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Chapter 2: Problem 50

There are four cis, trans isomers of 2-isopropyl-5-methylcyclohexanol: CC1CCC(C(C)C)C(O)C1 2-Isopropyl-5-methylcyclohexanol (a) Using a planar hexagon representation for the cyclohexane ring, draw structural formulas for the four cis,trans isomers. (b) Draw the more stable chair conformation for each of your answers in part (a). (c) Of the four cis, trans isomers, which is the most stable? (Hint: If you answered this part correctly, you picked the isomer found in nature and given the name menthol.)

Short Answer

Expert verified
Question: Identify and draw the cis, trans isomers of 2-isopropyl-5-methylcyclohexanol and determine the most stable isomer. Answer: The four possible cis, trans isomers of 2-isopropyl-5-methylcyclohexanol are cis-2-isopropylcis-5-methylcyclohexanol, cis-2-isopropyltrans-5-methylcyclohexanol, trans-2-isopropylcis-5-methylcyclohexanol, and trans-2-isopropyltrans-5-methylcyclohexanol. The most stable isomer is cis-2-isopropylcis-5-methylcyclohexanol (also known as menthol) as it has the least steric hindrance with both substituents in the equatorial positions.

Step by step solution

01

Draw the planar hexagon representation of cyclohexane ring

Draw a simple hexagon to represent the planar structure of the cyclohexane ring. Number the carbon atoms clockwise from 1 to 6; the numbering scheme is not essential here, but it will help identify the position of substituents on the ring.
02

Draw the structural formulas for the four cis, trans isomers

You need to consider the location of the isopropyl and methyl groups on the cyclohexane ring and their relative orientations (cis or trans). 1. cis-2-isopropylcis-5-methylcyclohexanol: Draw an isopropyl group at C-2 and a methyl group at C-5 with both groups on the same side (either both above or both below the hexagon). 2. cis-2-isopropyltrans-5-methylcyclohexanol: Draw an isopropyl group at C-2 and a methyl group at C-5 with one group above the hexagon and the other below the hexagon. 3. trans-2-isopropylcis-5-methylcyclohexanol: Draw an isopropyl group at C-2 and a methyl group at C-5 with one group above the hexagon and the other below the hexagon but with the opposite orientation compared to 2. 4. trans-2-isopropyltrans-5-methylcyclohexanol: Draw an isopropyl group at C-2 and a methyl group at C-5 with both groups on the opposite sides (one above and one below the hexagon).
03

Draw the more stable chair conformation for each isomer

For each of the cis,trans isomers from step 2, draw the chair conformation. Place the substituent in the equatorial position for better stability due to reduced steric hindrance. 1. For cis-2-isopropylcis-5-methylcyclohexanol, draw both groups in the equatorial positions. 2. For cis-2-isopropyltrans-5-methylcyclohexanol, draw the isopropyl group in the equatorial position and the methyl group in the axial position. 3. For trans-2-isopropylcis-5-methylcyclohexanol, draw the isopropyl group in the axial position and the methyl group in the equatorial position. 4. For trans-2-isopropyltrans-5-methylcyclohexanol, draw both groups in the axial positions.
04

Determine the most stable cis, trans isomer

To determine the most stable isomer, consider the steric hindrance between substituents and the positions (axial or equatorial) in each chair conformation. The most stable isomer will have the least steric hindrance and the substituents preferably in the equatorial position. The most stable isomer is the cis-2-isopropylcis-5-methylcyclohexanol, as both substituents are in the equatorial positions with the least steric hindrance. This isomer is found in nature and is called menthol.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Structural Formulas
Structural formulas are a way to visually represent the molecular structure of a compound, illustrating the arrangement of atoms within the molecule. When it comes to cyclohexanol isomers, the structural formula makes it easier to understand the spatial orientation of the substituents, which can be pivotal in determining physical and chemical properties. For instance, when examining the 2-Isopropyl-5-methylcyclohexanol molecule, the planar hexagon simplifies the complex three-dimensional structure and highlights the positions of the isopropyl and methyl groups. The cis or trans notation is then used to denote whether these groups are on the same side (cis) or on opposite sides (trans) of the hexagonal ring. This aids in visualizing the possible isomers of the compound and sets the stage for further analysis of its conformation and stability.
Chair Conformation
The cyclohexane ring can adopt several conformations, with the chair conformation being the most stable due to minimal steric strain and angle strain. When drawing 2-Isopropyl-5-methylcyclohexanol isomers, depicting them in chair conformations is crucial to illustrate which isomer would be energetically favored. In this conformation, carbon atoms are staggered, reducing the potential for torsional strain. Substituents on a cyclohexane ring in the chair conformation occupy either axial positions, projecting parallel to the ring's vertical axis, or equatorial positions, extending outward from the ring's horizontal plane. Typically, larger groups will prefer the equatorial position to minimize steric hindrance with neighboring atoms or groups. This principle guides the prediction of the most stable isomer among the four considered.
Stability of Cyclohexanol Isomers
The stability of cyclohexanol isomers, such as those of 2-Isopropyl-5-methylcyclohexanol, heavily depends on the steric interactions between the substituents. The axial positions on the hexagon are less favorable for bulky groups because they lead to increased steric clashes - known as 1,3-diaxial interactions. The equatorial position, on the other hand, provides more space and thus reduces these repulsive interactions. Among the four isomers, the one with both the isopropyl, which is bulkier, and the methyl group in the equatorial positions, namely the cis-2-isopropylcis-5-methylcyclohexanol, presents the most favorable energy profile. This translates to the highest stability, which directly correlates to its prevalence in nature as the compound known as menthol.
Menthol Structure
Menthol is the naturally occurring form of 2-Isopropyl-5-methylcyclohexanol with a specific chiral arrangement which exhibits a characteristic cool and refreshing sensation. Its structure is that of the most stable isomer of the cyclohexanol derivatives discussed, with both the isopropyl and methyl groups in the cis configuration and occupying equatorial positions. The three-dimensional chair conformation of menthol maximizes stability by minimizing steric hindrance, which is a hallmark of effective design in nature. Understanding menthol's molecular arrangement not only highlights the importance of conformational analysis in organic chemistry but also showcases the profound influence of molecular structures on the physical properties and sensory characteristics of biomolecules.

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Most popular questions from this chapter

Following are the structural formulas and names of four bicycloalkanes. Write the molecular formula of each compound. Which of these compounds are constitutional isomers?

Calculate the difference in Gibbs free energy in kilojoules per mole between the alternative chair conformations of: (a) trans-4-Methylcyclohexanol (b) cis-Methylcyclohexanol (c) trans-1,4-Dicyanocyclohexane

Draw structural formulas for all the following. (a) Alcohols with the molecular formula \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) (b) Aldehydes with the molecular formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\) (c) Ketones with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}\) (d) Carboxylic acids with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}\)

Consider 1-bromopropane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\). (a) Draw a Newman projection for the conformation in which \(-\mathrm{CH}_{3}\) and \(-\mathrm{Br}\) are anti (dihedral angle \(180^{\circ}\) ). (b) Draw Newman projections for the conformations in which \(-\mathrm{CH}_{3}\) and \(-\mathrm{Br}\) are gauche (dihedral angles \(60^{\circ}\) and \(300^{\circ}\) ). (c) Which of these is the lowest energy conformation? (d) Which of these conformations, if any, are related by reflection?

Draw a chair conformation of 1,4 -dimethylcyclohexane in which one methyl group is equatorial and the other is axial. Draw the alternative chair conformation, and calculate the ratio of the two conformations at \(25^{\circ} \mathrm{C}\).
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