Jor
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rxn of Hg(II) with hydroquinone in dil. HNO3
Today I did an experiment, and I don't know what happened.
Reference experiment (I did this after the actual experiment, but whatever):
-A spatula (about 100mg) of hydroquinone (1,4-dihydroxybenzene) was dissolved in about 3mL of water and 0,5mL of 2M HNO3 was added. Nothing happened.
Then 2 drops of 10M HCl were added, and again nothing happened.
Actual experiment:
-about 100mg of hydroquinone was dissolved in 3mL of water, giving a very pale yellow/brown solution. To this a little more than 0,5mL of
Hg(NO3)2-solution (about 10% Hg by weight) in dil. HNO3 was added. Very quickly a dirty brown suspension (looks orange/brown when observed in the
other light in my lab) was formed, and after shaking also some dirty green crystals could be seen on the test-tube wall. Finally there was also some
gas evolution.
I first thought these were nitrogen oxides, but considering how low the conc. of HNO3 actually is, this seems unlikely, especially because this was
not observed in the above experiment.
Next, to test for possibly free Hg2(2+), I added 2 drops of 10M HCl. When this is done, the solution becomes extremely dark at once, and I can see
some glittering crystals floating around, but i cannot say what the color is. When the solution was diluted about 25 times, it had a yellow color.
Now I can imagine that the solution was not acidic enough at first (because the solution of Hg(NO3)2 does not contain a lot of HNO3, its rather
dilute) and adding more HCl somehow made oxidation of the hydroquinone by Hg(II) possibly, to the highly colored benzoquinone.
However, I really cannot explain the dirty brown suspension formed. What is this? Does Hg(II) form and insoluble complex with hydroquinone? One would
expect it would oxidise it... Maybe some is oxidised, followed by formation of a Hg(i)-hydroquinone complex? However I have never heard of
hydroquinone as a ligand, and looking I think it is not really a good ligand.
Any ideas? Maybe someone could repeat the experiment  
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Paddywhacker
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Look up mercuration. It was a popular method in organic synthesis in days of yore. Now somewhat out of favour.
If it is mercuration, then you have replaced one of the hydrogens on the hydroquinone with a covalently-bound mercury, as the nitrate salt.
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woelen
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The dark green crystals must be quinhydrone, a complex of p-benzoquinone and hydroquinone. This forms needle-shaped dark green crystals and hardly
dissolves in water. This always is formed when a solution of hydroquinone is partially oxidized in acidic solution. The oxidizer in this reaction of
course is Hg(2+).
p-Benzoquinone itself does not have a really strong color, it is lemon-yellow and its solutions are light yellow, not dark at all. It is the
quinone/hydroquinone complex which has the dark green color.
The dark color, when conc. HCl is added most likely is due to formation of metallic Hg. Did you see any small droplets of metal on the bottom?
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I would not be surprised to see that hydroquinone acts as a ligand for mercury. It is a good ligand and forms complexes with many metals, but
admittedly I must say, more so at high pH. Just try adding hydroquinone to precipitates of copper hydroxide, chromium hydroxide, iron(III) hydroxide,
cobalt(II) hydroxide and you'll be surprised to see how many intensely colored complexes you can get. IIRC with chromium(III) an amazingly strongly
colored orange complex is formed. So, it could well be that with mercury also interesting complexes are formed.
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