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From the Russian State Chemistry Test:

enter image description here

It's easy to see $X$ is $\ce{H2O}$: water turns acetylene into acetaldehyde.

But what can turn acetaldehyde into acetic acid?

I checked: $\ce{NaOH}$ would turn it into sodium acetate, $\ce{CuO}$ would not react (you'd need $\ce{Cu(OH)2}$), ethanol would turn it into either an acetal or hemiacetal.

Would acetaldehyde react with formic acid to produce acetic acid? I tried to google for it, and found no mention.

Would water in the presence of a heat source somehow react with acetaldehyde? I googled and discovered that it might form hydrates, not acetic acid.

So I'm at a loss. If you consider the question too easy, you could give just a hint.

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    $\begingroup$ If you Google CuO oxidation of aldehydes you do get hits $\endgroup$
    – Waylander
    Apr 24 '17 at 13:08
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    $\begingroup$ I don't think it's a stretch to think that two copper(II) reagents would both do the trick, considering one is essentially just the hydrated form of the other... $\endgroup$
    – Zhe
    Apr 24 '17 at 13:41
  • $\begingroup$ chemistry.stackexchange.com/questions/50642/… $\endgroup$ Apr 24 '17 at 14:25
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Oxidation of aromatic aldehydes to acids by air oxidation catalysed by CuO is reported in Molecules 2008 13 948

None of the other reagents can oxidise aldehyde to acid.

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  • $\begingroup$ Acetaldehyde isn't aromatic. $\endgroup$ Apr 24 '17 at 13:45
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    $\begingroup$ I know!! But given that CuO oxidises aromatic aldehydes it is not such a stretch to think it probably will oxide acetaldehyde. Plus none of the other options look even remotely right $\endgroup$
    – Waylander
    Apr 24 '17 at 14:01
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  • $\begingroup$ Perhaps, but remember that we are supposed to solve the conceptual difficulties, not eliminate options like in a competitive examination. However I agree that $\ce{CuO}$ has a better chance of oxidizing. $\endgroup$ Apr 24 '17 at 16:01
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    $\begingroup$ A friend who teaches chemistry has just said to me that CuO will first turn into Cu(OH)2 and only then the reaction will proceed. I don't know whether that's true. $\endgroup$ Apr 25 '17 at 12:21

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