Practice With Stereochemical Relationships

Practice with Stereochemical Relationships

Chapter 5 – Practice with Stereochemical Relationships

  1. Identifying relationships: Classify each pair as enantiomers, diastereomers, identical, or constitutional isomers.

    • (R)-2-bromobutane vs (S)-2-bromobutane
    • (2R,3R)-dibromopentane vs (2R,3S)-dibromopentane
    • (R,R)-tartaric acid vs (S,S)-tartaric acid
    • cis-1,2-dichlorocyclohexane vs trans-1,2-dichlorocyclohexane
      Justify: all centers inverted → enantiomers; some inverted → diastereomers; same → identical.

  2. Assigning R/S: A Fischer projection of 2-chlorobutan-1-ol (Cl and OH on C2). Assign R or S at C2. Tip: OH > Cl > CH₂OH > H; horizontal = wedges.

  3. Meso identification: For 2,3-dibromobutane, draw (2R,3R), (2S,3S), and (2R,3S)/(2S,3R). Identify the meso isomer and explain its internal mirror plane; note the other two are enantiomers.

  4. Optical activity check: State if each is optically active, with reasoning:

    • (±)-1-phenylethanol
    • meso-2,3-butanediol
    • trans-1,2-dimethylcyclopropane

  5. Sequence puzzle: Order from “identical” to “most different”: identical molecule < enantiomers < diastereomers < constitutional isomers. Briefly explain.

Practice tips: assign priorities carefully, orient the lowest priority back, and compare R/S strings center by center to decide relationships.