4
$\begingroup$

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?

Improve this question
$\endgroup$
1
  • 8
    $\begingroup$ CuO does the job. $\endgroup$
    – Ivan Neretin
    Commented May 11, 2021 at 14:48

3 Answers 3

Reset to default
10
$\begingroup$

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.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ Thanks for the answer, just wanted to know if CuO is strong enough to oxidise aldehydes into acids? $\endgroup$
    – green_32
    Commented May 20, 2021 at 13:28
  • 2
    $\begingroup$ @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
    Commented May 20, 2021 at 19:09
5
$\begingroup$

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.

Improve this answer
$\endgroup$
3
$\begingroup$

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.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Here oxidation of compound take place because of escape of hydrogen gas from compound leading to rise of oxidation state of carbon by +1 $\endgroup$
    – Sanyam Goel
    Commented Jun 10, 2021 at 17:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.