Practice Problems

Chapter 5 Practice Problems

Chapter 5 Practice Problems (Stereochemistry)

  1. R/S assignment: Determine R or S for:
    a) 2-chlorobutane (at C2).
    b) 3-bromo-2-pentanol (at the carbon bearing OH).
    c) Chlorofluoroiodomethane (CHFClI).

  2. Relationship ID: Classify each pair:
    a) (R)-2-butanol vs (S)-2-butanol.
    b) cis-1,3-dichlorocyclohexane vs trans-1,3-dichlorocyclohexane.
    c) 1-bromo-1-chloroethane vs 1-chloro-2-bromoethane.
    d) (2R,3S)-dibromopentane vs (2S,3R)-dibromopentane.
    e) (R,R)-2,3-butanediol vs (S,S)-2,3-butanediol.

  3. Meso problem: Draw all stereoisomers of 2,3-dichlorobutane; label R/S for each carbon; identify the meso form and explain its achirality.

  4. Fischer projection challenge: For alanine: assign R/S to L-alanine (NH₂ left, COOH top in Fischer), draw D-alanine, and state the relationship (enantiomers).

  5. Optical activity of mixtures: A sample of 2-bromopentane is 75% R and 25% S.
    a) Compute ee.
    b) Is it optically active? Relative to pure R or racemate?
    c) What fraction is racemic pair vs excess enantiomer?

  6. Resolving enantiomers: Outline how to resolve racemic 1-phenylethanol via diastereomeric derivatives with a chiral acid; include separation and hydrolysis steps conceptually.

  7. Conformational stereochemistry: For 1,2-difluoro-1,2-dichloroethane, draw anti and gauche staggered conformers. Which (if any) is chiral? Which could rotate plane-polarized light if conformationally locked?

  8. True/False quick check:
    a) All R stereocenters rotate light clockwise.
    b) A meso compound has enantiomers.
    c) Diastereomers have different physical properties.
    d) A molecule with two stereocenters must have four stereoisomers.