Why can two molecules with the same formula behave very differently?
Geometric (cis-trans, E/Z) isomerism arising from restricted rotation about a double bond, optical isomerism arising from chirality, enantiomers and optical activity measured by polarimetry, racemic mixtures, and the importance of stereochemistry in pharmaceuticals.
An SQA Advanced Higher Chemistry answer on stereochemistry, covering geometric (cis-trans and E/Z) isomerism from restricted rotation about a double bond or ring, optical isomerism from chirality, enantiomers and optical activity measured by polarimetry, racemic mixtures, and why stereochemistry matters in pharmaceuticals.
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What this key area is asking
The SQA wants you to explain geometric (cis-trans and E/Z) isomerism from restricted rotation, optical isomerism from chirality, the existence of enantiomers and their optical activity measured by polarimetry, racemic mixtures, and the importance of stereochemistry in pharmaceuticals. Identifying when geometric or optical isomerism is possible and explaining optical activity are reliable exam earners.
Geometric isomerism
The cis isomer has the higher-priority groups on the same side of the double bond and the trans isomer has them on opposite sides; the more rigorous E/Z system assigns priorities by atomic number. Because the pi bond prevents rotation, the isomers cannot interconvert at room temperature, and they often have different melting points, boiling points and reactivity.
Optical isomerism and chirality
The two enantiomers are identical in most physical properties (melting point, boiling point, density) but differ in how they interact with other chiral molecules and with plane-polarised light.
Optical activity and polarimetry
A substance is optically active if it rotates the plane of plane-polarised light. The two enantiomers rotate it by equal amounts in opposite directions: one clockwise (dextrorotatory, ) and one anticlockwise (laevorotatory, ). The angle and direction of rotation are measured with a polarimeter.
Racemic mixtures
A racemic mixture contains equal amounts of the two enantiomers. Because the two opposite rotations cancel exactly, a racemic mixture shows no net optical rotation. Many ordinary laboratory syntheses produce racemic mixtures because the reaction has no preference for forming one enantiomer over the other.
Stereochemistry in pharmaceuticals
Examples in context
Stereochemistry has real consequences. Many natural products, including almost all amino acids and sugars, occur as a single enantiomer, which is why the body can process natural glucose but not its mirror image. In medicine, the thalidomide tragedy showed how two enantiomers can differ catastrophically, and modern drugs such as single-enantiomer painkillers and beta-blockers are sold as pure enantiomers for safety and potency. Geometric isomerism matters in nutrition and materials: the cis isomer retinal is essential for vision, and the difference between cis and trans fats affects how the body handles them. Polarimetry is used industrially to measure the concentration and purity of optically active substances such as sugar solutions.
Try this
Q1. State the condition needed for a molecule to show geometric isomerism. [1 mark]
- Cue. A carbon-to-carbon double bond (restricted rotation) with each double-bond carbon carrying two different groups.
Q2. Define a chiral centre. [1 mark]
- Cue. A carbon atom bonded to four different groups.
Q3. Explain why a racemic mixture shows no net optical rotation. [2 marks]
- Cue. It contains equal amounts of the two enantiomers, which rotate plane-polarised light equally in opposite directions, so the rotations cancel.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH 20193 marksBut-2-ene exists as two geometric isomers. (a) Explain why geometric isomerism is possible in but-2-ene. (b) Draw or describe the cis and trans isomers. (c) State why but-1-ene does not show geometric isomerism.Show worked answer →
Markers reward the restricted-rotation reason, the two isomers and the reason but-1-ene cannot show it.
(a) The carbon-to-carbon double bond has restricted rotation because of the pi bond, so the groups on each carbon are locked in position. With a different group on each side, two arrangements are possible.
(b) In the cis isomer the two methyl groups are on the same side of the double bond; in the trans isomer they are on opposite sides.
(c) But-1-ene has two hydrogen atoms on one of the double-bond carbons. Because that carbon carries two identical groups, swapping them gives the same molecule, so geometric isomerism is not possible.
SQA AH specimen2 marksExplain what is meant by a chiral centre and why a molecule with one chiral centre is optically active.Show worked answer →
The answer must define a chiral centre and link it to non-superimposable mirror images.
A chiral centre is a carbon atom bonded to four different groups. Such a carbon can be arranged in two ways that are non-superimposable mirror images of each other, called enantiomers.
Because the two enantiomers have different three-dimensional arrangements, they rotate the plane of plane-polarised light in equal but opposite directions. The molecule is therefore optically active, and the rotation is measured with a polarimeter.
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Sources & how we know this
- SQA Advanced Higher Chemistry Course Specification — SQA (2019)