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We use distillation with zinc dust to remove -OH group from phenol. $$\ce{Ph-OH + Zn \rightarrow Ph-H + ZnO }$$

What I want to know is that whether this reaction has any effect on other groups attached to the benzene ring, like $\ce{-CH_3,-NH_2,-NO_2,-CN,-CONH2}$ etc. or not?

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    $\begingroup$ Nitro and cyano will react $\endgroup$
    – Waylander
    Commented Aug 25, 2017 at 15:52
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    $\begingroup$ Zinc plus proton source and heating is a standard way of reducing aromatic nitro. I have run this with ammonium chloride as the proton source in methanol $\endgroup$
    – Waylander
    Commented Oct 2, 2017 at 9:18
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    $\begingroup$ @Waylander, I understand. But can you give the whole reaction? I mean, a NO2 group can be reduced to NHOH or NH2 group, and CN group can be reduced to CH=NH, or CH2NH2 groups. $\endgroup$
    – S R Maiti
    Commented Oct 10, 2017 at 7:59
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    $\begingroup$ I think at this point you should do you own research into the reductive properties of zinc. $\endgroup$
    – Waylander
    Commented Oct 10, 2017 at 12:17
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    $\begingroup$ To whoever is trying to repeatedly edit my question: please stop. I understand that C6H5 is a synonym for Ph but in this case, I feel that Ph is better because it explicitly mentions a phenyl ring instead of C6H5 which might be a number of other things. $\endgroup$
    – S R Maiti
    Commented Jul 6, 2021 at 13:41

2 Answers 2

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I did some research for this question many years ago. Nisarg's answer prompted me to post an answer.

Nisarg's answer showed how nitrobenzene can be reduced to form different compounds but my answer focuses on some different compounds. Doing some literature surveys gave me results from very old books. All the reaction involves only zinc dust. I tried quoting but the majority of the terms/compound names are obsolete, so I have wrote them as points:

  • formanilide and its derivatives is reduced benzenenitrile and aniline (and its derivatives) as major products and other side products such as N-Methyldiphenylamine. Yield of nitrile 10-20% weight of initial formanilide.
  • N-alkyl-o-toluidine and N-alkyl-p-toluidine yields corresponding nitrile: yield 15-20%
  • methyl-α- and methyl-β-naphthalamine yield α- and β-naphthonitrile
  • xylidine and its isomers yields xylonitrile (prone to hydrolysis to yield xylic acid(isomeric carboxylic acid of xylene)) : yield 12%
  • Phenyl isothiocyanate and thiocarboanilide forms benzonitrile and some aniline.

References:

  1. K.Gasiorowski and V.Merz, Journal of the Chemical Society, 1884
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  • $\begingroup$ Do these reactions occur in pure zinc dust distillation or do they require NaOH, NH4Cl etc.? $\endgroup$
    – S R Maiti
    Commented Aug 18, 2021 at 13:52
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    $\begingroup$ @SRMaiti only zinc dust is used. No other catalyst/reagent used. $\endgroup$
    – Nilay Ghosh
    Commented Aug 19, 2021 at 2:49
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Out of the groups you mention $\ce{-NO2}$, $\ce{-CN}$ will most certainly react, $\ce{-CH3}$ and $\ce{-NH2}$ not so much and I am ambiguous about $\ce{-CONH2}$.

Although I said that $\ce{-CN}$ will react, I can't pin point the product which will form in a certain condition. But in case of $\ce{-NO2}$, I certainly can predict the product based on a list I complied a couple of years ago.

The "List":

$$ \begin{array}{|ccc|} \hline \ce{Ph-NO2}&\xrightarrow[\ce{NH4Cl}]{\ce{Zn}}&\ce{Ph-NHOH}\\ \hline \ce{Ph-NO2}&\xrightarrow[\ce{NaOH}]{\ce{Zn}}&\ce{Ph-NH-NH-Ph}\\ \hline \ce{Ph-NO2}&\xrightarrow[\ce{NaOH}]{\ce{Zn+alcohol}}&\ce{Ph-N=N-Ph}\\ \hline \end{array} $$

References:

(1) Kamm, O. β-Phenylhydroxylamine". Organic Syntheses. ; Collective 1941, 1, 445.

(2) Bigelow, H. E.; Robinson, D. B. Azobenzene". Organic Syntheses. ; Collective 1955, 3, 103.

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    $\begingroup$ Why doesalcohol cause it to become double bond $\endgroup$
    – Brian
    Commented Aug 18, 2021 at 8:14
  • $\begingroup$ Nitrobenzene can also get reduced to amines through zinc reduction. Only the reaction conditions and reagents are slightly altered. (Here, Here, Here). $\endgroup$
    – Nilay Ghosh
    Commented Aug 18, 2021 at 9:10
  • $\begingroup$ @Buraian Possibly condensation. My guess: The reaction creates nitroso and hydroxylamine products which then condense to azoxy and then ultimately to azo product. $\endgroup$
    – Nilay Ghosh
    Commented Aug 18, 2021 at 10:02
  • $\begingroup$ @Nisarg As a side note: Although this doesn't count as reduction but cyanobenzene does react with zinc dust and bromoacetic acid to form a red crystalline compound. This reaction is somewhat similar to Blaise reaction. $\endgroup$
    – Nilay Ghosh
    Commented Aug 18, 2021 at 10:09
  • $\begingroup$ The original zn dust reduction of phenol is done by distilling with zinc dust. The reactions you have mentioned use other compounds as reactants/catalysts. I don't think this answers the question, sorry. $\endgroup$
    – S R Maiti
    Commented Aug 18, 2021 at 13:51

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