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.
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.