Conformation
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How can different conformations interconvert?
Through rotation about single bonds
How can stereoisomers be interconverted to different conformations?
By breaking bonds
What does a Newman projection show?
Arrangement of groups and emphasises interactions on adjacent atoms
What is a staggered conformation on the Newman projection?
The atoms are staggered in relation to each other, evenly spaced
What angle is there in a staggered conformation for a Newman projection?
Dihedral angle - spread out as far as the molecule can be 60 degrees
What are the two conformations for a Newman projection?
Staggered Eclipse
What is an eclipsed conformation?
The groups behind are covered by groups in front
What angle is there in an eclipsed conformation?
Parallel 0 degrees
Which conformation, staggered or eclipsed, is the least stable and has the most relative energy?
Eclipsed
Which conformation, staggered or eclipsed, is the most stable and has the lowest relative energy?
Staggered
What are the two reasons why the eclipsed conformation is less stable than the staggered?
• Electron-electron repulsion 2. Stabilising interaction in staggered form
How does electron-electron repulsion affect conformation stability?
The filled orbitals in the eclipsed conformation repel each other and this repulsion is maximum in eclipsed This is a destabilising interaction
How does stabilising interaction in the staggered form affect conformation stability?
There is an overlap of the filled sigma C-H and the empty sigma antibonding Orbitals are parallel which is stable
How does the energy profile differ for bond rotation in butane?
Methyl groups are larger than hydrogen and so the barrier to rotation is a bit higher
What is synperiplanar (eg. in butane)?
0 degrees between the two methyl groups Eclipsed conformation Worst as the groups are eclipsing each other
What is synclinal?
60degrees between groups Staggered formation also known as gauche
Out of all the conformations for butane, which is the worst with the highest relative energy?
Eclipsed syn-periplanar
Out of all the conformations for butane, which is the best with the lowest relative energy?
Staggered antiperiplanar
What is anticlinal?
120 degrees between the groups Eclipsed conformations
What is antiperiplanar?
180 degrees between the groups Staggered conformation
What is angle strain?
An angle being forced to be an angle that is not its optimum
Is there expected ring strain in a 3 membered ring?
Yes The angle is 60 degrees but as it is sp3 it would prefer 109degrees which means it is highly strained
What does ring strain depend on?
The size of the ring
Is there expected ring strain in a 5 membered ring?
No Angle is 108 and would want to be 109 so not much difference