IB DP Chemistry: AHL 20.3 Stereoisomerism : Study Notes

20.3 Stereoisomerism

Essential Idea:
Stereoisomerism involves isomers which have different arrangements of atoms in space but do not differ in connectivity or bond multiplicity (ie whether single, double or triple) between the isomers themselves.

Understandings:

  • Stereoisomers are subdivided into two classes—conformational isomers, which interconvert by rotation about a σ bond and configurational isomers that interconvert only by breaking and reforming a bond.

Configurational isomers are further subdivided into cis-trans and E/Z isomers and optical isomers.

  • Cis-trans isomers can occur in alkenes or cycloalkanes (or heteroanalogues) and differ in the positions of atoms (or groups) relative to a reference plane. According to IUPAC, E/Z isomers refer to alkenes of the form R1R2C=CR3R4 (R1 ≠ R2, R3 ≠ R4) where neither R1 nor R2 need be different from R3 or R4.
  • A chiral carbon is a carbon joined to four different atoms or groups.
  • An optically active compound can rotate the plane of polarized light as it passes through a solution of the compound. Optical isomers are enantiomers. Enantiomers are non-superimposeable mirror images of each other. Diastereomers are not mirror images of each other.
  • A racemic mixture (or racemate) is a mixture of two enantiomers in equal amounts and is optically inactive.

Applications and Skills:

  • Construction of 3-D models (real or virtual) of a wide range of stereoisomers.
  • Explanation of stereoisomerism in non-cyclic alkenes and C3 and C4 cycloalkanes.
  • Comparison between the physical and chemical properties of enantiomers.
  • Description and explanation of optical isomers in simple organic molecules.
  • Distinction between optical isomers using a polarimeter.

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