Aldehydes are organic substances that contain a carbonyl group> C = O, bound to at least one hydrogen atom. Aldehydes, as well as ketones similar in structure and properties to them, are called carbonyl or oxo compounds. Examples of aldehydes are formic, acetic, propionic aldehyde.
Nomenclature
The trivial names of aldehydes are formed from the trivial names of related carboxylic acids. Examples of aldehydes with names are shown in the figure. The first representative of the homologous series of aldehydes is formic aldehyde, or formaldehyde, during the oxidation of which formic acid is formed. The second representative is acetic aldehyde, acetaldehyde, during the oxidation of which acetic acid is formed.
According to the IUPAC nomenclature, the aldehyde group is designated with the suffix -al, which is added to the name of the corresponding hydrocarbon. Examples of aldehydes according to the IUPAC nomenclature are proposed in the image below.
If there are senior groups in the compound, for example, carboxyl, then the presence of the aldehyde group is indicated by the formyl prefix. An example of an aldehyde, which is more correctly called dicarboxylic acid :
- NOOS - CH (CHO) - CH 2 - COOH
This is 2-formylbutanedioic acid.
Description of substances
Aldehydes, unlike alcohols, do not have a mobile hydrogen atom, therefore their molecules are not associated, which explains the much lower boiling points. For example, formaldehyde aldehyde boils already at a temperature of -21 ° C, and methanol alcohol - at +65 ° C.
However, only formaldehyde has such a low boiling point; the next representative, acetaldehyde, boils at + 21 ° C. Therefore, at room temperature, of all aldehydes, only formaldehyde is gas, acetaldehyde is already a volatile liquid. An increase in the number of carbon atoms naturally increases the boiling point. So, benzaldehyde 6 5 boils only at +180 ° . Chain branching causes a lowering of the boiling point.
Lower aldehydes, for example formaldehyde, are highly soluble in water. A 40% formaldehyde solution is called formalin; it is often used to preserve biological preparations. Higher aldehydes are readily soluble in organic solvents - alcohol, ether.
Characteristic odors of aldehydes
Aldehydes have characteristic odors, and lower ones are sharp and unpleasant. Everyone knows the unpleasant smell of formalin - an aqueous solution of formaldehyde. Higher aldehydes have floral odors, they are used in perfumes.
An example of aldehydes - substances with a pleasant smell - is vanillin, which has a vanilla aroma, and benzaldehyde, which gives a characteristic aroma to almonds. Both substances are synthetically prepared and are widely used as flavorings in the confectionery industry and perfumery.
Getting
Consider methods for producing aldehydes.
Aldehydes are obtained by the oxidation of primary alcohols. For example, formaldehyde, which is used in the manufacture of polymeric materials, drugs, dyes, explosives. In industry, formaldehyde is obtained by oxygen oxidation of methanol: 2CH 3 OH + O 2 = 2CH 2 O + 2H 2 O.
The reaction is carried out on a hot silver grid, silver is a catalyst. Methanol vapors mixed with air are passed through the net. The reaction proceeds with the release of a large amount of heat, which is enough to maintain the grid in a hot state.
- Dehydrogenation of alcohols.
Aldehydes can be obtained from alcohols in the absence of oxygen. In this case, a copper catalyst and high temperatures (250 ° C.) are used: R — CH 2 —OH = R — CHO + H 2 .
- Recovery of acid chlorides.
Aldehydes can be prepared by hydrogen reduction of acid chlorides. As a catalyst, “poisoned” palladium is used, with reduced activity: RCClO + H 2 = RCHO + HCl.
Acetaldehyde is industrially produced by oxidizing ethylene with oxygen or air in a liquid phase. Palladium chloride (PdCl 2 ) is required as a catalyst : 2 CH 2 = CH 2 + O 2 = 2 CH 3 CHO.
Chemical properties
The following types of reactions are characteristic of aldehydes:
- carbonyl group additions;
- polymerization;
- condensation;
- reduction and oxidation.
Most reactions follow the nucleophilic attachment mechanism via the C = O bond.
Usually consider the chemical properties of aldehydes on the example of acetic aldehyde.
Addition reactions
In the carbonyl group C = O, the electron density is shifted to the oxygen atom; therefore, a partial positive charge is formed on the carbonyl carbon atom, which determines the chemical activity of aldehydes. A positive charge on the carbon atom of the C = O group ensures its activity in reactions with nucleophilic reagents - water, alcohol, magnesium, organic compounds. The oxygen atom of water can attack a carbonyl carbon atom, join it and cause a C = O bond to break.
Condensation Reactions
Aldehydes enter into reactions of aldol and croton condensation.
Acetaldehyde under the action of a weak solution of alkali in the cold turns into an aldol. The reaction product is a liquid miscible with water under reduced pressure. This substance contains both an aldehyde and an alcohol group (hence the name).
Qualitative reactions
Two qualitative reactions can be used to detect aldehydes:
- The reaction of the silver mirror. The reaction proceeds with a Tollens reagent - an ammonia solution of silver oxide. When mixing a solution of ammonia and a solution of silver nitrate, a silver hydroxide solution is first formed, and when an excess of ammonia is added, a solution of diamminsilver (I) hydroxide, which is an oxidizing agent. When interacting with an aldehyde, elemental silver is released as a black precipitate. If the reaction is carried out with slow heating, without shaking the tube, silver will cover the walls of the tube, creating the effect of a “mirror”.
- The reaction of the copper mirror. Another reagent that opens the aldehyde group is copper (II) hydroxide. When interacting with an aldehyde, it is reduced to copper oxide (I). The color changes from blue to orange first, then to yellow. If the reaction is carried out with slow heating, the oxide will form a thin orange-red coating on the walls of the tube - a "copper mirror": CH 3 CHO + 2 Cu (OH) 2 + NaOH = CH 3 COONa + Cu 2 O ↓ + 3H 2 O.