Cis-1-Bromo-4-tert-butylcyclohexane and...

Name: Problem 102, Chapter 9: Organic Chemistry
Uploaded: 2020-08-08T22:47:50-08:00
Duration: 2 min 51 s
Channel: Madeline Currie
Description: cis-1-Bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane both react with sodium ethoxide in ethanol to form 4-tert-butylcyclohexene. Explain why the cis isomer reacts much more rapidly than the trans isomer.

cis-1-Bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane both react with sodium ethoxide in ethanol to form 4-tert-butylcyclohexene. Explain why the cis isomer reacts much more rapidly than the trans isomer.

Key Concepts

E2 Elimination Mechanism

The E2 mechanism involves a concerted elimination where a base abstracts a proton while the leaving group departs, requiring an anti-periplanar arrangement between the hydrogen being abstracted and the leaving group. This necessitates a specific orientation of bonds in the substrate. If the leaving group is already positioned in an axial orientation, as it might be in one isomer, the substrate is preorganized for the elimination, resulting in a lower energy transition state and faster reaction.

Cyclohexane Conformations

Cyclohexane rings predominantly exist in the chair conformation, where substituents can occupy axial or equatorial positions. The relative positions of substituents in cyclohexane (cis versus trans) determine whether a substituent such as a leaving group is more likely to be in an axial or equatorial position. This in turn affects the ease with which a substrate can achieve the necessary orientation for certain reactions, as axial positions often provide the geometric alignment required for elimination reactions.

Stereoelectronic Effects

Stereoelectronic effects refer to the influence of orbital alignments on the rate and outcome of chemical reactions. In the context of E2 eliminations on cyclohexane rings, the alignment of the ? bond to the leaving group with the ?* orbital of the hydrogen being abstracted is a critical factor. When a substituent readily adopts an axial position, the required anti-periplanar geometry is more easily achieved, enhancing the reaction rate. Conversely, if the substituent is locked in an equatorial position, the molecule must undergo a conformational change, which can slow the reaction.

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