How can I draw trans-1,4-dimethylcyclohexane in its two chair conformations, and determine whether the two chairs are identical, conformational enantiomers, or conformational diastereomers? How can I do the same for the cis isomer?

1 Answer
Jun 20, 2015

Answer:

This is too long an answer for a one-sentence summary.

Explanation:

trans-1,4-Dimethylcyclohexane

1. Structure

Draw the two cyclohexane chairs, with #"C-1"# on the right.

In each structure, put the methyl group on #"C-1"# "down" and the methyl group on #"C-4"# "up".

This gives you trans-1,4-dimethylcyclohexane.

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2. Identical?

The two structures are not identical, because the two methyl groups are equatorial in one isomer and axial in the other.

I have never heard of conformational enantiomers or conformational diastereomers. But there are conformational isomers, enantiomers, and diastereomers.

3. Conformational isomers?

These are conformational isomers. because one form can be converted into another by a ring "flip" without breaking any bonds.

4. Enantiomers or diastereomers?

These are neither enantiomers nor diastereomers, because they have no chiral centres.

cis-1,4-Dimethylcyclohexane

1. Structure

Draw the two cyclohexane chairs, with #"C-1"# on the right.

In each structure, put the methyl groups on both #"C-1"# and #"C-4"# "down".

This gives you cis-1,4-dimethylcyclohexane.

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2. Identical?

The two structures are identical, because the there is one axial and one equatorial methyl group.

If you rotate the right hand structure 180 ° about a vertical axis passing through the centre of the ring, you get the left hand structure.

The two structures are superimposable on each other.

3. Conformational isomers?

These are not conformational isomers. because they are identical with each other.

4. Enantiomers or diastereomers?

These are neither enantiomers nor diastereomers, because they have no chiral centres.