CAIE A-Level Chemistry 29.4 Optical Isomerism

Practise identifying enantiomers, interpreting plane-polarised light and explaining chirality in drug synthesis.

Exam points

identify chiral centres and draw enantiomers as non-superimposable mirror images with correct 3D bonds
compare equal-concentration pure enantiomers as rotating plane-polarised light equally in opposite directions
explain why drug synthesis may require one enantiomer and how separation or a chiral catalyst provides it

Covered syllabus points

29.4.1—Enantiomers have identical physical/chemical 29.4.2—Terms: optically active and racemic mixture 29.4.3—Effect on plane polarised light of the two 29.4.4—Relevance of chirality to the synthetic

[Maximum number: 4]

The amino acid serine, $\mathrm{HOCH}_{2} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{COOH}$ , exists in two optically active forms. These optical isomers, isomer P and isomer Q, are shown in Fig. 8.1.

Fig. 8.1

(a)

Isomer P and isomer Q have identical physical and chemical properties, with the exception of two specific properties. One of these two properties is their differing effect on plane polarised light.

State the other property by which they differ.

[ 1 ]

(b)

A solution of pure isomer P of a particular concentration rotates plane polarised light by $5.0^{\circ}$ in a clockwise direction.

Describe how a solution of pure isomer Q of the same concentration affects plane polarised light.

[ 1 ]

rotates plane polarised light $5.0^{\circ}$ in anticlockwise direction

[1]

(c)

Give the term used to describe a mixture containing equal amounts of isomer P and isomer Q.

[ 1 ]

(d)

Describe one way in which a single pure optical isomer of serine can be produced, instead of making a mixture of isomer P and isomer Q.

[ 1 ]

[Maximum number: 1]

Describe what is meant by a racemic mixture.

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Covered syllabus points

29.4 Optical isomerism question 1

Question (a)

Question (b)

Question (c)

Question (d)

29.4 Optical isomerism question 2