20.3 Stereoisomerism

The methyl groups in Ethane can line up in two extreme positions, eclipsed or staggered, or anywhere in between.

QUESTION: Why might the Eclipsed version be at higher energy?

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In practise, even if we separated the two conformations our two samples of Ethane would soon go back to mixtures.

QUESTION: Why?

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So, these are not really isomers of each other in the conventional sense.

It’s not really possible to have a staggered version of Cyclopropene.

And this is why it is so unstable – the bonding is strained and liable to fall apart.

It’s possible to have planar Cyclobutane or for the molecule to take a ”puckered” conformation.

QUESTION: Which conformation would be favoured? Why?

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Unlike Conformational isomers, Configurational isomers are locked in place and can’t easily interconvert just by rotating a sigma bond.

These isomers can’t rotate because, usually, they have a C=C bond.

QUESTION: What stops C=C bonds rotating at low temperatures?

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Cis and Trans mean “on the same side” and “on the opposite side”

Ciskei is on the same side of the Great Kei river as Cape town, Transkei isn’t.

Don’t go to either of them.

It’s easy enough to see why these are Cis and Trans But-2-ene.

QUESTION : Draw the displayed formula of But-1-ene.

QUESTION: Why can’t there be Cis and Trans But-1-ene?

Once a cyclo- compound has been formed any side chains are locked on one side or the other of the molecule.

QUESTION: Which of the above 1,4-Dimethylcyclohexane molecules is the Trans version?

QUESTION: Draw the Cis Trans isomers of 1,2-Dibromocyclopentane

These two molecules are clearly isomers of each other.

But it’s not clear how to name them Cis or Trans.

Instead, they are named E and Z where Z stands for Zuzammen (Together) and is so the equivalent of Cis.

But we still need a rule to decide “what” together means?

The Cahn-Ingold Rules state that the heaviest group takes priority.

In the left example Bromines weigh more than Fluorines, Chlorines weigh more than Hydrogens and so the heaviest groups are together – it’s the E isomer.

QUESTION: Label each molecule as either E or Z

Optical Isomers can rotate polarised light.

We don’t need to know why or which way they rotate, but we do need to be able to recognise which molecules will have optical isomers (enantiomers)

Any molecule with a chiral centre has optical isomers – a simple chiral centre is a Carbon atom bonded to 4 different atoms or groups.

These can be rearranged to make a non-superimpossible mirror image.

QUESTION: Put a * by the chiral Carbons in each molecule.

What happens if we mix two optical isomers together?

If it’s 50:50 mix their rotations cancel out

This is called a racemic mixture.

When we make a molecule with a chiral centre we usually make a racemic mixture.

Only one enantiomer (optical isomer) has a medical effect but if we double the dosage of the drug we can avoid expensively separating the mixture.

QUESTION: When might this approach be a problem?

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QUESTION: Why is it usually difficult to separate enantiomers?

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These isomers are non-superimpossible.

But they are not mirror images.

They are diastereoisomers of each other.

In this case they both contain chiral centres, so each diastereoisomer has optical isomers of itself.

QUESTION: Draw the enantiomer of each diastereoisomer

Notice that the enantiomers are diasteriomers of each other.