Tollens' and Benedict's Test: Study with Video Lessons, Practice Problems & Examples

Tollens and Benedict tests are oxidation reactions used to identify aldehydes. Tollens test, known as the silver mirror test, uses silver oxide and ammonia to oxidize aldehydes into carboxylic acids, producing a silver precipitate. Benedict's test similarly oxidizes aldehydes in a basic solution, reducing copper(II) ions to form a brick red copper(I) oxide precipitate. Both tests confirm the presence of aldehydes through distinct visual changes, highlighting the importance of oxidation in organic chemistry.

Oxidation Reactions Concept 1

Now, when it comes to Tolan's test and Benedict's test, they basically function as oxidation reactions. Recall that oxidation uses an oxidizing agent to add as many carbon-oxygen bonds as possible without breaking any carbon-carbon bonds. When it comes to oxidations with Tollens' test and Benedict's test, we're paying attention to what's shaded within our box. Looking outside that box, initially we look at alkanes. It is said that alkanes can undergo oxidation to typically make alcohols. Alcohols themselves can be oxidized, where the carbon with the OH group becomes a carbonyl group. In this instance, this alcohol gets oxidized into an aldehyde, and that aldehyde can undergo further oxidation. We're trying to make as many carbon-oxygen bonds as possible. To do that, we're going to have to break one of these carbon-hydrogen bonds. Here, this H is going to be oxidized or removed, and in its place, we're going to have an oxygen placed. Oxygen needs to make two bonds, so it will be accompanied by a hydrogen. In this instance, we've gone from an aldehyde to a carboxylic acid. Now, aldehydes can be oxidized into carboxylic acids. Ketones, because they have a carbon on both sides, would not undergo oxidation. We're not trying to break any carbon-carbon bonds. We wouldn't be able to break this bond or this bond to add an oxygen to that carbonyl carbon. So, here we're just going from an aldehyde to a carboxylic acid. If we wanted to, we could go further on to a carbon dioxide molecule. But again, we are only focusing on this portion where we have an aldehyde being oxidized into a carboxylic acid.

Tollens' and Benedict's Test Example 1

Determine which of the following compounds cannot undergo an oxidation reaction. So, if we take a look at our choices here, we have hexanol, benzaldehyde, acetone, ethanol, and propanol. Now if we take a close look at their names, we see that a, d, and e end with "ol," meaning that they are alcohols. Remember, alcohols can be oxidized to aldehydes or ketones, so they can undergo oxidation. Now b and c, benzaldehyde, that is a benzene ring with an aldehyde attached to it. It's in the name "aldehyde." Since it's an aldehyde, it can also undergo oxidation to a carboxylic acid. The odd one out, the one that cannot undergo oxidation, is acetone. Acetone is a common name for ketone. Remember, acetone is a CH3 group connected to a carbonyl group, connected to another CH3 group. This cannot undergo oxidation because it would require us to cut through carbon-carbon bonds, which we're not allowed to do under typical oxidation reactions. So here, option c would be our correct answer. Acetone cannot undergo an oxidation reaction.

Tollens' Test Concept 2

Now, Tollen's test, it tests for the presence of an aldehyde within a basic solution. Here we're going to say that it's also known as the silver mirror test, because of the formation of a silver precipitate; remember precipitate just means a solid.

Now, here if we take a look at the reaction for Tollens' test, we have a typical aldehyde here and with it, we're reacting it with silver oxide and we have aqueous ammonia. "Aqueous ammonia" just means we have NH₃ dissolved in water. And then we have the addition of heat. This causes my aldehyde to be oxidized into a carboxylic acid. During this process, if we have an aldehyde and it gets oxidized to a carboxylic acid, we'd have the reduction of our silver, which would create a silver solid. So you make a silver solid as a product.

Now in the lab, you're doing this within a test tube and if your unknown happens to be an aldehyde, the reaction will be successful and give a positive result. The positive result will be the depositing of silver at the bottom of your test tube, making it look like a silver mirror. So that's where its name comes from. If you were to do this in a lab and you have a test tube, if you have an aldehyde as your unknown, look at the bottom of the test tube. You'll see silver deposits on the bottom, almost reflective. It's a silver mirror.

Now, here we're going to say the oxidizing agent can be presented in different ways. As a combination of silver, so Ag, and some type of ammonia. So some type of nitrogen-containing compound. Right? This is the way I've depicted it here, but we'll see other combinations that still qualify it as being a Tollen's test. So keep that in mind.

Tollens' Test Example 2

Draw the carboxylic acid produced when 2,3-dimethylpentanal is submerged in a Tollens solution. Alright. So here we know that Tollens' test will change our aldehyde into a carboxylic acid. Let's draw what 2,3-dimethylpentanal would look like. So "pent" means we have 5 carbons, and "al" means we have an aldehyde. So we're going to say that one of the end carbons is an aldehyde carbon. So it's that one. And where it's located is 1. So 1, 2, 3, 4, and 5. Then we have 2,3-dimethyl. Those are our substituents, and they're located on carbons 2 and 3. So on carbon 2, there's a methyl, and in carbon 3, there's a methyl. This is undergoing an oxidation by Tollens' test. So remember, it's a combination of silver with some type of ammonia compound. Here I'm just going to place O to show that it's undergoing oxidation. So our aldehyde group will just get oxidized into a carboxylic acid. So that would look like this. And then on carbons 2 and 3, we still have our methyl groups. So this would be the carboxylic acid formed from the oxidation of our aldehyde by Tollens' test.

Benedict's Test Concept 3

Here we're going to say that, like the Tollens' test, the Benedict's test tests for the presence of an aldehyde within a basic solution. So here we're going to say that a positive Benedict's test reduces copper(II) ion to copper(I) ion in the formation of a brick red copper(I) oxide precipitate. Remember, precipitate just means solid. Also remember that brick red is the exact name of the color you need to remember for a positive Benedict's test. If we take a look here at our equation, we're gonna say under Benedict's test, we have a typical aldehyde. It's going to be reacting with some copper(II) basic solution. As a result of this, the aldehyde gets oxidized into a carboxylic acid. So here's the formation of our carboxylic acid. And then we have copper(I) oxide as a byproduct. That gives us a brick red color to our test tube that would show that we have a positive Benedict's test that are unknown somewhere within it had an aldehyde that got oxidized to a carboxylic acid. So you'd have Cu2O. Right? So this would be a generic equation when it comes to the Benedict's test.

Benedict's Test Example 3

Here it says determine the product form when 3-ethylheptanal is treated with a basic copper(II) solution. So remember, a basic copper(II) solution would mean that we're undergoing the Benedict's test. And under the Benedict's test, we're just going to oxidize our aldehyde group into a carboxylic acid. So this structure, for the most part, would stay the same. We're just oxidizing that aldehyde group into a carboxylic acid group. So this represents our product from the oxidation of our aldehyde through the Benedict's test.

Determine the product formed when the following compound undergoes the Benedict's test.

• A.
• B.
• C.
• D. No Reaction

Under the following test, which structure represents the product formed?

• A.
• B.
• C.
• D. No Reaction

What was the starting material that created the following carboxylic acid product?

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the Tollens' test and how does it work?

The Tollens' test, also known as the silver mirror test, is used to detect the presence of aldehydes in a basic solution. It involves the reaction of an aldehyde with silver oxide (Ag₂O) and aqueous ammonia (NH₃). When heated, the aldehyde is oxidized to a carboxylic acid, and the silver ions are reduced to metallic silver, forming a silver precipitate. This precipitate deposits on the test tube, creating a reflective silver mirror. The reaction can be represented as:

${\mathrm{RCHO}}_{}+{\mathrm{Ag}}_{}\left({\mathrm{NH}}_{}\right)+H_{}O\to {\mathrm{RCOOH}}_{}+{\mathrm{Ag}}_{}\left(s_{}\right)$

What is the Benedict's test and how does it work?

The Benedict's test is used to detect the presence of aldehydes in a basic solution. It involves the reaction of an aldehyde with copper(II) ions (Cu²⁺) in a basic solution. The aldehyde is oxidized to a carboxylic acid, and the copper(II) ions are reduced to copper(I) oxide (Cu₂O), which forms a brick-red precipitate. This color change indicates a positive Benedict's test. The reaction can be represented as:

${\mathrm{RCHO}}_{}+{\mathrm{Cu}}_{}\left({\mathrm{OH}}_{}\right)\to {\mathrm{RCOOH}}_{}+{\mathrm{Cu}}_{}\left(O_{}\right)$

What are the visual indicators of a positive Tollens' test?

A positive Tollens' test is indicated by the formation of a silver mirror on the inner surface of the test tube. This occurs because the aldehyde in the solution is oxidized to a carboxylic acid, and the silver ions (Ag⁺) are reduced to metallic silver (Ag), which deposits as a reflective layer. The reaction can be summarized as:

${\mathrm{RCHO}}_{}+{\mathrm{Ag}}_{}\left({\mathrm{NH}}_{}\right)+H_{}O\to {\mathrm{RCOOH}}_{}+{\mathrm{Ag}}_{}\left(s_{}\right)$

What are the visual indicators of a positive Benedict's test?

A positive Benedict's test is indicated by the formation of a brick-red precipitate in the solution. This occurs because the aldehyde in the solution is oxidized to a carboxylic acid, and the copper(II) ions (Cu²⁺) are reduced to copper(I) oxide (Cu₂O), which precipitates out as a solid. The reaction can be summarized as:

${\mathrm{RCHO}}_{}+{\mathrm{Cu}}_{}\left({\mathrm{OH}}_{}\right)\to {\mathrm{RCOOH}}_{}+{\mathrm{Cu}}_{}\left(O_{}\right)$

How do Tollens' and Benedict's tests differ in their chemical reactions?

Both Tollens' and Benedict's tests are used to detect aldehydes, but they differ in their chemical reactions and visual indicators. Tollens' test uses silver oxide (Ag₂O) and aqueous ammonia (NH₃) to oxidize aldehydes to carboxylic acids, producing a silver mirror as a visual indicator. The reaction is:

${\mathrm{RCHO}}_{}+{\mathrm{Ag}}_{}\left({\mathrm{NH}}_{}\right)+H_{}O\to {\mathrm{RCOOH}}_{}+{\mathrm{Ag}}_{}\left(s_{}\right)$

Benedict's test uses copper(II) ions (Cu²⁺) in a basic solution to oxidize aldehydes to carboxylic acids, producing a brick-red precipitate of copper(I) oxide (Cu₂O). The reaction is:

${\mathrm{RCHO}}_{}+{\mathrm{Cu}}_{}\left({\mathrm{OH}}_{}\right)\to {\mathrm{RCOOH}}_{}+{\mathrm{Cu}}_{}\left(O_{}\right)$