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Consider for example ethanol being heated in the presence of copper at 300 °C. How does it give ethanal as a product?

Copper being in its elemental state (oxidation state 0). Since negative oxidation state is not possible for Cu, which means that it should reduce ethanol to ethane. Certainly, high temperature should have some role here. What could it be?

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    $\begingroup$ CuO does the job. $\endgroup$ May 11 at 14:48
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You likely had either or both of the following:

  1. Copper had some oxide on it, and this oxidized the alcohol leaving the metal behind.

  2. If there was air in your system, especially on heating copper could have reacted with it to generate the oxide (a relatively fast reaction with heating) and you're back to (1). The metal would then, of course, be available to react with more air. In effect this would be a catalytic oxidation.

Copper is actually known in negative oxidation states, but that does not happen or need to be invoked here.

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When heated in air, a piece of copper metal gets oxidized into black copper oxide $\ce{CuO}$ according to $$\ce{2 Cu + O2 -> 2 CuO}$$

When hot copper oxide $\ce{CuO}$ is in contact with some ethanol vapor $\ce{C2H5OH}$, it reacts according to : $$\ce{CuO + C2H5OH -> Cu + CH3CHO + H2O}$$ And the copper metal is regenerated. So if a copper plate is heated in a mixture air + ethanol, it is simultaneously oxidized to $\ce{CuO}$ and reduced to $\ce{Cu}$, so that the only visible effect in the experiment is that the ethanol molecules are transformed into ethanal $\ce{CH3CHO}$ molecules.

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  • $\begingroup$ Thanks for the answer, just wanted to know if CuO is strong enough to oxidise aldehydes into acids? $\endgroup$
    – green_32
    May 20 at 13:28
  • $\begingroup$ @green_32. At high temperature, CuO is able to oxidize all organic molecules to $\ce{CO2}$ and $\ce{H2O}$. This was even the principle of the traditional analysis of organic compounds in the 19th century. An unknown substance was mixed with a large excess of $\ce{CuO}$. The whole mixture was heated in a horizontal tube with a very gaz running along it. The produced $\ce{H2O}$ vapor was absorbed in $\ce{CaCl}$2 and $\ce{CO2}$ in $\ce{NaOH}$, for further analysis. $\endgroup$
    – Maurice
    May 20 at 19:09
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Copper is a very good dehydrogenating and dehydrating agent. When a primary alcohol is heated in presence of copper than one hydrogen connected to oxygen and another connected to carbon leave in the form of hydrogen gas converting it into an aldehyde.
Further oxidation of the aldehyde into a carboxylic acid is not favourable.
A secondary alcohol is converted into ketone showing same nature as above.

But tertiary alcohols show the dehydrating nature converting the alcohol into an alkene according Zaitsev's rule.

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  • $\begingroup$ Here oxidation of compound take place because of escape of hydrogen gas from compound leading to rise of oxidation state of carbon by +1 $\endgroup$ Jun 10 at 17:26

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