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 86

Chapter 4, Problem 86

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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Understand the context: The problem involves the preference of substituents (chloro and bromo) for the equatorial position in a cyclohexane chair conformation. This preference is influenced by steric and electronic factors.

Recall the steric hindrance concept: Bromine (Br) is a larger atom than chlorine (Cl), so you might initially expect bromine to have a stronger preference for the equatorial position to minimize steric interactions with axial hydrogens.

Consider electronic effects: Chlorine is more electronegative than bromine, which means it can withdraw electron density more effectively. This can influence the stability of the molecule in different conformations.

Analyze dipole-dipole interactions: The axial position of a substituent can lead to unfavorable dipole-dipole interactions with other axial groups. Chlorine's higher electronegativity may amplify these unfavorable interactions when in the axial position, making the equatorial position more favorable for chlorine.

Conclude the reasoning: The greater preference of chlorine for the equatorial position compared to bromine is likely due to a combination of steric and electronic effects, with chlorine's higher electronegativity playing a significant role in stabilizing the equatorial conformation.

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Key Concepts

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

Sterics and Steric Hindrance

Sterics refers to the spatial arrangement of atoms in a molecule and how this affects molecular interactions. Steric hindrance occurs when larger groups, like bromine, create more crowding around a central atom, making it less favorable for those groups to occupy certain positions, such as the equatorial position in cyclohexane rings.

Equatorial vs. Axial Positions

In cyclohexane, substituents can occupy equatorial or axial positions. Equatorial positions are generally more stable because they minimize steric interactions with other axial substituents. Chlorine, being smaller than bromine, can fit more comfortably in the equatorial position, leading to a greater preference for this arrangement compared to bromine.

Substituent Size and Electronic Effects

The size of a substituent affects its electronic interactions with the rest of the molecule. Chlorine, while larger than hydrogen, is smaller than bromine and has a stronger inductive effect, which can stabilize the equatorial position. This electronic effect, combined with steric factors, explains why chloro substituents prefer equatorial positions more than bromo substituents.