4.6.1Unit 4. Rates, Equilibria and Further Organic Chemistry
Chirality in Optical Isomers — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 42 of 56 Unit quiz
Part 1 of 3Chiral Centres
Optical isomerism is a type of stereoisomerism found in molecules that lack a plane of symmetry.
Chiral carbon
- A carbon atom bonded to four different atoms or groups is called a chiral centre or asymmetric carbon.
Examples
- Butan-2-ol, 2-bromobutane and 2-hydroxypropanoic acid each contain one carbon with four different groups attached.
Not chiral
- Propan-2-ol is not chiral because the central carbon carries two identical methyl groups.
Marking
- Chiral centres are often marked with an asterisk on the structural formula to help identify them.
Part 2 of 3Enantiomers
A chiral molecule exists as two forms called enantiomers.
Enantiomers
Mirror images
- Enantiomers are non-superimposable mirror images, like a left and right hand, and cannot be rotated into each other.
3D drawing
- Draw the chiral carbon with two bonds in the plane, one wedge toward the viewer and one dashed away.
Same properties
- Enantiomers have the same boiling point, melting point and chemical reactions with non-chiral reagents.
Biological difference
- They can behave differently in living systems because enzymes and receptors are themselves chiral.
Part 3 of 3Exam Checklist & Tips
4.6.1 – Key Points for the Exam
- Chiral centre Carbon atom attached to four different groups.
- Enantiomers Non-superimposable mirror image stereoisomers.
- Plane-polarised light Light that vibrates in only one plane.
- Optical activity The ability to rotate the plane of polarised light.
Common Mistakes
- Calling a carbon chiral when two of its four groups are the same.
- Saying enantiomers have different melting points.
- Drawing the mirror image as a simple rotation of the original molecule.
- Thinking all molecules containing a chiral carbon cannot be separated into optical forms.
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