Aldehydes are organic substances that contain a carbonyl group >C=O bonded to at least one hydrogen atom. Aldehydes, as well as ketones similar in structure and properties, 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, the oxidation of which produces formic acid. The second representative is acetaldehyde, acetaldehyde, the oxidation of which produces acetic acid.
According to the IUPAC nomenclature, the aldehyde group is denoted by the suffix -al, which is added to the name of the corresponding hydrocarbon. Examples of aldehydes according to the IUPAC nomenclature are suggested in the image below.
If incompound has senior groups, for example, carboxyl groups, then the presence of an aldehyde group is denoted by the prefix formyl. An example of an aldehyde, which is more correctly called a dicarboxylic acid:
NEOS – CH (SNO) – CH2 – COOH
This is 2-formylbutanedioic acid.
Description of substances
Aldehydes, unlike alcohols, do not have a mobile hydrogen atom, so their molecules do not associate, which explains the much lower boiling points. For example, aldehyde formaldehyde boils already at a temperature of -21 ° C, and alcohol methanol 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 a gas, acetaldehyde is already a highly volatile liquid. An increase in the number of carbon atoms naturally raises the boiling point. So, benzaldehyde C6H5CHO boils only at +180 °C. Chain branching causes the boiling point to drop.
Lower aldehydes, such as formaldehyde, are highly soluble in water. A 40% solution of formaldehyde is called formalin and is often used to preserve biological preparations. Higher aldehydes are highly soluble in organic solvents - alcohol, ether.
Characteristic odors of aldehydes
Aldehydes have characteristic odors, and the lower ones are sharp, unpleasant. Everyone knows the unpleasant smell of formalin - an aqueous solution of formaldehyde. Higher aldehydes have floral odors and are used inperfumery.
Examples of aldehydes - substances with a pleasant smell - are vanillin, which has the aroma of vanilla, and benzaldehyde, which gives the characteristic aroma of almonds. Both substances are obtained synthetically and are widely used as flavorings in the confectionery and perfume industries.
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Let's consider ways to obtain aldehydes.
Oxidation of alcohols
Aldehydes are produced by the oxidation of primary alcohols. For example, formaldehyde, which is used in the production of polymeric materials, drugs, dyes, explosives. In industry, formaldehyde is obtained by oxidation of methanol with oxygen: 2O.
The reaction is carried out on a hot silver grid, silver is a catalyst. Methanol vapor mixed with air is passed through the grid. The reaction proceeds with the release of a large amount of heat, which is enough to keep 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-CH2-OH=R-CHO + H2..
Recovery of acid chlorides
Aldehydes can be obtained by hydrogen reduction of acid chlorides. "Poisoned" palladium is used as a catalyst - with reduced activity: RCClO + H2=RCHO + HCl.
Production of acetaldehyde
Acetaldehyde is produced industrially by the oxidation of ethylene with oxygen orair in the liquid phase. Palladium chloride is required as a catalyst (PdCl2): 2 CH2=CH2 + O 2=2 CH3 CHO.
Chemical properties
The following types of reactions are typical for aldehydes:
- carbonyl additions;
- polymerization;
- condensation;
- reduction and oxidation.
Most of the reactions proceed by the mechanism of nucleophilic addition to the C=O bond.
Usually consider the chemical properties of aldehydes using the example of acetaldehyde.
Addition reactions
In the carbonyl group C=O, the electron density is shifted to the oxygen atom, so a partial positive charge is formed on the carbonyl carbon atom, which determines the chemical activity of aldehydes. The 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 the carbonyl carbon atom, attach to it and cause the C=O bond to break.
Condensation reactions
Aldehydes undergo aldol and croton condensation reactions.
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).
Quality reactions
To detect aldehydes, two qualitative reactions can be used:
Reaction of the "silver mirror". The reaction proceeds with Tollens' reagent - an ammonia solution of silver oxide. When mixing a solution of ammonia and a solution of silver nitrate, first a solution of silver hydroxide is formed, and when an excess of ammonia is added, a solution of diammine silver (I) hydroxide, which is the oxidizing agent. When interacting with aldehyde, elemental silver is released in the form of a black precipitate. If the reaction is carried out with slow heating, without shaking the tube, the silver will cover the walls of the tube, creating a "mirror" effect
