Question

As a rule, axial alcohols oxidize somewhat faster than equatorial alcohols. Which would you expect to oxidize faster, cis-4-tert-butylcyclohexanol or trans-4-tert-butylcyclohexanol? Draw the more stable chair conformation of each molecule.

Short answer

cis-4-tert-butylcyclohexanol will oxidize faster.

Step-by-step solution

Understanding Axial vs. Equatorial Positions

In a cyclohexane chair conformation, substituents can occupy either axial (perpendicular to the ring) or equatorial (in the plane of the ring) positions. Axial alcohols tend to oxidize faster than equatorial alcohols.

Analyzing cis-4-tert-butylcyclohexanol

In the cis configuration, both substituents (the hydroxyl group and the tert-butyl group) will be on the same side of the cyclohexane ring. To minimize steric strain, the bulky tert-butyl group will occupy the equatorial position in the most stable conformation. Thus, the hydroxyl group will be axial.

Analyzing trans-4-tert-butylcyclohexanol

In the trans configuration, one substituent will be axial, and the other will be equatorial. The most stable chair conformation places the bulky tert-butyl group equatorial, forcing the hydroxyl group to also be equatorial.

Comparing Oxidation Rates

Since the hydroxyl group is axial in cis-4-tert-butylcyclohexanol and equatorial in trans-4-tert-butylcyclohexanol, cis-4-tert-butylcyclohexanol will oxidize faster.

Key concepts

Oxidation of Alcohols

Oxidation of alcohols is an essential concept in organic chemistry. It involves the transformation of alcohols into carbonyl compounds like aldehydes, ketones, or carboxylic acids. The rate of oxidation can be influenced by factors such as the position of the alcohol group on a cyclohexane ring. In cyclohexane, alcohol groups can be positioned either axially or equatorially. Axial positions are more exposed, similar to standing up from the surface of the ring. This exposure allows a greater interaction with oxidizing agents, which increases the rate of the oxidation process.
Equatorial alcohols lie in the plane of the cyclohexane ring. This position offers more steric protection, making them less reactive compared to their axial counterparts.
In practical scenarios, when oxidizing an alcohol group present in a cyclohexane, it's essential to consider whether the group is in an axial or equatorial position to predict its reactivity and rate of oxidation.

Chair Conformation

The chair conformation is the most stable and commonly observed shape of the cyclohexane ring. It resembles a reclining chair, where the carbon atoms are positioned in a zigzag manner. This conformation minimizes torsional strain and allows substituents to lie in either axial or equatorial positions.
For a correct understanding of stereochemistry, it’s vital to draw chair conformations when analyzing cyclohexane derivatives. When a bulky substituent, like tert-butyl, is involved, it usually adopts the equatorial position due to reduced steric strain. This stability preference means other groups, like alcohols, may switch positions (axial or equatorial) depending on the configuration (cis or trans) of the compound.
By drawing the chair conformation for specific cyclohexane derivatives such as cis-4-tert-butylcyclohexanol, you can predict which groups are axial or equatorial and thus infer the oxidation rates as discussed.

Steric Strain in Cyclohexanes

Steric strain in molecules like cyclohexanes arises mainly due to the close proximity of bulky groups. When these groups are forced into positions that bring them too close to each other, it results in increased energy, making the molecules less stable. Cyclohexane in itself is generally free from steric strain in its chair form; however, substituents can introduce such strains.
For instance, in the chair conformation of cyclohexane, a bulky group such as tert-butyl prefers the equatorial position. This orientation is chosen to minimize steric clashes with other axial hydrogen atoms on the ring. The orientation directly impacts the stability and reactivity of cyclohexane derivatives.

• Axial positions offer less space, potentially causing crowding, especially for larger groups.
• Equatorial positions provide more room, hence are often more favorable for bulky groups.

Understanding and predicting steric strain is critical for anticipating chemical reactivity and stability in cyclohexane compounds.