carbocation rearrangement exercises v3.2

Prepared by Bill Weigel and Dr. Laurie Starkey [Version 3.2]

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For each carbocation below, propose a new structure that would result froma hydride shift and use curved arrows to account for the rearrangement.

A)

B)

C)

D)

BA C D

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For each carbocation below, propose a new structure that would result froma methyl shift and use curved arrows to account for the rearrangement.

A)

B)

C)

D)

BA C D

Check Your Answers

NextBack

For each carbocation below, propose a new structure that would result froma ring expansion and use curved arrows to account for the rearrangement. If no expansion is

expected, indicate so.

A) B) C)

D) E)

BA C D

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E NextBack

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The next slides contain only the answers. There are no more questions from this point forward.

Carbocation Rearrangements Involving a Hydride Shift

(Exercise A)Filling in omitted H’s on the line drawing can be useful to determine where the carbocation will be after a rearrangement by looking for the carbon that had 4 bonds before rearrangement and only 3 bonds after.

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A)

Hydrogens on the carbons involved in the shift have been drawn in to emphasize that the carbocation is located on the carbon with only 3 bonds (sp2).

Carbocation Rearrangements Involving a Hydride Shift

(Exercise B)Filling in omitted H’s on the line drawing can be useful to determine where the carbocation will be after a rearrangement by looking for the carbon that had 4 bonds before rearrangement and only 3 bonds after.

Return to Question

B)

The Hydrogens on the carbons involved in the shift have been drawn in to emphasize that the carbocation is located on the carbon with only 3 bonds (sp2).

Carbocation Rearrangements Involving a Hydride Shift

(Exercise C)Filling in omitted H’s on the line drawing can be useful to determine where the carbocation will be after a rearrangement by looking for the carbon that had 4 bonds before rearrangement and only 3 bonds after.

Return to Question

C)

Here only the hydrogen undergoing migration is shown.

Carbocation Rearrangements Involving a Hydride Shift

(Exercise D)Filling in omitted H’s on the line drawing can be useful to determine where the carbocation will be after a rearrangement by looking for the carbon that had 4 bonds before rearrangement and only 3 bonds after.

Return to Question

D)

Here only the hydrogens undergoing migration are shown.

Carbocation Rearrangements Involving Methyl Shift

(Exercise A)

A)

Return to Question

Carbocation Rearrangements Involving Methyl Shift

(Exercise B)

B)

Return to Question

Carbocation Rearrangements Involving Methyl Shift

(Exercise C)

C)

Return to Question

Carbocation Rearrangements Involving Methyl Shift

(Exercise D)

D)

Return to Question

Carbocation Rearrangements Involving a Ring Expansion

(Exercise A)

*Filling in omitted H’s may help in determining the new location of the carbocation.*Numbering the carbons in the initial structure may help you to visualize the rearranged product.

Return to Question

A)≡

2° → 3° (more stable)

Carbocation Rearrangements Involving a Ring Expansion

(Exercise B)

*Filling in omitted H’s may help in determining the new location of the carbocation.*Numbering the carbons in the initial structure may help you to visualize the rearranged product.

Return to Question

B)≡

1° → 2° (more stable)

Carbocation Rearrangements Involving a Ring Expansion

(Exercise C)

*Filling in omitted H’s may help in determining the new location of the carbocation.*Numbering the carbons in the initial structure may help you to visualize the rearranged product.

Return to Question

C) ≡

1° → 3° (more stable)

Carbocation Rearrangements Involving a Ring Expansion

(Exercise D)

*Filling in omitted H’s may help in determining the new location of the carbocation.*Numbering the carbons in the initial structure may help you to visualize the rearranged product.

Return to Question

D) ≡

2° → 3° (more stable)

Carbocation Rearrangements Involving a Ring Expansion

(Exercise E)

*Filling in omitted H’s may help in determining the new location of the carbocation.*Numbering the carbons in the initial structure may help you to visualize the rearranged product.

Return to Question

E)

3° is already the most stable location. Thus no ring expansion is expected.

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