Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Problem 74c

Chapter 4, Problem 74c

Draw the two chair conformers for each of the following and indicate which conformer is more stable:
c. trans-1-ethyl-2-isopropylcyclohexane

Verified step by step guidance

Step 1: Understand the problem. You are tasked with drawing the two chair conformers of trans-1-ethyl-2-isopropylcyclohexane and determining which conformer is more stable. The 'trans' designation means that the two substituents (ethyl and isopropyl) are on opposite sides of the cyclohexane ring (one axial and one equatorial in each conformer).

Step 2: Draw the basic chair structure of cyclohexane. Label the carbon atoms in the ring to help you place the substituents correctly. Remember that each carbon atom in the chair conformation has one axial bond (pointing straight up or down) and one equatorial bond (pointing outward and slightly angled).

Step 3: Place the substituents on the ring for the first conformer. Start by placing the ethyl group at carbon-1 in the axial position and the isopropyl group at carbon-2 in the equatorial position. Ensure that the groups are trans to each other (on opposite sides of the ring).

Step 4: Draw the second chair conformer by performing a ring flip. In the flipped chair, the axial bonds become equatorial, and the equatorial bonds become axial. Place the ethyl group at carbon-1 in the equatorial position and the isopropyl group at carbon-2 in the axial position. Again, ensure the trans relationship is maintained.

Step 5: Compare the stability of the two conformers. The stability of a chair conformer is influenced by steric hindrance. Larger groups prefer the equatorial position to minimize 1,3-diaxial interactions. Since the isopropyl group is bulkier than the ethyl group, the conformer where the isopropyl group is equatorial and the ethyl group is axial will be more stable.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Chair Conformation

Chair conformation is a three-dimensional representation of cyclohexane that minimizes steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, allowing for more stable interactions between substituents. Understanding chair conformations is crucial for analyzing the stability of cyclohexane derivatives, as different substituents can occupy axial or equatorial positions, affecting overall stability.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together, often due to the size of substituents. In cyclohexane derivatives, larger groups prefer equatorial positions to minimize steric interactions with other axial substituents. Recognizing the impact of steric hindrance is essential for determining the more stable conformer of a molecule, as it directly influences the energy and stability of different conformations.

Trans vs. Cis Substituents

In cyclohexane derivatives, 'trans' and 'cis' refer to the relative positions of substituents on the ring. Trans substituents are located on opposite sides of the ring, while cis substituents are on the same side. This spatial arrangement affects the steric interactions and overall stability of the conformers. For trans-1-ethyl-2-isopropylcyclohexane, analyzing the positions of the ethyl and isopropyl groups is key to determining which chair conformer is more stable.

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