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I was trying to synthesize 1-iodo-4-nitrobenzene in the lab today – however, the appearance of my product really confused me.

At first, I added conc. sulfuric acid to water and 4-nitroaniline. This was followed by diazotising the amine sulfate with sodium nitrite and water. (This was done at temperatures of below $10\ \mathrm{^\circ C}$).

Then, I added the diazonium salt to a solution of $\ce{NaI}$ and water. A large plume/foam formed ($\ce{N2}$). After, I performed a filtration using a Büchner funnel. That’s where things went wrong.

I used water to wet the filter paper. I added in the iodonitrobenzene. A suckback occurred within a few seconds as the product was quite viscous. After resumption, the filtration went as planned. However, the product on the filter paper was a molten brown mush.

I tried to let it dry for a further 5 minutes but once turned off it remained a sludge.

Do you have any suggestions on how I could improve on this synthesis or maybe an idea of what went wrong?

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    $\begingroup$ 1) How quickly\slowly did you add the diazonium salt to the NaI solution? 2) Did you maintain the reaction temperature below 10C during the addition? 3) Did you keep the diazonium salt cooled while adding it? 4) After addition was complete, did you let the reaction stir at 10C until nitrogen evolution ceased and then let the reaction slowly warm to room temp? Here is a tested lab procedure, see where you differed. $\endgroup$ – ron Jan 20 '15 at 16:14
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    $\begingroup$ Yes, I think a big issue was the fact that I didn't give enough time for the Nitrogen evolution to cease. You mention that I should have kept it at 10C even when the NaI was added. According to the manual I was following, this wasn't necessary.? $\endgroup$ – Edward Jan 20 '15 at 16:19
  • $\begingroup$ However, I didn't return it to the ice bath whenever I added the NaI. So, therefore, it didn't solidify. So, what was my brown sludge? Was it iodonitrobenzene, but in the wrong form? $\endgroup$ – Edward Jan 20 '15 at 16:21
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    $\begingroup$ Keeping the reaction cold during the addition is key. If it warms up too much, the diazonium can react with itself to give colored by-products. $\endgroup$ – ron Jan 20 '15 at 16:25
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Here are some things to focus on:

  • How quickly\slowly did you add the diazonium salt to the NaI solution?
  • Did you maintain the reaction temperature below 10°C during the entire addition?
  • Did you also keep the diazonium salt cooled while adding it?
  • After addition was complete, did you let the reaction stir at 10°C until nitrogen evolution ceased and then let the reaction slowly warm to room temp?

Here is a tested lab procedure for the preparation of 4-iodonitrobenzene. Compare it to your notes and see where you differed.

Keeping the reaction cold during the addition is key. If the diazonium salt warms up too much, the diazonium can react with itself (coupling) to give colored by-products. The diazonium ion can also eliminate nitrogen to generate the extremely reactive phenyl cation which will react with any nucleophile around, water for example (see page 6, the Mechanism section in this reference). Both pathways can contaminate and reduce the yield of the desired product.

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  • $\begingroup$ So the benzene ring of the diazonium could react with a Nitrogen triple bond? How could it ''react with itself''? Would the Nitrogen dioxide act as a nucleophile on the nitrogen triple bond? $\endgroup$ – Edward Jan 20 '15 at 19:43
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    $\begingroup$ Yes, the terminal nitrogen in the diazonium can react with another activated aromatic compound (starting material, final product, by-product) to produce coupling products. Your ring has a nitro group, so it is deactivated and less likely to couple. More likely, your diazonium salt thermally decomposes (eliminates nitrogen) to generate the extremely reactive phenyl cation which will react with any nucleophile around, water for example. $\endgroup$ – ron Jan 20 '15 at 19:56
  • $\begingroup$ @Maximilion Feel free to ask follow-up questions. BTW, if the answer was helpful, please mark it as accepted, thanks! $\endgroup$ – ron Jan 20 '15 at 20:20
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    $\begingroup$ No, if you wanted to make 4-bromo-nitrobenzene from p-nitroaniline, you'd use CuBr (Sandmeyer reaction). Iodide is reactive enough to react directly with the diazonium ion, bromine and chlorine need a little help from copper. Water is also reactive enough that generating the diazonium ion in the presence of excess water is a standard route to phenols. $\endgroup$ – ron Jan 20 '15 at 22:10
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    $\begingroup$ @Maximilion Yes, that method was used for the synthesis of 4-nitrophenylhydrazine $\endgroup$ – ron Jan 25 '15 at 20:18
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Wash the product (sludge) with 95% ethanol when filtering. The sludge is due to impurities.Your pure product should be light brown.

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