1. Determine which of the following compounds show cis-trans isomerism.
2. Draw and name the cis and trans (or Z and E) isomers of those that do.
e. 2,3-dimethylpent-2-ene
f. 3,4-dibromocyclopentene

Verified step by step guidance

1. Identify the structural features necessary for cis-trans isomerism: For alkenes, look for a double bond with different groups attached to each carbon of the double bond. For cyclic compounds, look for substituents on different carbons that can be positioned above or below the plane of the ring.
2. Examine compound e, 2,3-dimethylpent-2-ene: This compound has a double bond between the second and third carbon atoms. Check if the groups attached to these carbons are different, which is necessary for cis-trans isomerism.
3. For 2,3-dimethylpent-2-ene, the groups attached to the double-bonded carbons are: on C2, a methyl group and a hydrogen; on C3, a methyl group and the rest of the carbon chain. Since there are different groups on each carbon, this compound can exhibit cis-trans isomerism.
4. Examine compound f, 3,4-dibromocyclopentene: This is a cyclic compound with a double bond. Check if the bromine atoms are on different carbons and can be positioned above or below the plane of the ring, which allows for cis-trans isomerism.
5. For 3,4-dibromocyclopentene, the bromine atoms are on C3 and C4. Since these can be positioned differently relative to the plane of the ring, this compound can also exhibit cis-trans isomerism. Draw the structures for both compounds, labeling the isomers as 'cis' (same side) and 'trans' (opposite sides) for alkenes, or 'Z' and 'E' for more complex cases.

Key Concepts

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in compounds with restricted rotation around a bond, typically a double bond or a ring structure. It involves the arrangement of substituents around the bond, where 'cis' indicates substituents on the same side and 'trans' indicates substituents on opposite sides. This isomerism affects the physical and chemical properties of the compounds.

E-Z Nomenclature

E-Z nomenclature is a system used to describe the stereochemistry of double bonds in organic compounds. It is based on the Cahn-Ingold-Prelog priority rules, where 'E' (from German 'entgegen') indicates that the highest priority groups are on opposite sides of the double bond, and 'Z' (from German 'zusammen') indicates they are on the same side. This system is particularly useful for complex molecules where cis-trans terminology is insufficient.

Double Bond and Ring Structures

Double bonds and ring structures are key features that can lead to cis-trans isomerism. A double bond restricts rotation, creating a fixed spatial arrangement of substituents, while ring structures can also prevent rotation due to their cyclic nature. Understanding these structural elements is crucial for identifying potential geometric isomers in organic compounds.