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Monosaccharides - D and L Isomerism quiz

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  • How many stereoisomers are possible for a 2-ketohexose?
    A 2-ketohexose can have 8 stereoisomers, as it has three chiral centers, each contributing to the stereoisomer count.
  • What determines the D or L configuration of a monosaccharide?
    The D or L configuration is determined by the orientation of the hydroxyl group on the penultimate carbon.
  • What does the term 'penultimate carbon' refer to in monosaccharides?
    The penultimate carbon is the second to last carbon in a monosaccharide, which is crucial for determining its stereochemistry.
  • What is the relationship between D and L sugars?
    D and L sugars are enantiomers, meaning they are mirror images of each other.
  • How did Emil Fischer contribute to the understanding of monosaccharide chirality?
    Emil Fischer established the D and L nomenclature for sugars, although his assignments were based on guesswork and later found to have exceptions.
  • What is the mnemonic to remember the orientation of D and L configurations?
    The mnemonic is 'D is right, L is left,' referring to the direction the hydroxyl group points on the penultimate carbon.
  • Why is the correlation between D/L and optical rotation no longer valid?
    The correlation is invalid because historical assignments by Emil Fischer were based on guesswork, not actual optical measurements.
  • What is the significance of the stereo descriptor carbon in monosaccharides?
    The stereo descriptor carbon, synonymous with the penultimate carbon, determines the D or L configuration of the monosaccharide.
  • How does the orientation of the hydroxyl group affect monosaccharide classification?
    If the hydroxyl group on the penultimate carbon points to the right, the monosaccharide is classified as D-form; if it points to the left, it is L-form.
  • What is the typical configuration of the last chiral center in D and L sugars?
    In D sugars, the last chiral center usually has an R configuration, while in L sugars, it typically has an S configuration, though exceptions exist.