Question #16ebd

1 Answer
Oct 28, 2017

An aldehyde produces a brick red precipitate; a ketone does not react.


Benedict's solution

Benedict's solution contains copper(II) ions in a sodium carbonate solution.

The copper(II) ions are complexed with citrate ions to prevent the formation of a precipitate of copper(II) carbonate.

Benedict's test

A few drops of the aldehyde or ketone are added to the Benedict solution, and the mixture is warmed gently for a few minutes.

With a ketone, there will be no change in the blue solution.

In a positive test for an aldehyde, the blue solution produces a dark red precipitate of copper(I) oxide.

The equations

An aldehyde reduces the complexed #"Cu"^"2+"# ions to #"Cu"_2"O"# and is in turn oxidized to a carboxylate ion.

The equations for the reaction are

#"RCHO + 3OH"^"-" → "RCOO"^"-" + 2"H"_2"O" + 2"e"^"-"#
#ul(2"Cu"^"2+" + 2"OH"^"-" + 2"e"^"-" → "Cu"_2"O(s)" + "H"_2"O"color(white)(mmmmmmmmm))#
#"RCHO +"underbrace(2"Cu"^"2+")_color(red)("blue complex") + 5"OH"^"-" → "RCOO"^"-" + underbrace("Cu"_2"O(s)")_color(red)("brick red ppt") + "3H"_2"O"#