A chiral centre is one of the most important ideas in IB Chemistry Topic 10 (Organic Chemistry). It is the foundation of optical isomerism and explains why some molecules exist as non-superimposable mirror images called enantiomers. Chirality is especially important in pharmaceuticals and biological systems, where mirror-image molecules can behave very differently. Understanding what a chiral centre is—and how to spot one—is essential for mastering stereochemistry in the IB syllabus.
What Is a Chiral Centre?
A chiral centre (also called a stereocentre or asymmetric carbon) is a carbon atom bonded to four different atoms or groups.
Because the carbon is attached to four unique substituents, the molecule can exist in two distinct forms that are mirror images of each other. These mirror images cannot be superimposed, just like your left and right hands.
This is the origin of the word “chiral,” which comes from the Greek word for “hand.”
How to Identify a Chiral Centre
To determine whether a carbon is a chiral centre, check the groups directly attached to it.
A carbon is chiral if:
- It is bonded to four different substituents
- None of the groups are repeated
- The structure is three-dimensional (tetrahedral)
Steps to identify:
- Locate a carbon with four single bonds.
- Examine all four attached groups.
- Check if each group is different.
Even if two groups look similar, compare the atoms further along the chain. - If all four are different, the carbon is chiral.
Common examples of chiral centres appear in amino acids, sugars, alcohols, and many pharmaceuticals.
Why Chiral Centres Create Optical Isomers
A carbon with four different groups can form two non-superimposable mirror-image structures. These two forms are called enantiomers.
Enantiomers:
- Have identical physical properties (melting point, boiling point, density)
- Rotate plane-polarized light in opposite directions
- May have very different biological activity
Because their physical properties are so similar, identifying them requires spectroscopic or optical methods.
Chirality and 3D Molecular Structure
A chiral centre forces a tetrahedral arrangement around carbon.
This creates two possible orientations of the substituents in three-dimensional space.
The two mirror-image forms cannot be rotated to match, no matter how you twist the molecule. This property explains many differences in molecular behavior.
Examples of Chiral Centres
1. Lactic acid
CH₃–CH(OH)–COOH
The middle carbon has four different groups:
- H
- OH
- CH₃
- COOH
→ It is chiral.
2. Amino acids (except glycine)
Most amino acids have a carbon bonded to:
- NH₂
- COOH
- H
- A side chain (R)
→ This creates chirality.
3. 2-butanol
CH₃–CH(OH)–CH₂–CH₃
The carbon with the OH group is chiral because all four attachments differ.
Why Chirality Matters in Chemistry
1. Biological specificity
Enzymes, receptors, and proteins often recognize only one enantiomer.
For example:
- One enantiomer of a drug might be therapeutic
- The other might be inactive or harmful
2. Chemical reactivity
Chiral centres influence how molecules react in stereospecific reactions.
3. Optical activity
Chiral molecules rotate plane-polarized light:
- One enantiomer rotates right (dextrorotatory)
- The other rotates left (levorotatory)
IB students must understand how to identify this property.
Common IB Misunderstandings
“A carbon with double bonds can be chiral.”
Incorrect. A chiral centre must have four single bonds to four different groups.
“If two groups look similar, the carbon is achiral.”
Check the entire substituent, not just the first atom.
“Chiral molecules always have a chiral centre.”
Some molecules without traditional chiral centres can be chiral, but this is beyond IB scope.
“Only organic molecules can be chiral.”
Some inorganic compounds also show chirality, though not covered in IB Chemistry.
FAQs
How many chiral centres can a molecule have?
Any number—some molecules have dozens. More chiral centres increase the number of stereoisomers.
Do chiral centres always produce exactly two isomers?
Yes. Each chiral centre creates a pair of enantiomers, although multiple centres create more possibilities.
Is a carbon with two identical groups ever chiral?
No. If any two substituents are identical, it cannot be a chiral centre.
Conclusion
A chiral centre is a carbon atom bonded to four different groups, creating non-superimposable mirror-image molecules called enantiomers. Chiral centres are central to stereochemistry, influencing optical activity, biological behavior, and reactivity. Mastering chiral centres gives IB Chemistry students a strong foundation for understanding isomerism and molecular structure.
