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 53

Chapter 4, Problem 53

a. Draw Newman projections of the two conformers of trans-1,3-dimethylcyclohexane.
b. Which of the conformers predominates at equilibrium?

Verified step by step guidance

Step 1: Understand the structure of cis-1,3-dimethylcyclohexane. The 'cis' configuration means that both methyl groups are on the same side of the cyclohexane ring. Cyclohexane typically adopts a chair conformation, so visualize the molecule in this conformation with the methyl groups positioned accordingly.

Step 2: Identify the axial and equatorial positions for the substituents in the chair conformation. In one chair conformation, one methyl group will occupy an axial position while the other occupies an equatorial position. In the flipped chair conformation, the positions of the methyl groups will swap (axial becomes equatorial and vice versa).

Step 3: Draw the Newman projection for the first conformer. Focus on the carbon-carbon bond where the substituents are attached. Represent the axial and equatorial positions of the methyl groups in the projection, ensuring proper spatial orientation.

Step 4: Draw the Newman projection for the second conformer. Again, focus on the same carbon-carbon bond, but now represent the swapped positions of the methyl groups (axial and equatorial) in the projection.

Step 5: Analyze the steric hindrance in both conformers. The conformer with both methyl groups in equatorial positions will have less steric hindrance and will be more stable. At equilibrium, the conformer with the methyl groups in equatorial positions will predominate.

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Key Concepts

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

Newman Projections

Newman projections are a way to visualize the conformation of a molecule by looking straight down the bond connecting two atoms, typically carbon atoms. This representation helps in analyzing steric interactions and torsional strain between substituents attached to the carbons. In the case of cis-1,3-dimethylcyclohexane, drawing Newman projections allows for a clear comparison of the two conformers and their spatial arrangements.

Conformational Analysis

Conformational analysis involves studying the different spatial arrangements of a molecule that can be interconverted by rotation around single bonds. For cyclohexane derivatives like cis-1,3-dimethylcyclohexane, the chair conformation is typically more stable than the boat conformation due to minimized steric strain. Understanding the stability of these conformers is crucial for predicting which one predominates at equilibrium.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms or groups in a molecule that occurs when they are brought close together, leading to increased energy and decreased stability. In cis-1,3-dimethylcyclohexane, the positioning of the methyl groups can create steric strain in certain conformers. Analyzing steric hindrance helps determine which conformer is more stable and thus predominates at equilibrium.

a. Draw Newman projections of the two conformers of trans-1,3-dimethylcyclohexane.b. Which of the conformers predominates at equilibrium?