Organic Pulse notes · Stereochemistry
Chirality
Dr. Sandeep G. Sontakke · M.Sc., Ph.D., CSIR-NET (JRF), SET
- At a glance
- Chiral or achiral?
- Assigning R and S
- Wedges and hashes
- Optical activity
- Why chirality matters
- One-minute revision
At a glance
Chiral = not superimposable on its mirror image
- A chiral molecule cannot be laid exactly on its mirror image (IUPAC definition).
- The commonest cause is a stereocentre: a carbon carrying four different groups, marked C*.
- A chiral molecule and its mirror image are a pair of enantiomers.
- Each stereocentre is named R or S using the Cahn–Ingold–Prelog (CIP) rules.
- Enantiomers rotate plane-polarised light by equal amounts in opposite directions: (+) or (−).
The word comes from the Greek kheir, hand: your left and right hands are the everyday example of chirality.
Chiral or achiral?
Look at each carbon: if it carries four different groups, it is a stereocentre and the molecule is usually chiral. Ringed carbons below are the stereocentres.
- A molecule with a plane of symmetry or a centre of symmetry is achiral, even if it contains stereocentres (meso compounds).
- Most chiral molecules contain a stereocentre, but it is not strictly required: some allenes and biaryls are chiral without one.
Assigning R and S: the CIP rules
- Step 1. Rank the four groups by the atomic number of the atom attached to the stereocentre: higher atomic number, higher priority (Br > Cl > O > N > C > H).
- Step 2. Break ties by moving outwards: compare the sets of atoms attached to each tied atom, highest first, at the first point of difference.
- Step 3. Double bonds count twice: C=O is treated as a carbon bonded to two oxygens.
- Step 4. Point priority 4 away from you.
- Step 5. Trace 1 → 2 → 3: clockwise = R (rectus, right); anticlockwise = S (sinister, left).
| Group on C2 of lactic acid | Atom attached | Atoms on that atom | Priority |
|---|---|---|---|
| –OH | O (8) | H | 1 |
| –COOH | C (6) | O, O, O | 2 |
| –CH₃ | C (6) | H, H, H | 3 |
| –H | H (1) | none | 4 |
Try it on the 3D lactic acid in Quadrant I: tap the three buttons and watch the path run clockwise for (R) and anticlockwise for its mirror image.
Drawing a stereocentre: wedges and hashes
A solid wedge points towards you, a hashed bond points away, and plain lines lie in the page. Swapping any two groups turns R into S.
Structures drawn and R/S checked by computer (RDKit, CIP rules). This is the same molecule that turns inside out in the SN2 lesson.
Optical activity
A solution of one enantiomer rotates the plane of plane-polarised light; a polarimeter measures the angle α. Clockwise rotation is (+), dextrorotatory; anticlockwise is (−), laevorotatory.
| Enantiomer | Smell | Specific rotation [α]D |
|---|---|---|
| (R)-(−)-Carvone | spearmint | −61° |
| (S)-(+)-Carvone | caraway seeds | +61° |
Data: Wikipedia, 'Carvone'. Equal and opposite rotations: the signature of an enantiomer pair.
- A racemic mixture, written (±), contains equal amounts of both enantiomers. The rotations cancel, so it is optically inactive.
- Separating a racemate into its two enantiomers is called resolution. Louis Pasteur first did it in 1848, by sorting mirror-image crystals of a tartrate salt by hand.
Why chirality matters
- Living things build with one hand: the amino acids in proteins are the L forms, almost all of which are (S).
- Smell and taste can tell enantiomers apart, as the two carvones show.
- Drugs: (R)-thalidomide is a sedative and (S)-thalidomide is linked to birth defects. The two interconvert in the body, so even a pure single enantiomer would not have been safe.
One-minute revision
- Chiral: not superimposable on its mirror image; the mirror pair are enantiomers.
- Stereocentre: a carbon with four different groups; compare whole groups.
- CIP: rank by atomic number, break ties outwards, double bonds count twice.
- Priority 4 away: clockwise R, anticlockwise S (reverse it if 4 points towards you).
- (+)/(−) is measured, not predicted; a racemate (±) is optically inactive.
Organic Pulse · free notes by Dr. Sandeep G. Sontakke. Main sources: Clayden et al., Organic Chemistry (2nd ed.), ch. 14; OpenStax/LibreTexts Organic Chemistry, sections 5.1–5.5; IUPAC Gold Book, 'chirality'. Watch the shorts: youtube.com/@OrganicPulse-y9q