Chemical properties of aldehydes: reaction of a silver mirror

Aldehydes are functional derivatives of hydrocarbons, in the structure of which there is a group CO (carbonyl group). For simple aldehydes, trivial (historical) names, derived from the name of carboxylic acids, into which aldehydes are converted during oxidation, are traditionally preserved. If we talk about the nomenclature of IUPAC, then the longest chain containing an aldehyde group is taken as a basis. The beginning of the numbering of the hydrocarbon chain is derived from the carbon atom of the carbonyl group (CO), which itself receives the number 1. The term "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 compounds represented is due to the isomerism of the hydrocarbon skeleton.

Aldehydes are obtained in several ways: oxo synthesis, hydration of alkynes, oxidation and dehydrogenation of alcohols. The preparation of aldehydes from primary alcohols requires special conditions, since the resulting organic compounds are readily 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 the preparation of 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 takes place 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 substitution of halogen atoms for OH groups, the so-called hem-diol is intermediately formed, which is unstable and turns into a carboxyl compound with H20 cleavage.

The chemical property of aldehydes is a qualitative reaction to silver. During oxidation, aldehydes are converted to carboxylic acids (for example, C5H11COOH + O-C5H11COOH). In any specialized textbook one can find information that the reaction of a silver mirror is used to identify aldehydes. This group of organic substances can be oxidized not only by the action of special oxidizing agents, but also simply when stored under the influence of atmospheric oxygen. The ease with which aldehydes are oxidized to carboxylic acids has allowed the development of qualitative reactions (silver mirror reaction) on these organic compounds, which makes it possible to quickly and clearly determine the presence of aldehyde in a particular solution.

When heated with an ammoniacal solution of silver oxide, the aldehyde is oxidized to an acid. In this case, silver is reduced to metal and deposited on the walls of the 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 is a huge amount of substances that do not belong to aldehydes, but they too are able to enter into this reaction. To identify these compounds, another qualitative reaction to aldehydes is used - the reaction of a copper mirror. In the interaction of aldehydes with Fehling's reagent, which has a blue color (an aqueous solution of copper hydroxide, alkali and tartrate acid salts), copper from bivalent is reduced to a monovalent. In this case, a red-brown precipitate of copper oxide precipitates .

So, how does the silver mirror react? It would seem that nothing is easier: it is enough to heat an ammonia solution of silver with some aldehyde (for example, glucose solution or formaldehyde) in the dishes, but this approach is not always crowned with victory. Sometimes we observe the formation of a black suspension of silver in solution, rather than a mirror coating on the walls of glassware. What is the main reason for failure? To obtain 100% of the result, it is necessary to adhere to the reaction conditions, as well as carefully prepare the surface of the glass.