Question

Draw tetrahedral representations of the following molecules: (a) \((S)-2\) -Chlorobutane (b) \((R)-3\) -Chloropent-1-ene \(\left[\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}(\mathrm{Cl}) \mathrm{CH}_{2} \mathrm{CH}_{3}\right]\)

Short answer

(S)-2-Chlorobutane and (R)-3-Chloropent-1-ene have specific tetrahedral orientations ensuring their respective stereochemistry.

Step-by-step solution

Determine the Molecular Structure

Understand the base structure of the given molecules by identifying the main carbon chain and attached groups. (a) For (S)-2-Chlorobutane: The molecule has four carbon atoms. Chlorine is attached to the second carbon atom as well as other substituents like hydrogen (H) and methyl (CH₃). (b) For (R)-3-Chloropent-1-ene: The molecule has five carbon atoms with a chlorine atom attached to the third carbon atom. Additionally, there is a double bond starting at the first carbon.

Assign Priorities for Groups

To draw tetrahedral representations, prioritize the different groups attached to the chiral center based on atomic number and stereochemistry rules. (a) For (S)-2-Chlorobutane: Identify the priority of groups (Cl > CH₂CH₃ > CH₃ > H) based on atomic mass. (b) For (R)-3-Chloropent-1-ene: Consider the configuration at the chiral carbon: (Cl > H₂C=CHCH₂ > CH₂CH > H), again guided by atomic number and the C=C bond.

Draw the Tetrahedral Geometry

Represent each chiral carbon in a tetrahedral format, with bonds radiating from the central carbon atom. (a) For (S)-2-Chlorobutane: Draw a tetrahedron with Cl, CH₃CH₂, CH₃, and H around the second carbon. Follow the (S)-configuration, determined by descending priority order. (b) For (R)-3-Chloropent-1-ene: Draw a tetrahedron around the third carbon with Cl, vinyl, ethyl, and hydrogen, reflecting the (R)-configuration.

Confirm Stereochemistry Assignment

Verify if the drawn tetrahedral representation indeed corresponds to the given stereochemistry (R or S). (a) For (S)-2-Chlorobutane: Ensure that when looking from the least priority (H in this case), the priority order 1 -> 2 -> 3 goes counterclockwise confirming (S). (b) For (R)-3-Chloropent-1-ene: Check if from the least priority, the sequence 1 -> 2 -> 3 runs clockwise, confirming the (R) stereochemistry.

Key concepts

Chiral Center

In organic chemistry, a chiral center is a carbon atom that is bonded to four different groups. This unique configuration leads to the possibility of stereoisomers, which are molecules with the same chemical formula and sequence of bonded atoms, but different 3-dimensional orientations.

Chiral centers are central to understanding stereochemistry because they directly influence the molecular shape and behavior. A molecule can have one or more chiral centers, and each can contribute to multiple stereoisomers. When looking at a chiral center, the key is to identify the four distinct substituents attached to it. This setup makes the molecule asymmetric, meaning it cannot be superimposed onto its mirror image, similar to left and right hands.

To determine if a carbon is a chiral center:

• Check if it is bonded to four different atoms or groups.
• If any two groups are identical, the carbon is not chiral.

Identifying chiral centers is the first step in drawing tetrahedral representations of organic molecules, helping determine the molecule's stereochemical configuration, either R (rectus/right) or S (sinister/left).

Tetrahedral Geometry

Tetrahedral geometry is a fundamental concept in understanding the 3-dimensional arrangement of molecules, especially those with chiral centers. A tetrahedral shape arises when a central atom, such as a carbon, is bonded to four other atoms or groups.

This spatial arrangement occurs because the electrons around the central atom repel each other equally, creating an angle of about 109.5° between the bonds. This is the most spatially efficient arrangement, minimizing repulsion and stabilizing the molecule.

When drawing tetrahedral representations:

• The central atom, usually carbon, is depicted as the vertex.
• Bonds are drawn as lines radiating from this center, with consideration of 3D perspective.
• Use a wedge to indicate a bond coming out of the plane of the paper and a dashed line for one going behind.

This method of depicting molecular geometry helps to visualize and determine the stereochemical properties of chiral molecules. Understanding tetrahedral geometry is crucial for double-checking correct stereochemical assignments, such as the (R) or (S) configuration in stereochemistry.

Priority of Groups

In stereochemistry, the priority of groups is essential when determining the configuration of a chiral center as R or S. Groups attached to the chiral center are prioritized based on the Cahn-Ingold-Prelog (CIP) rules.

According to these rules:

• First, compare the atomic numbers of the atoms directly attached to the chiral center. The higher the atomic number, the higher the priority.
• If there's a tie, move to the next set of atoms along the chain and compare their atomic numbers.
• Double and triple bonds are accounted for by considering bonded atoms as multiple single bonds to phantom atoms.

Once the priorities are assigned, the molecule can be viewed from the side opposite the lowest-priority group. Observing the sequence of 1 -> 2 -> 3, the direction determines the configuration:

• Clockwise direction implies an (R) configuration.
• Counterclockwise direction implies an (S) configuration.

Understanding and applying the priority of groups is critical for accurately assigning the stereochemistry of chiral molecules.

Organic Molecules

Organic molecules form the basis of life and encompass a vast array of compounds primarily made of carbon and hydrogen, often with oxygen, nitrogen, and other elements. Their study involves understanding their structure, bonding, reactions, and properties.

Organic molecules are characterized by:

• Carbon backbone: chains or rings formed by carbon atoms bonded together.
• Diverse functional groups: specific groups of atoms that contribute to the molecules' unique properties and behaviors.
• Covalent bonding: strong bonds between atoms within the molecule, crucial for stability and reactivity.

The importance of these molecules lies in their diversity and complexity, which allow for a myriad of chemical reactions and functions.

When discussing stereochemistry in organic molecules, one focuses on how the spatial arrangement can affect the molecule's function and interact with biological systems. For example, (S)- and (R)- isomers may interact differently with biological targets, influencing efficacy and safety. Thus, understanding organic molecules and their stereochemistry is fundamental for applications in biochemistry, pharmaceuticals, and materials science.