Hard Chem Question

[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 :smile:

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.