Chapter 3: Conformations Flashcards
rotations around single bonds
- rotation is normally possible around single bonds, with the result that molecules can adapt various conformations
Newman projections
- 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)
Dihedral/torsion angles
4 bonded atoms
types of strains in molecules
- 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
angle strain
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
torsional strain
- 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
steric strain
- 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
eclipsed
- when the substituents are directly in front of each other
- highest energy conformation due to torsional strain
staggered
when the substituents are the maximum distance from each other (60) the conformation is staggered
- this is the lowest energy conformation
energy diagrams
- 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
naming conformation with different groups
- 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
antiperplanar
- 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
gauche
- lower energy
- only a littler steric strain
partially eclipsed conformation
- dihedral angle between the two groups is 120
- this is the second highers energy conformation because there is torsional strain and no steric strain
fully eclipsed conformation
- the dihedral angle between the group is 0
- this is the highest energy conformation because there is torsional strain and steric strain
naming conformations: generalizing
- more commonly these names are used as relative descriptors to describe the orientation of two groups with respect to each other
strain and conformation in cyclic molecules
- 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
3 membered rings
- 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
4 memebered rings
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
ring inversion
- two conformers can interconvert
5 membered rings
- atoms with sp3 hybridization want groups to be 109 apart
- the internal angles of a pentagon are 108
- very little strain
- envelope conformers
6 membered rings
- 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
axial and equatorial groups
- 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)
when ring inversion occurs
- the axial groups become equatorial groups
- the equatorial groups become axial groups
1-3 diaxial interactions and A values
- 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
1-3 diaxial interactions and equatorial conformations
- 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
size and 1-3 diaxial interactions
- 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
what if there is more than one substituent
- 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
