When cis-1-bromo-4-tert-butylcyclohexane is treated with...

Name: Problem 10, Chapter 7: Organic Chemistry
Uploaded: 2020-10-06T11:52:12-08:00
Duration: 3 min 12 s
Channel: Lottie Adams
Description: When cis-1-bromo-4-tert-butylcyclohexane is treated with sodium ethoxide in ethanol, it reacts rapidly; the product is 4 -tert-butylcyclohcxenc. Under the same conditions, trans1-bromo-4-tert-butylcyclohexane reacts very slowly. Write conformational structures and explain the difference in reactivity of these cis-trans isomers.

When cis-1-bromo-4-tert-butylcyclohexane is treated with sodium ethoxide in ethanol, it reacts rapidly; the product is 4 -tert-butylcyclohcxenc. Under the same conditions, trans1-bromo-4-tert-butylcyclohexane reacts very slowly. Write conformational structures and explain the difference in reactivity of these cis-trans isomers.

Key Concepts

Stereochemical Influence on Reactivity

The stereochemical relationship of substituents on a cyclohexane ring determines the availability of the reactive conformation needed for E2 eliminations. When the substituents are arranged so that the leaving group can assume an axial position, the molecule can more readily adopt the antiperiplanar conformation, leading to a faster reaction. Conversely, if the substituent is locked in an equatorial orientation, the geometrical requirements for the E2 mechanism are harder to meet, resulting in slower reactivity.

Antiperiplanar Geometry in E2 Elimination Reactions

E2 elimination reactions require that the leaving group and a beta hydrogen are aligned in an antiperiplanar fashion, meaning they lie in the same plane but on opposite sides of the molecule. In cyclohexane rings, this ideal spatial arrangement is more easily achieved when the leaving group occupies an axial position, as it allows for the necessary alignment with a beta hydrogen, thus facilitating the elimination process.

Axial versus Equatorial Substituent Effects

In cyclohexane derivatives, the orientation of substituents (axial or equatorial) has a profound impact on reaction pathways. Axial substituents, although less stable due to steric hindrance, are often required to achieve the proper geometric alignment for certain reactions, such as elimination reactions, while equatorial substituents, being more stable, may prevent the necessary orbital overlap for efficient reaction progress.

Cyclohexane Conformational Analysis

Cyclohexane rings mainly adopt chair conformations in which substituents can be positioned either axially or equatorially. The conformational placement significantly affects the molecule’s stability and reactivity, as axial substituents experience more steric strain and 1,3-diaxial interactions compared to their equatorial counterparts, thereby influencing the molecule’s ability to adopt reactive conformations.

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