Chirality and stereocentres
A molecule is chiral if it cannot be placed on top of its mirror image. This usually happens at a carbon with four different groups, called a stereocentre (chiral centre). A molecule with n stereocentres has up to 2ⁿ stereoisomers.
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Analogy
Your left and right hands: mirror images that never fit into the same glove.
Example
C2 holds: OH, CH₃, C₂H₅, HAll four different, so chiralTwo mirror forms exist2¹ = 2 stereoisomers
In practice
One mirror form of a drug can treat illness while the other does nothing or causes side effects, so drug makers control which form they make.
Practice
Flashcards
- What makes a carbon a stereocentre?Four different groups attached.
Interview questions
- Assign R or S to one form of 2-butanol, give the priority order, and explain what enantiomers are.
- Priorities: OH > ethyl > methyl > H
- Lowest priority at the back
- Clockwise is R, anticlockwise is S
- Enantiomers are mirror images with opposite labels
Questions people ask
How do I spot a stereocentre?
Look for a carbon with four different groups attached, as in 2-butanol or lactic acid. A carbon with two identical groups, like CH₂ or a carbon in a double bond, is not a stereocentre. Check each group by following the chain until a difference appears.
Do chiral molecules have different physical properties?
Mirror-image molecules (enantiomers) have identical melting points, boiling points and solubility. They differ in how they rotate plane-polarised light, in opposite directions, and in how they interact with other chiral things, like enzymes and receptors in the body. That is why one mirror form of a drug may work differently from the other.