You need to be careful when working with silver salts, as they leave black marks. Carry out the experiment wearing gloves.
How does the reaction occur?
?Reagents for the experiment can be found at any pharmacy. Silver nitrate is a lapis pencil. You can also buy formaldehyde and ammonia there. Among other things, you need chemical glassware. The substances you will have to deal with are non-aggressive, but any chemical experiments It is better to carry out in test tubes and flasks made of chemical glass. Of course, the dishes need to be washed thoroughly. Make an aqueous solution of silver nitrate AgNO3. Add ammonia to it, that is, ammonium hydroxide NH4OH. You form silver oxide Ag2O, which precipitates as a brown precipitate. The solution then becomes clear and an OH complex is formed. It is he who acts on the aldehyde during the redox reaction, which results in the formation of an ammonium salt. The formula for this reaction looks like this: R-CH=O + 2OH --> RCOONH4 + 2Ag +3NH3 + H2O. If you leave a glass rod or plate in the jar during the reaction, after about a day it will become covered with a shiny layer. The same layer is formed on the walls of the vessel.
The reaction can be written in a simplified way: R-CH=O + Ag2O --> R-COOH + 2Ag.
How mirrors were made
Before the advent of the sputtering method, the silver mirror reaction was the only way to produce mirrors on glass and porcelain. Currently, this method is used to obtain a conductive layer on glass, ceramics and other dielectrics. This technology is used to create coated optics for photographic lenses, telescopes, etc.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 the resulting ones 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 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.
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. It was the visual effect that gave this interaction the name “silver mirror”.
Determination of carbohydrates
The reaction of a silver mirror is qualitative to an aldehyde group, therefore 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 (at best) a 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 an undesirable result, 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 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.
Silver oxide dissolves to form a complex compound - diammine silver(I) hydroxide OH
when an aldehyde is added to which an oxidation-reduction 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 to distinguish 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.
Literature
- Nekrasov B.V. Fundamentals of general chemistry. - 3rd ed., rev. and additional - M.: “Chemistry”, 1973. - T. 2. - 688 p.
- Nesmeyanov A. N., Nesmeyanov N. A. The beginnings of organic chemistry. In 2 volumes. - 2nd ed., trans. - M.: “Chemistry”, 1974. - T. 1. - 624 p.
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- Jarisch-Herxheimer reaction
- Real (Castelo di Paiva)
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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 - diammine silver(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 into 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 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 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. 2 ml of Selivanov’s reagent are added 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.
