# Do formaldehyde and ethylene glycol give positive Fehling's test?

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:

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?

• Jun 6, 2021 at 2:19

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:
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)