Isomerism and conformation

• Isomers are compounds with the same formula but in a different structure
• Stereoisomers are isomers that differ only in spatial configuration
• Constitutional isomers are isomers with different atom connectivity
• Enantiomers are non-superimposable mirror image stereoisomers
• Diastereomers are non-superimposable non-mirror image stereoisomers
• Stereogenic carbon is a carbon that gives rise to stereoisomerism
• To specify configuration of each stereogenic C, CIP system is used:

  1. assign priority based on decreasing atomic number of first atom of attached group to C
  2. view in 3D with atom (4) behind C
  3. if 1-2-3 if anti-clockwise, it is S, if clockwise, it is R
• The double bond prevents rotation so E and Z isomers are possible
• Z isomers has groups with 1st priority on the same side
• E isomers has groups with 1st priority on different sides
• Z and E is used instead of cis and trans so tri- and tetra-substituted alkenes are not ambiguous
• Conformations are structures obtained by rotation about a single bond - no bonds are broken or altered in changing conformation
• Configuration is the arrangement of atoms or groups in space about a C or other atom - bonds can be broken and atoms/groups switched around
• Cycloalkanes are flat so 3D structure twists to a chair conformation as H atoms repel each other to place themselves as far away as possible. Chair conformation places H atoms in two different environments, axial above and below the ring plane, and equatorial plane