A chemist needs two compounds, each made from benzene in as few steps as she can manage…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A chemist needs two compounds, each made from benzene in as few steps as she can manage. All the reagents she may use are in the list below.
Reagents: conc. with conc. ; with ; with ; with ; hot acidified .
Compound 1 is m-bromonitrobenzene. Compound 2 is p-nitrobenzoic acid, whose ring carries a COOH group and a nitro group opposite each other.
Each step must be a reaction that you have met in this course, and a mixture of isomers may be separated after a step if that is needed. For each compound, give the order of the reagents, and explain why the other order would not give the same product. Then find the total number of reaction steps that the two syntheses need. Count the separation of ortho and para isomers as part of a step and not as a separate step. Write each answer as a list of steps in order, and say what each reagent does.
Need a reminder? The key ideaShow
A group on the ring directs the next substituent. , and alkyl groups activate and direct ortho and para. Halogens deactivate and direct ortho and para. , , , and deactivate and direct meta. Light and heat attack the side chain. A Lewis acid in the dark attacks the ring.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Compound 1. Nitrate first. The nitro group directs the bromine to the meta position, so brominating next gives m-bromonitrobenzene. That is steps. If bromine went first, it would direct the nitro group to the ortho and para positions and the meta isomer would be a very small part of the product.
Step 2. Compound 2. Put the methyl group on first, with and . Nitrate next: the methyl group directs the nitro group to ortho and para, and the para isomer is kept. Last, oxidise the methyl group to COOH with hot permanganate. That is steps. If the methyl group were oxidised first, the COOH group would direct the nitro group to the meta position, and the product would be m-nitrobenzoic acid.
Step 3. Total. steps.
Answer: compound 1: nitrate, then brominate. Compound 2: alkylate, nitrate, then oxidise. steps in all.
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