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QUESTION

How do you carry out Tollens' reagent test?

Here's how you do it…

Prepare the reagent

Prepare the reagent by adding ##"NaOH"## solution dropwise to a solution of ##"AgNO"_3## until a precipitate just forms; then add dropwise just enough dilute aqueous ##"NH"_3## to dissolve the precipitate.

Do the test

To carry out the test, add a few drops of the unknown to the freshly prepared reagent in a clean test tube, and warm gently for a few minutes in a hot water bath.

The formation of a grey-black precipitate or a silver mirror on the inside of the test tube is a positive result.

Why it works

The Tollens reagent is a colourless solution of ##"Ag"("NH"_3)_2^+## that is prepared in a two-step procedure.

1. ##"NaOH(aq)"## is added to ##"AgNO"_3("aq")##.

##"2AgNO"_3 + "2NaOH" → "Ag"_2"O" + "2NaNO"_3 + "H"_2"O"##

2. Aqueous ##"NH"_3## is added dropwise until the precipitated silver oxide dissolves completely.

##"Ag"_2"O"+ "4NH"_3 + "H"_2"O" → "2Ag"("NH"_3)_2^+ + "2OH"^"-"##

3. The ##"Ag"("NH"_3)_2^+## oxidizes aldehydes to the corresponding carboxylic acids, which are converted to carboxylate ions in the basic medium.

##"RCHO"color(white)(l) + "2Ag"("NH"_3)_2^+ + "3OH"^"-" → "RCOO"^"-" + "2Ag(s)" + "2NH"_3 + "2H"_2"O"##

If you are lucky, a silver mirror forms on the inside of the test tube; if you are unlucky, you get a grey-black precipitate of silver.

The Tollens reagent does not oxidize ketones, so the Tollens test distinguishes aldehydes from ketones.

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