Draw The Molecule Below After A Chair Flip

Flipping a cyclohexane ring, often visualized as a chair conformation, is a fundamental concept in organic chemistry that significantly influences the properties and reactivity of molecules. Mastering the ability to draw the molecule after a chair flip is crucial for understanding conformational analysis, predicting reaction outcomes, and interpreting spectroscopic data.

Understanding Chair Conformations

Cyclohexane rings are not planar; they adopt non-planar conformations to minimize torsional strain and steric hindrance. The most stable conformation is the chair conformation, which resembles a chair and has two distinct types of positions for substituents:

• Axial positions: These point directly up or down, perpendicular to the average plane of the ring.
• Equatorial positions: These point outward, roughly along the "equator" of the ring, and are slightly angled relative to the average plane.

The Chair Flip: A Dynamic Process

The chair flip, also known as ring inversion, is a dynamic process where the cyclohexane ring interconverts between two chair conformations. During this process, all axial substituents become equatorial, and all equatorial substituents become axial. The energy barrier for this interconversion is relatively low, meaning that chair flips occur rapidly at room temperature.

Drawing the Chair Conformation

Before we delve into the chair flip, let's establish how to draw the chair conformation. There are many ways to draw the chair conformation, here are a few tips to create a consistent and visually appealing representation:

1. Draw two parallel lines, slightly offset vertically and horizontally.
2. Connect the ends of these lines with two more lines, forming a parallelogram-like shape.
3. Add the remaining two lines to complete the chair. One end should point "up" and the other "down."

Drawing Substituents

1. Axial bonds: Draw vertical lines pointing directly up or down from each carbon atom.
2. Equatorial bonds: Draw lines that are slightly angled, pointing outwards from each carbon atom. These should be roughly parallel to the bonds of the ring carbons adjacent to the carbon you are drawing on.

Visualizing the Chair Flip

Imagine one end of the chair flipping up while the other end flips down, or vice versa. This motion converts all axial positions to equatorial positions and vice versa.

Step-by-Step Guide to Drawing the Molecule After a Chair Flip

Here's a detailed, step-by-step guide on how to draw a molecule after a chair flip:

Step 1: Draw the Initial Chair Conformation

Start by drawing the initial chair conformation of the molecule, ensuring that all substituents are correctly placed in either axial or equatorial positions according to the original structure. This is the foundation upon which the flipped structure will be built.

Step 2: Identify Axial and Equatorial Positions

Carefully identify which substituents are in axial positions and which are in equatorial positions on the initial chair conformation. Color-coding or labeling each substituent can be helpful to keep track of their positions during the flip.

Step 3: Draw the Flipped Chair Conformation

Draw the second chair conformation, ensuring that it is a mirror image of the first. This represents the ring after the chair flip has occurred.

Step 4: Convert Axial to Equatorial and Vice Versa

For each substituent, reverse its position from axial to equatorial or from equatorial to axial on the flipped chair conformation. An axial substituent in the initial conformation becomes an equatorial substituent in the flipped conformation, and vice versa.

Step 5: Maintain Up/Down Orientation

Pay close attention to the up or down orientation of each substituent. If a substituent is pointing up in the initial conformation (whether axial or equatorial), it must still be pointing up in the flipped conformation. Similarly, a substituent pointing down in the initial conformation must remain pointing down in the flipped conformation. The only thing that changes is whether it's axial or equatorial.

Step 6: Double-Check Your Work

After completing the flipped structure, double-check that all substituents are in the correct positions and orientations. Ensure that axial substituents have become equatorial and vice versa, and that the up or down orientation of each substituent remains consistent.

Example: Methylcyclohexane

Let's illustrate this process with methylcyclohexane.

1. Initial Chair Conformation: Draw cyclohexane in the chair conformation with a methyl group (CH3) in the axial position.
2. Flipped Chair Conformation: Draw the second chair conformation.
3. Convert Positions: The methyl group, which was axial, now becomes equatorial. Ensure the methyl group still points in the same general direction (up or down) as it did before the flip. In this case, since the methyl group was pointing up in the axial position, it should also point up in the equatorial position after the flip.

Factors Affecting Conformational Preference

While the chair flip occurs rapidly, the two chair conformations are not always equal in energy. The relative stability of the two conformations is influenced by several factors:

• Steric Hindrance: Axial substituents experience greater steric hindrance due to 1,3-diaxial interactions with other axial substituents on the same side of the ring. Bulky substituents prefer to be in the equatorial position to minimize this steric strain.
• Electronic Effects: In some cases, electronic effects can also influence conformational preference. For example, electronegative substituents may prefer the axial position due to the gauche effect.

Examples of Drawing Chair Flips with Multiple Substituents

Let's consider more complex examples with multiple substituents to illustrate the process of drawing chair flips:

Example 1: 1,2-Dimethylcyclohexane

Consider cis-1,2-dimethylcyclohexane. In this molecule, both methyl groups are on the same side of the ring. There are two possible chair conformations:

• Conformation A: Both methyl groups are axial.
• Conformation B: Both methyl groups are equatorial.

To draw the chair flip, follow these steps:

1. Draw Conformation A: Draw the chair conformation with both methyl groups in the axial positions on carbons 1 and 2. Ensure that both methyl groups are pointing in the same direction (either both up or both down relative to the ring).
2. Draw Conformation B: Draw the second chair conformation, which is the flipped version of Conformation A.
3. Convert Positions: The methyl groups, which were axial in Conformation A, now become equatorial in Conformation B. Maintain the cis relationship by ensuring that both methyl groups are still pointing in the same direction (either both up or both down).

In this case, Conformation B (both methyl groups equatorial) is more stable due to reduced steric hindrance.

Example 2: 1,4-tert-Butylcyclohexane

Consider trans-1,4-tert-butylcyclohexane. The tert-butyl group is very bulky, which significantly influences the conformational preference.

1. Draw Conformation A: Draw the chair conformation with the tert-butyl group in the axial position on carbon 1 and a hydrogen atom in the equatorial position. Since it is trans, the other group at position 4 is on the opposite side of the ring.
2. Draw Conformation B: Draw the second chair conformation, which is the flipped version of Conformation A. The tert-butyl group will now be in the equatorial position.
3. Convert Positions: The tert-butyl group, which was axial in Conformation A, now becomes equatorial in Conformation B.

Due to the large size of the tert-butyl group, Conformation B (with the tert-butyl group in the equatorial position) is overwhelmingly more stable. The tert-butyl group effectively "locks" the cyclohexane ring in this conformation because the energy barrier to flip to the axial conformation is very high.

Advanced Considerations

Disubstituted Cyclohexanes

For disubstituted cyclohexanes, understanding cis and trans relationships is crucial.

• Cis: Substituents are on the same side of the ring (both up or both down).
• Trans: Substituents are on opposite sides of the ring (one up and one down).

When drawing chair flips of disubstituted cyclohexanes, ensure that the cis or trans relationship is maintained after the flip.

Bulky Substituents

Bulky substituents, such as tert-butyl groups, strongly prefer the equatorial position. This is because the axial position causes significant 1,3-diaxial interactions, which destabilize the conformation. In molecules with bulky substituents, the conformation with the bulky group in the equatorial position is often the dominant one.

Common Mistakes to Avoid

• Incorrectly Drawing Axial and Equatorial Bonds: Ensure that axial bonds are drawn vertically (straight up or down) and equatorial bonds are drawn slightly angled.
• Forgetting to Maintain Up/Down Orientation: Substituents that are pointing up in one conformation must still be pointing up after the chair flip, and vice versa.
• Not Considering Steric Hindrance: Remember that bulky substituents prefer the equatorial position to minimize steric interactions.
• Misidentifying Cis and Trans Relationships: Double-check the cis or trans relationships of substituents and ensure they are maintained after the chair flip.
• Rushing the Process: Take your time and carefully follow each step. Drawing chair flips accurately requires attention to detail.

Practical Applications

Understanding chair flips and conformational analysis has numerous practical applications in chemistry and related fields:

• Drug Design: The conformation of a molecule can significantly affect its binding affinity to a target protein. Understanding conformational preferences is crucial in drug design to optimize the fit and activity of drug candidates.
• Polymer Chemistry: The properties of polymers are influenced by the conformations of their constituent monomers. Understanding conformational analysis helps in designing polymers with specific properties.
• Spectroscopy: Spectroscopic techniques, such as NMR spectroscopy, can provide information about the conformations of molecules. Analyzing the spectral data requires an understanding of conformational analysis.
• Reaction Mechanisms: Conformational effects can influence the rates and selectivities of chemical reactions. Understanding conformational analysis helps in predicting reaction outcomes and designing more efficient reactions.

Using Software for Visualization

While drawing chair conformations by hand is a valuable skill, several software tools can help visualize and manipulate molecules in 3D:

• ChemDraw: A popular chemical drawing program that allows you to draw and visualize molecules in various conformations.
• Avogadro: A free, open-source molecular editor and visualization tool.
• Molecular Modeling Software: Programs like Schrödinger Maestro, MOE (Molecular Operating Environment), and Gaussian can perform more advanced conformational analysis and energy calculations.

These tools can help you gain a better understanding of the spatial arrangement of atoms and the effects of chair flips on molecular properties.

Conclusion

Drawing the molecule after a chair flip is a fundamental skill in organic chemistry that enables you to understand and predict the behavior of cyclic molecules. By following the step-by-step guide outlined in this article, you can accurately draw chair conformations, identify axial and equatorial positions, and predict the outcome of chair flips. Understanding conformational preferences, steric hindrance, and other factors influencing conformational stability will enhance your ability to analyze and interpret chemical phenomena. Practice drawing various cyclohexane derivatives to reinforce your understanding and master this essential skill.