Conformation Flashcards
How can different conformations interconvert?
Through rotation about single bonds
How can stereoisomers be interconverted to different conformations?
By breaking bonds
What does a Newman projection show?
Arrangement of groups and emphasises interactions on adjacent atoms
What is a staggered conformation on the Newman projection?
The atoms are staggered in relation to each other, evenly spaced
What angle is there in a staggered conformation for a Newman projection?
Dihedral angle - spread out as far as the molecule can be
60 degrees
What are the two conformations for a Newman projection?
Staggered
Eclipse
What is an eclipsed conformation?
The groups behind are covered by groups in front
What angle is there in an eclipsed conformation?
Parallel
0 degrees
Which conformation, staggered or eclipsed, is the least stable and has the most relative energy?
Eclipsed
Which conformation, staggered or eclipsed, is the most stable and has the lowest relative energy?
Staggered
What are the two reasons why the eclipsed conformation is less stable than the staggered?
- Electron-electron repulsion
2. Stabilising interaction in staggered form
How does electron-electron repulsion affect conformation stability?
The filled orbitals in the eclipsed conformation repel each other and this repulsion is maximum in eclipsed
This is a destabilising interaction
How does stabilising interaction in the staggered form affect conformation stability?
There is an overlap of the filled sigma C-H and the empty sigma antibonding
Orbitals are parallel which is stable
How does the energy profile differ for bond rotation in butane?
Methyl groups are larger than hydrogen and so the barrier to rotation is a bit higher
What is synperiplanar (eg. in butane)?
0 degrees between the two methyl groups
Eclipsed conformation
Worst as the groups are eclipsing each other
What is synclinal?
60degrees between groups
Staggered formation
also known as gauche
Out of all the conformations for butane, which is the worst with the highest relative energy?
Eclipsed syn-periplanar
Out of all the conformations for butane, which is the best with the lowest relative energy?
Staggered antiperiplanar
What is anticlinal?
120 degrees between the groups
Eclipsed conformations
What is antiperiplanar?
180 degrees between the groups
Staggered conformation
What is angle strain?
An angle being forced to be an angle that is not its optimum
Is there expected ring strain in a 3 membered ring?
Yes
The angle is 60 degrees but as it is sp3 it would prefer 109degrees which means it is highly strained
What does ring strain depend on?
The size of the ring
Is there expected ring strain in a 5 membered ring?
No
Angle is 108 and would want to be 109 so not much difference
Is there expected ring strain in a 4 membered ring?
Yes
Angle is 90 degrees but as it is sp3 it wants to be 109 and therefore there is strain
Is there expected ring strain in a 6 membered ring?
Yes
Angle is 120 degrees which is larger than what it wants to be (109) so therefore there is strain
Why is the 5 membered ring strained when it’s not expected to be?
When you look down the molecule, it takes on a eclipsed conformation
Why is there no ring strain in a 6 membered ring when there is some to be expected?
Cyclohexane adopts a chair conformation
Angles in a chair conformation are 109 degrees so there is no angle strain
How do you draw chair conformations?
- Draw a ‘V’ leaning over
- Two parallel lines for the sides of the chair
- Complete the chair using the lines drawn
- Draw on axial and equatorial bonds
What is an axial bond?
They are vertically up or down and always come away from the point
What is an equatorial bond?
They are parallel to the next-but-one ring bond
What happens to the bonds when a ring inversion happens?
Axial groups become equatorial and equatorial groups become axial
The atoms stay above and below the atoms they were above or below - conformation of the atoms doesn’t change
Do large groups prefer to be axial or equatorial?
Equatorial
Why do large groups prefer to be equatorial?
1,3-diaxial interactions
Gauche interactions
What are 1,3-diaxial interactions?
Destabilising repulsion between the axial group X and the two axial hydrogen atoms on the same side of the ring
What are gauche interactions?
In the equatorial conformer, the C-X bond is antiperiplanar to the two C-C bonds
For the axial conformer, the C-X bond is synclinal (Gauche) to two C-C bonds
What arrangements of the groups results in a lower energy?
If both can be equatorial
What arrangements of the groups result in a higher energy?
If one is axial and one is equatorial
