Molecular Conformations

• What can flexible molecules adopt?

  Different shapes (conformations)
• How many conformations are possible with 5 rotatable bonds?

  6.0466 x 10¹² conformations
• What affects the energy of different conformations?

  Each conformation has a different energy
• Why do molecules adopt particular shapes?

  Electron pairs repel each other
• When are molecules most stable?

  When electron pairs are far apart
• What is strain in molecular terms?

  Energy associated with geometry
• What is steric strain?

  Strain due to non-bonded interactions
• What happens if bond angles are less than optimal?

  Destabilization occurs due to repulsion
• What is Van der Waals strain?

  Destabilization from close electron clouds
• What is torsional strain?

  Destabilization from close bond interactions
• What is a torsional angle?

  Angle between C-X and C-Y bonds
• What is ring strain?

  Extra energy in cyclic structures
• How does the energy of cyclic structures compare to non-cyclic structures?

  Cyclic structures are typically higher in energy
• What happens when a ring is broken?

  Extra energy is released
• Which rings are considered strained?

  3-membered and 7-membered rings
• What is the bond angle in cyclohexane?

  108°
• What are the two main conformations of cyclohexane?

  Chair and boat conformations

  
• Which cyclohexane conformation is the most stable?

  Chair form

  
• What is the torsional strain in the boat conformation?

  Associated with eclipsed bonds
• What happens during ring-flip in cyclohexane?

  Axial substituent becomes equatorial

  
• What do substituents in cyclohexane prefer?

  To be equatorial
• What are 1,3-diaxial repulsions?

  Repulsions experienced by axial substituents
• What happens when the energy barrier is high?

  Interconversion is slow or impossible
• Which form of an amide has lower energy?

  The trans form
• What is geometrical isomerism?

  Asymmetrical arrangement around a double bond
• What are the two types of geometrical isomers?

  E-isomer and Z-isomer
• How are E- and Z-isomers distinguished?

  By the position of higher-priority groups
• What is the bond angle in trans-but-2-ene?

  120°
• What is the difference in melting points between saturated and unsaturated fatty acids?

  Saturated have higher melting points