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 lapis pencil. You can also buy formaldehyde and ammonia. Among other things, you need chemical glassware. The substances you will have to deal with are non-aggressive, but any chemical experiments are best carried 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.It is necessary to understand what the silver mirror reaction is? 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 a silver complex compound, metallic silver is formed as a result of an oxidation-reduction reaction. As a result of the reaction, a beautiful mirror coating of silver or a mirror will form on the walls of the glass test tube.
R-CH=O + 2OH → 2Ag ↓ + R-COONH4 + 3NH3 + H2O
In any chemistry textbook you can read 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 react with silver mirror.
How can you carry out the silver mirror reaction in practice?
At first glance, it seems that it is very easy to carry out the silver mirror reaction, but this is not entirely true. Everything seemed simple, you can take an ammonia solution with some aldehyde, it could be a solution of formaldehyde or glucose, and carry out the silver mirror reaction. However, this is a simple and primitive approach to understanding what the silver mirror reaction is? This reaction can lead to disappointment. Instead of the expected mirror coating on the glass, a black or brown suspension of silver in solution may form.Usually the reaction is like this in a simple way, in most cases ends in failure. Even if they manage to create a mirror, it will be of very low quality. The silver layer turns out to be fragile and uneven. Why is this happening? There are many reasons for such an unsuccessful reaction. Of these, two main reasons can be distinguished: failure to comply with 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 tiny or colloidal silver particles. Such tiny particles can adhere tightly to the surface of the glass or remain in solution in the form of a silver suspension.
In order for colloidal silver particles 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 should not only be perfectly clean, but also as smooth as possible.
The main contaminant of glass is grease, which must be removed. To remove fat, use an alkali solution, a hot chrome mixture, and then the glass is washed repeatedly with distilled water. If there is no alkali, you can use a regular synthetic dishwashing detergent as a last resort. After degreasing, it is useful to rinse the glass with a solution of stannous chloride and distilled water.
All solutions must be made with distilled water. If distilled water is not available, as a last resort, you can use rainwater. 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.
Reaction of a silver mirror using formaldehyde
To demonstrate the experiment, the reaction of a 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.To silver glass, it is better to take silver, which contains silver salt - silver nitrate. Solutions of ammonia and alkali are added to silver nitrate. The deposition of silver on glass must take place in an alkaline solution. In this case, there should not be too much alkali solution, since its excess is also undesirable. Depending on the technique, the silver mirror reaction is carried out at room temperature or when heated.
When the solution turns color Brown color, this means that tiny colloidal particles of silver have formed in the solution. Later, a thin mirror coating of reduced silver forms on the surface of the glass. 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, and you need to be careful and very persistent.
For experiment, you can simply silver glass glass, for the purpose of preliminary familiarization and finding out what the reaction of a silver mirror is? When demonstrating this reaction, the result may be a mirror of not very good quality.
To carry out the reaction we will need: a clean glass flask with a capacity of 50 - 100 ml, an ammonia solution in a concentration of 2.5 to 4 percent, a 2 percent solution of silver nitrate and a formaldehyde solution.
Before silvering, we prepare the flask for a chemical reaction. Let's clean the flask from mechanical impurities by wiping it with a brush and soap, and then rinse the flask with distilled water. Then we rinse it with a chrome mixture, and then rinse it again with distilled water.
Pour a 2 percent solution into one quarter of the flask, then gradually add ammonia solution to this solution. An ammonia solution is prepared at the rate of taking a 25 percent ammonia solution and diluting it with distilled water 8 to 10 times. We gradually add ammonia solution to the silver nitrate until the precipitate that falls out is completely dissolved in its excess. To the solution that has formed, gradually add a solution of formalin - 0.5 - 1 ml along the wall. Place the glass flask in a container with hot or even better boiling water. Soon it will begin to form on the flask, which forms a nice silver mirror.
The main disadvantage of this technique is that to the silver nitrate solution, you need to add not only an ammonia solution, but also an alkali (this can be sodium hydroxide - NaOH or potassium hydroxide - KOH). To obtain a high-quality mirror coating, you must first add ammonia 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 stops - (Ag2O), 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 methenamine (or hexamethylenetetramine):
6CH2O + 4NH3 = (CH2)6N4 + 6H2O
The formation of a white precipitate is a bad sign and does not contribute to obtaining a high-quality mirror.
It should be especially noted that the reaction of a silver mirror must necessarily take place in an alkaline environment, and not in an acidic one. In an acidic environment, the reaction of the silver mirror will take place without the formation of a silver coating. Sometimes it happens that when mixing reagents, the medium suddenly becomes acidic. One has only to add excess alkali to the reaction mixture, and a silver mirror immediately forms on the walls of the glass test tube.
Silver mirror reaction using glucose
The silver mirror reaction using glucose can be carried out not only to demonstrate the 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 reduction of metallic silver from an ammonia solution of silver oxide (Tollens reagent).
The silver mirror reaction is the basis for the production of silver mirrors. To prevent metallic silver (Ag) from oxidizing and forming black (Ag2S), that is, from becoming dull and scratched, it is coated with a protective varnish. In order to silver a 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 add alkali to the resulting solution - 70 ml of a 3 percent solution of sodium hydroxide (NaOH), and again pour in 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 cool the solution and dilute it with the same volume of alcohol.Solutions: Mix A and B just 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 very important and a 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, a silver precipitate is formed.
Silver mirror reaction using sucrose
The silver mirror reaction can be carried out (if there is no formaldehyde or glucose) with the participation of sucrose. Before the reaction, sucrose is hydrolyzed using sulfuric and nitric acids diluted to 10 percent concentration. An acid solution is added to an aqueous sugar solution in the ratio: 10 ml of acid per 100 grams of sugar. Boil the resulting solution for 15 – 20 minutes. Subjected to hydrolysis, sucrose becomes a mixture of glucose and fructose.Silver mirror reaction using starch
Using starch in the reaction instead of glucose leads to failure. This happens because starch is not completely converted to glucose as a result of hydrolysis. With this partial hydrolysis of starch, dextrins are formed - polysaccharides, which, like starch, consist of glucose units that have a lower molecular weight, unlike starch. Dextrins, at the end of the chains, have aldehyde groups that reduce the silver ion, but this produces a black colloidal solution of silver, instead of the expected mirror coating. Metallic silver does not settle on the glass surface, apparently because the long linear molecules of dextrins stabilize the colloidal silver solution. These molecules, in other words, perform the function of a protective colloid. In order to prevent the formation of a black colloidal solution of silver, complete hydrolysis of starch is necessary.ALDEHYDES AND KETONES
Work 29
Reaction of silver mirror with formaldehyde
Reagents: 1. Formalin.
2. Ammonia solution of silver hydroxide.
Principle of the method. The method is based on the good reducing ability of aldehydes.
Reaction scheme:
H 2 C=O + 2*OH HCOONH 4 + 3NH 3 + H 2 O + 2Ag
Progress: Add a few drops of formaldehyde to 10 drops of an ammonia solution of silver hydroxide. Warm slightly. The contents of the test tube turn brown and a shiny silver coating forms on the walls. The reaction is also considered positive when silver simply precipitates (blackening of the solution). This reaction does not occur with ketones, since the oxidation of ketones requires more stringent conditions and is accompanied by the breaking of the carbon chain.
Work 30
Autoxidation of aqueous solutions of formaldehyde
(dismutation reaction)
Reagents: 1. Formalin.
9 Methylroth indicator (methyl red).
Principle of the method. The method is based on the increased ability of formaldehyde to oxidize. In an aqueous solution of formaldehyde, a spontaneous redox reaction, or dismutation reaction (Cannizzaro reaction), occurs. One molecule of formaldehyde is oxidized by X formic acid at the expense of another molecule of aldehyde, reducing it to methyl alcohol.
Reaction scheme:
Progress: Add 1 drop of methylroth indicator to the formaldehyde solution. The solution turns red, indicating an acidic reaction. In cases where neutral formalin is required, it must be neutralized immediately before work.
Note: the dismutation reaction is usually given by aldehydes that do not have an “H” in the α-position to the carbonyl group. Formaldehyde is an exception.
Work 37
Preparation of acetone from sodium acetate
Reagents: 1. Sodium acetate (dehydrated).
2. Lugol's solution (iodine solution in KJ).
Equipment: gas outlet pipe.
Principle of the method: The production of acetone is based on the decomposition of sodium acetate when it is heated (pyrolysis).
Reaction scheme:
The detection of acetone is based on the formation of a water-insoluble acetone derivative, iodoform.
Progress: first prepare a test tube with an alkaline solution of iodine in KJ. To do this, add 2 N to a few drops of iodine solution in K.J (Lugol's solution). NaOH solution until discolored. Place a pinch (0.1 g) of salt - sodium acetate - into another dry test tube. Close it with a stopper with a gas outlet tube and carefully heat it on an alcohol lamp. First, the salt will melt, then it will begin to foam due to the formation of acetone vapor.
Dip the lower end of the tube into the prepared Lugol's alkaline solution. A yellowish-white precipitate with a characteristic odor of iodoform immediately forms. The reaction of iodoform formation is widely used in clinical practice for the discovery of acetone, which is released from the body in case of metabolic disorders, in particular in diabetes. The iodoform test for acetone is very sensitive and allows you to detect acetone in aqueous solutions with a content of ~0.04%.
Work 39
Color reaction to acetone with sodium nitroprusside
Reagents: 1. Acetone, aqueous solution.
2. Sodium nitroprusside, 0.5 N. solution.
3. Sodium hydroxide, 2 N. solution.
4. Acetic acid, 2 N. solution.
Principle of the method: The method is based on the formation of a colored compound of acetone with sodium nitroprusside. This reaction, known as the Legal test, serves as an addition to the iodoform test for acetone and is widely used in clinical practice to detect acetone in the urine of patients with diabetes.
Progress: to a few drops of 0.5 N. sodium nitroprusside solution, add 3 drops of acetone solution and 1 drop of 2 N. NaOH solution. A red color appears, which is caused by adding 1 drop of 2 N. CH 3 COOH intensifies, taking on a cherry-red hue.
CONTROL QUESTIONS
Write and name structural nomenclatures using IUPAC and rational nomenclatures
formulas of three members of the homologous series of aliphatic aldehydes and ketones.
2. Indicate the main types of reactions for aldehydes and ketones.
3. Note the similarities and differences in the properties of aldehydes and ketones.
4. Explain the mechanism of the addition reaction for acetone using any example.
5. Write the reaction for the formation of acetals. Explain.the.mechanism.
6. What products are obtained by the oxidation of 2-pentanone and pentanal?
7. What is the difference between polymerization and condensation reactions? Bring
8. What reactions can distinguish acetone from propanal?
9. Determine the structure of a substance having the empirical formula C3H 6 O, if it
gives a silver mirror reaction, turning into propanoic acid.
10. Using chemical reactions, distinguish between propanal and acrylic aldehyde.
11. Write reaction equations that allow the transition from benzophenone to
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).
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 one is used qualitative reaction to aldehydes - reaction of a copper mirror. 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.
