Explain retention, inversion and racemisation with one example each. Why does an SN1 reaction of an optically active halide give racemisation?

1. 1 · Define the key terms. A chiral molecule is not superimposable on its mirror image and is optically active: it rotates plane polarised light.
2. 2 · Retention. (−)-2-Methylbutan-1-ol + HCl → (+)-1-chloro-2-methylbutane: no bond to the chiral carbon breaks, so the arrangement is kept.
3. 3 · Inversion. SN2: (−)-2-bromooctane + OH⁻ → (+)-octan-2-ol; backside attack flips the groups.
4. 4 · Draw the inversion. Wedge-dash drawing of the reactant, the transition state in brackets and the inverted product. Label backside attack and inversion.
5. 5 · Racemisation. SN1: the carbocation is sp² and planar; attack from both faces gives a 50:50 mixture, so there is no net rotation.
6. 6 · Link to real life. Medicines are often needed as one enantiomer, so chemists choose reactions that control configuration.
7. 7 · Close in one line. So SN2 inverts, SN1 racemises, and a reaction that breaks no bond at the stereocentre keeps it.

Scoring tip: Always write 'planar carbocation' and 'both faces' in a racemisation answer.