INTEC Chemistry Blog

Fehlings solution and benzaldehyde.

Posted on: November 9, 2012

There are some references around that say benzaldhyde does not give a positive test result with Fehlings reagent. Yet none of my respected organic books mention benzaldehyde as an exception amongst the aldehydes.

The REAL answer lies in the experimental results which has to power to throw tom dick and harry theory in the bin.

So I did it.

Guess what… it DID give a positive test result. I repeated it and still positive result.

Pics 1,3 and 4 of of the first test. A lot of Fehlings solution was used. Insoluble (in water) benzaldehyde floated on the top. I put the mixture in a water bath for a while. Pic 1 shows an orange precipitate. I was hoping for a really nice red colour, but it looked orange. Pic 3 was when I drained out the excess Fehlngs mixture. The “orange ppte” residue on the side of the boiling tube now does indeed look red. Pic 4 shows the red ppte filtered. It looks pretty much like the red ppte that other aldehydes give. Pic 2 was the test repeated this time only using a small amount of Fehlings solution. Again the ppte is clearly visible. Top layer = benzaldehyde, middle blue layer = unreacted Fehlings solution, bottom ‘layer’ = Cu2O ppte.

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2 Responses to "Fehlings solution and benzaldehyde."

your result same with me, so that is positive reaction between benzeldehyde and the fehling’s

Salam Afiqah.

I’m not too sure what to advise because different books say different things.
All I can do is say that because I got the result then it can be oxidized.
Practical results are ALWAYS more significant than what dry ‘whiteboard’ theory
because all theory comes from physical/practical observation/tests.

BUT…
1) Although i don’t think my benzaldehyde was contaminated, I guess there’s a possibility
that it was. However the amount of red precipitate I got was so much that I don’t think
it was from contamination. So I’ll stick with my observation that yes, it can be oxidized using Fehlings solution.

2) Your lecturer and the exam mark scheme they use might rely upon a book in which it says the compound cannot be oxidized, so it’s quite important you see your lecturer and ask them what THEY expect the result to be, and ultimately, that is what YOU will need to say in your exams.

Hope that helps.

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