L8/9 - Stereochemistry 1

• Structural isomers


  same Mr, different structure - (eg chain, positional, functional group)
• Stereoisomers


  same Mr, different arrangement of atoms in space
• Conformational isomers


  Different arrangement of atoms from sigma bond rotation. Staggered/Eclipsed.

  
• Constitutional isomers


  Different arrangement of atoms from bond breaking. Geometric/Optical
• Which conformation is more stable?

  Staggered conformation is lower in energy as atoms are furthest apart.

  
  

  (atoms like personal space - they don't like being eclipsed as this takes more energy)
• Chair conformations


  Axial protons (up and down)

  Equatorial protons (equator)

  
• Which chair conformation is favoured?

  Equatorial - lower in energy as axial conformations have steric repulsion

  
• Steric repulsion

  
  

  (What happens as R groups get bigger?)

  The increase in energy as atoms are crowded together.

  
  

  As R groups get bigger, there are more unfavourable interactions
• Geometric Isomers


  No rotation around pi bond. Trans (E) and Cis (Z)
• Optical Isomers


  Stereoisomers that are non-superimposable mirror images of each other
• Chiral molecules


  No plane of symmetry (non-superimposable mirror images). Sterocentre has four different groups of atoms

  
• What's the only chiral amino acid?

  Glycine

  
• Enantiomers


  Non-superimposable mirror images.

  
  

  Identical physical properties, EXCEPT the rotate plane polarised light (monochromic light) in equal but opposite directions

  
• What are the clockwise/anticlockwise rotations (of PPL) called?

  clockwise = dextrorotatory (+)

  
  

  anticlockwise = levorotatory (-)
• What's the equation for [α]D? And units?

  [α[D = α/l x c

  
  

  where [α] = specific rotation

  α = observed rotation

  l = pathlength (dm)

  c = concentration (g/cm3 og g/ml)

  
• What's the [α]D of a racemic mixture?

  0
• Enantiomeric Excess

  
  

  what is it? how can you calculate?

  the excess of one enantiomer over another. Calculate using a number line (% on top, degrees below)

  
  

  (x-y) /(x+y)