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I am conducting an experiment about synthesis of iodobenzene from aniline, referring to this site(and various sites) (http://www.prepchem.com/synthesis-of-iodobenzene/). Below is my experimental process.

  1. Put $\pu{9.5mL}$ of conc. $\ce{HCl}$, $\pu{9.5mL}$ of distilled water, $\pu{2mL}$ ($\pu{0.0215mol}$) of aniline in a $\pu{100mL}$ round flask with ice bath, and wait until the temperature of mixture drops below $\pu{5℃}$.

  2. Dissolve $\pu{1.56g}$ ($\pu{0.0226mol}$) of $\ce{NaNO2}$ in $\pu{50mL}$ of distilled water, and cool it with ice bath.

  3. Once the temperature of mixture dropped below $\pu{5℃}$, put the solution of $\ce{NaNO2}$. the solution must be poured very slowly not to raise temperature above $\pu{10℃}$.

  4. Stir the mixture for $\pu{10 min}$.

  5. Dissolve $\pu{ 3.58g}$ ($\pu{0.0216mol}$) of $\ce{KI}$ in distilled water. I used the water in very small quantities because $\ce{KI}$ is very soluble in water.

  6. Put solution of $\ce{KI}$ to the mixture, then color of the mixture turns to be black, and gas bubbles (may be nitrogen gas) rise from the mixture.

  7. After $\pu{1-2 h}$, gently heat the mixture (I set the temperature about $\pu{50-70 ℃}$).

  8. Again after $\pu{1-2 h}$, it seemed that there are no more gas bubbles, so I stopped stirring and heating.

  9. Layer separation occurred, (I guess) because iodobenzene is insoluble in water. so I moved it to separatory funnel and took the black oily organic layer presumed to be iodobenzene.

  10. Here is the stage I am stuck on. according to the reference site, it says "use $\ce{NaOH}$ to decolorize the iodobenzene" (iodobenzene is colorless liquid). I put $\ce{NaOH}$ and made the pH of mixture 7, but its color is still black.

Question 1. Do I have to make the mixture basic to decolorize it? (actually, I tried making it basic, but it was not decolorised. Should I wait a moment after making it basic?)

Question 2. According to the reference site, iodobenzene have to be distilled using steam-distillation, but should I distill it with steam-distillation, not just ordinary distillation?

Question 3. Are there any problems with my experimental process?

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This procedure looks basically OK. The key thing here is keeping the reaction mixture below 5C. It is good practice to cool your KI soln to 5C, and add it slowly so that the temperature of the reaction mixture stays below 10C until addition is complete. Using very concentrated KI soln may be an issue as the reaction is exothermic, better to have it more dilute and cold.

I would use an organic solvent (EtOAc or Et2O) at stage 9 to extract the product simply because you are doing this on a relatively small scale. On this scale chromatography is a better option than distillation for final purification.

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  • $\begingroup$ Thank you for your answer. If I have to use distillation, Can I use ordinary distillation, not steam-distillation? (question 2), and can I ask you about question 1, too? +) If I use organic solvent, can I separate iodobenzene by separatory funnel and rotatory evaporator? (and some anhydrous magnesium) $\endgroup$ – Krang Lee Nov 22 '18 at 12:00
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    $\begingroup$ Yes to both questions, ordinary distillation will do it. The organic phase containing the iodobenzene will most likely still require purification after drying. I don't see what you are removing by base washing except excess HCl, but that is not a reason to omit the step. The procedure says that the dark colour is due to iodine, so a wash with sodium metabisufite will remove that. If you are happy with the answer please accept it. $\endgroup$ – Waylander Nov 22 '18 at 12:28
  • $\begingroup$ @Waylander: I think you mean extraction of the product using an organic solvent at stage 9. $\endgroup$ – Mathew Mahindaratne Nov 22 '18 at 18:02
  • $\begingroup$ The boiling point of iodobenzene is about $\mathrm{188~^oC}$, thus, it's better to purify using fractional didtillation under reduced pressure (if you are reluctant to purify it by chromatography). $\endgroup$ – Mathew Mahindaratne Nov 22 '18 at 18:08
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    $\begingroup$ @MathewMahindaratne You are correct, text edited to correct this $\endgroup$ – Waylander Nov 22 '18 at 22:57

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