Aldehydes are functional derivatives of hydrocarbons that contain a CO group (carbonyl group) in their structure. For simple aldehydes, trivial (historical) names are traditionally preserved, derived from the name of carboxylic acids, into which aldehydes are converted during oxidation. If we talk about the IUPAC nomenclature, then the longest chain containing the aldehyde group is taken as the basis. The beginning of the numbering of the hydrocarbon chain is made from the carbon atom of the carbonyl group (CO), which itself receives the number 1. The ending "al" is added to the name of the main hydrocarbon chain. Since the aldehyde group is at the end of the chain, the number 1 is usually not written. The isomerism of the presented compounds is due to the isomerism of the hydrocarbon skeleton.
Aldehydes are obtained in several ways: oxosynthesis, hydration of alkynes, oxidation and dehydrogenation of aldehydes from primary alcohols require special conditions, since the resulting ones are easily oxidized to carboxylic acids. Aldehydes can also be synthesized by dehydration of the corresponding alcohols in the presence of copper. One of the main industrial methods for obtaining aldehydes is the oxosynthesis reaction, which is based on the interaction of alkene, CO and H2 in the presence of catalysts containing Co at a temperature of 200 degrees and a pressure of 20 MPa. This reaction proceeds in the liquid or gas phase according to the scheme: RCH=CH2 + C0 + H2 - RCH2CH2C0H + RCH(CH)3C0H. Aldehydes can be obtained by hydrolysis of dihalogen derivatives of hydrocarbons. In the process of replacing halogen atoms with OH groups, the so-called gem-diol is intermediately formed, which is unstable and turns into a carboxyl compound with elimination of H20.
The chemical property of aldehydes is their quality. In the process, they are converted into carboxylic acids (for example, C5H11SON + O - C5H11COOH). In any specialized textbook, you can find information that the silver mirror reaction is used to identify aldehydes. This group of organic substances can be oxidized not only under the action of special oxidizing agents, but also simply during storage under the influence of atmospheric oxygen. The ease with which aldehydes are oxidized to carboxylic acids made it possible to develop qualitative reactions (silver mirror reaction) for these organic compounds, which makes it possible to quickly and clearly determine the presence of aldehyde in a particular solution.
When heated with an ammonia solution of silver oxide, the aldehyde is oxidized to an acid. In this case, silver is reduced to metallic and is deposited on the walls of the test tube in the form of a dark layer with a characteristic mirror shine - the reaction of a silver mirror. It should be noted that there are a huge number of substances that do not belong to aldehydes, but they are also capable of entering into this reaction. To identify these compounds, another qualitative reaction for aldehydes is used - the reaction of a copper mirror. When aldehydes interact with Fehling's reagent, which has a blue color (an aqueous solution of alkali and salts of tartrate acid), copper is reduced from divalent to monovalent. In this case, a red-brown precipitate of copper oxide precipitates.
So how does the silver mirror reaction work? It would seem that there is nothing simpler: it is enough to heat silver in a bowl with any of the aldehydes (for example, or formaldehyde), but this approach is not always crowned with victory. Sometimes we observe the formation of a black suspension of silver in solution, and not a mirror coating on the walls of glassware. What is the main reason for failure? To obtain a 100% result, it is necessary to adhere to the reaction conditions, as well as carefully prepare the glass surface.
Very visual is the experience with a beautiful effect of formation on the glass of a mirror coating. For this reaction, you need to stock up on experience and patience. In this article, you will learn about the necessary and specific features of equipment preparation, and also see what reaction equations this process follows.
The essence of the silver mirror reaction is the formation of metallic silver as a result of a redox reaction during the interaction of an ammonia solution of silver oxide in the presence of aldehydes.
"Silver Mirror" (test tube on the left)To create a durable silver layer you will need:
- glass flask with a capacity of up to 100 ml;
- ammonia solution (2.5-4%);
- silver nitrate (2%);
- aqueous solution of formaldehyde (40%).
Instead, you can take the ready-made Tollens reagent - an ammonia solution of silver oxide. To create it, you need to add 1 gram of silver nitrate to 10 drops of water (if the liquid will be stored for a long time, you need to place it in a dark place or in a glass container with dark walls). Immediately before the experiment, the solution (about 3 ml) must be mixed in a 1:1 ratio with a 10% aqueous solution of sodium hydroxide. Silver can precipitate, so it is diluted by slowly adding an ammonia solution. We recommend that you conduct another spectacular experiment with an ammonia solution and print a “chemical photograph”.
The reaction is carried out at room temperature. Required condition a successful finale - these are perfectly clean and smooth walls of a glass vessel. If there are the smallest particles of pollution on the walls, the sediment resulting from the experiment will become a loose layer of black or dark gray.
To clean the flask, use different types alkali solutions. So, for processing, you can take a solution that, after cleaning, has to be washed off with distilled water. It is necessary to rinse the flask from the cleaning agent many times.
Why is cleanliness so important?
The fact is that the colloidal silver particles formed at the end of the experiment must firmly adhere to the glass surface. On its surface there should be no fats and mechanical particles. water does not contain salts and is ideal for the final cleaning of the flask. It can be prepared at home, but it is easier to buy ready-made liquid.
Silver mirror reaction equation:
Ag₂O + 4 NH₃ H₂O ⇄ 2OH + 3H₂O,
where OH is diammine silver hydroxide, obtained by dissolving the metal oxide in an aqueous solution of ammonia.
Diamminesilver complex molecule
Important! The reaction works at low concentrations of ammonia - carefully observe the proportions!
This is how the final stage of the reaction proceeds:
R (any aldehyde)-CH=O + 2OH → 2Ag (precipitated silver colloid) ↓ + R-COONH₄ + 3NH₃ + H₂O
The second step of the reaction is best carried out by carefully heating the flask over a burner flame - this will increase the chances that the experiment will be successful.
What can a silver mirror reaction show?
This interesting chemical reaction demonstrates not only certain states of matter - it can be used to perform a qualitative determination of aldehydes. That is, such a reaction will solve the question: is there an aldehyde group in the solution or not.
General structural formula of aldehydes
For example, in a similar process, you can find out what is contained in the solution: glucose or fructose. Glucose will give a positive result - you will get a “silver mirror”, and fructose contains a ketone group and you cannot get a silver precipitate. In order to carry out the analysis, instead of a formaldehyde solution, it is necessary to add a 10% glucose solution. Consider why and how dissolved silver turns into a solid precipitate:
2OH + 3H₂O + C₆H₁₂O₆ (glucose) = 2Ag↓+ 4NH₃∙H₂O + C₆H₁₂O₇ (gluconic acid is formed).
Author L.A. TsvetkovThis experience should be demonstrated to the students in such a way that it simultaneously serves as an instruction for the subsequent practical session. The nature of the ammonia solution of silver oxide can be explained to students in detail, or you can only tell them that the silver hydroxide formed during the reaction is a fragile substance and easily decomposes into silver oxide and water:
AgNO 3 + NH 4 OH à AgOH + NH 4 NO 3
2 AgOH à Ag 2 O + H 2 O
Silver oxide has the ability to dissolve in ammonia
Ag 2 O + 4NH 4 OH à 2OH + H 2 O
The oxidative effect of silver oxide is explained by the fact that this substance is an oxide of a noble metal; therefore, the oxide is also unstable in the presence of a reducing agent, i.e. a substance that is easily oxidized, it easily gives off oxygen, as a result of which the release (reduction) of metallic silver occurs. The reaction equation can be given in the usual form:
HC-OH + Ag 2 O -> HCOOH + 2Ag
and you can write the full equation for the reaction:
HC-OH + 2OH -> HCOOH + 2Ag + 4NH 3 + H 2 O
A silver mirror is formed when reducing silver is deposited on the smooth walls of a vessel from solutions that are not too concentrated. The slightest contamination prevents the recovering silver from "cling" to the glass and causes it to stand out as a loose precipitate. The nature of heating has a much smaller effect on the success of the experiment. If the vessel is not clean enough, then even the most careful heating will not give a mirror, and vice versa, if the vessel is carefully prepared, then even heating the mixture over an open fire can give the desired result.
The first experience of obtaining a silver mirror should be carried out in a flask, and not in a test tube. Before the lesson, the flask with a capacity of 50-100 ml is cleaned of mechanical impurities, washed with a brush with soapy water or an alkali solution is heated in the flask, then rinsed with water, washed with a chromium mixture and finally washed clean with distilled water.
A 2% solution of silver nitrate is poured into the flask by a quarter of the volume, then an ammonia solution is gradually added (25% ammonia should be diluted 8-10 times) until the precipitate formed at the beginning dissolves in its excess. 0.5-1 ml of formalin is carefully added to the resulting solution along the wall and the flask is placed in a glass of hot (preferably boiling) water.
Soon a beautiful silver mirror forms in the flask. The flask can be heated without a water bath, directly on a small flame, carrying the flame around the flask without shaking it. When demonstrating experience, instead of a silver mirror, a black precipitate is sometimes formed. The teacher in this case usually completely rejects the experience. Meanwhile, with such a result, it should be explained to the students that silver was also reduced here only in the form of a loose black precipitate.
Lab #5
Propertiescarbohydrates
Experiment 1. Silver mirror reaction is a recovery reaction silver from ammonia solution silver oxide (Tollens reagent).
in aqueous solution ammonia silver oxide dissolves to form a complex compound - diamminesilver(I) hydroxide OH
when added to which aldehyde a redox reaction occurs with the formation of metallic silver:
If the reaction is carried out in a vessel with clean and smooth walls, then silver precipitates in the form of a thin film, forming a mirror surface.
In the presence of the slightest contamination, silver is released in the form of a gray loose precipitate.
The silver mirror reaction can be used as a qualitative test for aldehydes. Thus, the reaction of the "silver mirror" can be used as a distinguishing between glucose And fructose. Glucose belongs to aldoses (containing an open aldehyde group), and fructose belongs to ketoses (containing an open keto group). Therefore, glucose gives a "silver mirror" reaction, while fructose does not. But if an alkaline medium is present in the solution, then ketoses isomerize into aldoses and also give positive reactions with an ammonia solution silver oxide (Tollens reagent).
Qualitative reaction of glucose with ammonia solution of silver oxide. You can prove the presence of an aldehyde group in glucose using an ammonia solution of silver oxide. Add a glucose solution to the ammonia solution of silver oxide and heat the mixture in a water bath. Soon, metallic silver begins to deposit on the walls of the flask. This reaction is called the silver mirror reaction. It is used as a quality for the discovery of aldehydes. The aldehyde group of glucose is oxidized to a carboxyl group. Glucose is converted to gluconic acid.
CH 2 OH - (CHOH) 4 – SLEEP+Ag 2 O= CH 2 OH - (CHOH) 4 – COOH + 2Ag↓
The order of the work.
Pour 2 ml into two test tubes. ammonia solution of silver oxide. 2 ml is added to one of them. 1% glucose solution, in another - fructose. Both tubes are boiling.
An ammonia solution of silver hydroxide is obtained by reacting silver nitrate with sodium hydroxide and ammonium hydroxide:
AgNO3+ NaOH → AgOH↓+ NaNO3,
AgOH + 2 NH4 OH→[Ag(NH3)2] OH + H2O,
ammonia solution
OH + 3 H2 → Ag2O + 4 NH4 OH.
The principle of the method. A mirror is formed on the walls of the test tube with glucose as a result of the release of metallic silver.
Work design: Write the conclusion, as well as the course and equations of the reaction in a notebook.
Experience 3. Qualitative reaction to fructose
The principle of the method. When a sample with fructose is heated in the presence of resorcinol And of hydrochloric acid up to 80 ° C after a while a bright red color appears in a test tube with fructose.
When a sample with fructose is heated in the presence of resorcinol And of hydrochloric acid a cherry red color appears. The assay is also applicable to the detection of other ketosis. Aldoses under the same conditions, they interact more slowly and give a pale pink color or do not interact at all. open F. F. Selivanov in 1887. It is used in the analysis of urine. The test is positive for fructosuria of metabolic or transport origin. In 13% of cases, the test is positive with a food load of fruits and honey. Chem. formula fructose - C 6 H 12 O 6
The cyclic formula of fructose
Acyclic form 
fructose
Painted connection
R-residue
hydroxymethylfurfural
The order of the work.
2 ml are poured into two test tubes: in one - 1% glucose solution, in the other - 1% fructose solution. Both tubes are filled with 2 ml of Selivanov's reagent: 0.05 g of resorcinol is dissolved in 100 ml of 20% hydrochloric acid. Both tubes are gently heated to 80°C (before boiling). A red color appears.
Conclusions: the results of the experiment and the reaction equation are written in a notebook.
It is necessary to understand what is the reaction of a silver mirror? The silver mirror reaction is the process of reducing metallic silver from an ammonia solution of silver oxide.
Ag2O + 4NH4OH ↔ 2OH + H2O
Silver oxide in an aqueous solution of ammonia dissolves to form a complex silver compound - silver diamine hydroxide (I) OH.
By adding any aldehyde (formaldehyde) to the silver complex compound, metallic silver is formed as a result of the redox reaction. As a result of the reaction on the walls of a glass test tube, a beautiful mirror coating of silver or a mirror will form.
R-CH=O + 2OH → 2Ag ↓ + R-COONH4 + 3NH3 + H2O
You can read in any chemistry textbook that the silver mirror reaction can be used to detect aldehydes. For example, glucose gives a "silver mirror" reaction, but fructose does not. However, there are many chemicals that, like aldehydes, can undergo a silver mirror reaction.
How, in practice, can a silver mirror reaction be carried out?
At first glance, it seems that it is very easy to conduct a silver mirror reaction, but this is not entirely true. Everything seemed simple, you can take an ammonia solution with some aldehyde, it can be a solution of formaldehyde or glucose and carry out a silver mirror reaction. However, this is a simple and primitive approach to understanding what is the reaction of a silver mirror? This reaction can lead to disappointment. Instead of the expected specular coating on the glass, a black or brown suspension of silver in solution may form.Usually the reaction is in a simple way, in most cases ends in failure. Even if you manage to create a mirror, then it comes out of very poor quality. The silver layer is fragile and uneven. Why is this happening? There are many reasons for such an unsuccessful reaction. Of these, two main reasons can be distinguished: non-observance of the necessary conditions for the reaction or a poorly prepared glass surface for silvering.
As a result of the reaction, a positively charged silver ion is formed, which combines with the aldehyde group to form the smallest or colloidal silver particles. Such tiny particles can adhere strongly to the glass surface or remain in solution as a suspension of silver.
In order for the colloidal particles of silver to reliably adhere to the glass and form a strong and uniform layer of silver, that is, a mirror, the glass surface must first be degreased before silvering. The surface of the glass must not only be perfectly clean, but also as smooth as possible.
The main contaminant of glass is fatty deposits, which must be removed. To remove fat, an alkali solution, a hot chromium mixture are used, and then the glass is repeatedly washed with distilled water. If there is no alkali, as a last resort, you can use the usual synthetic detergent for dishes. After degreasing, it is useful to rinse the glass with a solution of tin chloride and distilled water.
All solutions should be made with distilled water. If distilled water is not available, rainwater can be used as a last resort. To reduce metallic silver, in the silver mirror reaction, reducing agents are often used: formaldehyde or glucose. The choice of these two substances depends on the purpose for which the chemical reaction will be used.
Silver mirror reaction using formaldehyde
To demonstrate the experience, the reaction of the silver mirror can be carried out with the participation of formaldehyde. If you need to make a high-quality mirror with a durable and uniform surface, it is better to use glucose.For silvering glass, it is better to take silver, as part of the silver salt - silver nitrate. Ammonia and alkali solutions are added to silver nitrate. The deposition of silver on glass must take place in an alkaline solution. In this case, the alkali solution should not be too much, since its excess is also undesirable. Depending on the technique, the silver mirror reaction is carried out at room temperature or under heating.
When the solution turns brown, this means that tiny colloidal particles of silver have formed in the solution. Later, on the surface of the glass, a thin mirror coating is formed, made of reduced silver. It is very easy to carry out the reaction of a silver mirror, but it can be very difficult to obtain a high-quality mirror. In order to obtain a high-quality silver coating - a silver mirror, you need to spend a lot of work, while you need to be careful and very persistent.
For experience, you can carry out a simple silvering of glass, in order to first familiarize yourself and find out what the reaction of a silver mirror is? When demonstrating this reaction, a mirror of not very good quality may turn out.
To carry out the reaction, we need: a clean glass flask with a capacity of 50 - 100 ml, an ammonia solution at a concentration of 2.5 to 4 percent, a 2 percent solution of silver nitrate and a formalin solution.
Before silvering, prepare the flask for a chemical reaction. We clean the flask from mechanical impurities by wiping it with a brush with soap, and then rinse the flask with distilled water. Then we wash it with a chromium mixture, and then rinse it again with distilled water.
Pour a 2% solution into one quarter of the flask, then gradually add ammonia solution to this solution. An ammonia solution is prepared from the calculation, we take a 25% ammonia solution and dilute it with distilled water 8-10 times. We gradually add an ammonia solution to the silver nitrate, until the precipitation is completely dissolved in its excess. To the solution that has formed, gradually along the wall, add formalin solution - 0.5 - 1 ml. We place the glass flask in a container with hot or even better with boiling water. Soon, it will begin to form on the flask, which forms a pretty silver mirror.
The main disadvantage of this technique is that not only ammonia solution, but also alkali (this can be sodium hydroxide - NaOH or potassium hydroxide - KOH) must be added to the silver nitrate solution. To obtain a high-quality mirror coating, it is necessary to add ammonia first, and then alkali.
To simply demonstrate the reaction of a silver mirror, you can do the opposite: first, add alkali to silver nitrate until the formation of a brown precipitate (Ag2O) stops, and then add an ammonia solution until the precipitate is completely dissolved:
2Ag+ + 2OH - = Ag2O + H2O
Ag2O + 4NH3 + H2O = 2OH
When carrying out this technique, a white precipitate may form, most likely it will be urotropine (or hexamethylenetetramine):
6CH2O + 4NH3 = (CH2)6N4 + 6H2O
The formation of a white precipitate is a bad sign that does not contribute to obtaining a high-quality mirror.
It should be especially noted that the reaction of the silver mirror must necessarily take place in an alkaline environment, and not in an acidic one. In an acidic environment, the silver mirror reaction will proceed without the formation of a silver coating. Sometimes it happens that when mixing reagents, the medium suddenly becomes acidic. One has only to add an excess of alkali to the reaction mixture, as a silver mirror is immediately formed on the walls of a glass test tube.
Silver mirror reaction using glucose
The silver mirror reaction using glucose can be carried out, not only to demonstrate a chemical reaction, but also to obtain a high-quality silver mirror or conductive surface.Everyone is well aware that the silver mirror reaction is a chemical process of reducing metallic silver from an ammonia solution of silver oxide (Tollens' reagent).
The reaction of the silver mirror is at the heart of the production of silver mirrors. In order for metallic silver (Ag) not to oxidize, not form black (Ag2S), that is, it does not become dull and scratched, it is coated on top with a protective varnish. In order to silver the mirror, you need to prepare two freshly prepared solutions (A and B).
Solution - A
In 100 ml of distilled water, dissolve 6 grams of silver nitrate - (AgNO3), add an aqueous solution of ammonia to this solution, until the initially formed precipitate dissolves. Then we add alkali to the resulting solution - 70 ml of a 3% solution of sodium hydroxide (NaOH), and again pour an aqueous solution of ammonia until the solution is completely clear (without excess). The resulting solution is diluted with distilled water to 500 ml.Solution - B
In 25 ml of distilled water, dissolve 1.3 grams of glucose (one drop of concentrated nitric acid - HNO3 is added to the resulting solution) and boil the resulting solution for two minutes. Then the solution is cooled and diluted with the same volume of alcohol.Solutions: A and B are mixed before use in a ratio of 10:1. After mixing the solutions, a thick film of silver forms on the glass within 30 minutes.
Before silvering a mirror, you need to clean the glass well. This is a very important and necessary condition. To obtain a high-quality mirror coating, this condition cannot be neglected. The glass surface is cleaned with a hot mixture - HNO3 + K2Cr2O7, then the glass is rinsed in distilled water and treated with alcohol.
In order to obtain a thicker layer of silver, the surface treatment of silvered glass is repeated again with freshly prepared portions of solutions, one or two more times. Then washed with water and alcohol, the resulting precipitate of silver.
