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Chapter 26: Problem 639

During the heat of an important Organic Chemistry examination, a student proposed the following route for the synthesis of p-bromocumene from p-aminocumene. The professor, upon seeing the student's synthesis, laughed and gave the chagrined student half credit, promising full credit if the student could carry out the experiment in good yield in the lab. Try as he might, the student continued to obtain a mixture of products. What went wrong, and what is the nature of the mixture?

Short Answer

Expert verified
The issue with the synthesis of p-bromocumene from p-aminocumene is the high reactivity of the amine group, which leads to multiple bromination events. The resulting mixture of products likely contains the desired p-bromocumene, as well as ortho-brominated and/or di- or tri-brominated products. To improve the yield and avoid a mixture of products, an alternative synthesis route should be considered, possibly involving the deactivation or protection of the amine group during the bromination step.

Step by step solution

01

Understand the proposed synthesis route

The problem states that the student proposed a synthesis route for p-bromocumene from p-aminocumene. We need to analyze this route to find out any mistakes or side reactions that would lead to obtaining a mixture of products. Since it's not provided, it's hard to say what went wrong. However, let's look at some common issues faced in electrophilic aromatic substitution reactions like this one.
02

Analyze the electrophilic aromatic substitution reactions

The conversion of p-aminocumene to p-bromocumene involves a change in the substituent on the aromatic ring. The possible reaction mechanism could involve an electrophilic substitution reaction, where the amine group (-NH2) on the aromatic ring of p-aminocumene is replaced by the bromine atom. Both the bromine and amine substituents are ortho-para directing and activating groups, which means that they encourage reactivity at the ortho and para positions of the benzene ring. Thus, any mistakes or side reactions that form a mixture of products might arise from competition between these activated positions.
03

Identify potential issues with the suggested synthesis route

In the case of p-aminocumene, the amine substituent is a very strong activator for the ortho and para positions of the aromatic ring. Therefore, any attempt to perform an electrophilic bromination reaction with p-aminocumene could lead to multiple bromination events. The most likely mixture of products would contain not only the desired p-bromocumene but also ortho-brominated and/or di- or tri-brominated products due to the high reactivity of the ring.
04

Alternative synthesis route

A more viable synthesis route for p-bromocumene could involve a multi-step process that deactivates or protects the amine group such that it would not undergo multiple substitution reactions during the bromination step. Amination of the aromatic ring could be carried out later in the synthesis process to avoid the competitive reactions and improve the yield of the desired product.
05

Conclusion

In this exercise, we analyzed the issue with the synthesis of p-bromocumene from p-aminocumene. The nature of the mixture is most likely due to multiple bromination products of p-aminocumene, including ortho-brominated and/or di- or tri-brominated products. An alternative synthesis route should be considered to improve the yield and avoid the formation of a mixture of products.

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