Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

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

Bruice 8th Edition

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Problem 102

Chapter 10, Problem 102

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.

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Analyze the structure of cis-1-bromo-4-tert-butylcyclohexane and trans-1-bromo-4-tert-butylcyclohexane. In both cases, the cyclohexane ring adopts a chair conformation, and the bulky tert-butyl group will preferentially occupy the equatorial position to minimize steric strain.

In the cis isomer, the bromine atom is in the axial position when the tert-butyl group is equatorial. This arrangement allows for a favorable anti-periplanar geometry between the β-hydrogen (on the adjacent carbon) and the leaving group (bromine), which is a requirement for the E2 elimination mechanism.

In the trans isomer, the bromine atom is in the equatorial position when the tert-butyl group is equatorial. This arrangement does not provide the necessary anti-periplanar geometry between the β-hydrogen and the leaving group, making the E2 elimination less favorable.

The anti-periplanar geometry is critical for the E2 mechanism because it allows for the proper orbital alignment for the base (sodium ethoxide) to abstract the β-hydrogen and for the leaving group (bromine) to depart simultaneously, forming the double bond.

As a result, the cis isomer reacts much more rapidly than the trans isomer because the anti-periplanar geometry is readily available in the cis isomer, facilitating the elimination reaction to form 4-tert-butylcyclohexene.

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Key Concepts

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

Steric Hindrance

Steric hindrance refers to the repulsion between atoms that occurs when they are brought close together, which can affect the reactivity of molecules. In the case of cis-1-bromo-4-tert-butylcyclohexane, the bulky tert-butyl group is positioned on the same side as the bromine atom, allowing for a more favorable transition state during the reaction. In contrast, the trans isomer has the tert-butyl group on the opposite side, leading to increased steric hindrance and a less favorable reaction pathway.

E2 Elimination Mechanism

The E2 elimination mechanism involves the simultaneous removal of a leaving group and a hydrogen atom from adjacent carbon atoms, resulting in the formation of a double bond. For the cis isomer, the spatial arrangement allows for the necessary anti-periplanar orientation of the leaving bromine and the hydrogen atom, facilitating a faster reaction. The trans isomer, however, may not achieve this optimal orientation as easily, leading to a slower reaction rate.

Conformational Analysis

Conformational analysis studies the different spatial arrangements of a molecule that can occur due to rotation around single bonds. In cyclohexane derivatives, the chair conformation is the most stable, but the positioning of substituents can significantly influence reactivity. The cis isomer can adopt a conformation that minimizes steric strain and maximizes the accessibility of the bromine for elimination, while the trans isomer may have a less favorable conformation, contributing to its slower reaction rate.

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Related Practice

Textbook Question

What substitution products are obtained when each of the following compounds is added to a solution of sodium acetate in acetic acid?

a. 2-chloro-2-methyl-3-hexene

b. 3-bromo-1-methylcyclohexene

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Textbook Question

What are the products of the following reactions?

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Textbook Question

Draw the substitution and elimination products.

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Textbook Question

tert-Butyl chloride undergoes solvolysis in both acetic acid and formic acid. Solvolysis occurs 5000 times faster in one of these two solvents than in the other. In which solvent is solvolysis faster? Explain your answer. (Hint: Formic acid is more polar than acetic acid.)

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Textbook Question

a. Draw the structures of the products obtained from the reaction of each enantiomer of cis-1-chloro-2-isopropylcyclopentane with sodium methoxide in methanol.

b. Are all the products optically active?

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Textbook Question

When the following compound undergoes solvolysis in ethanol, three products are obtained. Propose a mechanism to account for the formation of these products.

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