Sciencemadness Discussion Board

Fehlings Solution

Rich2189 - 24-9-2006 at 06:22

I know that fehilings solution will not give a result when reacted with an aromatic aldehyde but why doesn't it have an effect? In other words how come tollens reagent will work with both aliphatic and aromatic aldehydes but not fehlings.

Thanks

Rich.

guy - 24-9-2006 at 09:19

It doesn't work with aromatic aldehydes? I don't see why not. I've never read about that.

Rich2189 - 24-9-2006 at 10:22

Quote from: http://www.wpbschoolhouse.btinternet.co.uk/page13/ChemicalTe...
Code:
Aldehydes are stronger reducing agents and reduce (i) silver(I) ion to silver metal, (ii) the blue solution of the Cu2+ complex changes to the brown/brick red colour of insoluble copper(I) oxide Cu2O (Note: aromatic aldehydes do NOT give a result with Benedict's/Fehling's reagent). So (b)(i) and (ii) can be used to distinguish aldehydes and ketones. Note that reducing sugars give a positive test too.


It is also mentioned in several other books I have read. As you said I don't see why it wouldn't work.

Magpie - 24-9-2006 at 10:30

I tested some fructose in a 5% aqueous solution with Fehling's test. D-fructose is a ketohexose which in alkaline solution converts to 5-hydroxymethylfurfural, an aldehyde. This gave a good positive test. I realize that this is not an aromatic aldehyde but thought this information might be useful for confirming your test reagents.

In organic class I was given an aromatic aldehyde as an unknown. I did not test it with Fehling's solution but did use the Tollens test. It gave a positive test but only slowly and with heating in a water bath.

Rich2189 - 24-9-2006 at 10:48

Sure helps, it seems that fehlings wont work with an aromatic aldehyde, but why doesnt it work is the major question. This ones got me stumped, i've looked everywhere. Its obviously got to do with the delocalised electrons in the benzene ring but i have no idea how that effects the fehlings solution.

Magpie - 24-9-2006 at 11:08

I understand your question but have no answer nor have I heard of this before.

I will pass on one more experience. I tried to do a Fehling's test on the weak benzaldehyde almond flavoring sold for cooking. It is a solution with alcohol and water. When I added just a few drops to the Fehling's solution a gel formed immediately and I could not complete the test. I was very curious as to what caused this but just assumed it was the alcohol - but have no basis for this assumption. I just abandoned the test. The Tollen's test worked well on this benzaldehyde.

guy - 24-9-2006 at 14:51

Ok here is my guess.

The phenyl group is electron withdrawing because it will increase resonance, so it is harder to remove electrons from it, whereas aliphatic aldehydes have electron donating groups ie., methyl groups, which help increase reducing power of the the aldehyde.

Tollens reagent is able to reduce it due to the fact that Ag+ is a stronger oxidizing agent than Cu2+ is.

Magpie - 24-9-2006 at 20:18

Under "aromatic aldehydes" Vogel's 3rd says that they are reduced by Tollens reagent but not Fehlings. With Fehlings there is instead a Cannizzaro reaction. This is where an aldehyde having no alpha-hydrogen, under alkaline conditions, undergoes oxidation and reduction to form an alcohol and an acid.

This explains the gel formation when I attempted to perform a Fehling's test on benzaldehyde.

[Edited on 25-9-2006 by Magpie]

unionised - 25-9-2006 at 09:50

"I tested some fructose in a 5% aqueous solution with Fehling's test. D-fructose is a ketohexose which in alkaline solution converts to 5-hydroxymethylfurfural, an aldehyde. This gave a good positive test. I realize that this is not an aromatic aldehyde "
Sure about that?

guy - 25-9-2006 at 12:09

Quote:
Originally posted by unionised
"I tested some fructose in a 5% aqueous solution with Fehling's test. D-fructose is a ketohexose which in alkaline solution converts to 5-hydroxymethylfurfural, an aldehyde. This gave a good positive test. I realize that this is not an aromatic aldehyde "
Sure about that?


That furfural compound is aromatic. It probably gave a positive test because the fructose did not 100% convert to the furfural.

Rich2189 - 25-9-2006 at 12:31

Thanks Guy and magpie! I need to invest in a load of chem books. Trying to get the ramsdon but that didn't have anything about that in. Can you recomend a good chemistry book? On organic chemistry and reaction kinetics? Nothing too advanced.