Understanding Geometric (Cis-Trans) Isomerism in Cycloalkanes

Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism? Select all that apply.

1-ethyl-3-methylcycloheptane

3-ethyl-1,1-dimethylcyclohexane

1-ethyl-3-methylcyclopentane

1-cyclopropyl-2-methylcyclohexane

Answer:

Only 1-ethyl-3-methylcycloheptane and 1-ethyl-3-methylcyclopentane are capable of exhibiting cis-trans isomerism since they have substituents on non-adjacent carbon atoms within sufficiently large rings.

Explanation:

Cis-Trans Isomerism in Cycloalkanes

Cycloalkanes with two substituents on different carbons can often exhibit geometric isomerism, commonly referred to as cis-trans isomerism. For such isomerism to occur, each carbon atom that is part of the isomerism must have two different groups attached to it. In the case of cycloalkanes, this means that for geometric isomerism to occur, the ring must be large enough to allow for the different spatial arrangements of the substituents and it must have substituents that prevent free rotation.

Considering the given examples:

1-ethyl-3-methylcycloheptane: This compound can exhibit cis-trans isomerism because it has two different substituents on non-adjacent carbon atoms within a sufficiently large ring.

3-ethyl-1,1-dimethylcyclohexane: This compound cannot exhibit cis-trans isomerism because the two methyl groups are on the same carbon atom.

1-ethyl-3-methylcyclopentane: Like the cycloheptane compound, this cyclopentane can also show cis-trans isomerism due to having different substituents on non-adjacent carbons.

1-cyclopropyl-2-methylcyclohexane: Although this compound has two different groups, the cyclopropyl group is not considered a substituent that allows for cis-trans isomerism in the context of this discussion. Therefore, this compound likely does not show cis-trans isomerism.