PRPM112: ISOMERISM AND CHIRALITY

• isomers


  compounds having the same numbers and atoms but differ in atom's arrangement
• structural / constitutional isomers


  different bond pattern or connectivity
• skeletal (CSI)


  different carbon skeletons
• functional (CSI)


  different functional groups
• positional (CSI)


  different position of functional groups
• stereoisomers


  same molecular formula and constitution but different spatial (3D) arrangement of atoms
• stereoisomers


  same bond pattern / connectivity
• stereochemistry


  chemistry concerned with the three dimensional (3d) structures of molecules
• conformers (rotamers)


  stereoisomers interconvertible by single bond rotation
• configurational isomers


  stereoisomers not interconvertible by single bond rotation
• conformers (rotamers)


  different arrangement due to rotation around a SINGLE BOND
• sawhorse representation


  views the C-C bond from an oblique angle and indicates spatial relationships by showing all the C-H bonds

  
• newman projection presentation


  a method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis.

  
• staggered (99%)


  the lowest-energy, most stable conformation

  
• staggered (99%)


  all six C-H bonds are as far away from one another as possible

  
• eclipsed (1%)


  the highest-energy, least stable conformation

  
• eclipsed (1%)


  six C-H bonds are as close as possible

  
• steric strain


  repulsion as result of trying to occupy the same space
• geometric (cis-trans) isomers


  - Lack of rotation in C=C bonds

  - For disubstituted alkenes

  
• cis isomers


  the two Xs are on the same side

  
• trans isomers


  the two x's are on opposite sides

  
• there cannot be cis-trans isomerism.


  Question: Can there be cis-trans isomerism if two identical groups are connected to the same position?
• cis-but-2-ene

  
  

  - cis alkenes are less stable than their trans isomers because of steric strain


  Question: Which is less stable?

  
  

  cis-but-2-ene or trans-but-2-ene?
• chirality


  handedness (Greek cheir = "hand")
• chirality


  any object can be viewed in a mirror, revealing
• superimposable chirality


  mirror image is identical to the actual object.
• nonsuperimposable chirality


  object and its mirror image are different.
• stereocenter (chirality center)or chiral carbon


  The presence of a carbon atom bonded to four different groups

  
• plane of symmetry


  a mirror plane that cuts a molecule in half, so that one half of the molecule is a mirror of the other half
• not chiral (achiral)


  Question: Is a molecule a chiral if it contains a plane of symmetry?
• optical activity


  the rotation of plane-polarized light by a chiral molecule
• polarimeter


  measures the amount of rotation
• Dextrorotatory (+)- (D-glucose)


  rotate plane polarized light to the right (clockwise)
• Levorotatory (-) - (L-mannose)


  rotate plane polarized light to the left (counterclockwise)
• racemetes (racemic mixtures)


  50:50 mixture of enantiomers
• 2^n

  
  

  n = number of chirality centers


  maximum number of stereoisomers
• enantiomers


  stereoisomers that are like a mirror-image (nonsuperimposable)
• diastereomers


  stereoisomers that are NOT mirror images of each other
• mesocompounds


  compounds that are achiral, yet contain stereocenters

  
  

  multiple chiral centers and have internal plane of symmetry
• chiral objects


  are not superimposable on their mirror images