relationship and superimposability.
Compare my reasoning
- The structures are mirror images.
- They cannot be superimposed with all corresponding atoms matching.
- They have the same connectivity and formula.
They must be non-superimposable mirror images with the same atom connectivity and molecular formula.
Look for these in your work
- Mirror relationship checked
- Superimposability checked
- Connectivity retained
A learner says enantiomers have different boiling points because they are different molecules. Correct the statement.
Give me a hint
Apply the syllabus comparison in an achiral environment.
Compare my reasoning
- Enantiomers have identical ordinary physical properties in an achiral environment.
- They do rotate plane-polarised light in opposite directions.
- Biological interactions may differ because biological sites can be chiral.
Enantiomers have the same ordinary physical properties in an achiral environment; their optical rotation differs, and their biological activity may differ in chiral systems.
Look for these in your work
- Physical-property claim corrected
- Optical difference stated
- Biological qualification retained
Why might two enantiomers of a medicine have different effects even though their ordinary properties match?
Give me a hint
Compare an achiral environment with a chiral receptor.
Compare my reasoning
- Their ordinary properties match in an achiral setting.
- A biological receptor has a chiral three-dimensional shape.
- The enantiomers may fit or interact differently with that receptor.
A chiral biological receptor can interact differently with each mirror image, so their biological activity may differ even when ordinary achiral properties match.
Look for these in your work
- Environment distinguished
- Receptor chirality stated
- Effect appropriately qualified
Common mistakes
- Calling mirror-image molecules identical without checking superimposability
- Assuming all measurable properties differ
- Claiming all enantiomers have different biological activity
- Confusing enantiomers with any pair of structural isomers
What can you explain now?
What relationship defines enantiomers?
Compare with the explanation
Non-superimposable mirror images
Both mirror-image relationship and non-superimposability are required.
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Make it stick
Which properties of enantiomers are identical, and which effects may depend on chirality?
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