Your hands are mirror images, yet a left glove will not fit a right hand. Molecules with a chiral centre behave the same way, and the difference matters: the body can treat two mirror-image drugs completely differently.
A chiral centre is a carbon atom bonded to four different groups. A molecule with one chiral centre exists as two enantiomers: non-superimposable mirror images of each other. Enantiomers have identical physical properties and identical reactions with non-chiral reagents, but they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture contains equal amounts of the two and has no net effect on plane-polarised light. Living systems are chiral, so enantiomers can act very differently as drugs: one enantiomer of thalidomide is a sedative, the other caused birth defects. A molecule with n chiral centres has at most 2n stereoisomers.
Pick a molecule with four different groups on one carbon in the simulator, look at its mirror image, then try to rotate the mirror image so it fits the original.
| Molecule | Groups on the key carbon | Chiral? |
|---|---|---|
| butan-2-ol, CH3CH(OH)CH2CH3 | H, OH, CH3, C2H5 | yes, one chiral centre |
| propan-2-ol, CH3CH(OH)CH3 | H, OH, CH3, CH3 | no, two groups are the same |
| 2-aminopropanoic acid, CH3CH(NH2)COOH | H, NH2, CH3, COOH | yes |
| aminoethanoic acid, H2NCH2COOH | H, H, NH2, COOH | no, the only amino acid that is not chiral |
Large molecules often have several chiral centres. Check every carbon: skip any with a double bond, two H atoms or two identical groups.
| Molecule | Chiral centres, n | Maximum stereoisomers, 2n |
|---|---|---|
| butan-2-ol | 1 | 2 |
| 3-chlorobutan-2-ol, CH3CHClCH(OH)CH3 | 2 | 4 |
| open-chain ribose, HOCH2(CHOH)3CHO | 3 | 8 |
| open-chain glucose, HOCH2(CHOH)4CHO | 4 | 16 |
A chiral centre needs four different groups, not four different atoms: in butan-2-ol the CH3 and C2H5 both join through carbon but are different groups. A carbon in a C=C or C=O is never a chiral centre. Enantiomers have the same boiling point, density and reactions with non-chiral reagents; only the direction of rotation of plane-polarised light (and their behaviour with other chiral molecules) differs. 2n is a maximum: symmetrical molecules can have fewer.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is a chiral centre?
Which compound has a chiral centre?
How do two enantiomers differ?
Why does a racemic mixture not rotate plane-polarised light?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Open-chain glucose, HOCH2(CHOH)4CHO, has four chiral centres. What is the maximum number of stereoisomers?
How many chiral centres are there in CH3CHClCH(OH)CH3?
2-hydroxypropanenitrile is made from ethanal and HCN. Why is the product optically inactive?
Why are many modern drugs made as a single enantiomer?
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