Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis

Problem 54f

Chapter 2, Problem 54f

For each pair of conformations shown, choose which is most stable. [If both conformations have the same number of gauche interactions, choose the one where the interactions are between smaller groups.]
(f)

Verified step by step guidance

Step 1: Analyze the Newman projections for conformations A and B. Both conformations represent staggered arrangements of substituents around a central carbon-carbon bond.

Step 2: Identify gauche interactions in each conformation. Gauche interactions occur when two substituents are 60° apart in a staggered conformation. In conformation A, the methyl group (CH₃) on the front carbon is gauche to the ethyl group on the back carbon. In conformation B, the methyl group on the front carbon is gauche to the methyl group on the back carbon.

Step 3: Compare the steric hindrance caused by the gauche interactions. In conformation A, the interaction is between a methyl group and an ethyl group, which is less sterically hindered compared to the interaction between two methyl groups in conformation B.

Step 4: Consider the rule provided in the problem: If both conformations have the same number of gauche interactions, the conformation with smaller groups involved in the interactions is more stable. Since conformation A involves a methyl-ethyl interaction and conformation B involves a methyl-methyl interaction, conformation A is more stable.

Step 5: Conclude that conformation A is the most stable due to fewer steric hindrance in its gauche interactions compared to conformation B.

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Key Concepts

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

Conformational Analysis

Conformational analysis involves studying the different spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds. In alkanes, these conformations can significantly affect the molecule's stability due to steric interactions and torsional strain. Understanding how to visualize and compare these conformations is crucial for determining which is more stable.

Gauche Interactions

Gauche interactions occur when two substituents on adjacent carbon atoms in a chain are positioned 60 degrees apart in a staggered conformation. These interactions can lead to increased steric strain, making the conformation less stable. When comparing conformations, the number and type of gauche interactions are key factors in assessing stability, with fewer gauche interactions generally indicating a more stable conformation.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together, often due to their size. In conformational analysis, larger groups can create more steric hindrance, leading to increased energy and decreased stability of a conformation. When evaluating the stability of conformations, it is important to consider the size of substituents and their spatial arrangement to minimize steric interactions.

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Related Practice

Textbook Question

Using the numbers shown in the chair conformation on the left, label the carbons of the flipped chair on the right. [Assume that the angle through which you view the chair conformation doesn't change.]

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Textbook Question

Given the following structures, show the Newman projection that would result from looking down the indicated bond in the direction shown. [Orient yourself as if you were the eyeball looking down the bond. Some of the examples have been partially completed for you to fill in the rest.]

(l) <IMAGE>

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Textbook Question

For each pair of conformations shown, choose which is most stable. [If both conformations have the same number of gauche interactions, choose the one where the interactions are between smaller groups.]

(e)

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Textbook Question

Looking down the indicated bond, show the three most stable conformations and choose the one that is most stable. Be sure that the first Newman projection you show is the one you see initially (before rotation). [Why should none of your three Newman projections show eclipsed conformations?]

(b) <IMAGE>

(e) <IMAGE>

(k) <IMAGE>