Chapter 3: Conformations

Question 1

rotations around single bonds

Answer 1

• rotation is normally possible around single bonds, with the result that molecules can adapt various conformations

Question 2

Newman projections

Answer 2

• are a type of drawing where we imagine looking down a signma bond
• this makes it easier to easier to see angles between groups on each side
• sometime sgroups in the front are directly in front of groups in the back (we cant draw this properly, instea we offset them slighty)

Question 3

Dihedral/torsion angles

Answer 3

4 bonded atoms

Question 4

types of strains in molecules

Answer 4

• molecules would like to reset in the lowest possible energy state, which is not always possible
• when a molecule is forced to adopt some condition that is not idea they experience a strain

Question 5

angle strain

Answer 5

bond angels (defined by three bonded atoms in a row deviating from their ideal values
- this costs energy because now the orbitals do not overlap as well
- the most common reason for angle strain is a geometric constraint

Question 6

torsional strain

Answer 6

• arises when the torsion agle deviates from an ideal value
• most commonly occurs when the torsion angle is close to 0
• very common and only has small costs associated with it

Question 7

steric strain

Answer 7

• two non bonded atoms being in close proximity
  atoms are considered non bonded if they are separated by more than three bonds or if they are on different molecules
• this costs energy because each atom is surrounded by an electron cloud and negative charges repel each other

Question 8

eclipsed

Answer 8

• when the substituents are directly in front of each other
• highest energy conformation due to torsional strain

Question 9

staggered

Answer 9

when the substituents are the maximum distance from each other (60) the conformation is staggered
- this is the lowest energy conformation

Question 10

energy diagrams

Answer 10

• as we continue to rotate we go back and forth between eclipse and staggered
• if all of the substituents are the same, then all the strains are the same

Question 11

naming conformation with different groups

Answer 11

• the most important tyoes of conformation have names
• if all the substituents are not tthe same this is harder
• usually they are named for : where the large/ important grouprs are relative to each other

Question 12

antiperplanar

Answer 12

• when the substituents are not all the same, there are different staggered conformations and different eclipse conformations
• this is the lowest energy conformation because there is no torsional strain and no steric strain

Question 13

gauche

Answer 13

• lower energy
• only a littler steric strain

Question 14

partially eclipsed conformation

Answer 14

• dihedral angle between the two groups is 120
• this is the second highers energy conformation because there is torsional strain and no steric strain

Question 15

fully eclipsed conformation

Answer 15

• the dihedral angle between the group is 0
• this is the highest energy conformation because there is torsional strain and steric strain

Question 16

naming conformations: generalizing

Answer 16

• more commonly these names are used as relative descriptors to describe the orientation of two groups with respect to each other

Question 17

strain and conformation in cyclic molecules

Answer 17

• many molecules experience strain because they cannot avoid angle, torsional or steric strain
• the situation is very common in cyclic molecules
• being in a ring limits flexibility by tethering the two ends together so they are not able to otate independently

Question 18

3 membered rings

Answer 18

• atoms with sp3 hybridization want groups to be 109 apart
• the internal angles of triangle are 60
• A 3 membered ring must be a planar triangle
• 3 membered sings have a significant amount of angle strain
• when rings are planar they must eclipse each other
• have a significant amount of torsional strain

Question 19

4 memebered rings

Answer 19

atoms with sp3 hybridization want groups to be 109 apart
- the internal angles of a square are 90
- a four membered ring doesnt have to be planar
- have a significant amoung of angle strain
become butterfly conformations
- must eclipse each other
moderate amount of torsional strain

Question 20

ring inversion

Answer 20

• two conformers can interconvert

Question 21

5 membered rings

Answer 21

• atoms with sp3 hybridization want groups to be 109 apart
• the internal angles of a pentagon are 108
• very little strain
• envelope conformers

Question 22

6 membered rings

Answer 22

• the most common in nature and in synthetic molecules
• atoms with sp3 hybridization want groups to be 109 apart
• the internal angles of a hexagon are 120
• no angle strain
• chair conformations

Question 23

axial and equatorial groups

Answer 23

• there are names to describe the orientation of groups in chair conformations
• the 6 groups pointing directly up/down are called axial groups (aligned with the z axis)
• the 6 groups pointing outwards are equatotial groups (point to the molecules equator)

Question 24

when ring inversion occurs

Answer 24

• the axial groups become equatorial groups
• the equatorial groups become axial groups

Question 25

1-3 diaxial interactions and A values

Answer 25

• chair conformations have no angle strain and no torsional strain
• there is another possible strian = steric strain
• any axial substituent larger than H (anything but hydrogen) can interact with the other axial substituents

Question 26

1-3 diaxial interactions and equatorial conformations

Answer 26

• substituents in the equatorial orientation do not suffer from 1-3 diaxial interactions
• two chair conformations are not equal in energy
• the equatorial conformation is favoured

Question 27

size and 1-3 diaxial interactions

Answer 27

• it is possible to quantify the energy cost of a group being axial
• this is refered to as an A value
• the larger the substituent the larger the A value and the less it wants to be in Axial formation

Question 28

what if there is more than one substituent

Answer 28

• in the left structure the methyls are on the same face of the ring
• in each one equatorial and one axial substituent
• the two conformations have the same energy
• the right structure the methyls are on different faces of the ring - two equitorial and two axial
• the left is lower in energy and favoured