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My NCERT book(Class 12) said the following for reduction of nitrobenzene using $\ce{Sn/HCl}$ and $\ce{Fe/HCl}$ : enter image description here Now, I have been following this mechanism for this particular reduction, and it seems to me that the metal is getting oxidized to +2 state wherever it is shown that 2 electrons are being transferred. If that is the case, then:

•Why is $\ce{Fe + HCl}$ being preferred?

•Because I looked it up on Wikipedia and it says that $\ce{SnCl2}$ also gets hydrolysed to liberate $\ce{HCl}$ as follows: $\ce{SnCl2 (aq) + H2O (l) ⇌ Sn(OH)Cl (s) + HCl (aq)}$

Or have I interpreted the mechanism incorrectly?

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    $\begingroup$ Probably preferred for cost reasons, iron being cheaper than tin $\endgroup$ – Waylander Mar 8 '19 at 10:14
  • $\begingroup$ @Waylander But the way the book has stated it,it seems that this regeneration of HCl takes place only with Fe. So is there a problem with the yield in case of Sn+HCl? $\endgroup$ – Yusuf Hasan Mar 8 '19 at 10:37
  • $\begingroup$ I can see no problem with your. My guess is that the preference the authors express is for Fe/HCl over catalytic hydrogenation. $\endgroup$ – Waylander Mar 8 '19 at 10:39
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The book's reasoning is correct. Fe/HCl gives $\ce{FeCl2}$ on reaction and it gets hydrolyzed by steam vapors to produces more hydrochloric acid and hydrogen to push the reaction forward thus making the reaction self-sustaining to produce more aniline from nitrobenzene.

$$\ce{Fe + 2HCl ->[160°C,CH3OH] FeCl2 + H2}$$

$$\ce{3FeCl2 + 4H2O(steam) ->[Hydrolysis] Fe3O4 + 6HCl + H2 }$$

whereas $\ce{SnCl2}$ gives $\ce{Sn(OH)Cl}$ and 1 mole of hydrogen chloride which is insufficient and it gets exhausted after sometime.

The other thing is cost factor as @Waylander said. Iron scrap is way cheaper than tin.

Both Sn/HCl and Fe/HCl reduction(Bechamp reduction) is replaced by catalytic hydrogenation due to the latter being more efficient and more productive in industial production. Fe/HCl reduction is now an academic interest and is studied as a route to produce iron oxide based pigments.

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