# Question #16ebd

Oct 28, 2017

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

#### Explanation:

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 $\text{Cu"^"2+}$ ions to $\text{Cu"_2"O}$ and is in turn oxidized to a carboxylate ion.

The equations for the reaction are

$\text{RCHO + 3OH"^"-" → "RCOO"^"-" + 2"H"_2"O" + 2"e"^"-}$
$\underline{2 \text{Cu"^"2+" + 2"OH"^"-" + 2"e"^"-" → "Cu"_2"O(s)" + "H"_2"O} \textcolor{w h i t e}{m m m m m m m m m}}$
$\text{RCHO +"underbrace(2"Cu"^"2+")_color(red)("blue complex") + 5"OH"^"-" → "RCOO"^"-" + underbrace("Cu"_2"O(s)")_color(red)("brick red ppt") + "3H"_2"O}$