What are optical isomers?

Stereoisomers that are non-superimposable mirror images of each other (enantiomers).

Why is propan-2-ol not chiral?

Its central carbon has two identical CH₃ groups, so it is not bonded to four different groups.

Define optical activity.

The ability of a single optical isomer to rotate the plane of plane-polarised monochromatic light.

What is plane-polarised monochromatic light?

Light of a single wavelength that vibrates in only one plane.

How do two enantiomers differ in their effect on polarised light?

They rotate the plane by equal angles in opposite directions.

What is a racemic mixture?

A mixture containing equal amounts of the two enantiomers.

Why is a racemic mixture optically inactive?

The equal and opposite rotations of the two enantiomers cancel.

Which physical properties do enantiomers share?

Boiling point, melting point and density (the same in a non-chiral environment).

What does an SN1 reaction of a single enantiomer give?

A racemic mixture, because the planar carbocation is attacked from either side.

What does an SN2 reaction of a single enantiomer give?

A single enantiomer with the configuration inverted, so the product is optically active.

Why is the SN1 product racemic?

The carbocation is planar, so the nucleophile attacks from either side equally.

What would you see in a polarimeter for an SN1 reaction of an optically active tertiary halogenoalkane?

The rotation falls to zero as the racemic product forms.

Chirality and optical isomerismEdexcel A-Level Chemistry: Flashcards

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What is a chiral centre?

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What is a chiral centre?: A carbon atom bonded to four different atoms or groups.
What are optical isomers?: Stereoisomers that are non-superimposable mirror images of each other (enantiomers).
Why is propan-2-ol not chiral?: Its central carbon has two identical CH₃ groups, so it is not bonded to four different groups.
Define optical activity.: The ability of a single optical isomer to rotate the plane of plane-polarised monochromatic light.
What is plane-polarised monochromatic light?: Light of a single wavelength that vibrates in only one plane.
Which instrument measures optical rotation?: A polarimeter.
How do two enantiomers differ in their effect on polarised light?: They rotate the plane by equal angles in opposite directions.
What is a racemic mixture?: A mixture containing equal amounts of the two enantiomers.
Why is a racemic mixture optically inactive?: The equal and opposite rotations of the two enantiomers cancel.
Which physical properties do enantiomers share?: Boiling point, melting point and density (the same in a non-chiral environment).
What does an SN1 reaction of a single enantiomer give?: A racemic mixture, because the planar carbocation is attacked from either side.
What does an SN2 reaction of a single enantiomer give?: A single enantiomer with the configuration inverted, so the product is optically active.
Why is the SN1 product racemic?: The carbocation is planar, so the nucleophile attacks from either side equally.
What would you see in a polarimeter for an SN1 reaction of an optically active tertiary halogenoalkane?: The rotation falls to zero as the racemic product forms.

Exam questions on Chirality and optical isomerism

A student examines four alcohols: propan-2-ol, butan-2-ol, 2-methylpropan-2-ol and pentan-3-ol. Only one of them exists as a pair of optical isomers.
Explain why butan-2-ol shows optical isomerism.2 marks
A polarimeter measures the rotation of plane-polarised monochromatic light by solutions of 2-chlorobutane. Sample P is a pure single enantiomer and rotates the plane of polarised light by +20°. Sample Q is the other enantiomer of 2-chlorobutane. Sample R is made by mixing equal amounts of P and Q, with all solutions at the same total concentration.
Explain why Sample R shows no optical activity.2 marks
Lactic acid, CH₃CH(OH)COOH, exists as two optical isomers. In a polarimeter, a solution of one pure enantiomer of lactic acid at a concentration of 0.10 mol dm⁻³ rotates plane-polarised monochromatic light by +6.0°. The other enantiomer rotates it by the same angle in the opposite direction. A student prepares a second solution, also with total concentration 0.10 mol dm⁻³, containing 75% of the (+) enantiomer and 25% of the (−) enantiomer by amount.
Identify the chiral centre in lactic acid and explain why this molecule has two enantiomers. State how the two enantiomers differ in their effect on plane-polarised light.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).