Flashcards · 9620 · A Level Topic 25

Optical isomerism (A Level) flashcards

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Asymmetric carbon atom (chiral centre): A carbon atom with four different atoms or groups attached.
Enantiomers (optical isomers): Non-superimposable mirror-image forms of a molecule with a chiral centre.
How do two enantiomers differ in their effect on plane-polarised light?: They rotate the plane of plane-polarised light by the same angle in opposite directions. Their other physical properties are the same.
Racemic mixture (racemate): A mixture of equal amounts of the two enantiomers.
Why is a racemate optically inactive?: The two enantiomers rotate the plane of polarised light equally in opposite directions; with equal amounts, the rotations cancel.
Why does nucleophilic addition of CN⁻ to an aldehyde give a racemate?: The C=O carbon is planar, so CN⁻ can attack from either side with equal probability, forming the two enantiomers in equal amounts.
Which carbon atoms can never be chiral centres?: Carbons in C=C, C=O or C≡N, any CH₃ or CH₂ carbon, and any carbon in a benzene ring.