9701 · 13.4
Isomerism: structural isomerism and stereoisomerism flashcards
Revision flashcards for Cambridge 9701 Isomerism: structural isomerism and stereoisomerism (syllabus 13.4). Flip, recall, then mark a real past-paper question.
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What are isomers?
Molecules that have the same molecular formula but a different arrangement of atoms in space.
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What is structural isomerism?
When molecules have the same molecular formula but a different structural formula (i.e., the atoms are connected in a different order).
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What is stereoisomerism?
When molecules have the same molecular and structural formula but a different spatial arrangement of atoms.
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Define chain isomerism with an example.
A type of structural isomerism where the carbon skeleton is arranged differently. Example: Butane and 2-methylpropane (both C₄H₁₀).
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Define positional isomerism with an example.
A type of structural isomerism where the functional group is attached to a different position on the same carbon skeleton. Example: Propan-1-ol and propan-2-ol.
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Define functional group isomerism with an example.
A type of structural isomerism where the molecules have different functional groups. Example: Propanal (an aldehyde) and propanone (a ketone), both C₃H₆O.
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What are the two conditions required for E/Z isomerism?
1. Restricted rotation around a bond (usually a C=C double bond). 2. Each carbon atom in the double bond must be attached to two different groups.
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What is a chiral centre (or asymmetric carbon)?
A carbon atom that is bonded to four different atoms or groups of atoms.
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What are enantiomers?
A pair of optical isomers that are non-superimposable mirror images of each other.
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What is the key physical property that distinguishes between a pair of enantiomers?
They rotate the plane of plane-polarised light by equal amounts but in opposite directions.
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What is a racemic mixture (or racemate)?
An equimolar mixture of two enantiomers. It is optically inactive because the rotations of the two enantiomers cancel each other out.
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Why does a C=C double bond have restricted rotation?
A C=C double bond consists of a σ-bond and a π-bond. The π-bond is formed by the sideways overlap of p-orbitals. Rotation would break this π-bond, which requires a large input of energy.