1
$\begingroup$

While I understand why aromatic aldehydes and ketones do not give positive Fehling's test, I do not know why Glyoxal and Glyoxylic Acid give a negative test too. Nothing has been written about these two compounds in my study material and I was hoping someone could help me out with the reasoning. I figure it's something to do with presence of alpha-H but I'm not sure.

$\endgroup$
2

1 Answer 1

4
$\begingroup$

There is a good diagram of the mechanism here that I have reproduced below.

The first step is the formation of an enolate anion of the aldehyde by removal of the alpha proton by hydroxide. The enolate then binds to Cu2+. If the enolate cannot be formed then the reaction cannot proceed. Glyoxal and Glyoxylic Acid cannot form enolates so they give a negative test. The only proton that can be removed from glyoxal to form an enolate is the proton on a -CHO group. They are far less acidic than the alpha protons of an aldehyde. Glyoxalic acid obviously deprotonates at the carboxylic acid group, so no enolate forms.

enter image description here

$\endgroup$
2
  • $\begingroup$ Why can't glyoxal form the enolate? $\endgroup$ Jun 13 at 10:27
  • $\begingroup$ @DhruvKaushik I have edited to answer this $\endgroup$
    – Waylander
    Jun 13 at 11:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.