INTEC Chemistry Blog

Archive for November 2012

INTEC has asked me to take 10G5 again for at least 1 semester 🙂
[and hopefully all the way to the final exams] 😀

See you in January.

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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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