Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics

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

Bruice 8th Edition

Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics

Problem 20b

Chapter 6, Problem 20b

Why is the percentage of molecules with the substituent in an equatorial position greater for isopropylcyclohexane than for fluorocyclohexane?

Verified step by step guidance

Understand the concept of axial and equatorial positions in cyclohexane: Cyclohexane adopts a chair conformation where substituents can occupy either axial (parallel to the ring's axis) or equatorial (pointing outward from the ring) positions. Substituents in the equatorial position experience less steric hindrance compared to axial positions.

Consider the size of the substituent: Isopropyl is a bulky group, while fluorine is relatively small. Larger substituents, like isopropyl, experience significant steric interactions when placed in the axial position due to crowding with axial hydrogens on the same side of the ring.

Analyze the steric hindrance: For isopropylcyclohexane, the steric hindrance caused by the axial position is much greater than for fluorocyclohexane because the isopropyl group is bulkier. This makes the equatorial position more favorable for isopropylcyclohexane to minimize steric strain.

Consider electronic effects: Fluorine is an electronegative atom, and its small size means that steric hindrance is less pronounced. While fluorine may have some electronic preferences, steric factors dominate in determining the preferred position for substituents in cyclohexane.

Conclude the reasoning: The percentage of molecules with the substituent in the equatorial position is greater for isopropylcyclohexane because the bulky isopropyl group strongly favors the equatorial position to reduce steric hindrance, whereas the smaller fluorine substituent does not experience as much steric strain in the axial position.

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Key Concepts

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

Chair Conformation

Cyclohexane can adopt a chair conformation that minimizes steric strain. In this conformation, substituents can occupy either axial or equatorial positions. Equatorial positions are generally more stable because they reduce steric hindrance with other axial substituents, making it crucial to understand how substituents affect stability.

Steric Hindrance

Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together. In cyclohexane derivatives, bulky substituents prefer equatorial positions to minimize interactions with other axial substituents. The degree of steric hindrance varies with the size and shape of the substituents, influencing their preferred positions.

Substituent Effects

Different substituents have varying effects on the stability of cyclohexane conformations. For example, isopropyl groups are larger and create more steric strain when in axial positions compared to smaller groups like fluorine. Understanding these effects helps explain why isopropylcyclohexane has a higher percentage of equatorial substituents compared to fluorocyclohexane.

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Related Practice

Textbook Question

Calculate the percentage of isopropylcyclohexane molecules that have the isopropyl substituent in an equatorial position at equilibrium. (Its ∆G° value at 25 °C is -2.1 kcal/mol.)

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Textbook Question

a. For a reaction with ∆H° = -12 kcal/mol and ∆S° = 0.01 kcal mol^-1 K^-1, calculate the ∆G° and the equilibrium constant at:

1. 30 °C and 2. 150 °C.

b. How does ∆G° change as T increases?

c. How does K_eq change as T increases?

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Textbook Question

What alkene would you start with if you wanted to synthesize

a. pentane?

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Textbook Question

Calculate ∆G° for the conversion of “axial” methylcyclohexane to “equatorial” methylcyclohexane at 25 °C.

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Textbook Question

The ∆G° for conversion of “axial” fluorocyclohexane to “equatorial” fluorocyclohexane at 25 °C is -0.25kcal/mol. Calculate the percentage of fluorocyclohexane molecules that have the fluoro substituent in an equatorial position at equilibrium.

1151

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Textbook Question

a. For which reaction in each set will ∆S° be more significant?

b. For which reaction will ∆S° be positive?

1. A ⇌ B or A + B ⇌ C

2. A ⇌ B + C or A + B ⇌ C + D

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