Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism? Draw the cis and trans isomers.
a. 3-ethyl-1,1-dimethylcyclohexane

Verified step by step guidance

1. Understand the concept of geometric (cis-trans) isomerism: Geometric isomerism occurs in cycloalkanes when there are two substituents on different carbon atoms of the ring, and these substituents can be positioned on the same side (cis) or opposite sides (trans) of the ring.
2. Identify the structure of 3-ethyl-1,1-dimethylcyclohexane: The cyclohexane ring has six carbon atoms. The substituents are located at the 1st and 3rd positions. At the 1st position, there are two methyl groups, and at the 3rd position, there is an ethyl group.
3. Determine if geometric isomerism is possible: For geometric isomerism to occur, the substituents must be on different carbon atoms and not on the same carbon atom. In this case, the substituents are on different carbon atoms (1st and 3rd), which allows for cis-trans isomerism.
4. Draw the cis isomer: In the cis isomer, the ethyl group at the 3rd position and one of the methyl groups at the 1st position are on the same side of the cyclohexane ring. Use wedge and dash bonds to represent the 3D orientation of the substituents.
5. Draw the trans isomer: In the trans isomer, the ethyl group at the 3rd position and one of the methyl groups at the 1st position are on opposite sides of the cyclohexane ring. Again, use wedge and dash bonds to show the 3D orientation.

Key Concepts

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, occurs in compounds with restricted rotation around a bond, typically a double bond or a ring structure. In cycloalkanes, this isomerism arises when two substituents are attached to different sides of the ring, leading to distinct spatial arrangements known as cis (same side) and trans (opposite sides) isomers.

Cycloalkanes

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring structure. They have the general formula CnH2n and can exhibit geometric isomerism if they have substituents that allow for different spatial arrangements. The rigidity of the ring structure restricts rotation, making it possible for cis and trans configurations to exist.

Substituent Positioning in Cycloalkanes

The positioning of substituents on a cycloalkane ring is crucial for determining the possibility of geometric isomerism. For isomerism to occur, the ring must have at least two substituents on different carbon atoms. The spatial arrangement of these substituents, either on the same side (cis) or opposite sides (trans) of the ring, defines the isomeric form.