Aldehydes are functional derivatives of hydrocarbons, in the structure of which there is a CO group (carbonyl group). For simple aldehydes, trivial (historical) names are traditionally retained, derived from the names of carboxylic acids into which aldehydes are converted upon oxidation. If we talk about the IUPAC nomenclature, then the longest chain containing an aldehyde group is taken as the basis. The numbering of the hydrocarbon chain begins 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 requires special conditions, since those formed are easily oxidized into 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 producing aldehydes is the oxosynthesis reaction, which is based on the interaction of an 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 occurs 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 dihalogenated hydrocarbons. In the process of replacing halogen atoms with OH groups, the so-called heme-diol is intermediately formed, which is unstable and turns into a carboxyl compound with the elimination of H20.
The chemical property of aldehydes is that they are qualitatively 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 into carboxylic acids has made it possible to develop qualitative reactions (silver mirror reaction) to 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 into an acid. In this case, silver is reduced to metallic and 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 are not aldehydes, but they are also capable of entering into this reaction. To identify these compounds, another qualitative reaction to aldehydes is used - the copper mirror reaction. When aldehydes react 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 take place? It would seem that nothing is simpler: it is enough to heat silver in a bowl with any of the aldehydes (for example, formaldehyde), but this approach is not always crowned with victory. Sometimes we observe the formation of a black suspension of silver in the solution, rather than a mirror coating on the walls of the glassware. What is the main reason for failure? To obtain 100% results, you must adhere to the reaction conditions and carefully prepare the glass surface.
The experiment with the beautiful effect of the formation of a mirror coating on glass is very visual. This reaction requires experience and patience. In this article you will learn about the necessary and specific preparation of equipment, and also see what reaction equations this process takes place.
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 a 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 may precipitate, so it is diluted by slowly adding an ammonia solution. We recommend conducting another spectacular experiment with an ammonia solution and printing a “chemical photograph”.
The reaction is carried out at room temperature. Required condition a successful finale is perfectly clean and smooth walls of the glass vessel. If there are the slightest particles of contaminants on the walls, the sediment obtained as a result of the experiment will become a loose layer of black or dark gray color.
To clean the flask you need to use different types alkali solutions. So, for processing, you can take a solution, which after cleaning has to be washed off with distilled water. It is necessary to rinse the flask of the cleaning agent many times.
Why is vessel cleanliness so important?
The fact is that the colloidal silver particles formed at the end of the experiment must adhere firmly to the surface of the glass. There should be no fat or mechanical particles on its surface. the water does not contain salts and is ideal for 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·Н₂О ⇄ 2ОН + 3Н₂О,
where OH is diammine silver hydroxide, obtained by dissolving the metal oxide in an aqueous ammonia solution.
Diammine silver 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
It is better to carry out the second stage of the reaction by carefully heating the flask over the burner flame - this will increase the chances that the experiment will be successful.
What can the reaction of a silver mirror show?
This interesting chemical reaction not only demonstrates certain states of matter - it can be used to perform qualitative definition aldehydes. That is, such a reaction will solve the question: whether there is an aldehyde group in the solution or not.
General structural formula of aldehydes
For example, in a similar process you can find out whether a solution contains glucose or fructose. Glucose will give positive result- you will get a “silver mirror”, but fructose contains a ketone group and it is impossible to obtain a silver precipitate. In order to carry out the analysis, instead of a formaldehyde solution, it is necessary to add a 10% glucose solution. Let's look at why and how dissolved silver turns into a solid precipitate:
2OH + 3H₂O + C₆H₁₂O₆ (glucose) = 2Ag↓+ 4NH₃∙H₂O + C₆H₁₂O₇ (gluconic acid is formed).
The silver mirror reaction is a fancy name for a chemical reaction that results in the precipitation of a thin layer of silver on the walls of the vessel where the process took place. Once upon a time, all surfaces where a mirror coating was needed were treated this way.
Now this method of obtaining a thin metal deposit on glass or ceramics is used only if it is necessary to create a conductive layer on dielectrics, as well as in the production of optics for telescopes, cameras, etc. This reaction can also be used to obtain. Such a poetic name for a simple chemical reaction is based on the excitement that arises when it comes to precious metals - gold and silver.
In order to carry out the reduction of silver from its oxide not in laboratory conditions, it is necessary to dissolve silver nitrate in water. You can get it at the pharmacy. This lapis pencil. It is better to use distilled water. You can get it by simply condensing water evaporating from a boiling kettle. If we proceed from a half-liter container, then in this amount of silver nitrate solution it is necessary to dissolve ammonia (1 tsp). Here you need to add 2-3 drops of formaldehyde - formaldehyde.
All reagents do not react immediately, so shake the solution thoroughly and leave it alone for about a day. If everything goes well, then during this period your jar will be covered with a thin metal layer. The same layer will cover the item that you place in the jar.
Sometimes something goes wrong and instead of a mirror, the reaction produces gray precipitated flakes. This suggests that the reagents were not entirely pure. Most often, complaints should be made about water and cleanliness of dishes. Particular attention should be paid to the acidity of the water, since most surprises occur in an alkaline environment.
Reaction indicator function
Using this reaction, the presence of aldehydes in a solution is determined. This group includes organic substances that have an aldehyde group. Otherwise they are called alcohols devoid of hydrogen. The presence of aldehyde in the solution gives the mirror effect.
An ammonia solution of silver oxide is used for the determination of monosaccharides and disaccharides. The first group includes glucose in all its isomeric states, the second group includes lactose and maltose. The reaction of the silver mirror is especially characteristic of glucose, which is reflected in the methods for detecting glucose and fructose.
Despite the similarity of these substances and the fact that fructose is isomeric to glucose, they are still different. In open form, the aldehyde group is present only in glucose. Accordingly, silver will precipitate only in the presence of glucose, while fructose will not give such a reaction. But in an alkaline environment, fructose can give a positive reaction.
Thus, silver oxide as a reagent can be used as an indicator of the presence of a certain group of substances in a solution. In addition, with the help of the described reaction you can get pure silver, a silver mirror and a plate coated on both sides with a metallic coating, which is not only amusing, but also often useful.
Laboratory work No. 5
Propertiescarbohydrates
Experiment 1. Reaction of a silver mirror 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 to form metallic silver:
If the reaction is carried out in a vessel with clean and smooth walls, then the 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 sediment.
The "silver mirror" reaction can be used as a qualitative reaction for aldehydes. Thus, the “silver mirror” reaction can be used as a distinctive one between glucose And fructose. Glucose is an aldose (contains an aldehyde group in an open form), and fructose is a ketose (contains a keto group in an open form). Therefore, glucose gives a “silver mirror” reaction, but 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 an ammonia solution of silver oxide. The presence of an aldehyde group in glucose can be proven using an ammonia solution of silver oxide. Add 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 compound for the discovery of aldehydes. The aldehyde group of glucose is oxidized to a carboxyl group. Glucose is converted to gluconic acid.
CH 2 HE – (SNON) 4 – SLEEP +Ag 2 O= CH 2 HE – (SNON) 4 – COOH + 2Ag↓
The order of work.
2 ml is poured into two test tubes. ammonia solution of silver oxide. Add 2 ml to one of them. 1% glucose solution, the other - fructose. Both test tubes are boiling.
An ammonia solution of silver oxide hydrate 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.
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.
Design of the work: Write the conclusion, as well as the course and equations of the reaction in a notebook.
Experiment 3. Qualitative reaction to fructose
Principle of the method. When heating a sample containing fructose in the presence resorcinol And of hydrochloric acid up to 80 o C after some time a bright red color appears in the test tube with fructose.
When heating a sample containing fructose in the presence resorcinol And of hydrochloric acid a cherry-red color appears. The sample is also applicable to detect other ketosis. Aldose 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. Used for urine analysis. 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
Cyclic formula of fructose
Acyclic form 
fructose
Painted connection
R- residues
hydroxymethylfurfural
The order of work.
2 ml are poured into two test tubes: into one - a 1% glucose solution, into the other - a 1% fructose solution. Add 2 ml of Selivanov’s reagent to both test tubes: 0.05 g of resorcinol is dissolved in 100 ml of 20% hydrochloric acid. Both test tubes are carefully heated to 80 o C (before boiling). A red color appears.
Conclusions: the results of the experiment and the reaction equation are written down in a notebook.
Let's start with the fact that each class of organic substances has a certain reaction with the help of which its representatives can be distinguished from other substances. The school chemistry course involves studying all high-quality reagents for the main classes of organic substances.
Aldehydes: structural features
Representatives of this class are derivatives of saturated hydrocarbons in which the radical is connected to an aldehyde group. Ketones are isomers of aldehydes. Their similarity lies in their belonging to the class of carbonyl compounds. When performing a task that involves isolating an aldehyde in a mixture, a “silver mirror” reaction will be required. Let us analyze the features of this chemical transformation, as well as the conditions for its implementation. The silver mirror reaction is the reduction process of silver metal from silver diamine(1) hydroxide. In a simplified form, it is possible to write this complex compound in the simplified form of silver oxide (1).
Separation of carbonyl compounds
To form a complex compound, silver oxide is dissolved in ammonia. Considering that the process is a reversible reaction, the silver mirror reaction is performed with a freshly prepared ammonia solution of silver oxide (1). When a complex compound of argentum is mixed with an aldehyde, a redox reaction occurs. The completion of the process is indicated by the precipitation of metallic silver. When the interaction of ethanal and an ammonia solution of silver oxide is carried out correctly, the formation of a silver coating is observed on the walls of the test tube. Exactly visual effect gave this interaction the name “silver mirror”.
Determination of carbohydrates
The reaction of a silver mirror is qualitative to an aldehyde group, so in organic chemistry courses it is also mentioned as a way to recognize carbohydrates such as glucose. Considering the specific structure of this substance, which exhibits the properties of an aldehyde-alcohol, thanks to the “silver mirror” reaction, it is possible to distinguish glucose from fructose. Thus, this is not only a qualitative reaction to aldehydes, but also a way to recognize many other classes of organic substances.
Practical application of the “silver mirror”
It would seem, what difficulties may arise with the interaction of aldehydes and an ammonia solution of silver oxide? You just need to purchase silver oxide, stock up on ammonia and select an aldehyde - and you can safely begin the experiment. But such a primitive approach will not lead the researcher to the desired result. Instead of the expected mirror surface on the walls of the test tube, you will see (in best case scenario) dark brown silver suspension.
The essence of interaction
A high-quality reaction to silver implies adherence to a certain algorithm of actions. Often, even when signs of a mirror layer appear, its quality clearly leaves much to be desired. What are the reasons for such a failure? Is it possible to avoid them? Among the many problems that can lead to undesirable results, there are two main ones:
- violation of the conditions for chemical interaction;
- poor surface preparation for silvering.
During the interaction of the starting substances in the solution, silver cations are formed, combining with the aldehyde group, ultimately forming colloidal small particles of silver. These grains are able to adhere to glass, but can be preserved in solution as a silver suspension. In order for the precious metal particles to adhere to the glass and a uniform and durable layer to form, it is important to pre-degrease the glass. Only if there is a perfectly clean initial surface of the test tube can one count on the formation of a uniform silver layer.
Possible problems
The main contaminant of glassware is greasy deposits, which must be removed. An alkali solution, as well as a hot chrome mixture, will help solve the problem. Next, the test tube is washed with distilled water. If there is no alkali, you can use a synthetic dishwashing detergent. After degreasing is completed, the glass is washed with a solution of tin chloride and rinsed with water. Distilled water is used to prepare solutions. If it is not available, you can use rainwater. Glucose and formaldehyde are used as reducing agents that allow the precipitation of a pure substance from a solution. With aldehyde it is difficult to count on obtaining a high-quality silver coating, but a monosaccharide (glucose) gives a uniform and durable silver layer on the surface of the mirror.
Conclusion
To silver glass, it is advisable to use silver nitrate. Alkali and ammonia solution are added to the solution of this salt. The condition for a complete reaction and deposition of silver on glass is the creation of an alkaline environment. But if there is an excess of this reagent, side effects. Depending on the chosen experimental technique, a high-quality reaction is obtained by heating. Coloring the solution in Brown color indicates the formation of tiny colloidal particles of silver. Next, a mirror coating appears on the surface of the glass. If the process is successful, the metal layer will be smooth and durable.
