INTEC Chemistry Blog

CHM 412 Page

Posted on: October 9, 2012

Please use this page to post questions/discuss CHM412 labs. Monday 6:00 – 9:00pm MK2

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8 Responses to "CHM 412 Page"

Salam sir. this question was on carbonyl compound’s experiment before. Actully, im getting confused with the colour change that occur in Fehling’s test. From readings, i found out that Fehling test will be positive for an aldehyde. thus, the colour will turn to brick-red indicating the reduction of Cu2+. however, from the experiment that ive done, only acetaldehyde turns out to be like that. benzaldehyde turns out to be light green in colour. same goes to acetone (dark green), while no reaction occured with acetophenon. so, my question is:

i) does this means that benzaldehyde doesnt react with fehling’s solution?
ii) why the solution turns out to be green in colour?

tq ๐Ÿ™‚

Salam.
Sometimes it’s hard to see the red ppte as it can be dispersed in the muddy green solution and requires a reasonable amount of time if ‘low heat’ is used. If you heat it for some time and let it settle (don’t shake or stir it) and look at the very bottom of the test tube (or boiling tube) you should notice the red ppte. Here’s a video of the test being done

You may notice a LOT of heat is sued and gives an obvious and large amount of red ppte.

i) does this means that benzaldehyde doesnt react with fehlingโ€™s solution? – It certainly should have done. The benzoate anion is the organic product (deprotonated benzoic acid). Acetone (or propanone to give it its IUPAC name) should not react as it’s a ketone

ii) why the solution turns out to be green in colour? – I’m not totally sure, but I’d guess the intermediate green colour is some kind of short lived copper compound (a copper complex) which can obscure or mask the presence of the red ppte, especially if the ppte is present only in small amounts.

but sir, this gentleman here said that benzaldehyde doesnt react with fehling’s solution

http://www.moleclues.org/forums/open-forum/why-doesnt-benzaldehyde-reduce-fehlings-solution

Actually I assumed it does.

As I assumed the reduction potential for benzaldehyde is more negative than that of the copper complex in the conditions used in the reaction.

OK, I didn’t look them up, so I may be wrong, but no recollection of benzaldehyde failing to undergo oxidation in the Fehlings reaction.

I’d would expect Cu to have a less positive reduction potential than silver (compared to standard conditions and silver aqua complex) but still, I’d expect it to work.

I can’t remember the last time I did this test so just have to go from memory and reference texts. The texts I have here say R-CHO -> RCOO- for Fehlings and do not say R cannot be a phenyl group.

I don’t thin the hydroxide is so concentrated that a Cannizzaro reaction would occur in the benzaldehyde instead, but I don’t know how strong the alkali was in the prepared solution you used in the lab.

Hummm… I’ve found a reference which suggests I am wrong…

p131 of: http://books.google.co.uk/books?id=yWnCkDVJDpQC&lpg=PA130&ots=edrGfoytqP&dq=benzaldehyde%20and%20fehling's%20test&pg=PA131#v=onepage&q=benzaldehyde%20and%20fehling's%20test&f=false

I’ll have to investigate further. Sorry about this, and thanks for bringing it to my attention.

Well, I looked up more trusted references and no mention that benzaldehyde cannot be oxidised by Fehlings, but no direct reference to it being able to be oxidised by Fehlings either. So there’s only a few internet references saying benzaldehyde cannot undergo oxidation with this reagent. Last night I saw a reference on the web that said an exam board said it couldn’t be oxidised but I say some person say they tried it and it worked.
Sadly I”ve not had time to check it out in the lab myself today. Will try on Monday – I have 2 lab sessions then.

ohh thanks a lot sir..sorry for messing up your head..hiiihih ๐Ÿ˜€

LOL. ๐Ÿ˜€

Just want to say…

It’s tempting to think that science has a definitive answer for everything. It doesn’t. It only has suggestions or models to try and fit everyday and experimental observations.

What makes something a ‘good model’ is not only does it fit those everyday and experimental observations, but predictions based on the model will be correct when further experimental tests are done on the system.

However, even if the predictions do turn out to be correct, the model could still be wrong, because it could be that the test which would show the flaw in the model hasn’t been thought of yet. This isn’t so much of an important factor as far as the oxidation of benzaldehyde with Fehlings solution is concerned but it’s connected to the next point:

When doing an experimental test, you must be very careful to exclude factors which could influence the result generated and give a ‘false positive’. When students do work in the lab it often happens that they do not do things in the way reported in the books. This is one possible reason why it sometimes happens that the ‘standard’ result (which has tested very specifically a chemical system) doesn’t happen for the student. Sadly contamination amongst students working in the lab is common and can affect results in unexpected ways.

I tried Fehlings solution and benzaldehyde. I got an orangey precipitate. It too some time. I used a water bath. The benzaldehyde didn’t dissolve in the aq. Fehlings mixture, I avoided adding ethanol to dissolve both layers as there was a chance the ethanol would interfere with the test (primary alcohols, like ethanol, can also be oxidised).

I’m wondering why the orangey ppte didn’t look as red as I expected, but the filtrate looked quite reddish brown (greater intensity on a white filter paper). I should have used a comparison with ethanal but I didn;t ๐Ÿ˜ฆ

Conclusion: Benzaldehyde can be oxidised by Fehlings solution, but the reaction is quite slow.

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