Problem 2 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

The following questions relate to meso-butane-2,3-diol. (Sections 18.2 and 18.4 ) (a) Draw sawhorse and Newman projections of the synperiplanar conformation of meso-butane-2,3-diol. (b) Assign \(R\) or \(S\) configuration to each of the chiral centres in meso- butane-2,3-diol. (c) Draw a Fischer projection of meso-butane- 2,3 -diol and use this to help to explain why this is a meso compound. (d) Draw a Fischer projection of a diastereomer of mesobutane-2,3-diol.

Step-by-Step Solution

Answer

Meso-butane-2,3-diol is (2R,3S) with symmetrical sawhorse, Newman, and Fischer projections. Its diastereomer breaks this symmetry.

Step 1: Understand the Structure of Meso-butane-2,3-diol

Meso-butane-2,3-diol is an organic compound with two chiral centers at the second and third carbon atoms. The compound is meso because it has an internal plane of symmetry.

Step 2: Draw the Sawhorse Projection

For the sawhorse projection, visualize the compound along the C2-C3 bond. Arrange the two hydroxyl groups (-OH) adjacent to each other (synperiplanar conformation) with the remaining hydrogen atoms and methyl groups attached to the second and third carbons.

Step 3: Draw the Newman Projection

In the Newman projection, view down the C2-C3 bond. With the front carbon (C2) showing a -OH and a methyl group at 60-degree angles, position the back carbon (C3) directly opposite with its -OH and methyl group, showing the synperiplanar alignment.

Step 4: Assign R or S Configuration

Use the Cahn-Ingold-Prelog rules to assign priorities to the substituents on each chiral center. For C2, the configuration is R. For C3, the configuration is S. The compound's symmetry leads to the internal plane that makes it meso.

Step 5: Draw the Fischer Projection

In the Fischer projection, draw C2 and C3 vertically with the -OH groups on either side, and the methyl groups in vertical positions, indicating the plane of symmetry.

Step 6: Explain Meso Compound Nature

The Fischer projection shows that the molecule has a symmetrical reflective plane, inherently showcasing its meso nature as it maps onto itself.

Step 7: Draw a Fischer Projection of a Diastereomer

A diastereomer of meso-butane-2,3-diol would have one chiral center with the opposite configuration. Switch one of the chiral centers in the Fischer projection to create a different spatial arrangement, breaking the symmetry required for a meso compound.

Key Concepts

chiral centers

In organic chemistry, chiral centers are pivotal for determining the spatial arrangement of atoms in a molecule, particularly when assessing compounds like meso-butane-2,3-diol. A chiral center typically features a carbon atom bonded to four different substituents—this arrangement prevents mirror images from being superimposable, resulting in chirality or handedness.

For meso-butane-2,3-diol, the second and third carbon atoms serve as chiral centers. Despite having these chiral centers, the compound is not fully chiral. Why? Because it has an internal plane of symmetry that divides it into mirrored halves.

• The existence of chiral centers gives rise to the potential for stereoisomerism, where molecules might present as enantiomers (non-superimposable mirror images) or meso forms.
• Meso compounds uniquely possess chiral centers but are internally symmetrical—rendering them achiral overall due to this symmetry.

Understanding chiral centers is crucial for deciphering molecular configurations like R (rectus) and S (sinister), which follow specific rules: the Cahn-Ingold-Prelog priority system. These configurations define the spatial arrangement and relationship between substituents.

sawhorse projection

Sawhorse projections are helpful in visualizing molecules through a skewed, three-dimensional perspective. For meso-butane-2,3-diol, this projection helps better understand the spatial arrangement of atoms related to the synperiplanar conformation.

In practice, the sawhorse projection is drawn by looking along the bond between the key carbon atoms. For meso-butane-2,3-diol, focus on the bond between the second and third carbon atoms (C2 and C3). Here's how you craft the sawhorse view:

• The hydroxyl groups (-OH) in a synperiplanar orientation are adjacent.
• The methyl groups and hydrogen atoms arrange themselves around these -OH groups, with the projection showing a slight skew to help illustrate 3D geometry.

The sawhorse method is especially beneficial when relating to other visual connotations like Newman projections, offering varying perspectives of the same molecular arrangement, aiding in stereochemical comprehension.

Newman projection

Newman projections offer a direct line of sight down a molecular bond, providing a clear view of the arrangement of substituents around the two carbon atoms forming the bond. In the case of meso-butane-2,3-diol, viewing along the C2-C3 axis allows for a critical interpretation of the molecular conformation.

To draw a Newman projection here:

• The front carbon (C2) should prominently display its -OH and methyl group positioned around a central circle, typically represented at 60-degree angles to each other.
• The back carbon (C3) arranges its -OH and methyl group as mirror images, establishing synperiplanar alignments.

Newman projections are effective for quickly gauging steric interactions and sigma-bond conformations. Similarly, they provide an excellent framework for comparing geometrical conformations with other molecular visualization methods, like the sawhorse and Fischer projections.

Fischer projection

Fischer projections are very specific, flat representations of molecules, ideal for displaying stereochemistry in complex organic compounds like meso-butane-2,3-diol.

Constructing a Fischer projection involves positioning the most oxidized carbon at the top. For meso-butane-2,3-diol, the second and third carbons run vertically, and their substituents are arranged:

• The horizontal lines represent bonds coming out of the plane, usually holding the -OH groups.
• The vertical lines represent bonds going into the plane, typically holding the methyl groups.

This orientation clearly underscores the plane of symmetry for a meso compound, where the molecule can fold into two mirror-like halves.

Fischer projections showcase why meso-butane-2,3-diol is a meso compound—despite having chiral centers, its reflective symmetry causes it to be internally symmetrical. This type of projection is also used to visualize modifications leading to diastereomers, reflecting minor changes in spatial configuration at a single chiral center.