Structural Formulae for Isomer P, Q and R: Explained

[MA103]

With an aqueous solution containing Cu(I) ions complexed by ammonia, isomer P gave a red precipitate. On vigorous oxidation by aqueous manganate (VII) ions, isomer Q gave only ethanoic acid whereas isomer R gave an unbranched chain, dicarboxylic acid.
Write structual formulae for isomers P, Q and R: explain your reasoning.

This is a past old examination question, I'm finding this difficult to answer. Anyone have any ideas?

 

[chem_tr]

Hello, this is not an easy question, as you say.

The first one involving red precipitate is very likely to be cuprous oxide, Cu₂O. This is an indicator of the positive Fehling test; and any oxidizable substance, but especially aldehydes. So ammoniacal cuprous solution is best reacted by an aldehyde; this is very similar to Tollens' test in which silver ions are used and a silver mirror is produced as a visual indicator.

The second one seems to be ethanol, since vigorous oxidation only gives the highest oxidation state; not the aldehyde stage, I think.

The last one is similar to the one before; but dialcohol is likely to be the solution of this problem.

It's difficult to draw the exact formulae for the aldehyde and dialcohol though.

 

[GCT]

I suggest that you go through the carbohydrates chapter in your organic chem text, this is what the question pertains to (glucose, fructose, mannose etc...).

 

[chem_tr]

GCT, you are a genius

Three isomers with one alcohol, one aldehyde and one dicarboxylic acid, involving Fehling test also are nothing but carbohydrates in Fischer conformation... I totally agree with you about this.