Tollens Silver Mirror Test for Aldehydes: Experiment

Before we get into the details of this experiment lets first start by having a look at aldehydes and ketones.

a Ketone

Aldehydes and ketones are members of the same type of compound known as carbonyl compounds. Carbonyl compounds are those that have carbon to oxygen double bonds attached. In ketones the molecule is normally arranged so that the carbon to oxygen double bond can be the first thing seen on the molecule (usually in the middle).

an Aldehyde

Aldehydes on the other hand have hydrogen attached to the carbon to oxygen double bond. It is this hydrogen that accounts for the difference between aldehydes and ketones. Also it is important to note that this hydrogen is what makes an aldehyde such a great reducing agent and thus can be oxidized easily. Ketones are however resistant to oxidation meaning they aren’t that easily oxidized. When an aldehyde is oxidized in an acidic environment a carboxylic acid will be formed but when oxidized in an alkaline environment a salt of a carboxylic acid will be formed. Try to remember these facts because it is this difference that this entire experiment is based upon. Also Tollen's reagent comprises diamminesilver (i) ion which is an oxidizing agent, the formula for diamminesilver (i) ion is:
[Ag (NH3)2]+

Title of Lab/Experiment: Tollens Test

Aim of Lab/Experiment: Using Tollens reagent to distinguish Aldehydes from ketones.

Things you’ll need:

• silver nitrate solution
• Dilute ammonia
• Glucose (Aldehyde)
• Acetone (Ketone)
• Distilled Water
• Test Tube
• Bunsen Burner

Tollens apparatus

Procedure:

1. Accurately measure out 3ml of silver nitrate solution and place in a clean test tube, then using the dropper add dilute ammonia to the test tube drop by drop until no precipitate is left. Add another 5ml of ammonia to the test tube.
2. Accurately measure 4g of glucose and place in a beaker, after which you’ll measure 10ml of distilled water and place in the beaker as well.
3. Pour the contents of the beaker and the test tube in a large boiling tube.
4. Immerse the boiling tube into water at 75 degrees. Record observations in a suitable table.
5. Repeat the above procedures using acetone instead of glucose.

Results:

Table Showing Results Obtained From Tollens Test

	Glucose(Aldehyde)	Acetone(Ketone)
Observations	After a few hours the colorless solution formed a silver mirror in the boiling tube.	No change took place. The solution remained colorless.
Interpretation of what happened	The diamminesilver ion was reduced to metal silver while the aldehyde in turn was oxidized to a carboxylic acid (gluconic acid).	Because the acetone is a ketone it wasn’t oxidized and therefore didn’t do any reducing.

The equation for the overall reaction of the aldehyde and the diamminesilver is:

RCHO + 2[Ag(NH₃)₂]⁺+ 2OH^- ---> RCOOH + 2Ag + H₂O