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3: Conformations and Stereochemistry

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  • Stereochemistry is the study of how bonds are oriented in three-dimensional space. It is difficult to overstate the importance of stereochemistry in nature, and in the fields of biology and medicine in particular. As Pasteur so convincingly demonstrated, life itself is chiral: living things recognize different stereoisomers of organic compounds and process them accordingly.

    • 3.1: Prelude to Conformations and Stereochemistry
      In 1848, a 25 year old chemist named Louis Pasteur made a startling - and some thought brash - claim to the scientific community. Pasteur was inexperienced, to say the least: he had only earned his doctorate the previous year, and had just started his first job as an assistant to a professor at the Ecole normale superieure, a university in Paris.
    • 3.2: Conformations of open-chain organic molecules
      Before we begin our exploration of stereochemistry and chirality, we first need to consider the subject of conformational isomerism, which has to do with rotation about single bonds.
    • 3.3: Conformations of cyclic organic molecules
      Browse through a biochemistry textbook and you will see any number of molecules with cyclic structures. Many of these cyclic structures are aromatic, and therefore planar. Many others, though, are composed of sp3-hybridized atoms, and it is these cyclic structures that are the topic of discussion in this section.
    • 3.4: Chirality and stereoisomers
      We turn now to concept of chirality that formed the basis of the story about Louis Pasteur in the beginning of this chapter. Recall that the term chiral, from the Greek work for 'hand', refers to anything which cannot be superimposed on its own mirror image.
    • 3.5: Naming chiral centers- the R and S system
      Chemists need a convenient way to distinguish one stereoisomer from another. The Cahn-Ingold-Prelog system is a set of rules that allows us to unambiguously define the stereochemical configuration of any stereocenter, using the designations 'R ’ (from the Latin rectus, meaning right-handed) or ' S ’ (from the Latin sinister, meaning left-handed).
    • 3.6: Optical Activity
      Chiral molecules, as we learned in the introduction to this chapter, have an interesting optical property. You may know from studying physics that light waves are oscillating electric and magnetic fields. In ordinary light, the oscillation is randomly oriented in an infinite number of planes. When ordinary light is passed through a polarizer, all planes of oscillation are filtered out except one, resulting in plane-polarized light.
    • 3.7: Compounds with multiple chiral centers
      So far, we have been analyzing compounds with a single chiral center. Next, we turn our attention to those which have multiple chiral centers. We'll start with some stereoisomeric four-carbon sugars with two chiral centers.
    • 3.8: Meso Compounds
      The levorotatory and dextrorotatory forms of tartaric acid studied by Louis Pasteur were, as we now know, the (S,S) and (R,R) enantiomers.
    • 3.9: Fischer and Haworth projections
      While organic chemists prefer to use the dashed/solid wedge convention to show stereochemistry, biochemists often use drawings called Fischer projections and Haworth projections to discuss and compare the structure of sugar molecules.
    • 3.P: Problems for Chapter 3
    • 3.10: Stereochemistry of alkenes
      When we talk about stereochemistry, we are not always talking about chiral compounds and chiral centers. Consider cis- and trans-2-butene.
    • 3.11: Stereochemistry in biology and medicine
      While challenging to understand and visualize, the stereochemistry concepts we have explored in this chapter are integral to the study of living things. The vast majority of biological molecules contain chiral centers and/or stereogenic alkene groups.
    • 3.12: Prochirality
      When a tetrahedral carbon can be converted to a chiral center by changing only one of the attached groups, it is referred to as a ‘prochiral' carbon. The two hydrogens on the prochiral carbon can be described as 'prochiral hydrogens'.
    • 3.13: Solutions to Chapter 3 exercises