Lecture 07

• What is the definition of polarimetry?

  Measurement of optical rotation in compounds
• What are the basic principles of polarimetry?

  It measures the rotation of plane-polarised light
• How does a polarimeter work?

  It measures the angle of optical rotation
• What is the relationship between structure and rotation of plane-polarised light?

  Chiral structures rotate plane-polarised light
• What is specific rotation?

  Rotation of light per unit concentration and path length
• What are the applications of polarimetry in research and the pharmaceutical industry?

  Confirming compound identity and estimating optical purity
• What are optically active compounds?

  Molecules with chiral carbon atoms or centres
• What defines chiral carbon atoms?

  Four different atoms or groups bound to them
• What are enantiomers?

  Non-superimposable mirror images of compounds
• What does polarimetry measure?

  Degrees of optical rotation in compounds
• What happens to racemic mixtures in polarimetry?

  They show no net optical rotation
• How can polarimetry confirm the identity of compounds?

  By estimating the optical purity of a compound
• What is unpolarised light?

  Light with oscillations in all directions
• What is plane-polarised light?

  Light with electric field oscillating in one direction
• What is a polarimeter?

  Instrument to measure optical rotation
• How is optical rotation measured?

  By filling a sample tube and measuring angle
• What is the initial position of the polariser and analyser?

  When they are aligned at 0 degrees
• What is specific rotation used for?

  To compare samples on different polarimeters
• What factors affect specific rotation?

  Path length, concentration, solvent, temperature
• What is optical rotation?

  Amount a chiral material rotates light
• What happens with optically inactive samples?

  They do not affect polarised light
• What is the result of a perfect racemic mixture?

  It gives a reading of 0 degrees
• How do enantiomers affect plane-polarised light?

  They rotate it in opposite directions
• What is dextrorotary designated as?

  (+) clockwise rotation
• What is levorotary designated as?

  (-) anti-clockwise rotation
• What indicates optical purity?

  Presence of only one enantiomer
• If (S)-2-bromobutane has +23.1° and (R)-2-bromobutane has -23.1°, what is the dominant isomer in a mixture with -9.2°?

  (R)-2-bromobutane is dominant
• What should be stated when measuring specific rotation?

  Wavelength and temperature should be specified
• What are common solvents used in polarimetry?

  Water, methanol, ethanol, dichloromethane
• What are the main sources of errors in polarimetry?

  Filling conditions, temperature, concentration gradients
• How is polarimetry used in the pharmaceutical industry?

  To check if a compound is racemic or pure
• How is polarimetry used in the food processing industry?

  For quality control of syrup solutions
• What is a limitation of polarimetry?

  Not specific; many compounds have similar values
• What must be done to confirm the identity of a compound using polarimetry?

  Use other methods alongside polarimetry
• What are the principles behind polarimetry and the design of the instrument?

  • Measures optical rotation of plane-polarised light
  • Uses a polarimeter to measure angles
  • Relates structure of compounds to optical activity
• How do you calculate specific rotation?

  • Specific rotation = observed rotation / (concentration × path length)
  • Units: degrees mL per dg per dm
• How do you calculate optical purity of a mixture of isomers?

  • Optical purity = (observed rotation / specific rotation) × 100%
  • Compare with pure enantiomer's specific rotation
• What are the applications and limitations of polarimetry?

  Applications:

  • Confirming identity of compounds
  • Estimating optical purity
  • Quality control in industries

  
  

  Limitations:

  • Not specific for all compounds
  • Requires calibration for accuracy