Introduction to conformational analysis

Question 1

What is a conformer?

Answer 1

Structures that only differ through the rotation of single bonds

Question 2

What is conformational analysis

Answer 2

the study of the energy chnages that occur during these rotations

Question 3

Conformational analysis allows us to predict the 3D shapes of organic molecules, which has consequences for:

Answer 3

• Reactivity
• Stereoselectivity
• J-Coupling in ¹H NMR Spectroscopy

Question 4

What is a conformer?

Answer 4

Molecular structures that can be interconverted by rotation around one or more single bonds
(same connectivity + same chiral centre)

Question 5

What are enantiomers?

Answer 5

Molecules that are non-superimposable mirror images of each other
(no way you can convert one to the other)

Question 6

What are diasteroisomers?

Answer 6

They are Steroisomers that are ‘superimposable mirror images of eadch other
(they have different chiral centre orientations

Question 7

For the S-isomer, in which direction is the rotation?

Answer 7

Anticlockwise

Question 8

For the R-isomer, which direction is the rotation?

Answer 8

clockwise

Question 9

In ethane, the barrier to rotation is 2.5kcal/mol, while it is 6kcal in butane
What is the reason for this difference?

Answer 9

Clash between the two terminal carbons (Sterics)

Question 10

The barrier to rotation in ethane is 2.5kcal/mol, while the barrier to rotation in cyclohexane between two chain isomers is 11kcal/mol
What is the reason for this difference?

Answer 10

Ring strain results in a higher energy barrier and hene a much slower equilibrium

Question 11

The barrier to rotation in butane is 6kcal/mol, while the barrier to rotation in butadiene is 7kcal/mol
What is the reason for this difference

Answer 11

The single bond in the centre has partial double bond character due to resonance
Hence making rotation higher in energy as it is more difficult

Question 12

The barrier to Rotation in DMF is 20 kcal/mol
Why is it so high

Answer 12

Single C-N bond has partial double bond character due to nitrogen donating its LP (more electronegative carbon)
high more energy is required for rotation

this is why two methyl groups are seen in NMR cuz rotation is so slow

Question 13

The following schematic shows rotation about a single bond
What is the name of these conformers?

Answer 13

Question 14

What is the difference in the energy profile for single bond rotation between ethane and butane

Answer 14

• All conformers apart from antiperiplanar are higher in energy in butane
• The synperiplanar is the highest energy conformer in butane
• Followed by the anticlinal conformer

Question 15

What are the 4 factors which contributes to the maxiumum of energy seen in the synperiplanar conformer?

Answer 15

• Sterics
• Pitzer (Torsional) strain
• Hyperconjugation
• Hydrogen bonding

Question 16

Why does Sterics contribute to conformation energy?

Answer 16

Repulsion between substituents destabilises eclipsed conformations
e.g. neither of these big substituents in blue want to clash

Question 17

What is Pitzer (Torsional) Strain contribution to conformation energy

Answer 17

Electrostatic repulsions between electrons in filled orbitals destablilises eclipsed conformers

Question 18

What is the Hyperconjugation contributions to conformational energy

Answer 18

Interaction between the filled σ orbitals and empty σ’ orbital leads to stabilisation
This stabilises the antiperiplanar conformation
(Hyperconjugation is most relevant when you have highly electronegative substituents)

Question 19

Why does hydrogen bonding contribute to the conformational energy

Answer 19

Hydrogen bonding stablilises the synclinal and synperiplanar conformation due to the formation of hydrogen bonds when the hydroxyl groups as in these positions
(predominantly for staggered conformations)