Isomerism: optical flashcards

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What is a chiral centre?

A carbon atom that is bonded to four different atoms or groups of atoms. It is also known as an asymmetric carbon.

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What are enantiomers?

A pair of optical isomers that are non-superimposable mirror images of each other.

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What property defines optical isomers?

Their ability to rotate the plane of plane-polarised light. One enantiomer rotates it clockwise (+), the other rotates it anti-clockwise (-) by the same magnitude.

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What is a racemic mixture (or racemate)?

An equimolar (50:50) mixture of two enantiomers. It is optically inactive.

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Why is a racemic mixture optically inactive?

The equal and opposite rotations of the plane of polarised light by the two enantiomers cancel each other out, resulting in no net rotation.

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What is plane-polarised light?

Light in which the oscillations of the electric field are restricted to a single plane.

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What does 'non-superimposable' mean?

It means that no matter how you rotate or move one object, you cannot make it identical to its mirror image.

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Identify the chiral centre in 2-chlorobutane.

The carbon at position 2 (C2). It is bonded to H, Cl, CH₃, and CH₂CH₃.

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Why is propan-2-ol achiral (not chiral)?

The central carbon (C2) is bonded to two identical methyl (CH₃) groups, so it does not have four different groups attached.

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How is a racemic mixture often formed in a lab reaction?

Through nucleophilic addition to a planar carbonyl group (C=O) in an unsymmetrical aldehyde or ketone. The nucleophile can attack from above or below the plane with equal probability.

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What is the biological significance of chirality?

Enzymes and receptors in biological systems are themselves chiral. They often interact with only one enantiomer of a substrate or drug, leading to different biological effects. For example, one enantiomer of limonene smells of oranges, the other of lemons.