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Fehling's test follows a single electron transfer mechanism, with electron transfer form the substrate to $\ce{Cu^2+}$, which forms $\ce{Cu^+},$ finally giving reddish brown precipitate of $\ce{Cu2O}$. The mechanism is as follows:

Fehling's test mechanism

It can be easily concluded from this mechanism that formaldehyde without α-hydrogen does not give this test as it is the enolate which forms the complex intermediate with $\ce{Cu^2+}$. Formation of enolate is only possible if α-hydrogen is present.

If we look at formaldehyde, there are no α-hydrogens present, and in ethylene glycol there is no aldehyde group, so neither should give a positive Fehling's test.

However, the answer key from my organic chemistry exercise inexplicably states that both formaldehyde and ethylene glycol give positive Fehling's test. Is this correct? If so, what would be an explanation for it?

andselisk
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sameed hussain
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    See: https://chemistry.stackexchange.com/questions/127463/fehlings-solution-turning-yellow-and-then-black?r=SearchResults – Nilay Ghosh Jun 06 '21 at 02:19

1 Answers1

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Fehling’s test is given by aldehydes, hemi-acetals, hemi-ketals , and alpha hydroxy ketones (due to enolization, one form is like an aldehyde). Note that aromatic aldehydes do not give Fehling test. Also an alpha hydrogen isn't needed, what is needed is the carbonyl group to be present ($\ce{-CHO}$). The mechanism for aldehydes with no alpha hydrogen is given below:

enter image description here

As for ethylene glycol, It is sort of an exception. You can refer to this video. https://m.youtube.com/watch?v=Wt1VzZxWv3Y (it’s in Hindi though, but you’ll get the idea, reacting $OH^-$ and then enolisation)

Please see: Does an acetal give a positive Tollens test?. Also you can refer to Test for functional groups in organic compound, NCERT Lab Manual for Class XII.

PSR_123
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  • "As for ethylene glycol, I believe it should not give the positive Fehling test, though I am not sure why it is given so in your textbook." Don't believe. If you know, you know. If necessary, provide a reference where you learned it and explain why OP's textbook is wrong. 2. Don't recommended NCERT textbook as references.
    • – Nilay Ghosh May 28 '22 at 11:34
  • @NilayGhosh I actually found out that ethylene glycol gives positive Fehling test, not sure of the mechanism, it is a sort of exception – PSR_123 Jun 02 '22 at 10:29