Question

For each of the following substituted cyclohexanes, draw the two possible chair conformations, label each substituent as axial or equatorial, and identify the more stable conformer. (a) cyclohexanol (b) trans-3-methylcyclohexanol

Short answer

For cyclohexanol, the more stable chair conformation is the one where the OH group is in an equatorial position, avoiding steric interaction. For trans-3-methylcyclohexanol, the more stable chair conformation is where the methyl group is in an equatorial position, also minimizing steric interaction because it is larger than hydrogen.

Step-by-step solution

Draw Chair Conformations of Cyclohexanol

Draw the two possible chair conformations for cyclohexanol - one where the hydroxyl (OH) group is in an axial orientation and one where it is in an equatorial orientation. Label each OH group as axial or equatorial.

Identify the More Stable Conformation of Cyclohexanol

Compare the two chair conformations for cyclohexanol. The conformation with the OH group in an equatorial position will be more stable because equatorial substituents experience fewer steric interactions with other hydrogens on the cyclohexane ring.

Draw Chair Conformations of trans-3-Methylcyclohexanol

Now draw the two possible chair conformations for trans-3-methylcyclohexanol - one where the methyl group is in an axial position and one where it is in an equatorial position. Remember the 'trans' designation means the hydroxyl group is on the opposite side of the ring to the methyl group.

Identify the More Stable Conformation of trans-3-Methylcyclohexanol

Compare the two chair conformations for trans-3-methylcyclohexanol. The conformation where the larger methyl group is in the equatorial position, thus minimizing steric interaction, will be the more stable chair conformation.

Key concepts

Chair Conformation

One of the most stable forms that cyclohexane takes is known as the chair conformation. This is because it minimizes torsional strain and steric hindrance.
Imagine the shape of a classic wooden chair — this is somewhat what the cyclohexane ring looks like in its chair conformation. The bonds are staggered in such a way that they are as far apart as possible. This minimizes repulsion between electron clouds, thus stabilizing the molecule.
Chair conformations are crucial when analyzing molecules like cyclohexane because they give us an insight into how various substituents (like hydroxyl or methyl groups) interact. This can determine their positions and ultimately, their stability.

Axial and Equatorial Positions

In the chair conformation, the positions of substituents are categorized as either axial or equatorial. These positions are fundamental to understanding how molecules like cyclohexane behave.

• Axial positions: These are parallel to the axis of the ring and alternate up and down along the ring. They tend to be less stable due to increased interactions with other axial substituents.
• Equatorial positions: These lie along the equator of the molecule and are positioned more sideways. This orientation allows substituents to avoid potential clashes with other parts of the molecule, making them generally more stable than their axial counterparts.

For example, in chair conformations of cyclohexanol or 3-methylcyclohexanol, substituents prefer to be in equatorial positions when possible, to minimize steric interactions.

Steric Interactions

Steric interactions refer to the physical hindrance that atoms or groups in a molecule can exert on one another. These interactions are particularly significant in cyclohexane chair conformations. They happen when atoms are close enough that their electron clouds overlap, leading to repulsive forces.
Think of steric interactions as parties. The closer you pack party-goers (or atoms), the more they might jostle each other, similarly creating an uncomfortable situation. This analogy is useful when discussing axial substituents; in these cases, adjacent axial groups on cyclohexane come close enough to experience steric hindrances.
Reducing the number of steric interactions increases the stability of the molecule significantly. That's why equatorial positions are preferred over axial ones — substituents occupy more space without coming into conflict with each other. Understanding these interactions helps predict which chair conformation of a cyclohexane derivative is more stable.