What is optical isomerism?

Stereoisomerism in which two isomers are non-superimposable mirror images, caused by a chiral centre.

What are enantiomers?

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

How are enantiomers drawn in 3D?

With the chiral carbon at the centre, two bonds in the plane, one wedge and one dash, the mirror image swapping the wedge and dash groups.

How do enantiomers differ in physical properties?

They do not: melting point, boiling point and density are identical.

What is optical activity?

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

What is plane-polarised light?

Light whose waves vibrate in one plane only.

What is a racemic mixture?

An equal mixture of two enantiomers, which has no net rotation of plane-polarised light.

Why is a racemic mixture optically inactive?

The equal and opposite rotations cancel.

What does an optically inactive product from a pure enantiomer indicate?

An SN1 mechanism: a planar carbocation is attacked from either side.

What does an optically active product from a pure enantiomer indicate?

An SN2 mechanism: attack from the opposite side gives inversion and a single enantiomer.

Why is HCN addition to ethanal racemic?

The carbonyl group is planar, so CN⁻ attacks from either side equally and both enantiomers form.

Is 2-hydroxypropanenitrile chiral?

Yes: C2 is bonded to H, CH₃, OH and CN.

Chirality and optical isomerismEdexcel International 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 groups.
What is optical isomerism?: Stereoisomerism in which two isomers are non-superimposable mirror images, caused by a chiral centre.
What are enantiomers?: A pair of optical isomers that are non-superimposable mirror images.
How are enantiomers drawn in 3D?: With the chiral carbon at the centre, two bonds in the plane, one wedge and one dash, the mirror image swapping the wedge and dash groups.
How do enantiomers differ in physical properties?: They do not: melting point, boiling point and density are identical.
What is optical activity?: The ability of a single enantiomer to rotate the plane of plane-polarised monochromatic light.
What is plane-polarised light?: Light whose waves vibrate in one plane only.
What is a racemic mixture?: An equal mixture of two enantiomers, which has no net rotation of plane-polarised light.
Why is a racemic mixture optically inactive?: The equal and opposite rotations cancel.
What does an optically inactive product from a pure enantiomer indicate?: An SN1 mechanism: a planar carbocation is attacked from either side.
What does an optically active product from a pure enantiomer indicate?: An SN2 mechanism: attack from the opposite side gives inversion and a single enantiomer.
Why is HCN addition to ethanal racemic?: The carbonyl group is planar, so CN⁻ attacks from either side equally and both enantiomers form.
Is 2-hydroxypropanenitrile chiral?: Yes: C2 is bonded to H, CH₃, OH and CN.

Exam questions on Chirality and optical isomerism

Lactic acid (2-hydroxypropanoic acid), CH₃CH(OH)COOH, is made by muscle cells during hard exercise and by bacteria in yoghurt production.
Explain why lactic acid shows optical isomerism and what is meant by its optical isomers.2 marks
Ibuprofen, (CH₃)₂CHCH₂C₆H₄CH(CH₃)COOH, is a painkiller that is sold as an equal mixture of its two optical isomers, although only one of them is pharmacologically active. Optical activity is measured in a polarimeter using plane-polarised monochromatic light.
Explain why the equal mixture of ibuprofen isomers has no effect on plane-polarised light.2 marks
Two samples of halogenoalkane, each consisting of a single optical isomer, are hydrolysed by heating with aqueous sodium hydroxide. Compound X, 3-bromo-3-methylhexane, CH₃CH₂C(CH₃)(Br)CH₂CH₂CH₃, gives an alcohol product that does not rotate the plane of plane-polarised light. Compound Y, 2-bromobutane, gives butan-2-ol that does rotate the plane of plane-polarised light.
Explain what the result for compound X indicates about the mechanism of its hydrolysis.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).