Question

The skeletal structure of an organic molecule is provided in the figure. Analyze the structure and identify the compound and its stereochemistry, providing the E/Z designation and IUPAC name.

• A. cis-3-chloro-but-2-ene
• B. E-3-chloro-but-2-ene
• C. Z-3-chloro-but-2-ene
• D. 3-chloro-but-2-ene

Answer 1

Answer: (B)

E-3-chloro-but-2-ene

Answer 2

The provided skeletal structure represents 3-chloro-but-2-ene. The double bond between the second and third carbon atoms allows for stereoisomerism.

To determine the E/Z configuration, we apply the Cahn-Ingold-Prelog priority rules to the substituents on each carbon of the double bond:

For the second carbon ($C_2$):

• Attached groups are a methyl group ($CH_3$) and a hydrogen atom (H).
• Priority: $CH_3$ (higher) > H (lower).

For the third carbon ($C_3$):

• Attached groups are a chlorine atom (Cl) and a methyl group ($CH_3$).
• Priority: Cl (higher) > $CH_3$ (lower).

Observing the skeletal structure, the methyl group on $C_2$ is depicted pointing downwards, and the methyl group on $C_3$ is also depicted pointing downwards. The hydrogen on $C_2$ would be pointing upwards, and the chlorine on $C_3$ is pointing upwards.

In the E/Z nomenclature, the E isomer has the higher priority groups on opposite sides of the double bond, while the Z isomer has them on the same side.

In this representation:

• The higher priority group on $C_2$ is $CH_3$ (down).
• The higher priority group on $C_3$ is Cl (up).

Since the higher priority groups ($CH_3$ on $C_2$ and Cl on $C_3$) are on opposite sides of the double bond, the configuration is E.

The IUPAC name, including the stereochemistry, is E-3-chloro-but-2-ene.

The cis designation refers to the relative positions of identical or similar groups. In this case, the two methyl groups are on the same side (both downwards), which would traditionally be called cis. However, the E/Z system is more rigorous. For this specific molecule, cis corresponds to E.

Therefore, the correct answer is E-3-chloro-but-2-ene.