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6: Stereochemistry at Tetrahedral Centers

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  • Learning Objectives

    After reading this chapter and completing the exercises and homework, a student can be able to:

    • recognize and classify molecules as chiral or achiral and identify planes of symmetry - refer to section 6.1
    • draw, interpret, and convert between perspective formulae and Fischer projections for chiral compounds - refer to section 6.2
    • name chiral compounds using (R) & (S) nomenclature - refer to section 6.3
    • recognize and classify diastereomers and meso compounds - refer to section 6.4 and 6.5 respectively
    • explain how physical properties differ for different types of stereoisomers - refer to section ?????
    • distinguish and discern the structural and chemical relationships between isomeric compounds - refer to section 6.6
    • define and explain the lack of optical activity of racemic mixtures - refer to section 6.7
    • determine the percent composition of an enantiomeric mixture from polarimetry data and the for specific rotation formula - refer to section 6.7
    • explain how to resolve (separate) a pair of enantiomers - refer to section 6.8
    • interpret the stereoisomerism of compounds with three or more chiral centers - refer to section 6.9
    • compare and contrast absolute configuration with relative configuration - refer to section 6.10
    • interpret the stereoisomerism of compounds with nitrogen, phosphorus, or sulfur as chiral centers - refer to section 6.11
    • recognize and explain biochemical applications of chirality - refer to section 6.12
    • describe Jean Baptiste Biot and Louis Pasteur's contributions to the understanding of optical isomers - refer to section 6.13

    • 6.1: Chirality
      Chiral carbons are tetrahedral carbons bonded to four unique groups.  At first glance, many carbons may look alike, but upon closer inspection, we can discern their differences.
    • 6.2: Fischer Projections to communicate Chirality
      Converting between perspective formula structures (wedges and dashes) and Fischer projections can be useful when evaluating stereochemistry, especially for carbohydrate chemistry.
    • 6.3: Absolute Configuration and the (R) and (S) System
      The absolute configuration of chiral centers as R or S is determined by applying the Cahn-Ingold-Prelog rules.
    • 6.4: Diastereomers - more than one chiral center
      Diastereomers are stereoisomers with two or more chiral centers that are not enantiomers. Diastereomers have different physical properties (melting points, boiling points, and densities). Depending on the reaction mechanism, diastereomers can produce different stereochemical products.
    • 6.5: Meso Compounds
      A meso compound is an achiral compound that has two or more chiral centers.  Molecular symmetry allows the mirror images to super-impose so that they are not enantiomers.
    • 6.6: Isomerism Summary Diagram
      A simple diagram is helpful in distinguishing between the different types of isomers that are possible.
    • 6.7: Optical Activity and Racemic Mixtures
      Optical activity is one of the few ways to distinguish between enantiomers.  A racemic mixture is a 50:50 mixture of two enantiomers.  Racemic mixtures were an interesting experimental discovery because two optically active samples were combined to create an optically INACTIVE sample.
    • 6.8 Resolution (Separation) of Enantiomers
      The most commonly used procedure for separating enantiomers is to convert them to a mixture of diastereomeric salts that can be  separated based on their differences in their physical properties.  After separation,  the isolated D or the L enantiomer can be recovered.
    • 6.9: Stereochemistry of Molecules with Three or More Asymmetric Carbons
      As the number of chiral carbons increases, the number of stereoisomers also increases.  This sections shows a short cut for compounds with three or more stereocenters.
    • 6.10: Absolute and Relative Configuration - the distinction
      The absolute configuration at a chiral center in a molecule is a time-independent and unambiguous symbolic description of the spatial arrangement of ligands (atoms) around it.  The relative configuration is the experimentally determined relationship between two enantiomers even though we may not know the absolute configuration.
    • 6.11: Chirality at Nitrogen, Phosphorus, and Sulfur
      Chirality can also occur with central atoms other than carbon.
    • 6.12: Biochemistry of Enantiomers
      Biological activity and chirality are strongly correlated.  The section explores a few examples.
    • 6.13 The Discovery of Enantiomers
      The initial work carried out by Biot and Pasteur contributed to the concepts of chirality.
    • 6.14: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 6.15: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.