Practice ExercisesFor Elimination Reactions (E1

& E2)

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D)

Alkene Preparation via Elimination

For each transformation shown, propose an acceptable reaction mechanism. Be sure to use proper arrow pushing, and include all lone pairs and formal charges. When more than one product is possible specify the major product.

A)

B)

C)

BA C D

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Eliminations Involving Alkyl Halides

Propose an acceptable reaction mechanism for each of the elimination products in the following reaction. Be sure to use proper arrow pushing, and include all lone pairs and formal charges. Which do you expect to be the major product(s)?

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Eliminations Involving Alkyl Halides

Predict the major elimination product(s) in each of the following reactions.

A)

B)

C)

D)

BA C D

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Alkene Preparation via Elimination(Exercise A)

Eliminations can occur via two different mechanisms (E1 vs E2) based on various conditions.*E2 mechanisms involve three separate arrows occurring in a single step (anti- elimination).*E1 mechanisms involve first losing a LG to form a carbocation intermediate followed by the deprotonation of the β-hydrogen.

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The tert-Butoxide anion is a strong base which means that this reaction will occur via the E2 mechanism. The conversion of reactant to product is concerted (single-step) mechanism. In this reaction, there is only a single β-hydrogen which is abstracted by the base to yield only one possible elimination product.

*The major/minor products are determined by Zaitsev’s Rule.

Major (trisubstituted)

Minor (disubstituted)

Alkene Preparation via Elimination(Exercise B)

Eliminations can occur via two different mechanisms (E1 vs E2) based on various conditions.*E2 mechanisms involve three separate arrows occurring in a single step (anti- elimination).*E1 mechanisms involve first losing a LG to form a carbocation intermediate followed by the deprotonation of the β-hydrogen.

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Methanol is weakly basic (like water) so this reaction will proceed via the E1 mechanism. The first step involving the Br LG proceeds very slowly since it leaves behind an unstable primary carbocation. Rearrangement through hydride shift then occurs extremely quickly to shift the carbocation to a much more stable tertiary position. Finally, two distinct types of β-hydrogen are deprotonated by the methanol to generate two products of which the more substituted one is more stable.

Minor (monosubstituted)

Major (trisubstituted)

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Hydroxide is a strong base so this reaction will proceed via E2. There are 2 β-hydrogens that each yield two products of which the more substituted one is more stable.

Alkene Preparation via Elimination(Exercise C)

Minor (disubstituted)

Major (tetrasub.)

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Hydroxide is a strong base so this reaction will proceed via E2. There are 2 β-hydrogens that each yield two products of which the more substituted one is more stable.

Alkene Preparation via Elimination(Exercise D)

Eliminations Involving Alkyl HalidesThis problem is based on the wide range of carbocation rearrangements that E1 (and Sn1) undergo.

*Consider the stability of the carbocation and any hydride/alkyl shifts that may occur. *Identify and consider elimination of each β-H in any intermediates.

[Very Minor] Unstable carbocation undergoes fast rearrangement before elimination can occur

[Minor] Zaitsev’s Rule,

[Minor] Zaitsev’s Rule

[Major] Zaitsev’s Rule, conjugated with existing pi-bond

[Major] Zaitsev’s Rule, conjugated with existing pi-bond

Vs.

Vs.

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Eliminations Involving Alkyl Halides(Exercise A)

Consider the chemical mechanism (even though only the major product is asked for). Drawing the whole mechanism out may assist you.

A) NaOH is a strong base (E2) thus rearrangement is not possible. The most substituted trans product is major.

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Eliminations Involving Alkyl Halides(Exercise B)

Consider the chemical mechanism (even though only the major product is asked for). Drawing the whole mechanism out may assist you.

B)Ethanol is a weak base (E1). No favorable carbocation rearrangement is possible. The most substituted trans product is major.

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Eliminations Involving Alkyl Halides(Exercise C)

Consider the chemical mechanism (even though only the major product is asked for). Drawing the whole mechanism out may assist you.

C) Ethanol is a weak base (E1). Carbocation rearrangement from 2° to 3° occurs. The most substituted product of the rearranged carbocation is major.

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Eliminations Involving Alkyl Halides(Exercise D)

Consider the chemical mechanism (even though only the major product is asked for). Drawing the whole mechanism out may assist you.

D)EtO- is a strong base (E2) thus rearrangement is not possible. The most substituted product is major.

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