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) cis-1-isopropyl-3-methylcyclohexane (b) \(c\) is- 4 -tert-butylcyclohexanol

Short answer

For cis-1-isopropyl-3-methylcyclohexane, the more stable conformation is the one with the isopropyl group in the equatorial position. For cis-4-tert-butylcyclohexanol, the conformation with the tert-butyl group in the equatorial position is more stable.

Step-by-step solution

Drawing Chair Conformations for cis-1-isopropyl-3-methylcyclohexane

Start by drawing a simple cyclohexane chair conformation, remembering that each carbon has two potential bonding sites - one axial (vertical) and one equatorial (horizontal). For cis-1-isopropyl-3-methylcyclohexane, place the isopropyl group on carbon 1 and the methyl group on carbon 3. Since it’s cis, both groups must be on the same side of the plane (both up or both down). In the first conformation, place the isopropyl group axial and the methyl group equatorial, in the second flip them.

Identifying More Stable Conformation for cis-1-isopropyl-3-methylcyclohexane

The stability of a conformation is determined by the steric hindrance of the substituents. Substituents prefer the equatorial position due to less steric hindrance. Thus, the chair conformation with the larger isopropyl group in the equatorial position is more stable.

Drawing Chair Conformations for cis-4-tert-butylcyclohexanol

For cis-4-tert-butylcyclohexanol, begin again by drawing a simple cyclohexane chair conformation. On carbon 4, place the tert-butyl group and on a carbon adjacent to that (either 3 or 5), place the hydroxyl group (which due to being cis must be on the same side). In the first conformation, place the tert-butyl group axial and the hydroxyl group equatorial, in the second flip them.

Identifying More Stable Conformation for cis-4-tert-butylcyclohexanol

In this molecule, the bulky tert-butyl group strongly prefers the equatorial position due to reduced steric hindrance, so the chair conformation with the tert-butyl group in the equatorial position is more stable.

Key concepts

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, is a form of stereoisomerism describing the orientation of functional groups within a molecule. In cyclohexane rings, this relates to the placement of substituents either on the same side (cis) or on opposite sides (trans) of the ring. When dealing with cyclohexanes, understanding cis/trans isomerism is essential since it influences molecular stability and reactivity.

For example, in the exercise's cis-1-isopropyl-3-methylcyclohexane, both substituents are on the same side of the plane. The physical placement of these groups can create significant differences in how the molecule interacts with other compounds and how it behaves under various conditions. Understanding their orientation helps in predicting the molecule's chemical properties and its interactions in biological systems.

Cyclohexane Conformations

Cyclohexane conformations refer to the spatial orientation of atoms in a cyclohexane molecule, which directly influences its chemical and physical properties. The chair conformation is the most stable due to the relatively low strain on bonds. In it, the hydrogens (or other substituents) alternate between axial (perpendicular to the ring) and equatorial (approximate to the plane of the ring) positions.

In our exercise with cis-1-isopropyl-3-methylcyclohexane and cis-4-tert-butylcyclohexanol, analyzing the two chair conformations allows us to understand how different placements of substituents can alter molecular stability. The steric hindrance, or spatial conflict, between groups can be assessed by envisioning these conformations, providing insight into the most stable form of the molecule.

Steric Hindrance

Steric hindrance is a phenomenon where atoms or groups of atoms impede the physical access of other atoms due to their size. In the context of cyclohexane rings, bulky substituents can interfere with each other, leading to increased energy and decreased stability of the molecule.

In the step-by-step solutions, we see that larger substituents like the isopropyl and tert-butyl groups prefer the equatorial position to minimize steric clashes. Steric hindrance is a crucial factor in the stability of chair conformations of cyclohexane, as it can force the molecule into a less favorable position, introduce strain, and potentially lead to unexpected chemical reactions or a lack of reactivity.

Molecular Stability

Molecular stability in cyclohexane conformations is determined by the sum of internal forces, including bond angles, torsional strain, and non-bonded interactions such as steric hindrance. A molecule is most stable when it experiences the least strain and when steric hindrance is minimized. This explains why, in both parts of our exercise, the more stable conformer is the one with the bulkier substituents in the equatorial positions.

Understanding molecular stability is vital for predicting the behavior of organic compounds. It allows chemists to anticipate the most favorable conformations and to design synthesis pathways that optimize yields and reduce unwanted side reactions. When studying chair conformations, recognizing the interplay between all forms of strain is key to mastering organic chemistry.