Stereochemistry 1

• What is a chiral centre?

  A carbon with 4 non-identical groups
• What defines a chiral molecule?

  A molecule with one or more chiral centres
• What are enantiomers?

  Non-superimposable mirror images
• How do enantiomers differ in their interaction with plane-polarised light?

  They differ in their rotation of plane-polarised light
• What is a racemate?

  A 50:50 mixture of enantiomers
• Why do different enantiomers of the same drug show different pharmacological properties?

  They interact differently with chiral environments
• What are the steps to assign R or S configurations using Cahn-Ingold-Prelog (CIP) Rules?

  1. Prioritize atoms by atomic number
  2. Lowest priority group must point away
  3. Link up highest three priority atoms
  4. Determine orientation: clockwise = R, anticlockwise = S
• What is the first step in the Cahn-Ingold-Prelog (CIP) Rules?

  Prioritize each atom by atomic number
• How do you determine priority if the first rule does not yield a clear order?

  Compare atomic numbers of subsequent atoms
• What is the priority order for isotopes based on atomic mass?

  Tritium > deuterium > hydrogen
• What are the properties of enantiomers regarding physical properties and biological interactions?

  • Same physical properties (mp, bp, density)
  • Differ in behavior towards plane-polarized light
  • Differ in interactions with biological systems
• What is the biological effect of L-Dopa?

  It is an anti-Parkinson’s disease agent
• What is optical activity and how is it measured?

  • Ability to rotate plane-polarized light
  • Measured using a polarimeter
  • Rotation is denoted as [α]
• What is the term for clockwise rotation of plane-polarized light?

  Dextrorotatory (d)
• What is the specific rotation of a drug with +1.9° rotation in a 5 cm tube?

  $+$$38°$
• How do you calculate the enantiomeric composition of a mixture?

  1. Use observed rotation and pure isomers' [α]D
  2. Calculate excess of one isomer
  3. Determine remaining percentages of both isomers
• What is the enantiomeric excess formula?

  $ee =$$\frac{\alpha \times 100}{\alpha_D}$