3.3.7
Optical Isomerism Flashcards
Active recall practice for stereocentres, enantiomer pairs, polarimetry, and racemic mixtures.
Chirality & Enantiomers: read all 6 questions and answers
- What is stereoisomerism?Isomers with the same structural formula but a different spatial arrangement of atoms.
- What is optical isomerism?A form of stereoisomerism where two molecules are non-superimposable mirror images of each other.
- Define chiral centre (asymmetric carbon atom).A carbon atom bonded to four different groups.
- What are enantiomers?A pair of optical isomers (non-superimposable mirror images).
- Does propan-2-ol show optical isomerism?No, because the central carbon atom is bonded to two identical methyl (-CH3) groups, so it is not chiral.
- Does butan-2-ol show optical isomerism?Yes, because the central carbon (carbon-2) is bonded to four different groups: -H, -OH, -CH3, and -CH2CH3.
Optical Activity & Polarimetry: read all 3 questions and answers
- What is plane-polarised light?Light that oscillates in only a single plane.
- Explain how optical isomers differ in their interaction with plane-polarised light.Enantiomers rotate the plane of plane-polarised light by equal angles but in opposite directions (one clockwise, one anticlockwise).
- What is a polarimeter?An instrument used to measure the direction and angle of rotation of plane-polarised light by an optically active substance.
Racemic Mixtures & Synthesis: read all 5 questions and answers
- What is a racemic mixture (racemate)?A 50:50 mixture of two enantiomers.
- Why is a racemic mixture optically inactive?The clockwise rotation of plane-polarised light by one enantiomer is exactly cancelled out by the anticlockwise rotation by the other enantiomer.
- Why does the reaction of ethanal with HCN form a racemic mixture?The carbonyl group (C=O) in ethanal is planar. The cyanide nucleophile (CN-) has an equal probability of attacking the delta+ carbon from above or below the plane, producing equal amounts of the two enantiomers.
- Why is separating enantiomers difficult in chemical synthesis?Enantiomers have identical physical properties (e.g. boiling point, solubility) and identical chemical properties in reactions with achiral reagents, making standard separation methods like distillation useless.
- Why is stereospecific synthesis important in the pharmaceutical industry?Often only one enantiomer of a drug is therapeutically active, while the other may be inactive or harmful (e.g. thalidomide).