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A-Level Structural isomerism and stereoisomerism
What the A-Level syllabus expects for Structural isomerism and stereoisomerism, and how to practise it.
What the syllabus expects
- Explain what constitutional, or structural, isomerism is.
- Describe cis-trans isomerism in alkenes and trace its origin to the way a pi bond blocks rotation.
Scope: E, Z labels are not required - Say what a chiral centre is.
- Decide whether a molecule is chiral by checking for chiral centres and for any plane of symmetry.
- Recognise that a sample which turns plane-polarised light is optically active and holds chiral molecules.
- Recognise that a pair of enantiomers share every physical property except for which way each one rotates a beam of plane-polarised light.
Scope: the term diastereomers is not required - Recognise that enantiomers behave chemically alike except when reacting with some other chiral molecule.
- Recognise that stereoisomers can differ in their biological effects, as drug action illustrates.
- Given a molecular formula, work out which isomers are possible.
- Pick out any chiral centre or cis-trans isomerism in a molecule whose structural formula is supplied.
How it's examined
Questions on this topic most often ask you to explain, sketch, compare, name. About 3% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Explain how the use of chiral auxiliaries assists in the preparation of drugs that are obtained as a single pure enantiomer.
Show the worked answer
A single, enantiomerically pure chiral auxiliary is covalently attached to the starting material. This places a fixed chiral centre next to the reacting site, so the subsequent reaction now produces two diastereomers rather than two enantiomers, and one diastereomer is strongly favoured (the reaction is diastereoselective). Because diastereomers have different physical properties, the desired one can be separated by ordinary means such as crystallisation or chromatography. The auxiliary is then cleaved off to release the required single pure enantiomer (and can often be recovered and reused).
Example 2 (3 marks)
Naloxone carries a terminal alkene in the form of an N-allyl group (-CH2-CH=CH2). Representing naloxone as RCH2CH=CH2, draw the structural formulae of the optical isomers produced when naloxone reacts with bromine in an inert solvent. Label the stereochemistry of each optical isomer with its R or S configuration and justify your assignments.
Show the worked answer
Br2 adds across the terminal C=C of RCH2-CH=CH2 to give the 1,2-dibromide RCH2-CHBr-CH2Br. The internal carbon (the CHBr) now carries four different groups: -CH2R, -Br, -H and -CH2Br, so it is a single stereocentre and the product exists as a pair of optical isomers (enantiomers). The terminal -CH2Br carbon is not a stereocentre (two H). CIP priorities at the stereocentre: Br (Z=35) is highest; then compare -CH2Br vs -CH2R at the first atom - both are C, so go to the next atoms: -CH2Br gives (Br,H,H) while -CH2R gives (C,H,H); Br beats C, so -CH2Br > -CH2R; H is lowest. Priority order: Br > CH2Br > CH2R > H. Draw the two mirror-image tetrahedra: with H pointing away, if Br->CH2Br->CH2R is clockwise the centre is R, and the enantiomer (anticlockwise) is S.
Example 3 (2 marks)
Sketch two possible structures for the complex anion present in Reinecke's salt, and name the kind of isomerism that this anion displays.
Show the worked answer
Reinecke's salt contains the octahedral anion [Cr(NCS)4(NH3)2]^-, with four thiocyanate and two ammine ligands about Cr(III). Two arrangements are possible: - trans: the two NH3 ligands on opposite vertices (180 deg apart), the four NCS in the equatorial plane. - cis: the two NH3 ligands on adjacent vertices (90 deg apart). Because the two forms differ only in the relative positions of identical ligands, the anion shows cis-trans (geometric) isomerism.
More worked questions on this topic
- It is found that the value obtained for optical purity matches the enantiomeric excess exactly. (2 marks)
- With a suitable catalyst, C2H2 polymerises to polyacetylene, -[CH=CH]n-, an organic semiconduct (2 marks)
- Psilocin can be described as a tryptamine carrying a hydroxyl group at position 4: the indole f (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry