Elimination Reactions

E2 Elimination Practice Problems

In this post, we will practice E2 elimination reactions, focusing on the mechanism and how to determine the major and minor products based on the structure of the substrate and the nature of the base. Common bases we will consider include sodium hydroxide, methoxide, ethoxide, tert-butoxide, DBU, and DBN.

We will also explore how factors such as stereoselectivity and stereospecificity influence the outcome of E2 reactions. Additionally, special attention will be given to the elimination reactions of cyclohexanes, where conformational analysis and the presence of antiperiplanar relationships play a critical role.

 

 

Practice

1.

The following reaction exhibits a first-order rate in each reactant. What happens to the rate of the reaction when:

 

 

a) The concentration of 2-bromobutane is doubled, and [EtONa] remains the same?

b) The concentration of 2-bromobutane is tripled, and [EtONa] is halved?

Answer

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2.

Identify the β hydrogens and draw the curved arrow mechanism for each of the following E2 reactions.

 

Answer

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Solution

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3.

Do you think any of the products will be the major outcome of the following E2 reaction, as they are both yielding a double bond with the same degree of substitution? Explain your answer.

 

a)
Answer

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4.

Despite using a sterically unhindered base (–OH), the Hoffman product appears to be predominant in the following E2 reaction. Identify the Hofmann and Zaitsev products and explain this observation.

 

Answer

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5.

Determine the major and minor products in each E2 elimination reaction when EtONa or tBuOK is used:

 

Answer

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Solution

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6.

Provide a base to prepare the following major products for each of the following E2 reactions:

 

Answer

a) The major product here is the less substituted hence less stable alkene, so any bulky base such as tBuOK, DBN will achieve the desired transformation.

b)

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7.

Consider the regiochemistry and stereochemistry to identify the major product for the E2 reaction that occurs when each of the following substrates is treated with a strong base, such as ethoxide (EtO–) and tert-butoxide (tBuO–) ion:

 

a)
Answer

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b)
Answer

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c)
Answer

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d)
Answer

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e)
Answer

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f)
Answer

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g)
Answer

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h)
Answer

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i)
Answer

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j)
Answer

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Solution

These problems are slightly modified; however, you can find the video solutions to the original ones and some more under the corresponding topics:

 

8.

Explain why the following cyclohexane does not undergo an E2 reaction when treated with a strong base:

Answer

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9.

Determine the major product in the following elimination reaction of anorborane:

 

Answer

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10.

Explain why the following compound does not undergo either El or E2 elimination. In fact, it does not undergo SN1 or SN2 either.

 

Answer

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11.

Draw the major product of each elimination reaction:

Answer

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12.

Draw a plausible curved arrow mechanism to explain the formation of each product when the following bicyclic compounds are treated with tBuOK:

 

Answer

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13.

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