conformational analysis: cyclic systems

• Strain: Destabilizing effects present within molecules due to electrostatic repulsions between electrons in close proximity
• Angle strain is strain from deviations from optimal angle bonds
• Torsional strain: repulsion between electrons in adjacent bonds as they twist and move into close proximity. Therefore, atoms experience torsional strain in the eclipsed conformation and are relieved in the staggered conformation
• Which newman projection conformation would have the least torsional strain?

  Staggered because this orientation has the least adjacent interference
• Which newman projection has the most torsional strain?

  Eclipsed because the molecules are directly adjacent to each other and those are in closest proximity
• Van der waals strain is due to atoms or groups being too close to each other
• Steric strain is the sum of all strains present in a molecule
• Ring strain?

  Strain that is present in a cyclic structure compared to an analogous acyclic structure. Energy is released when ring is broken. Strain is mainly due to angle and torsional strain.
• smaller rings have more ring strain
• Constitutional isomers differ in the order in which the atoms are connected so they can contain different functional groups and / or bonding patterns (e.g. branching)
• Stereoisomers have the same functional groups and connectivities, they differ only in the arrangement of atoms and bonds in space
• Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about σ bonds, and are often rapidly interconverting at room temperature
• Anti?

  Description given to two substitutents attached to adjacent atoms when their bonds are at 180 degrees with respect to each other.
• Eclipsed?

  A high energy conformation where the bonds on adjacent atoms are directly aligned with each other.
• Staggered?

  A low energy conformation where the bonds on adjacent atoms bisect each other (60 degrees), maximising the separation.
• Gauche?

  Description given to two substitutents attached to adjacent atoms when their bonds are at 60 degrees with respect to each other.
• Syn?


  Description given to two substitutents attached to adjacent atoms when their bonds are at 0 degrees with respect to each other.
• Ring flipping?

  Equatorial and axial H atoms in a chair conformation switch
• Most stable conformation for cyclohexane?

  chair;  The C-C-C bonds are very close to 109.5 degrees, so it is almost free of angle strain. It is also a fully staggered conformation and so is free of torsional strain.
• Least stable conformation for cyclohexane?

  Boat; optimal angles so no angle strain but there are 2 H atoms (flagpole H's) in close proximity causing steric interactions known as the flagpole interaction. Also, 4 of the H atoms are eclipsed causing torsional strain