Question

Three of the four structures below are chiral. Assign \(R /\) S designations to all chiral centers, and identify the achiral molecule.

Short answer

Question: Identify the chiral centers in the given molecular structures, assign R/S designations as needed, and determine the achiral molecule. Answer: To solve this problem, examine the provided molecular structures, identify the chiral centers, assign R/S designations using the Cahn-Ingold-Prelog rules, and determine the achiral molecule by looking for planes of symmetry or centers of inversion.

Step-by-step solution

Identify chiral centers

Chirality arises when a carbon center is bonded to four unique substituents. To identify chiral centers, look for carbons where there are no two identical ligands attached.

Assign R/S designations using CIP rules

To assign R/S designations, follow the Cahn-Ingold-Prelog rules: 1. Assign priority to the substituents around the chiral center based on atomic number (higher atomic number = higher priority). 2. If there is a tie, consider the next atoms in the substituent until the tie is broken. 3. Orient the molecule such that the lowest priority group (4) is pointing away from you. 4. Determine if the remaining three groups are in clockwise (R) or counterclockwise (S) order.

Determine the achiral molecule

An achiral molecule is one that is superimposable on its mirror image. An achiral molecule can possess chiral centers, but its overall structure lacks chirality. Look for a plane of symmetry or center of inversion for the given structure. Now, examine the provided molecular structures, place the chiral centers and achiral molecule into their respective slots, and assign R/S designations as needed.

Key concepts

Chiral Centers

Chiral centers play a crucial role in determining the chirality of molecules. A chiral center is typically a carbon atom bonded to four distinct substituents. This arrangement ensures that there are no two identical ligands attached to it. Think of a chiral center as a unique crossroad in a molecule where each road leads to a different destination.
When identifying chiral centers, you need to use the following simple steps:

• Check each carbon atom to see if it is connected to four different substituents.
• If you find such carbons, you've identified a chiral center.
• Remember that an achiral molecule is one that can be superimposed on its mirror image. It might have chiral centers but is symmetrical overall.

Identifying these centers is the groundwork for assigning stereochemistry, such as R/S configurations. Many molecules in biochemistry, pharmaceuticals, and organic chemistry possess chiral centers, which can greatly influence their function and interaction.

R/S Configuration

The R/S configuration is a systematic way to describe the spatial arrangement of atoms around a chiral center. To understand this, we use the Cahn-Ingold-Prelog priority rules to assign an absolute configuration of either 'R' (Rectus) or 'S' (Sinister).
Here's how to assign the R/S configuration:

• Assign priorities to the substituents surrounding a chiral center based on their atomic number. Higher atomic numbers get higher priority.
• If there's a tie in atomic numbers, move outward to the next atoms in the chains until the tie is broken.
• Reorient the molecule so that the lowest-priority group is pointing away from you.
• Look at the three higher-priority groups and determine their order of arrangement: clockwise means 'R' and counterclockwise means 'S'.

Using these steps helps you communicate the specific 3D configuration of a molecule, essential for understanding its chemical properties and behavior.

Cahn-Ingold-Prelog Rules

The Cahn-Ingold-Prelog (CIP) rules provide a logical method to assign priorities to chiral centers. These rules help ensure consistency when determining the R/S configuration for chiral molecules. They are necessary because different substituents can make identifying the correct order challenging without a standardized approach.
The core steps of the CIP rules are:

• Firstly, assign priorities based on atomic number: the higher the atomic number, the higher the priority.
• If two substituents have the same atomic number, compare the next set of atoms along the chain until you find a difference.
• Multiple bonds are treated as if each bond is a separate single bond to a "phantom" atom with the same atomic number.
• The lowest priority group should be pointing away from the viewer to accurately assess the 3D orientation of the substituents.

These rules aid in clearly defining and communicating the complex stereochemistry of organic compounds, ensuring clarity in everything from synthesis to drug development.