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Chemistry · What happens in 3D

Stereochemistry: inversion and racemisation

A mechanism leaves its fingerprint on the shape of the product. Know the mechanism and you can predict the stereochemistry.

JEE Main: medium priority · JEE Advanced: high priority · about 20 minutes

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Chirality

Your left and right hands are mirror images that cannot be superposed. A molecule like that is chiral. The usual cause is a stereocentre, a carbon with four different groups.

Enantiomers have the same melting point, boiling point and density. They differ only in how they turn plane-polarised light and in how they react with other chiral molecules. Dextro (+) means clockwise, and laevo (-) means anticlockwise. These signs are measured, and R and S do not predict them.

Explore in 3D: Mirror images

LightPolariserSample tubeAnalyserEyeThe analyser turns by the angle of rotation
A polarimeter measures the angle by which a sample turns plane-polarised light.

R and S in four steps

  1. Rank the four groups by atomic number of the atom attached to the stereocentre. If tied, move outward to the next atoms.
  2. Point the lowest priority group away from you.
  3. Trace 1, 2, 3.
  4. Clockwise is R, and anticlockwise is S. If the lowest group points towards you, reverse the answer.

Explore in 3D: Name it: R or S

Practise this in the 3D lab, where the page draws a random stereocentre and checks your answer.

Worked example: count stereoisomers

How many stereoisomers does (a) 2-bromo-3-chlorobutane and (b) 2,3-dibromobutane have?

Show the solution

Both have two stereocentres, so start from 22=4.

(a) The two ends differ, so no mirror plane can exist. All 4 are real: two pairs of enantiomers.

(b) The two ends match. The (R,S) and (S,R) forms are one meso compound, so the count is 3.

What each mechanism does

ReactionStereo outcomeWhy
SN2 at the stereocentreInversionAttack from the back only
SN1 at the stereocentreRacemisationFlat cation, two open faces
Reaction not touching the bond to the stereocentreRetentionThe arrangement never changes
abcAttack from aboveAttack from belowEmpty p orbital
Flat cation, two open faces: equal attack gives a racemic mixture.

Explore in 3D: A flat cation has two faces

Real SN1 reactions often give a little more inversion than retention, because the leaving group partly shields one face just after it leaves. Books call this partial racemisation.

The letter trap

(R)-2-Bromobutane with OH− gives (S)-butan-2-ol, because OH takes the same top priority that Br had. But if the incoming group ranks differently from the leaving group, inversion of shape can leave the letter unchanged. Work out priorities again. Never just flip the letter.

Rotation and enantiomeric excess

%ee=observed rotationrotation of pure enantiomer×100

If a pure sample rotates +40∘ and a mixture rotates +10∘, the excess is 25%. So 62.5% is the major enantiomer and 37.5% the minor.

Try the simulator: Optical purity

Diastereomers and meso compounds

With two stereocentres, up to 22=4 stereoisomers exist. A meso compound has stereocentres and an internal mirror plane, so it is optically inactive, and the count falls. 2,3-Dichlorobutane has three stereoisomers: a pair of enantiomers and one meso form. The JEE Advanced syllabus lists enantiomers, diastereomers and meso compounds.