Question

Named bombykol, the sex pheromone secreted by the female silkworm moth has the formula \(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\) and the systematic name \((10 E, 12 Z)\) hexadeca-10,12-dien-1-ol. Draw bombykol showing correct geometry for the two double bonds.

Short answer

The structure shows a 16-carbon chain with correct double bond configurations (E,Z) and an -OH group at the first carbon.

Step-by-step solution

Identify the Structure

The systematic name \(10 E, 12 Z)\) hexadeca-10,12-dien-1-ol indicates a 16-carbon chain with a hydroxyl group (\(-\mathrm{OH}\)) at position 1. The 'dien' suggests two double bonds: one at carbon 10 and another at carbon 12.

Understand the Geometrical Isomerism

The terms \(E\) and \(Z\) refer to the configuration of the double bonds according to the Cahn-Ingold-Prelog priority rules. \(E\) means the higher priority groups on either side of the double bond are on opposite sides, while \(Z\) means they are on the same side.

Place the Double Bonds

On a skeletal structure, place a double bond between carbon 10 and 11 and between carbon 12 and 13. Ensure the bond between carbon 10 and 11 is in the \(E\) configuration, meaning that the highest priority substituents on each carbon are on opposite sides. For the bond between carbon 12 and 13, the \(Z\) configuration requires them on the same side.

Add Remaining Groups

Once the double bonds and their configurations are set, ensure the remaining bonds are single, and add the hydroxyl group at position 1 of the carbon chain.

Draw the Full Structure

Using the above instructions, draw the carbon chain starting with a hydroxyl group, then positioning the \(E\) double bond at carbons 10 and 11, and the \(Z\) double bond at carbons 12 and 13. Confirm the line-angle structural representation reflects correct geometry for these bonds.

Key concepts

Geometrical Isomerism

Geometrical isomerism is a fascinating concept of organic chemistry. It describes a situation where molecules with the same molecular formula have different spatial orientations of their atoms. In the context of bombykol, geometrical isomerism refers to the different possible arrangements of the atoms around its double bonds.

These configurations are primarily labeled as either the "E" (from the German word 'entgegen,' meaning 'opposite') or "Z" (from 'zusammen,' meaning 'together'). These terms indicate whether similar priority groups are on opposite sides or the same side of the double bond respectively. This effect is crucial because these different spatial arrangements can lead to molecules with very different physical and chemical properties, even though the base formula is the same.

Skeletal Structure

In organic chemistry, skeletal structures provide a simplified way of representing organic compounds. These diagrams use lines to represent carbon-carbon bonds, omitting most hydrogens to minimize clutter while still conveying essential molecular information clearly.

For bombykol, the skeletal structure includes a long chain of 16 carbon atoms. The structure helps in visualizing the placement and relationships of functional groups like double bonds and hydroxyl groups. It serves as a scaffold, ensuring the chemist understands which atoms attach where, maintaining the correct sequence and connectivity in an otherwise potentially confusing molecule.

Cahn-Ingold-Prelog Priority Rules

The Cahn-Ingold-Prelog priority rules are a set of guidelines used to determine the order of groups attached to a double bond. This is crucial for understanding the spatial arrangement of molecules as mentioned in geometrical isomerism.

These rules assign priorities based on atomic numbers; the higher the atomic number, the higher the priority. When dealing with double bonds like those in bombykol, these rules assist in correctly identifying which configuration, either "E" or "Z," is present. Understanding these rules allows scientists to correctly label and predict the behaviors of molecules, an essential part of chemical analysis and synthesis.

Double Bonds

Double bonds play a central role in adding complexity to organic molecules. In a carbon double bond, two pairs of electrons are shared between two carbon atoms. This creates a situation where those bonds are unable to rotate freely, leading to the distinct arrangements seen in geometrical isomerism.

For bombykol's structure, there are specific carbon atoms where double bonds are positioned, precisely at carbons 10 to 11 and carbons 12 to 13. Each double bond location has unique implications for the molecule's geometry and physical properties, influenced by whether each is in an "E" or "Z" configuration. Double bonds not only affect how atoms are arranged but also play a role in the chemical reactivity and interactions of the molecule with other substances.