What causes optical isomerism?

Chirality: a molecule that cannot be superimposed on its mirror image, usually from a carbon atom bonded to four different groups.

What is a chiral centre?

A carbon atom bonded to four different atoms or groups (an asymmetric carbon).

What are enantiomers?

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

Is propan-2-ol chiral? Why?

No. C2 is bonded to two identical CH₃ groups, so it does not have four different groups.

Why is butan-2-ol chiral?

C2 is bonded to four different groups: H, OH, CH₃ and CH₂CH₃.

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

One rotates the plane clockwise and the other anticlockwise, by the same angle.

What is plane-polarised light?

Light whose waves vibrate in one plane only.

What is a racemic mixture?

A mixture containing equal amounts of two enantiomers (a racemate).

Why is a racemic mixture optically inactive?

Equal and opposite rotations from the two enantiomers cancel out, so there is no overall rotation.

How are enantiomers shown in a displayed formula?

With wedges and dashed bonds around the chiral carbon, the second enantiomer being the mirror image.

Why is glycine, H₂NCH₂COOH, not optically active?

Its central carbon is bonded to two hydrogen atoms, so it does not have four different groups.

Optical isomerism and chiralityAQA A-Level Chemistry: Flashcards

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What is stereoisomerism?

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What is stereoisomerism?: Isomers with the same structural formula but a different arrangement of atoms in space.
What causes optical isomerism?: Chirality: a molecule that cannot be superimposed on its mirror image, usually from a carbon atom bonded to four different groups.
What is a chiral centre?: A carbon atom bonded to four different atoms or groups (an asymmetric carbon).
What are enantiomers?: A pair of optical isomers that are non-superimposable mirror images of each other.
Is propan-2-ol chiral? Why?: No. C2 is bonded to two identical CH₃ groups, so it does not have four different groups.
Why is butan-2-ol chiral?: C2 is bonded to four different groups: H, OH, CH₃ and CH₂CH₃.
How do two enantiomers differ in their effect on plane-polarised light?: One rotates the plane clockwise and the other anticlockwise, by the same angle.
What is plane-polarised light?: Light whose waves vibrate in one plane only.
What is a racemic mixture?: A mixture containing equal amounts of two enantiomers (a racemate).
Why is a racemic mixture optically inactive?: Equal and opposite rotations from the two enantiomers cancel out, so there is no overall rotation.
How are enantiomers shown in a displayed formula?: With wedges and dashed bonds around the chiral carbon, the second enantiomer being the mirror image.
Why is glycine, H₂NCH₂COOH, not optically active?: Its central carbon is bonded to two hydrogen atoms, so it does not have four different groups.

Exam questions on Optical isomerism and chirality

Lactic acid, CH₃CH(OH)COOH, builds up in muscle tissue during hard exercise and is also the acid that makes milk taste sour. Its molecules show optical isomerism.
Explain why lactic acid exists as a pair of optical isomers.2 marks
A chemist has two samples, X and Y, of the same chiral compound. When each is placed separately in a polarimeter, X rotates the plane of plane-polarised light by 12° clockwise and Y rotates it by 12° anticlockwise. When equal masses of X and Y are dissolved together in the same volume of solvent, the solution produces no rotation at all.
Explain why the mixture of X and Y produces no rotation of plane-polarised light.2 marks
The amino acid alanine, CH₃CH(NH₂)COOH, is found in proteins. Glycine, H₂NCH₂COOH, is the simplest amino acid. A student synthesises alanine in the laboratory from starting materials that are not chiral.
Alanine is chiral. Identify the chiral centre in alanine and the four groups bonded to it, and state how its 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).