OChem topic 2- conformation of molecules

conformation

• the 3D shape of a molecule that can change as a result of rotation about bonds

• results in a molecule adopting a number of different shapes while the localised connectivity remains unchanged

when is rotation about bonds possible

• sigma bonds that have radical symmetry (ie. bond is not changed by rotating the group at either end)

• at room temperature in solution single bonds in a molecule can be rotating

• BONDS ARE NOT BROKEN

• rotations of groups about double bonds are not changing conformers as the π system would have to be broken and

do different conformations of a molecules have the same energy?

• no!

• different conformation have different energies as a result of strain

what are the three components of strain?

• torsional strain

• van der waals strain

• angle strain

torsional strain

• distance between groups ie. changes in torsion angles leads to interactions as the groups change their relative positions

• strain caused by electron repulsion when atoms on neighbouring bonds are forced into an eclipsed conformation

Vand der waals strain

• groups are placed closer than the sum of their van der waals radii

• ie. non bonding atoms placed too close together - steric hinderance

angle strain

• molecules are forced to have bond angles that deviate from the ideal for that hybridisation

• eg in rings

what are the two extremes of conformation?

• staggered

• eclipsed

why is an eclipsed conformation higher in energy than a staggered conformation?

1. steric clash - outer groups being brought within the sum of their van der waals radii

2. pairs of electrons within C-? bonds repel each other and are brought closest together in the eclipsed conformation

3. when C-H bonds (or other groups) are 180˚ to each other (antiperiplanar) as in the staggered conformation the sigma bond of one of the C-H bonds overlaps with the sigma * orbital of the other - this is a stabilising interaction - called hyperconjugation

barriers to rotation

• it is easier to rotate some bonds than others bc there is an energy barrier to rotation about bonds - the activation energy needed to interconvert between conformations

• ie. the energy difference between conformations at energy minima and maxima

• eg. barrier to rotation is higher in propane than ethane reflecting the greater repulsive interactions in eclipsed conformation

useful number to remember

• a barrier of 73 kj mol^-1 allows one rotation every second at 25˚c (rate is 1 s^-1)

• every 6 kJ mol^-1 changes the rate at 25˚c by a factor of 10

• to see separate peaks on NMR for different conformations, rate of interconversion must be <1000 s^-1 (energy barrier ≈ 55 kJ mol^-1 at 25˚c)

• if the barrier is >100 kJ mol^-1, conformation interconvert slowly enough to exist as different compounds

synperiplanar

two groups of interest are on:

• same face

• same plane

antiperiplanar

two groups of interest are on:

• opposite faces

• same plane

guache (synclincal)

two groups of interest are:

• at 60˚ to each other

trend for strain/heat of combustion for cyclic alkanes

• three membered ring - most strained therefore highest heat of combustion

• six membered ring - least strained therefore lowest heat of combustion

• strain starts to go down on higher systems eg 12 membered rings onwards as the effect of the ring structure reduces and becomes closer to a straight chain with conformational freedom

three membered rings

• planar

• always possible to draw a plane through three points

• all bonds are eclipsed making it a high energy conformation

• angle strain + torsional strain

• eg. epoxides, strain makes them very reactive

four membered rings

• not completely planar

• ring distorts/puckers in order to reduce eclipsing interactions and therefore torsional strain

• leads to a slightly higher angle strain than in the planar equivalent but is more stable overall + lower energy

five membered rings

heat of combustion shows they’re not completely strain free despite 109.5 degree bond angles in planar form - but planar has unfavourable eclipsing C-H etc bonds

• ring pucker to reduce the number of eclipsing bonds

• one carbon atom goes above the plane of the remaining 4 in an envelope form

• this increases the angle strain compared to the planar conformation but is overall lower in energy

6 membered rings

• according to combustion data cyclohexane is strain free

• there are two conformations with no angle strain - chair + boat

• chair - all bonds are staggered therefore favoured lower energy conformer

• boat - many eclipsing interactions - has torsional strain - ‘flagpole’ interactions between the top two C-H bonds

cyclohexane chair conformation

• two different environments for hydrogen atoms - axial and equatorial

• axial and equatorial hydrogen atoms are not degenerate ie. not isoenergetic

ring flipping/ring inversion

• converting between the two conformations of cyclohexane by rotating about C-C bonds - rapidly interconvert at room temperature

• barrier to ring flipping is 43 kJ mol^-1 so only see one signal of ¹H NMR

why does an axial position of a substituent of a cyclohexane ring result in a higher energy conformer?

• unfavourable guache interactions (increase in repulsive steric interactions than antiperiplanar conformer) - when axial dihedral angle to next CH2 in ring is 60 degrees but 180 in equatorial

• repulsive 1,3-diaxial interactions - close in space to other axial groups - as the groups get bigger the repulsive steric interactions increase

^tBu

• a locking group

• forces the cyclohexane ring into a particular conformer - the one with the tBu in an equatorial position

A-values

• energy difference between axial and equatorial conformations for a cyclohexane ring with a single substituent

• halide values are all similar (apart from fluorine) as although iodine is bigger for example the C-I bond is longer than C-Cl so the factors balance out

• other factors like dipole moment, electrostatic interactions and stabilising intramolecular interactions and solvent used also have effects

how to calculate the number of stereoisomers

2ⁿ

lis the conformations of cyclohexane in order of decreasing energy

half chair > boat > twist boat > chair

(order in graph = chair A → half chair → twist boat → true boat → twist boat → half chair → chair B (not in order of energy just in order of reaction coordinate))