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Are phenoquinone and benzoquinone the same thing? When I searched for the colour of benzoquinone I found that it's yellow. But when I searched oxidation of phenol in air. It states that it turns pink/red due to formation of phenoquinone. Also when aniline is oxidised with $\ce{K2Cr2O7}$ quinone is formed (perhaps it is aniline black). Is it the same compound everywhere?

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  • $\begingroup$ & what about aniline. It also forms quinone on oxidation but it is aniline black. It it the same case as a complex? $\endgroup$ – Yashwini Jun 28 '20 at 20:04
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Benzoquinone($\ce{C6H4O2}$) is a quinone with a single benzene ring. There are two types of benzoquinone:

  1. 1,4-Benzoquinone also called p-benzoquinone. More common type of benzoquinone. It is the main product of oxidation of phenol.
  2. 1,2-Benzoquinone also called o-benzoquinone. Less common type of benzoquinone.

Benzoquinones

Phenoquinone is a crystalline complex made of one quinone molecule and 2 phenol molecules. It is most likely formed from the polymerization of benzoquinone, hence the color change from yellow to reddish pink. See the mechanism below, taken from this answer:

Formation of phenoquinone

It is formulated as $\ce{C6H4O2.2(C6H5OH)}$. See: http://scripts.iucr.org/cgi-bin/paper?S0567740868002451

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  • $\begingroup$ Such adducts are not hydrogen bound, but charge transfer complexes. $\endgroup$ – Mithoron Jun 28 '20 at 17:29
  • $\begingroup$ @Mithoron From ref 4 of my answer: "Different molecules in each complex are connected by hydrogen bond between the hydroxyl bond and carbonyl bond [...] In phenoquinone, one quinone molecule is placed between two phenol molecules to form a short complex chain. In spite of difference of complex chain, no significant difference was observed between complex due to short interatomic distance". I changed the "adduct" to "crystalline complex" but there is indeed a hydrogen bond between them. $\endgroup$ – Nilay Ghosh Jun 28 '20 at 17:50
  • $\begingroup$ What "ref 4"? I've seen this misconception earlier and it's quite obvious. $\endgroup$ – Mithoron Jun 28 '20 at 18:02
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    $\begingroup$ Here is a bit about CT complexes of quinones and how they are related to color. Thing is h-bonds don't change color to any significant extent and CT complexes do. $\endgroup$ – Mithoron Jun 28 '20 at 18:05
  • $\begingroup$ While then sure can be (even should be!) h-bonds between individual phenol-quinone complexes, color change cannot be attributed to them, but to change transfer between the rings stacked one over another. $\endgroup$ – Mithoron Jun 28 '20 at 18:10

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