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Three Dimensional Representations:
- Lines with wedges (forward) and slashes (backward)
- Perspective (looking down at things)
- Ball and tube
- Space filled
- Fischer projections
- Haworth projections
- Newman projections
Exam question 16: What is the
corresponding systematic name or line drawing for a given Newman Projection?
- Structures that vary only by "twisting" sigma bonds are called
conformers (or rotamers) or different conformations of the same compound.
- The angle difference between two bonds on the front and back atoms
in a Newman projection is called the dihedral, vicinal or torsional
- When the dihedral angle is 0°, the atoms are said to be eclipsed.
- When the dihedral angle is 60°, the atoms are said to be gauche.
- When the dihedral angle is 180°, the atoms are said to be anti.
- When looking at a Newman projection of ethane there is the eclipsed (0°)
and staggered (60°, no eclipsing) conformations and everything in between.
- The energy difference (torsional strain) between eclipsed and
staggered conformations of ethane is 2.9 kcal/mole.
- The energy difference is due to an anti-bonding interaction between
adjacent sigma bonds that have large p character (simulation).
- When looking at a Newman projection of butane (C2-C3) there are various
- The lowest energy conformation is when methyls are anti (180°) to
each other, which is a staggered conformation.
- 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.
- 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.
- 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.
- Repulsive interactions between atoms is called a steric interaction.
- Combining the energy values of ethane and butane conformations we can do
- Relative to when hydrogens are gauche, the hydrogen-hydrogen
eclipsing energy is 2.9/3 = 1.0 kcal/mole.
- Relative to when methyls are anti, the methyl-methyl gauche energy
is 0.8 kcal/mole.
- 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.
- 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.
- Assuming that methyl and methylene interactions are approximately
the same, the zigzag or fully staggered conformation has the lowest
energy for normal alkanes.
- 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.
- 2,3-dimethylbutane conformers.
- Chair and boat cyclohexanes.
- Axial and equatorial methylcyclohexane.
- 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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Thursday, April 06, 2017 13:18