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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Darren on June 19, 2012, 08:59:52 AM

Title: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 08:59:52 AM
Why do methanoic acid and ethanedioic acid decompose so easily? What makes them so unstable that they must decompose to produce carbon dioxide and water? Arent there resonance structures of the acids which can stabilise them?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 10:46:27 AM
Why do you think they are instable?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 10:52:33 AM
Why do you think they are instable?


Because from my organic chemistry teachings in sch, the teachers say that whenever a qn asks you to give the products from oxidation of an organic compound, never give methanoic acid or ethanedioic acid as the final organic products because they will be oxidised to form carbon dioxide and water instead. So from this they said that those two products are unstable and will decompose very quickly.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 10:58:21 AM
First what is sch an qn.

Both acids stable itself under normal circumstances, but if an oxidising compound is present the they will easily oxidized to the carbondioxide and water. They are reducing agents.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 11:04:26 AM
First what is sch an qn.

Both acids stable itself under normal circumstances, but if an oxidising compound is present the they will easily oxidized to the carbondioxide and water. They are reducing agents.

Oops so sorry. Sch is school :D and qn is question
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 11:05:54 AM
Oh I see kids language. Not my mother language.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 11:06:09 AM
If they are acids, shouldnt they remain as acids? What makes them oxidise to carbon dioxide and water? If you oxidise alcohols, the most they can go to will be acids. And those acids dont get oxidised further. Why the special case for these two acids?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 11:09:06 AM
What can you see if you draw the formula of formic acid? What group you can see? And what happens after oxidising. Which acid is resulting to it?
For Oxalic acid the same thing.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Borek on June 19, 2012, 11:40:19 AM
sch qn i

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Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Borek on June 19, 2012, 11:42:16 AM
If you oxidise alcohols, the most they can go to will be acids. And those acids dont get oxidised further.

Yes they do, it is just a matter of how harsh conditions are.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 12:46:47 PM
But i thought you cant oxidise formic acid to anything else except to carbon dioxide? Unless there is some kind of intermediate structure to it which i havent seen before. Same goes for oxalic acid?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 01:07:25 PM
Compare Alcohol go to which compound if you oxidis smoothly and if you oxidise strong.

Compare the first product with Formic acid.

Did you dont see the forest because of to much trees?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 01:21:08 PM
Compare Alcohol go to which compound if you oxidis smoothly and if you oxidise strong.

Compare the first product with Formic acid.

Did you dont see the forest because of to much trees?

Alcohols are oxidised to aldehydes then to acids. I still got get how this leads to the instability or the ease of decomposition of the two acids.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 19, 2012, 01:57:30 PM
Compare the formula of an aldehyde with the formula of formic acid. What do you can see?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 19, 2012, 09:34:53 PM
Compare the formula of an aldehyde with the formula of formic acid. What do you can see?


One more oxygen atom in the acid? The C=O double bond remains there
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 20, 2012, 12:43:43 AM
What is the functional group of an aldehyde? Compare this with the formula of formic acid.
What is the formula of Hydroxy-Methanal. ;D ;D ;D
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 20, 2012, 01:09:46 AM
What is the functional group of an aldehyde? Compare this with the formula of formic acid.
What is the formula of Hydroxy-Methanal. ;D ;D ;D

Ahhhhh i seee now i get it!! Haha the 'alcohol' group of the carboxylic acid group is further oxidised to produce another carbon oxygen double bond right? And this loses the hydrogen to form carbon dioxide while the hydrogen is oxidised by the oxidising agent to produce water!

Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 20, 2012, 01:15:09 AM
Something like that.

H-COOH you can write also as HO-CHO . The formic acid contains still an Aldehyde function. So it is easy to oxidise further to Hydroxy-formic acid HO-COOH, also known as carbonic acid H2CO3. This decompose to carbondioxide and water as we know.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 20, 2012, 01:26:54 AM
Something like that.

H-COOH you can write also as HO-CHO . The formic acid contains still an Aldehyde function. So it is easy to oxidise further to Hydroxy-formic acid HO-COOH, also known as carbonic acid H2CO3. This decompose to carbondioxide and water as we know.

Now that is cool... What about ethanedioic acid? Can it end up to be carbonic acid also? But there are two carbon atoms so how will it rearrange itself after further oxidation?

And why dont other organic acids have this further oxidation reaction? Why are the other organic acids more stable?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 20, 2012, 01:33:19 AM
In oxalic acid we have HOOC-COOH.  What can you see if you look at this molecule? it jumpes directly in your eyes, I hope. :D :D

Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 20, 2012, 01:41:28 AM
In oxalic acid we have HOOC-COOH.  What can you see if you look at this molecule? it jumpes directly in your eyes, I hope. :D :D
It splits up to form two molecules of carbon dioxide? :D so in this case, i can say that in the decomposition of organic acids, not all will be further oxidised to form carbonic acid first before decomposition?
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Hunter2 on June 20, 2012, 01:50:32 AM
HOOC-COOH expells one CO2 and then we have.....Bingo my friend. The rest as before.
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 20, 2012, 01:52:39 AM
HOOC-COOH expells one CO2 and then we have.....Bingo my friend. The rest as before.

Cool!! Thank you so much for explaining this to me :) really appreciate it because ive never had a proper explanation by anyone for many months already since i first had this question. Finally i understand it :)
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: AWK on June 20, 2012, 08:15:15 AM
Why do you think they are instable?


Because from my organic chemistry teachings in sch, the teachers say that whenever a qn asks you to give the products from oxidation of an organic compound, never give methanoic acid or ethanedioic acid as the final organic products because they will be oxidised to form carbon dioxide and water instead. So from this they said that those two products are unstable and will decompose very quickly.

During oxidation of sugar (sucrose, Saccharose) with concentrated nitric acid you can obtain the oxalic acid with good yield
Title: Re: Instability of methanoic acid and ethanedioic acid
Post by: Darren on June 20, 2012, 10:41:38 AM
Why do you think they are instable?


Because from my organic chemistry teachings in sch, the teachers say that whenever a qn asks you to give the products from oxidation of an organic compound, never give methanoic acid or ethanedioic acid as the final organic products because they will be oxidised to form carbon dioxide and water instead. So from this they said that those two products are unstable and will decompose very quickly.

During oxidation of sugar (sucrose, Saccharose) with concentrated nitric acid you can obtain the oxalic acid with good yield

Why doesnt the oxalic acid oxidise further to produce carbon dioxide? Why is a good yield obtained? Or does it depend on the strength of the oxidising agent? How can you determine the strength of an oxidising agent?