Isomerism: Conformational Isomers Flashcards
Can the two molecules be superimposed on one another after rotating about a single bond
If the answer is yes to this question, the molecules are…
… conformational isomers
Are these two compounds conformational isomers?
These two compounds look different but if you rotate around the cental carbon-carbon single bond of the structure on the LHS, you can convert it to the structure on the RHS
Hence they are an example of conformational isomers
Are these two compounds conformational isomers?
If we focus on the chiral centre on the bottom of the structure, in each case, the hydrogen atom is pointing backwards, the methyl group forwards and carboxylate group is in the plane
However, if we look at the chiral centre at the top of the molecule, the arrangement of groups is different
However, if we rotate around the central carbon single bond, on the structure on the LHS, we can in fact convert it to the structure on the RHS
So are conformational isomers
Are these two compounds conformational isomers
These two are conformational isomers which can be interconverted by rotating around single bonds
Why is rotation around the single C-C bond in ethane termed ‘free rotation’
Rotation around the bond has an energy barrier of 12kJ per mole
This is very small
You can rotate around the central carbon bond in ethane, but not in ethene, why is this the case?
It takes 260kJ per mole of energy in ethene, compared to 12kJ per mole in ethane
This is too much energy to permit this to happen
Why do the following molecules, have a higher barrier to rotation than ethane, but a lower barrier to rotation than ethene
Because there is restrictions to their rotation, hence the energy required to rotate them is higher
What is the difference between the two extreme conformations of ethane, staggered and eclipsed?
In the eclipsed version the groups have been rotated to be exactly in line with another
In the staggered orientation the groups have been rotated to where they are outline of line with each other
These two forms can be interconverted by just rotating around the carbon-carbon bond - they are conformational isomers of each other
What is a Newman projection?
Give ethane as an example
Newman projections are used to represent conformation
In order to draw a Newman projection, we first draw a circle and then to represent the atoms attached to the nearest carbon atoms, we draw the bonds going to the centre of the circle
To represent the bonds going to the carbon atom at the rear, we show the bonds disappearing behind the circle
Describe the graph for the rotation around the central C-C bond in ethane
It has a maximum with an angle of 0, a minimum with an angle of 60, another maximum at 120 and a minimum at 180, etc
This pattern is repeated
The dihedral angle (angle between the two hydrogen atoms) is zero when we have the high energy eclipsed conformation and 60° when we have the low energy staggered conformation
Why does the energy of ethane fluctuate?
As we rotate around the carbon-carbon bond the conformation changes from eclipsed through to intermediates to staggered, then intermediates back to eclipsed again, and so on
The energy required to do this is 12kJ per mole, and with being very low the processes happens with no restrictions at room temperature
Why is there an energy difference between the eclipsed and staggered forms of ethane
The effect is caused by orbital interactions
In the eclipsed formation: the fulled orbitals are aligned - which repel another as they both contain electrons hence is an unfavourable situation
In the staggered conformation: the alignment is between the filled sigma C-H bonding orbital and empty C-H sigma antibonding orbital of the next atom - hence is favourable and the interaction lowers energy of the system
Comparing rotation around the central carbon-carbon bond of butane with ethane, why is butane more complicated
Because intead of just having just hydrogen atoms substituted on the two central carbon atoms like we had in ethane, in butane we have a methyl group substituted onto each of the internal carbon atoms
Considering the rotation of the central C-C bond of butane, what is the syn-periplanr conformation
Where the two methyl groups are eclipsed
Considering the rotation of the central C-C bond of butane, what is the synclinal conformation
A staggered intermediate where the methyl groups are adjacent to each other
When comparing the different conformations of butane, which one is the highest energy conformation
is the synperiplanar form where the two methyl groups are ecplised
When comparing the different conformations of butane, which one is the highest energy conformation
is the synperiplanar form where the two methyl groups are ecplised
Moving from the highest energy form of butane, which conformation is formed at the miminium next to it
The synclinal form, where the two methyl groups are adjacent to another
What is the lowest energy minimum conformation of butane
The anti-periplanar form, where the two methyl groups are opposite to each other
What is the lowest energy minimum conformation of butane
The anti-periplanar form, where the two methyl groups are opposite to each other
Which conformation of cyclohexane is the lowest energy
The chair conformation
This is usually how cyclohexane is drawn
What is the other conformation of cyclohexane
The boat conformation
What is ring flipping in cyclohexane and how does it affect axial and equatorial hydrogens
Ring flipping: is rotating single bond in cyclohexane, creating conformational isomers
Atoms which are equatorial before the ring flipping, become axial
And atoms which are axial before the ring flipping become equatorial
Going from the chair conformation to the boat conformation, what intermediates have to be formed, and are they high or lower in energy than the boat conformation
A half chair conformation and a twist boat conformation
The half chair is the highest energy conformation possible
The twist boat is lower in energy than the true boat conformation
BUT … notice that going from chair A structure to chair B, the ring has flipped
We have seen that when ring flipping occurs in cyclohexane, the hydrogen atoms attached move from axial to equatorial OR equatorial to axial
How does this relate to energy level
Atoms in the equatorial position are lower in energy than atoms in the axial position
Therefore equilibrium in this example, lies to the RHS
Why is it the case, that the lowest energy conformation is when the substituent is in the equatorial position
We can see that there is the possibility of 1,3-diaxial interactions, if the substituent is in the axial position
This doesn’t happen in the equatorial conformation
This is an unfavourable interaction, and the bigger X, the bigger the effect is
What are the two conformational isomers possible for cis-1,4-cyclohexanediol
Represented as a 3D chair drawing, you will have one of the OH groups in an axial position and one in an equatorial position
If we flip the ring this causes the two to exchange, one was equatorial becomes axial and one axial becomes equatorial
What are the two conformational isomers of trans-1,4-cyclohexanediol
In the chair conformation, you can either: (1) have both of the OH groups being axial or both being equatorial
So here there is a preference for the conformation with both OH groups in the equatorial position
why in cis-4-t-butylcyclohexanol, does the hydroxyl group only exist in the axial position
Because of the size of the large tertiary butyl group, it must exclusively go in the equatorial postion
