[Aqualad300]

Hey there,
                So we did this lab right and we tested two cabonyl compounds under the aim differentiating between aldehydes and ketones using their reactions with Tollens Reagent, Fehling’s Solution and acidified potassium dichromate.

Carbonyl A tested positive for tollens', fehling's while carbonyl B tested negative for all three.

The question is, why did Carbonyl A test negative for the reaction with acidified potassium dichromate if it was pointing towards being an aldehyde.

The lecturer gave the hint that is in its cyclic form.

If you wish to get the methods feel free to contact me

[pgk]

Is compound A a sugar (carbohydrate), as probably indicated by the hint about the cyclic form?

[rolnor]

If it is a carbohydrate it should be named a hemiacetal, not aldehyde?

[pgk]

Typically, yes!
However, carbonyl compounds in solutions are usually in equilibrium with their acetal or hemiacetal forms and roughly, the names “aldehydes” and “ketones” can still be used.
Furthermore and as being in equilibrium, acetals and hemiacetals are usually positives to all characteristic tests of their mother carbonyl compounds.

[rolnor]

It is given in the question that it is in its cyclic form, not that it is in equilibrium with its cyclic form?

[pgk]

Nevertheless and as being hemiacetals, aqueous solutions of carbohydrates are also in equilibrium of pyranose, furanose and their linear form at ratios that mainly depend on the pH.
Besides, α- and β- anomerization, as well as epimerization of cyclic carbohydrates in aqueous solutions, occur via ring opening due the said, dynamic equilibrium

[pgk]

By the way Aqualad300, what do you know about epimerization (isomerization) of carbohydrates in solutions?
Hint: Carbohydrates epimerization is fast under alkaline conditions but very slow under acidic conditions.

[pgk]

Or else, which monosaccharide is a ketone but it can be epimerized to an aldehyde one?