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 85

Chapter 4, Problem 85

One of the chair conformers of cis-1,3-dimethylcyclohexane is 5.4 kcal/mol less stable than the other. How much steric strain does a 1,3-diaxial interaction between two methyl groups introduce into the conformer?

Verified step by step guidance

Step 1: Understand the problem. The question asks us to determine the steric strain introduced by a 1,3-diaxial interaction between two methyl groups in one of the chair conformers of cis-1,3-dimethylcyclohexane. The given information states that one conformer is 5.4 kcal/mol less stable than the other.

Step 2: Recall the concept of 1,3-diaxial interactions. In cyclohexane chair conformations, substituents in axial positions experience steric strain due to interactions with axial hydrogens on the same side of the ring. For cis-1,3-dimethylcyclohexane, the methyl groups in axial positions will interact with each other, introducing steric strain.

Step 3: Analyze the stability difference. The stability difference of 5.4 kcal/mol between the two conformers is due to the steric strain caused by the 1,3-diaxial interaction of the methyl groups in the less stable conformer. This strain is the value we are trying to determine.

Step 4: Relate the steric strain to the stability difference. Since the stability difference is entirely attributed to the steric strain from the 1,3-diaxial interaction, the steric strain introduced by the interaction is equal to the given stability difference of 5.4 kcal/mol.

Step 5: Conclude the reasoning. The steric strain introduced by the 1,3-diaxial interaction between the two methyl groups in the less stable conformer is 5.4 kcal/mol, as this accounts for the entire stability difference between the two conformers.

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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 the most stable arrangement of cyclohexane rings, minimizing steric strain and torsional strain. In this conformation, carbon atoms adopt a staggered arrangement, allowing substituents to occupy equatorial or axial positions. The stability of different chair conformers can vary significantly based on the positioning of substituents, such as methyl groups, which can lead to steric interactions.

Steric Strain

Steric strain arises when atoms are forced closer together than their atomic radii allow, leading to increased energy in a molecule. In cyclohexane derivatives, substituents in axial positions can experience steric strain due to 1,3-diaxial interactions, where axial substituents interact unfavorably with hydrogen atoms on the same side of the ring. This strain contributes to the overall stability of the conformer.

1,3-Diaxial Interactions

1,3-diaxial interactions occur in cyclohexane when axial substituents interact with hydrogen atoms located two carbons away on the same side of the ring. These interactions create steric hindrance, which destabilizes the conformer. In the case of cis-1,3-dimethylcyclohexane, the presence of two axial methyl groups leads to significant steric strain, affecting the energy difference between the chair conformers.

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Related Practice

Textbook Question

Name the following compounds:

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Textbook Question

The most stable form of glucose (blood sugar) is a six-membered ring in a chair conformation with its five substituents all in equatorial positions. Draw the most stable conformer of glucose by putting the OH groups and hydrogens on the appropriate bonds in the structure on the right.

Explain the following:

a. 1-Hexanol has a higher boiling point than 3-hexanol.

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Textbook Question

Name the following compounds:

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Textbook Question

What is each compound’s systematic name?

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Textbook Question

Bromine is a larger atom than chlorine, but the equilibrium constants in Table 3.9 indicate that a chloro substituent has a greater preference for the equatorial position than does a bromo substituent. Suggest an explanation for this fact.

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