10 min readConcept sheet1-mark MCQ on chirality and racemic mixtures; 2 or 3-mark question on why SN2 gives inversion and SN1 gives racemisation; also a NEET favourite

Stereochemistry of substitution: optical activity, retention, inversion and racemisation

A chiral molecule is not superimposable on its mirror image. SN2 on a chiral carbon causes inversion; SN1 causes racemisation, because the flat carbocation is attacked from both sides.

Optical activityRotation of the plane of plane polarised light; measured with a polarimeter

d and l formsDextrorotatory (d or +) turns the plane clockwise; laevorotatory (l or −) turns it anticlockwise

Chiral (asymmetric) carbonA carbon joined to four different groups, e.g. C-2 of butan-2-ol

Racemic mixtureEqual amounts of the two enantiomers; zero rotation; written dl or (±)

Optical activity

Ordinary light vibrates in all planes. Passing it through a Nicol prism gives plane polarised light, which vibrates in one plane only. Some compounds rotate this plane; they are optically active. The angle of rotation is measured with a polarimeter. A compound that turns the plane to the right (clockwise) is dextrorotatory (d or +). One that turns it to the left is laevorotatory (l or −). The d and l forms of a compound are called optical isomers.

Chirality and enantiomers

An object that is not superimposable on its mirror image is chiral, like your left and right hands. One that is superimposable is achiral.
A carbon joined to four different groups is an asymmetric carbon or stereocentre. A molecule with one such carbon is chiral.
Butan-2-ol, CH₃CH(OH)CH₂CH₃, is chiral: C-2 carries H, OH, CH₃ and C₂H₅. Propan-2-ol, CH₃CH(OH)CH₃, is achiral: C-2 carries two CH₃ groups.
Enantiomers are the two non-superimposable mirror-image forms. They have the same physical properties, except that they rotate plane polarised light by equal amounts in opposite directions.
A racemic mixture has equal amounts of both enantiomers. Their rotations cancel, so it is optically inactive. Turning one enantiomer into a racemic mixture is racemisation.

Retention, inversion and racemisation

Term	What happens at the chiral carbon	Example
Retention	The arrangement of bonds round the chiral carbon stays the same, usually because no bond to that carbon breaks	(−)-2-Methylbutan-1-ol + HCl, heat → (+)-1-chloro-2-methylbutane (the sign changes, but the arrangement is kept)
Inversion	The new group takes the position opposite to the leaving group, so the arrangement flips	SN2: (−)-2-bromooctane + OH⁻ → (+)-octan-2-ol
Racemisation	A 50:50 mixture of the retained and inverted products forms	SN1: optically active 2-bromobutane + OH⁻ → (±)-butan-2-ol

Why SN1 gives racemisation

1
R-X → R⁺ + X⁻
The optically active alkyl halide ionises in the slow step
2★
The carbocation is sp² hybridised and planar (flat), so it is achiral.
3
The nucleophile can attack the flat carbocation from either face, about equally often.
4
Attack from the front gives the retained product; attack from the back gives the inverted product. A roughly 50:50 mixture forms, so the rotations cancel.

★ key step

How to draw inversion in an SN2 reaction

Draw the chiral carbon with three groups, using one wedge (towards you) and one dash (away from you), and X on the right.
Draw the nucleophile (OH⁻) on the left, with a curved arrow from it to the carbon, and another from the C-X bond to X.
Draw the transition state in square brackets: HO···C···X with dotted bonds and the three groups in one flat plane.
Draw the product with OH on the left and the three groups flipped to the right, like an umbrella blown inside out.
Label: backside attack, transition state, inversion of configuration.

In real life

Many medicines are chiral, and often only one enantiomer works well; the other may do nothing or even cause harm. So medicine makers choose reactions that give inversion or retention, not a racemic mixture.

Exam angle

2 marks: “What is a racemic mixture? Why is it optically inactive?” An equimolar mixture of the d and l enantiomers. They rotate the plane of polarised light equally in opposite directions, so the rotations cancel.

3 marks: “Optically active 2-bromobutane gives an optically inactive alcohol by SN1, but (−)-2-bromooctane gives (+)-octan-2-ol by SN2. Explain both.” SN1: planar sp² carbocation, attack from both faces, about 50:50 retained and inverted products, racemic. SN2: backside attack in one step, so only the inverted alcohol forms.

1 marks: “Which is chiral: butan-2-ol or propan-2-ol?” Butan-2-ol: its C-2 carries four different groups (H, OH, CH₃, C₂H₅).

Remember it

SN2 flips, SN1 mixes, and breaking no bond at the chiral carbon keeps it.