Conformation

Question 1

How can different conformations interconvert?

Answer 1

Through rotation about single bonds

Question 2

How can stereoisomers be interconverted to different conformations?

Answer 2

By breaking bonds

Question 3

What does a Newman projection show?

Answer 3

Arrangement of groups and emphasises interactions on adjacent atoms

Question 4

What is a staggered conformation on the Newman projection?

Answer 4

The atoms are staggered in relation to each other, evenly spaced

Question 5

What angle is there in a staggered conformation for a Newman projection?

Answer 5

Dihedral angle - spread out as far as the molecule can be

60 degrees

Question 6

What are the two conformations for a Newman projection?

Answer 6

Staggered

Eclipse

Question 7

What is an eclipsed conformation?

Answer 7

The groups behind are covered by groups in front

Question 8

What angle is there in an eclipsed conformation?

Answer 8

Parallel

0 degrees

Question 9

Which conformation, staggered or eclipsed, is the least stable and has the most relative energy?

Answer 9

Eclipsed

Question 10

Which conformation, staggered or eclipsed, is the most stable and has the lowest relative energy?

Answer 10

Staggered

Question 11

What are the two reasons why the eclipsed conformation is less stable than the staggered?

Answer 11

1. Electron-electron repulsion

2. Stabilising interaction in staggered form

Question 12

How does electron-electron repulsion affect conformation stability?

Answer 12

The filled orbitals in the eclipsed conformation repel each other and this repulsion is maximum in eclipsed
This is a destabilising interaction

Question 13

How does stabilising interaction in the staggered form affect conformation stability?

Answer 13

There is an overlap of the filled sigma C-H and the empty sigma antibonding
Orbitals are parallel which is stable

Question 14

How does the energy profile differ for bond rotation in butane?

Answer 14

Methyl groups are larger than hydrogen and so the barrier to rotation is a bit higher

Question 15

What is synperiplanar (eg. in butane)?

Answer 15

0 degrees between the two methyl groups
Eclipsed conformation
Worst as the groups are eclipsing each other

Question 16

What is synclinal?

Answer 16

60degrees between groups
Staggered formation
also known as gauche

Question 17

Out of all the conformations for butane, which is the worst with the highest relative energy?

Answer 17

Eclipsed syn-periplanar

Question 18

Out of all the conformations for butane, which is the best with the lowest relative energy?

Answer 18

Staggered antiperiplanar

Question 19

What is anticlinal?

Answer 19

120 degrees between the groups

Eclipsed conformations

Question 20

What is antiperiplanar?

Answer 20

180 degrees between the groups

Staggered conformation

Question 21

What is angle strain?

Answer 21

An angle being forced to be an angle that is not its optimum

Question 22

Is there expected ring strain in a 3 membered ring?

Answer 22

Yes

The angle is 60 degrees but as it is sp3 it would prefer 109degrees which means it is highly strained

Question 23

What does ring strain depend on?

Answer 23

The size of the ring

Question 24

Is there expected ring strain in a 5 membered ring?

Answer 24

No

Angle is 108 and would want to be 109 so not much difference

Question 25

Is there expected ring strain in a 4 membered ring?

Answer 25

Yes

Angle is 90 degrees but as it is sp3 it wants to be 109 and therefore there is strain

Question 26

Is there expected ring strain in a 6 membered ring?

Answer 26

Yes

Angle is 120 degrees which is larger than what it wants to be (109) so therefore there is strain

Question 27

Why is the 5 membered ring strained when it’s not expected to be?

Answer 27

When you look down the molecule, it takes on a eclipsed conformation

Question 28

Why is there no ring strain in a 6 membered ring when there is some to be expected?

Answer 28

Cyclohexane adopts a chair conformation

Angles in a chair conformation are 109 degrees so there is no angle strain

Question 29

How do you draw chair conformations?

Answer 29

1. Draw a ‘V’ leaning over
2. Two parallel lines for the sides of the chair
3. Complete the chair using the lines drawn
4. Draw on axial and equatorial bonds

Question 30

What is an axial bond?

Answer 30

They are vertically up or down and always come away from the point

Question 31

What is an equatorial bond?

Answer 31

They are parallel to the next-but-one ring bond

Question 32

What happens to the bonds when a ring inversion happens?

Answer 32

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

Question 33

Do large groups prefer to be axial or equatorial?

Answer 33

Equatorial

Question 34

Why do large groups prefer to be equatorial?

Answer 34

1,3-diaxial interactions

Gauche interactions

Question 35

What are 1,3-diaxial interactions?

Answer 35

Destabilising repulsion between the axial group X and the two axial hydrogen atoms on the same side of the ring

Question 36

What are gauche interactions?

Answer 36

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

Question 37

What arrangements of the groups results in a lower energy?

Answer 37

If both can be equatorial

Question 38

What arrangements of the groups result in a higher energy?

Answer 38

If one is axial and one is equatorial