What makes a molecule chiral?
A molecule is chiral when it cannot be superimposed on its own mirror image, the way a left hand cannot be turned into a right hand. Most often this comes from a carbon bonded to four different groups, but the real test is the absence of an internal mirror plane (or other improper symmetry).
Watch it explained
What makes a molecule chiral?
An interactive 3D scene of a carbon holding four different groups and its reflection: lifted and turned, the mirror image never matches all four groups, so the two are non-superimposable enantiomers, like hands; swap bromine for a second chlorine and the image overlaps exactly, so the molecule is achiral.
Labeled in the scene:
- Four different groups
- Mirror image
- Try to overlap
- Spin it around
- Enantiomers
- Two groups alike
How to spot it
- Find stereocenters: sp3 carbons bonded to four different groups. CH2 and CH3 carbons never qualify, and neither does a carbon in a double bond.
- No stereocenter usually means achiral. One stereocenter always means chiral.
- Two or more stereocenters: look for an internal mirror plane. If there is one, the molecule is a meso compound and achiral despite its stereocenters.
Example: 2-butanol is chiral, because its C2 carries OH, CH3, CH2CH3 and H. 2-propanol is not, because its central carbon carries two identical CH3 groups. Meso-tartaric acid has two stereocenters but a mirror plane between them, so it is achiral.
A few molecules are chiral with no stereocenter at all, such as some substituted allenes and twisted biaryls, where the whole shape lacks a mirror plane.
Assigning R and S
- Rank the four groups by the Cahn-Ingold-Prelog rules: higher atomic number at the first point of difference wins.
- Point the lowest-priority group (often H) away from you.
- Trace 1 to 2 to 3. Clockwise is R; counterclockwise is S.
If the lowest-priority group points toward you on a wedge, trace as usual and reverse the answer. R and S say nothing about which way the compound rotates plane-polarized light; (+) and (−) are measured, not predicted from the letter.
Why it matters
Enantiomers have the same melting point, boiling point and solubility, and rotate plane-polarized light by equal amounts in opposite directions. They differ when they meet something else that is chiral, such as an enzyme or a receptor. (R)-carvone smells like spearmint and (S)-carvone like caraway; in drugs, one enantiomer can be active while the other is weaker or does something else. A molecule with n stereocenters has at most 2^n stereoisomers, fewer when meso forms exist.
Turning the mirror image
Most students get chirality once they try to stack a molecule on its mirror image and fail. Ask Isotoon "Why is a carbon with four different groups chiral?" and, on Pro, open it as a 3D model you can turn to line up the two forms yourself. Related: SN1 vs SN2, chair conformation, and the organic chemistry page.
Questions
- What is the difference between enantiomers and diastereomers?
- Enantiomers are non-superimposable mirror images of each other, with every stereocenter inverted. Diastereomers are stereoisomers that are not mirror images, usually differing at some stereocenters but not all, and they have different physical properties.
- What is a racemic mixture?
- A 50:50 mix of two enantiomers. Their optical rotations cancel, so the mixture does not rotate plane-polarized light.