Molecular Conformation
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Three Dimensional Representations:

  1. Lines with wedges (forward) and slashes (backward)
  2. Perspective (looking down at things)
  3. Ball and tube
  4. Space filled
  5. Fischer projections
  6. Haworth projections
  7. Newman projections

Exam question 16:  What is the corresponding systematic name or line drawing for a given Newman Projection?

Conformers

  1. Structures that vary only by "twisting" sigma bonds are called conformers (or rotamers) or different conformations of the same compound.
    1. The angle difference between two bonds on the front and back atoms in a Newman projection is called the dihedral, vicinal or torsional angle.
    2. When the dihedral angle is 0, the atoms are said to be eclipsed.
    3. When the dihedral angle is 60, the atoms are said to be gauche.
    4. When the dihedral angle is 180, the atoms are said to be anti.

Conformational Energy

  1. When looking at a Newman projection of ethane there is the eclipsed (0) and staggered (60, no eclipsing) conformations and everything in between.
    1. The energy difference (torsional strain) between eclipsed and staggered conformations of ethane is 2.9 kcal/mole.
    2. The energy difference is due to an anti-bonding interaction between adjacent sigma bonds that have large p character (simulation).
  2. When looking at a Newman projection of butane (C2-C3) there are various conformations:
    1. The lowest energy conformation is when methyls are anti (180) to each other, which is a staggered conformation.
    2. The next lowest energy conformation (0.8 kcal/mole above the anti conformation) is when methyls are gauche (60) to each other, which is also a staggered conformation.
    3. The next higher energy conformation (3.4 kcal/mole above the anti conformation) is when both methyls are eclipsing a hydrogen or are 120 from each other.
    4. The highest energy conformation (6.1 kcal/mole above the anti conformation) is when methyls are eclipsing (0) and methyl hydrogens bump into each other.
    5. Repulsive interactions between atoms is called a steric interaction.
  3. Combining the energy values of ethane and butane conformations we can do conformational analysis.
    1. Relative to when hydrogens are gauche, the hydrogen-hydrogen eclipsing energy is 2.9/3 = 1.0 kcal/mole.
    2. Relative to when methyls are anti, the methyl-methyl gauche energy is 0.8 kcal/mole.
    3. Given that when both methyls are eclipsing a hydrogen in butane two hydrogens are also eclipsing, the methyl-hydrogen eclipsing energy is (3.4-1.0)/2 = 1.2 kcal/mole.
    4. Given that when methyls are eclipsing in butane two pairs of hydrogens are also eclipsing, the methyl-methyl eclipsing energy is 6.1- 2 x 1.0 = 4.1 kcal/mole.
    5. Assuming that methyl and methylene interactions are approximately the same, the zigzag or fully staggered conformation has the lowest energy for normal alkanes.
    6. These energies are estimated by DG = -R T ln(K), K = e(-DG/RT), where K is the ratio of the concentration of two conformers.
  4. Examples:
    1. 2,3-dimethylbutane conformers.
    2. Chair and boat cyclohexanes.
    3. Axial and equatorial methylcyclohexane.
    4. Cis and trans dimethylcyclohexanes.

Exam question 17:  What is the energy difference between the two given conformations to a tenth of a kcal/mole?

 

 

 

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