Cycloalkanes

Naming cycloalkanes:

Nomenclature of cycloalkanes is simple, and follows the same rules as for any other alkane. The prefix cyclo- is added to the basic alkane name, and any substituents are added in the front.

For example:

Name the following cycloalkanes:

Questions taken from Solomans "Fundamentals of Organic Chemistry" 1994

 

Cast your minds back to types of isomerism, and you’ll remember that we discussed conformational isomers. These are particularly important in cycloalkanes, as the molecule is locked in a cyclic arrangement and cannot rotate out of an unfavourable conformation.

 

The most stable conformation for cyclohexane is the “chair conformation”, called so because it resembles a reclining chair. This conformation is the most stable as all carbon-carbon bond angles are 109.5 degrees, which minimises strain and, as the Newman projection shows, all of the substituents are in staggered conformations.

See? If you squint a bit, it looks like a cyclohexane ring

The most comfortable conformation

By simple rotation, the chair conformation can become the boat conformation, which is also free of strain, but is less stable as there are interactions between the substituents. The Newman projection shows that the boat conformation is an eclipsed conformation, and therefore less stable.

Avast! A more strained conformation

 

 

At room temperature, cyclohexanes rapidly flip, via the boat conformation, from one chair conformation to another. This ring flipping is important, because when the ring flips, all of the bonds that were axial become equatorial and vice versa.

If all of the substituents are hydrogens, this doesn’t matter too much as both chair conformations are equivalent and the ring will flip equally between the two. However, if there is a non-hydrogen substituent, the chair conformation that has the substituent in an equatorial position will be the most stable.

 

 

 

If there is more than one substituent on the ring, you want the bulkiest group to be in an equatorial position. The gold standard example of this is a tertiary butyl group:

This group is, comparatively, huge, (try making a cyclohexane ring with a tert-butyl group on it with a molecular modelling kit. You’ll see that the tert-butyl group is almost as large as the ring itself), and must always be in the equatorial position.

 

 

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Filed under Chem 1, Organic Chemistry

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