Aldehydes are called organic compounds, in which the carbonyl group (C-O) is bonded to hydrogen and radical R (residues of aliphatic, aromatic and heterocyclic compounds):
The polarity of the carbonyl group ensures the polarity of the molecule as a whole, so aldehydes have higher boiling points than nonpolar compounds of comparable molecular weight.
Since the hydrogen atoms in aldehydes are bonded only to the carbon atom (close relative electronegativity), no intermolecular hydrogen bonds are formed. Therefore, the boiling points of aldehydes are lower than those of the corresponding alcohols or carboxylic acids. As an example, we can compare the boiling points of methanol (T^ 65 °C), formic acid (Gbp 101 °C) and formaldehyde (7^, -21 °C).
Lower aldehydes are soluble in water, probably due to the formation hydrogen bonds between solute and solvent molecules. Higher aldehydes are highly soluble in most common organic solvents (alcohols, ethers). Lower aldehydes have a pungent odor, aldehydes with C3-C6 have a very unpleasant odor, while higher aldehydes have floral odors and are used in perfumery.
Chemically, aldehydes are very reactive compounds. The most typical reactions for aldehydes are nucleophilic addition reactions, which is due to the presence in the molecule of an electrophilic center - the carbonyl carbon atom of the C=0 group.
Many of these reactions, such as the formation of oximes, semicarbazones and other compounds, are used in qualitative and quantitative analysis Drugs from the aldehyde group because the addition products of aldehydes are characterized by a melting point specific for each aldehyde. Thus, aldehydes, when shaken with a saturated solution of sodium hydrogen sulfite, easily enter into an addition reaction:
The addition products are salts that have a certain melting point and are highly soluble in water, but insoluble in organic solvents.
When heated with dilute acids, hydrosulfite derivatives hydrolyze to their parent compounds.
The ability of aldehydes to form hydrosulfite derivatives is used both to determine the authenticity of a drug with an aldehyde group in the molecule, and to purify aldehydes and isolate them from mixtures with other substances that do not react with sodium hydrosulfite.
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Aldehydes also readily add ammonia and other nitrogen-containing nucleophiles. The addition products are usually unstable and easily undergo dehydration and polymerization. The cyclic compounds formed as a result of polymerization, when heated with dilute acids, easily decompose, again releasing the aldehyde:
| r-ch-nh2 | g z | -NH R-CC |
| -зн2о " | ||
| He |
Aldehydes are easily oxidized. Silver(I) oxide and other oxidizing agents with a low oxidation potential are capable of oxidizing aldehydes. For example, aldehydes are characterized by the formation reaction silver mirror, which flows with an ammonia solution of AgN03:
AgN03 + 3NH3 - OH + NH4N03
Tollens reagent
In this case, a mirror coating of metallic silver is formed on the walls of the test tube:
2OH + RCOH 2Agi + RCOOH + 4NH3T + H20
Likewise, aldehydes can reduce copper(II) to copper(I). To carry out the reaction, Fehling's reagent (an alkaline solution of copper(II) tartrate complex) is added to the aldehyde solution and heated. First, a yellow precipitate of copper(1) hydroxide, CuOH, is formed, and then a red precipitate of copper(1) oxide, Cu20:
2KNa + RCOH + 3NaOH + 2KOH -
2CuOHi + RCOONa + 4KNaC4H406 + 2H20 2CuOH - Cu20 + H20
The redox reaction also includes the reaction of aldehydes with Nessler's reagent in an alkaline medium; in this case, a dark precipitate of reduced mercury precipitates:
K2 + RCOH + ZKON - RCOOK + 4KI + Hgl + 2Н20
It should be borne in mind that the reaction with Nessler's reagent is more sensitive, so it is used to detect aldehyde impurities in drugs. Authenticity medicines containing an aldehyde group are confirmed by less sensitive reactions: a silver mirror or with Fehling’s reagent. Some other compounds, such as polyphenols, are also oxidized by Ag(I) and Cu(P) compounds, i.e. the reaction is not specific.
Formaldehyde and acetaldehyde are prone to polymerization. Formaldehyde polymerizes to form cyclic trimers, tetramers or linear polymers. The polymerization reaction occurs as a result of the nucleophilic attack of the oxygen of one molecule of the carbonyl carbon atom of another:
Thus, from a 40% aqueous solution of formaldehyde (formalin) a linear polymer is formed - paraform (u = 8 - 12), trimer and tetramer.
Aldehydes are characterized by narcotic and disinfectant properties. Compared to alcohols, the aldehyde group increases the toxicity of the substance. The introduction of a halogen into the aldehyde molecule increases its narcotic properties. For example, the narcotic properties of chloral are more pronounced than those of acetaldehyde:
s!3s-ss
Receipt. Aldehydes can be produced by oxidation primary alcohols chromic acid (Na2Cr04, H2S04) when boiling or potassium permanganate in an alkaline medium:
Dehydrogenation of primary alcohols is carried out over a copper catalyst (Cu, Cr2O3) at 300-400 °C.
The industrial production of methanal is based on the vapor-phase oxidation of methanol with an iron-molybdenum catalyst:
2CH3OH + 02 500 ~600 2CH2=0 + H20
Formaldehyde solution (formalin)
Receipt. Formalin is an aqueous solution of formaldehyde (40%) stabilized with methanol (6-10%). The European Pharmacopoeia contains the FS “Formaldehyde solution (35%)” (see Table 9.1). In laboratory conditions, formaldehyde can be obtained by dehydrogenation of methanol over copper or depolymerization of paraform.
Determination of authenticity. Pharmacopoeial method - silver mirror reaction.
Since formaldehyde easily enters into condensation reactions, for example, with hydroxyl-containing aromatic compounds with the formation of colored compounds, the State Fund also recommends using a reaction with salicylic acid for its identification, as a result of which a red color appears:
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The reaction with chromotropic acid proceeds similarly with the formation of blue-violet and red-violet products (EP).
To determine the identity of pharmaceuticaldehyde, reactions with nitrogen-containing nucleophiles, such as primary amines, can be used:
H-Ctf° + H2N-R - n-s^^K + H20
The resulting N-substituted imines (Schiff bases) are slightly soluble, some of them are colored, others give colored compounds with heavy metal ions. EF suggests a reaction with phenylhydrazine. In the presence of potassium ferricyanide in an acidic environment, intensely red reaction products are formed.
Purity tests. Control of formic acid impurities is carried out by determining acidity. According to the Global Fund, the concentration of formic acid in the preparation should not exceed 0.2%; The content of formic acid is determined by the neutralization method (NF). According to the EF, methanol is determined by gas chromatography (9-15% vol.). Sulfated ash - no more than 0.1% in a 1.0 g sample.
I2 + 2NaOH - Nal + NaOI + H20
Hypoiodite oxidizes formaldehyde to formic acid. When the solution is acidified with excess sulfuric acid, unreacted hypoiodite is converted into iodine, which is titrated with sodium thiosulfate:
НСО + NaOI + NaOH - HCOONa + Nal + H20 NaOI + Nal + H2S04 -*■ I2 + Na2S04 + H20 I2 + 2Na2S203 - Na2S406 + 2NaI
It is possible to use other titrating agents in the determination of formaldehyde: hydrogen peroxide in an alkaline solution, cerium (IV) sulfate, sodium sulfite.
The drug can be considered as a prodrug, since the physiological effect is not exerted by hexamethylenetetramine itself, but by formaldehyde, which is released when the drug decomposes in an acidic environment. This is precisely why it is included in this section (see Table 9.1).
Receipt. Hexamine (tetraazaadamantane) is obtained by condensation of methanal and ammonia from aqueous solutions. The reaction intermediate is hexahydro-1,3,5-triazine:
| ll Hexahydro-Hurotropine 1,3,5-trnazine |
Determination of authenticity. When heating a mixture of the drug with diluted sulfuric acid, an ammonium salt is formed, from which ammonia is released when an excess of alkali is added:
(CH2)6N4 + 2H2S04 + 6H20 - 6HSON + 2(NH4)2S04 (NH4)2S04 + 2NaOH - 2NH3t + Na2S04 + 2H20
Hexamethylenetetramine can also be detected by the red coloration of the solution when salicylic acid is added after preheating with sulfuric acid (see identification of formaldehyde).
Purity tests. The presence of impurities of organic compounds, paraform, and ammonium salts is not allowed in the preparation. The State Fund specifies the permissible limits for the content of impurities of chlorides, sulfates, and heavy metals.
Quantification. For the quantitative determination of hexamethylenetetramine, the GF suggests using the neutralization method. To do this, a sample of the drug is heated with an excess of 0.1 M sulfuric acid solution. Excess acid is titrated with an alkali solution with a concentration of 0.1 mol/l (methyl red indicator).
The iodometric method of quantitative determination is based on the ability of hexamethylenetetramine to give tetraiodides with iodine.
Aldehydes and ketones are characterized by the presence of a carbonyl group in the molecule. In aldehydes, the carbonyl group is bonded to one hydrogen atom and one hydrocarbon radical. All aldehydes contain a group
called an aldehyde group.
General formula of aldehydes:
An aldehyde molecule contains two fewer hydrogen atoms than the corresponding alcohol molecule
i.e., an aldehyde is a dehydrogenated (oxidized) alcohol. This is where the name “aldehyde” comes from - from the combination of two abbreviated Latin words alcohol dehydrogenatus (dehydrogenated alcohol).
Saturated aldehydes and ketones have the same total formula
Nomenclature and isomerism. The names of aldehydes come from the names of the saturated acids into which they turn during oxidation. This is explained by the fact that many acids were discovered and named earlier than their corresponding aldehydes.
The names and formulas of some of the simplest aldehydes are given below:
To compile the names of aldehydes according to the Geneva nomenclature, the ending al is added to the name of the hydrocarbon with the same number of carbon atoms. IN difficult cases The position of the aldehyde group is indicated by a number that is placed after this ending:
The isomerism of aldehydes is due to the isomerism of the chain of carbon atoms of the hydrocarbon radical:
According to rational nomenclature, the names of ketones are derived from the names of the radicals included in their molecule, with the addition of the ending ketone, for example:
Some ketones have historical names, for example dimethyl ketone is called acetone.
According to the Geneva nomenclature, ketones are named by adding the ending he to the name of the corresponding hydrocarbon. In the case of a branched ketone chain, the numbering of carbon atoms begins from the end to which the branch is closest (according to the rules for numbering hydrocarbons). Place
occupied by a carbonyl group is indicated in the name by a diphro after the end, for example:
Physical properties. The first member of the homologous series of aldehydes is formic aldehyde - a gas; average representatives of liquid; higher aldehydes are solids. Lower aldehydes have a pungent odor and mix well with water. Medium aldehydes are moderately soluble in water; higher aldehydes are insoluble. All aldehydes are highly soluble in alcohol and ether.
Lower ketones are liquids with a characteristic odor that easily mix with water. Higher ketones are solids. All ketones are highly soluble in alcohol and ether.
Chemical reactions of aldehydes and ketones. Aldehydes and ketones are extremely reactive organic substances. Many of their reactions occur without heat or pressure. Particularly characteristic of aldehydes and ketones are reactions that occur with the participation of a carbonyl group. There are, however, some differences in the reactions of aldehydes and ketones. In general, aldehydes are more reactive than ketones.
Addition reactions: A number of different substances can add to the carbonyl group of aldehydes and ketones. In this case, one of the bonds connecting the oxygen and carbon atoms in the carbonyl group is broken, and parts of the reactant are added to the resulting free valences. If the joining substance contains hydrogen, then the latter is always directed towards carbonyl oxygen; the carbonyl group is converted into a hydroxyl group:
From an electronic point of view, this "reactive feature of carbonyl oxygen in aldehydes and ketones is explained by the fact that electronic clouds, forming a bond between the carbon and oxygen atoms in the carbonyl group, are shifted towards the oxygen atom, since the latter attracts electrons more strongly than the carbon atom. As a result, the double bond becomes highly polarized:
Towards a polarized double bond various substances join in a certain direction. Let's consider some addition reactions characteristic of aldehydes and ketones.
Addition of hydrocyanic acid The bond in the hydrocyanic acid molecule is also polarized, and therefore hydrogen, which has some positive charge, is added to the oxygen atom, and the group to the carbon atom:
The resulting compounds are called cyangiorins (or oxynitriles) and are compounds with mixed functions (containing both hydroxyl and cyano groups). Oxynitriles serve as starting materials for the synthesis of various organic compounds.
Addition of sodium bisulfite (acidic sodium sulfide
Resulting compounds (bisulfite compounds) - crystalline substances. They are used in laboratory practice to isolate aldehydes and ketones in a pure state from their mixtures with other substances, since they easily decompose when
boiling with soda or dilute acids to form the initial aldehydes and ketones.
The addition of organometallic compounds to the carbonyl group of aldehydes and ketones is discussed on page 165.
The reduction of aldehydes and ketones can be considered as the reaction of the addition of a hydrogen molecule to the carbonyl group. When aldehydes are reduced, primary alcohols are formed, and when ketones are reduced, secondary alcohols are formed:
Substitution reactions in the series of aldehydes and ketones lead to the replacement of the oxygen of the carbonyl group with other atoms or radicals.
Action of phosphorus pentahalide. When exposed to, for example, phosphorus pentachloride, the carbonyl oxygen is replaced by two chlorine atoms and a dihalogenated hydrocarbon is formed:
These dihalide compounds, when reacting with water, are capable of producing the original aldehydes and ketones again.
Action of hydroxylamine. When hydroxylamine acts on aldehydes and ketones, aldoximes and ketoximes are formed, respectively (hydroxylamine can be considered as ammonia in which one hydrogen atom is replaced by hydroxyl):
The resulting oximes in most cases are crystalline substances and serve to open and isolate aldehydes and ketones in their pure form.
Oxidation reactions. Aldehydes are easily oxidized by various oxidizing agents, turning into carboxylic acids:
For example, aldehydes easily remove oxygen from the oxides of some metals. The so-called silver mirror reaction is based on this property. It lies in the fact that when the aldehyde is heated with an ammonia solution of silver oxide, the aldehyde is oxidized into an acid and the silver oxide is reduced to metallic silver:
Metallic silver settles on the walls of the vessel and forms a shiny mirror surface.
Ketones are much more difficult to oxidize. Only with very vigorous oxidation does their rupture occur carbon chain two acids are formed, for example:
Reactions involving a hydrogen atom in the -position relative to the carbonyl group.
Action of halogens. The carbonyl group in waldehydes and ketones greatly affects the mobility of hydrogen atoms located at carbon, standing nearby with the carbonyl group -position). For example, when aldehydes or ketones are exposed to bromine or chlorine, they easily replace hydrogen atoms in the - position:
Halogen atoms that have entered the -position to the carbonyl group of aldehydes or ketones also have very high reactivity.
Condensation reactions. Condensation reactions are densification reactions in which new carbon-carbon bonds are formed. Condensation reactions can occur without the release of simple molecules (water, ammonia, hydrogen chloride, etc.) or with their release.
Aldehydes easily undergo condensation reactions. So, for example, a molecule of acetaldehyde, under the influence of small amounts of dilute alkali in the cold, condenses with another molecule of the same aldehyde:
The resulting compound, containing aldehyde and alcohol groups, is called aldol (short for aldehyde alcohol), and the above reaction is called aldol condensation. As can be seen from the reaction equation, aldol condensation occurs due to the mobile hydrogen atom in the -position to the carbonyl group.
Under slightly different conditions, condensation can occur with the formation of a new carbon-carbon double bond:
The resulting compound is called crotonaldehyde, and the reaction is called croton condensation.
Ketongs are also capable of condensation reactions, which are somewhat more complex than for aldehydes.
Characteristic reactions of aldehydes. For aldehydes, as compounds more reactive than ketones, the following reactions are also characteristic:
Formation of esters. If a small amount of aluminum alkoxide is added to an aldehyde, a vigorous reaction occurs, in which the oxidation of one aldehyde molecule occurs due to the reduction of another aldehyde molecule, and ester:
This reaction is called the Tishchenko reaction, after the Russian scientist who discovered it.
Formation of acetals. When aldehydes are heated with alcohols in the presence of small amounts of mineral acids, the following reaction occurs:
The resulting compound is called an acetal and is like an ether of an unstable dihydric alcohol:
The reaction of acetal formation is reversible. When hydrolyzed in the presence of acids, acetals easily decompose to form the parent aldehydes and alcohols. 4
Polymerization. Aldehydes can form linear or cyclic polymers, and in both cases, the residues of aldehyde molecules are linked to each other through an atom
Mineral acids are used as substances that accelerate the process of polymerization of aldehydes. When heated, cyclic polymers split into molecules of the original aldehydes.
Methods of obtaining. Oxidation of alcohols. As we already know, the oxidation of primary alcohols produces aldehydes, and the oxidation of secondary alcohols produces ketones. Oxidation can be carried out using various oxidizing agents, for example, potassium dichromate in an acidic environment or air oxygen in the presence of catalysts - platinum, copper, etc. In both cases, reactions proceed according to the following scheme:
Preparation from dihalogenated hydrocarbons. If both halogen atoms are located at the same carbon atom, then when such halogen derivatives are heated with water or better with alkali, the formation of aldehydes or ketones occurs:
The effect of water on acetylene hydrocarbons (Kucherov reaction). When water acts on acetylene in the presence of divalent mercury salts, acetaldehyde is obtained:
Homologues of acetylene under these conditions form ketones:
Oxosynthesis. Oxosynthesis is a method of producing oxygen-containing organic compounds by reacting unsaturated hydrocarbons with carbon monoxide and hydrogen at elevated temperature, in the presence of a cobalt catalyst and at pressure. This process produces aldehydes containing one more carbon atom than the original olefin:
Formic aldehyde (formaldehyde) Colorless gas with a sharp, specific odor; soluble in water. An aqueous solution of formaldehyde containing formaldehyde in solution is called formalin. When the solution is evaporated, formaldehyde polymerizes to form a solid mixture of low molecular weight polyoxymethylenes (paraformaldehyde), which again gives formaldehyde under the action of acids.
Formaldehyde is the first member of the homologous series of aldehydes. In the general formula
formaldehyde has a hydrogen atom instead of an alkyl radical. Therefore some chemical properties formaldehyde differ sharply from the properties of other aldehydes of this series. Thus, for example, under the action of alkalis, formaldehyde, unlike other fatty aldehydes that are resinous by alkalis, forms methyl alcohol and a salt of formic acid;
In this reaction, one molecule of formaldehyde is reduced to alcohol, and the other is oxidized to acid.
Formaldehyde is used in huge quantities for the production of phenol-formaldehyde, urea and other synthetic polymers. The high molecular weight polymer of formaldehyde, polyformaldehyde, has exceptionally valuable properties (p. 327).
A significant amount of formaldehyde is used to prepare isoprene (2-methylbutadiene-1,3) - the starting material for synthetic rubber.
The process of producing isoprene from formaldehyde and isobutylene proceeds in two stages according to the following scheme:
The second stage of the process takes place at 200-220 °C in the presence of phosphoric acid derivatives as a catalyst.
Formaldehyde serves as a starting material for the production of dyes, pharmaceuticals, synthetic rubber, explosives and many other organic compounds. Formaldehyde is toxic and even in small concentrations is irritating to mucous membranes.
Formalin (an aqueous solution of formaldehyde) is quite widely used as an antiseptic (disinfectant). It is interesting that the preservative effect of smoke when smoking food (fish, meat) is explained by the strong antiseptic effect of formaldehyde, which is formed as a result of incomplete combustion of fuel and is contained in small quantities in smoke.
An industrial method for producing formaldehyde is the catalytic oxidation of methanol. Methanol is oxidized in the gas phase with atmospheric oxygen at 500-600 °C:
Metallic copper or silver deposited on an inert porous support or in the form of a metal mesh are used as catalysts. (Recently they began to use more effective iron oxide molybdenum
catalyst.) To lower the process temperature, which favors the oxidation reaction and increase the yield of formaldehyde, 10-12% water is added to methanol.
In Fig. Figure 15 shows a schematic diagram of the production of formaldehyde by oxidation of Methanol.
Evaporator 2 receives methanol from meter 1 and purified air through blower 4. In the evaporator, liquid methanol evaporates and mixes with air, resulting in the formation of a steam-air mixture containing methanol in the mixture. The steam-air mixture heated to 100 °C enters the contact apparatus 6, in which the oxidation of methanol occurs at
Rice. 15. Scheme of formaldehyde production by methanol oxidation: 1 - measuring tank; 2 - evaporator; 3 - filter; 4 - blower; 5 - heater; 6 - contact device; 7 - refrigerator; 8, 10 - absorbers; 9 - intermediate refrigerator.
The reaction products are sent to refrigerator 7, where they are cooled to 100-130 °C. Then they enter absorbers 8 and 10, in which the formed formaldehyde is absorbed. Absorber 8 is irrigated with a dilute solution of formaldehyde coming from absorber 10, irrigated with water. Thus, the resulting formaldehyde leaves the absorber in the form of an aqueous solution containing 37.6% formaldehyde and about 10% methanol. Formaldehyde yield is about 80%. The gases leaving the absorber 10 contain nitrogen (about 70%), hydrogen (about 20%) and small amounts of methane, oxygen, carbon monoxide and carbon dioxide.
Recently, a method for the synthesis of formaldehyde by incomplete oxidation of concentrated methane with atmospheric oxygen has gained industrial application:
The catalyst is nitrogen oxides. (Oxidation is carried out at a temperature of about 600 °C.
Acetaldehyde (acetaldehyde) CH3-CHO. Colorless liquid with a pungent odor, highly soluble in water; pace. kip. +21°С. Under the influence of acids, it easily polymerizes into cyclic polymers - paraldehyde (liquid) and metaldehyde (solid).
Acetaldehyde is the most important starting compound for the production of acetic acid, synthetic polymers, medicinal compounds and many other substances.
The following methods for producing acetaldehyde are most widely used in industry:
1. Direct hydration of acetylene with water vapor in the presence of liquid mercury catalysts (according to the Kucherov reaction).
3. Direct oxidation of ethylene with atmospheric oxygen in the presence of liquid palladium catalysts.
Hydration of acetylene in the presence of mercury catalysts is carried out by passing acetylene mixed with water vapor at 90-100°C into a hydrator filled with a catalyst, the so-called “contact” acid (a solution of mercury sulfate in sulfuric acid). Metallic mercury also enters the hydrator continuously or periodically, forming mercury sulfate with sulfuric acid. A mixture of acetylene and water vapor bubbles through the acid layer; In this case, acetylene hydrates and acetaldehyde is formed. The vapor-gas mixture leaving the hydrator is condensed and the separated acetaldehyde is separated from impurities. The yield of acetaldehyde (counting acetylene) reaches 95%.
When hydrating acetylene in the presence of non-mercury catalysts, acetylene is diluted with nitrogen, mixed with water vapor, and the resulting vapor-gas mixture is passed at high temperature over a non-mercury catalyst, for example oxides of zinc, cobalt, chromium or other metals. The duration of contact of the vapor-gas mixture with the catalyst is a fraction of a second, as a result of which there are no side reactions, which leads to an increase in the yield of acetaldehyde and a more pure product.
A very promising industrial method for producing acetaldehyde is the direct oxidation of ethylene with atmospheric oxygen in the presence of liquid palladium catalysts:
The reaction proceeds over much more complex scheme than is shown above, and a number of by-products are formed. The process is carried out in tubular reactors at a temperature of about 120 °C and pressure.
Acetone (dimethyl ketone) Colorless liquid with a characteristic odor, highly soluble in water, temp. kip. 56.1 °C.
Acetone is an excellent solvent for many organic matter, and therefore is widely used in various industries (production of artificial fiber, medicines, etc.). Acetone is also used for the synthesis of various organic compounds.
A. E. Favorsky obtained isoprene from acetone and acetylene. The reaction occurs in three stages:
The main industrial method for producing acetone is to obtain it from isopropylbenzene simultaneously with phenol (p. 234).
Some acetone is obtained by oxidative dehydrogenation or dehydrogenation of isopropyl alcohol.
Oxidative dehydrogenation of isopropyl alcohol can be carried out over a silver catalyst at 450-500 °C:
Carbon dioxide, propylene and acetic acid are formed as by-products. This process can also be carried out in the liquid phase at atmospheric pressure and a temperature of about 150 °C:
The resulting hydrogen peroxide is used for various syntheses, for example to obtain glycerol from acrolein (p. 96).
Dehydrogenation of isopropyl alcohol is carried out in the vapor phase at 350-400 °C in the presence of a copper catalyst:
(for the simplest aldehyde R=H)
Classification of aldehydes
According to the structure of the hydrocarbon radical:
Limit; For example:
Unlimited; For example:
Aromatic; For example:
Alicyclic; For example:
General formula of saturated aldehydes
Homologous series, isomerism, nomenclature
Aldehydes are isomeric to another class of compounds, ketones.
For example:
Aldehydes and ketones contain a carbonyl group ˃C=O and are therefore called carbonyl compounds.
Electronic structure of aldehyde molecules
The carbon atom of the aldehyde group is in a state of sp 2 hybridization, therefore all σ bonds in this group are located in the same plane. The clouds of p electrons forming a π bond are perpendicular to this plane and are easily displaced towards the more electronegative oxygen atom. Therefore, the C=O double bond (unlike the C=C double bond in alkenes) is highly polarized.
Physical properties
Chemical properties
Aldehydes are reactive compounds that undergo numerous reactions. Most characteristic of aldehydes:
a) addition reactions at the carbonyl group; HX type reagents are added as follows:
b) oxidation reactions C-H bonds aldehyde group, resulting in the formation of carboxylic acids:
I. Addition reactions
1. Hydrogenation (primary alcohols are formed
2. Addition of alcohols (hemiacetals and acetals are formed)
In excess alcohol in the presence of HCl, hemiacetals are converted to acetals:
II. Oxidation reactions
1. The “silver mirror” reaction
Simplified:
This reaction is qualitative reaction on the aldehyde group (a mirror coating of metallic silver is formed on the walls of the reaction vessel).
2. Reaction with copper(II) hydroxide
This reaction is also a qualitative reaction to the aldehyde group y (a red precipitate of Cu 2 O precipitates).
Formaldehyde is oxidized by various O-containing oxidizers, first to formic acid and then to H 2 CO 3 (CO 2 + H 2 O):
III. Di-, tri- and polymerization reactions
1. Aldol condensation
2. Trimerization of acetaldehyde
3. Polymerization of formaldehyde
During long-term storage of formaldehyde (40% aqueous solution of formaldehyde), polymerization occurs in it with the formation of a white paraform precipitate:
IV. Polycondensation reaction of formaldehyde with phenol
Contents of the article
ALDEHYDES AND KETONES– organic compounds containing a >C=O fragment (carbon linked by a double bond to oxygen, it is called carbonyl). In aldehydes, the carbonyl carbon is connected to the H atom and the organic group R (general formula RHC=O), and in ketones - to two organic groups(general formula R 2 C=O).
Nomenclature of aldehydes and ketones. The –(H)C=O group is called aldehyde; it has only one free valence to bind with organic groups, this allows it to be located only at the end of the hydrocarbon chain (but not in the middle). When compiling the name of an aldehyde, the name of the corresponding hydrocarbon is indicated, to which the suffix “al” is added, for example, methanal H 2 C=O, ethanal H 3 CC(H)=O, propanal H 3 CCH 2 C(H)=O. In more complex cases, the carbon chain of the R group is numbered, starting with the carbonyl carbon, then using numerical indices to indicate the position of the functional groups and various substituents.
Rice. 1. NOMENCLATURE OF ALDEHYDES. Substituting and functional groups, as well as their corresponding digital indices, are highlighted in different colors.
For some aldehydes, trivial (simplified) names that have developed historically are often used, for example, formaldehyde H 2 C=O, acetaldehyde H 3 CC(H)=O, crotonaldehyde CH 3 CH=CHC(H)=O.
Unlike the aldehyde group, the ketone group >C=O can also be located in the middle of the hydrocarbon chain, therefore simple cases indicate the names of organic groups (mentioning them in ascending order) and add the word “ketone”: dimethyl ketone CH 3 –CO–CH 3, methyl ethyl ketone CH 3 CH 2 –CO–CH 3. In more complex cases, the position of the ketone group in the hydrocarbon chain is indicated by a digital index, adding the suffix “ He" The numbering of the hydrocarbon chain starts from the end that is closer to the ketone group (Fig. 2).
Rice. 2. NOMENCLATURE OF KETONES. Substituting and functional groups and their corresponding digital indices are highlighted in different colors.
For the simplest ketone CH 3 –CO–CH 3 the trivial name is accepted - acetone.
Chemical properties of aldehydes and ketones
are determined by the characteristics of the carbonyl group >C=O, which has polarity - the electron density between the C and O atoms is unevenly distributed, shifted to the more electronegative O atom. As a result, the carbonyl group acquires an increased reactivity, which manifests itself in a variety of addition reactions at the double bond. In all cases, ketones are less reactive than aldehydes, in particular, due to the steric hindrance created by the two organic R groups, formaldehyde H 2 C=O most easily participates in reactions.
1. Addition via the double bond C=O.
When interacting with alcohols, aldehydes form hemiacetals - compounds containing both an alkoxy and a hydroxy group at one carbon atom: >C(OH)OR. Hemiacetals can further react with another alcohol molecule, forming full acetals - compounds where one carbon atom simultaneously contains two RO groups: >C(OR) 2. The reaction is catalyzed by acids and bases (Figure 3A). In the case of ketones, the addition of alcohols to the double bond in C=O is difficult.
In a similar way, aldehydes and ketones react with hydrocyanic acid HCN, forming hydroxynitriles - compounds containing an OH and CN group on one carbon atom: >C(OH)Cє N (Fig. 3B). The reaction is noteworthy in that it allows the carbon chain to increase (a new C-C bond appears).
In the same way (opening the C=O double bond), ammonia and amines react with aldehydes and ketones, the addition products are unstable and condense with the release of water and the formation of a C=N double bond. In the case of ammonia, imines are obtained (Fig. 3C), and from amines so-called Schiff bases are formed - compounds containing the fragment >C=NR (Fig. 3D). The product of the interaction of formaldehyde with ammonia is somewhat different - it is the result of the cyclization of three intermediate molecules, resulting in the framework compound hexamethylenetetramine, used in medicine as the drug urotropine (Fig. 3D).
2. Condensation reactions. For aldehydes and ketones, condensation is possible between two molecules of the same compound. With such condensation of aldehydes, the double bond of one of the molecules opens, forming a compound containing both an aldehyde and an OH group, called an aldol (aldehyde alcohol). The condensation that occurs is called aldol, and this reaction is catalyzed by bases (Fig. 4A). The resulting aldol can further condense to form a C=C double bond and release condensation water. The result is an unsaturated aldehyde (Fig. 4A, crotonaldehyde). This condensation is called crotonic condensation after the name of the first compound in the series of unsaturated aldehydes. Ketones are also capable of participating in aldol condensation (Fig. 4B), but the second stage, croton condensation, is difficult for them. Molecules of various aldehydes, as well as both an aldehyde and a ketone, can jointly participate in aldol condensation; in all cases, the carbon chain lengthens. The crotonaldehyde obtained at the last stage (Fig. 4A), possessing all the properties of aldehydes, can further participate in aldol and croton condensation when interacting with the next portion of acetaldehyde from which it was obtained (Fig. 4B). In this way, it is possible to lengthen the hydrocarbon chain, obtaining compounds in which single and double bonds alternate: –CH=CH–CH=CH–.
The condensation of aldehydes and ketones with phenols involves the removal of the carbonyl O atom (in the form of water), and the methylene group CH2 or a substituted methylene group (CHR or CR2) is inserted between two phenol molecules. This reaction is most widely used to produce phenol-formaldehyde resins (Fig. 5).
Rice. 5. CONDENSATION OF PHENOL WITH FORMALDEHYDE
3. Polymerization of carbonyl compounds occurs with the opening of the C=O double bond and is characteristic mainly of aldehydes. When aqueous solutions of formaldehyde are evaporated in vacuum, a mixture of cyclic compounds (mainly trioxymethylene) and linear products with a small chain length n = 8–12 (paraforms) is formed. By polymerizing the cyclic product, polyformaldehyde is obtained (Fig. 6), a polymer with high strength and good electrical insulating properties, used as a structural material in mechanical and instrument making.
Rice. 6. FORMALDEHYDE POLYMERIZATION PRODUCTS
4. Reduction and oxidation. Aldehydes and ketones are intermediate compounds between alcohols and carboxylic acids: reduction leads to alcohols, and oxidation leads to carboxylic acids. Under the action of H2 (in the presence of a Pt or Ni catalyst) or other reducing reagents, for example, LiAlH4, aldehydes are reduced, forming primary alcohols, and ketones - secondary alcohols (Fig. 7, schemes A and B).
The oxidation of aldehydes to carboxylic acids occurs quite easily in the presence of O 2 or under the action of weak oxidizing agents, such as an ammonia solution of silver hydroxide (Fig. 7B). This spectacular reaction is accompanied by the formation of a silver mirror on inner surface reaction device (usually an ordinary test tube), it is used for qualitative detection of the aldehyde group. Unlike aldehydes, ketones are more resistant to oxidation; when heated in the presence of strong oxidizing agents, for example, KMnO 4, mixtures of carboxylic acids are formed that have a shortened (compared to the original ketone) hydrocarbon chain.
Rice. 7. REDUCTION AND OXIDATION OF ALDEHYDES AND KETONES
Additional confirmation that aldehydes occupy an intermediate position between alcohols and acids is the reaction that results in an alcohol and a carboxylic acid from two aldehyde molecules (Fig. 8A), i.e. one aldehyde molecule is oxidized and the other is reduced. In some cases, the two resulting compounds—an alcohol and a carboxylic acid—further react with each other, forming an ester (Fig. 8B).
Rice. 8 . SIMULTANEOUS OXIDATION AND REDUCTION OF ALDEHYDES
Preparation of aldehydes and ketones.
The most universal method is the oxidation of alcohols, in which aldehydes are formed from primary alcohols, and ketones from secondary alcohols (Fig. 9A and B). These are the opposite reactions to those in Fig. 7A and B. The reaction is reversed if the active reagent (oxidizing agent instead of the reducing agent) and the catalyst are changed; a copper catalyst is effective in the oxidation of alcohols.
In industry, acetaldehyde is obtained by the oxidation of ethylene (Fig. 9B); at the intermediate stage, an alcohol is formed in which the OH group is “adjacent” to the double bond (vinyl alcohol); such alcohols are unstable and immediately isomerize into carbonyl compounds. Another method is the catalytic hydration of acetylene (Fig. 9D), the intermediate compound being vinyl alcohol. If you take methyl acetylene instead of acetylene, you get acetone (Fig. 9E). The industrial method for producing acetone is the oxidation of cumene. Aromatic ketones, such as acetophenone, are prepared by the catalytic addition of an acetyl group to aromatic nucleus(Fig. 9E).
Application of aldehydes and ketones.
Formaldehyde H 2 C=O (its aqueous solution is called formalin) is used as a leather tanning agent and a preservative for biological preparations.
Acetone (CH 3) 2 C=O is a widely used extractant and solvent for varnishes and enamels.
Aromatic ketone benzophenone (C 6 H 5) 2 C=O with the smell of geranium, used in perfume compositions and for flavoring soap.
Some of the aldehydes were first found in plant essential oils and later artificially synthesized.
Aliphatic aldehyde CH 3 (CH 2) 7 C (H) = O (the trivial name is pelargonaldehyde) is found in the essential oils of citrus plants, has the smell of orange, and is used as a food flavoring.
The aromatic aldehyde vanillin (Fig. 10) is found in the fruits of the tropical vanilla plant; now synthetic vanillin is more often used - a well-known flavoring additive in confectionery(Fig. 10).
Rice. 10. VANILLIN
Benzaldehyde C 6 H 5 C (H) = O with the smell of bitter almonds is found in almond oil and essential oil eucalyptus. Synthetic benzaldehyde is used in food flavor essences and perfume compositions.
Benzophenone (C 6 H 5) 2 C=O and its derivatives are capable of absorbing UV rays, which determined their use in suntan creams and lotions; in addition, some benzophenone derivatives have antimicrobial activity and are used as preservatives. Benzophenone has a pleasant geranium scent, and therefore it is used in perfume compositions and for flavoring soap.
The ability of aldehydes and ketones to participate in various transformations determined their main use as starting compounds for the synthesis of various organic substances: alcohols, carboxylic acids and their anhydrides, drugs (urotropine), polymer products (phenol-formaldehyde resins, polyformaldehyde), in the production of all kinds of fragrant substances ( based on benzaldehyde) and dyes.
Mikhail Levitsky
Aldehydes are compounds whose molecules contain a carbonyl group connected to a hydrogen atom, i.e. the general formula of aldehydes can be written as
where R is a hydrocarbon radical, which can be of varying degrees of saturation, for example, saturated or aromatic.
The –CHO group is called aldehyde.
Ketones – organic compounds whose molecules contain a carbonyl group connected to two hydrocarbon radicals. General formula ketones can be written as:
where R and R’ are hydrocarbon radicals, for example, saturated (alkyl) or aromatic.
Hydrogenation of aldehydes and ketones
Aldehydes and ketones can be reduced with hydrogen in the presence of catalysts and heating to primary and secondary alcohols, respectively:
Aldehyde oxidation
Aldehydes can be easily oxidized even by such mild oxidizing agents as copper hydroxide and ammonia solution of silver oxide.
When copper hydroxide and aldehyde are heated, the initial blue color of the reaction mixture disappears, and a brick-red precipitate of cuprous oxide is formed:
In the reaction with an ammonia solution of silver oxide, instead of the carboxylic acid itself, its ammonium salt is formed, since the ammonia in the solution reacts with acids:
Ketones do not react with copper (II) hydroxide and an ammonia solution of silver oxide. For this reason, these reactions are qualitative for aldehydes. Thus, the reaction with an ammonia solution of silver oxide, when carried out correctly, leads to the formation of a characteristic silver mirror on the inner surface of the reaction vessel.
Obviously, if mild oxidizing agents can oxidize aldehydes, then stronger oxidizing agents, for example, potassium permanganate or potassium dichromate, can naturally do the same. When these oxidizing agents are used in the presence of acids, carboxylic acids are formed:
Chemical properties of carboxylic acids
Carboxylic acids are hydrocarbon derivatives containing one or more carboxyl groups.
Carboxyl groupsA:
As you can see, the carboxyl group consists of a carbonyl group –C(O)- connected to a hydroxyl group –OH.
Due to the fact that a carbonyl group is directly attached to the hydroxyl group, which has a negative inductive effect O-H connection is more polar than in alcohols and phenols. For this reason, carboxylic acids are noticeably more pronounced than alcohols and phenols, acidic properties. In aqueous solutions they exhibit properties weak acids, i.e. reversibly dissociate into hydrogen cations (H+) and anions of acid residues:
Salt formation reactions
To form salts, carboxylic acids react with:
1) metals to hydrogen in the activity series:
2) ammonia
3) basic and amphoteric oxides:
4) basic and amphoteric hydroxides metals:
5) salts of weaker acids - carbonates and bicarbonates, sulfides and hydrosulfides, salts of higher (with a large number of carbon atoms in the molecule) acids:
The systematic and trivial names of some acids and their salts are presented in the following table:
| Acid formula | Acid name trivial/systematic | Salt name trivial/systematic |
| HCOOH | formic / methane | formate/methanoate |
| CH3COOH | acetic/ethane | acetate/ethanoate |
| CH3CH2COOH | propionic/propane | propionate/ propanoate |
| CH 3 CH 2 CH 2 COOH | oil/butane | butyrate/butanoate |
The opposite should also be remembered: strong mineral acids displace carboxylic acids from their salts as weaker ones:
Reactions involving the OH group
Carboxylic acids enter into an esterification reaction with monohydric and polyhydric alcohols in the presence of strong inorganic acids, resulting in the formation of esters:
This type of reaction is reversible, and therefore, in order to shift the equilibrium towards the formation of an ester, they should be carried out by driving off the more volatile ester when heated.
The reverse of the esterification reaction is called ester hydrolysis:
This reaction occurs irreversibly in the presence of alkalis, since the resulting acid reacts with the metal hydroxide to form a salt:
Reactions of substitution of hydrogen atoms in a hydrocarbon substituent
When carrying out reactions of carbonates with chlorine or bromine in the presence of red phosphorus, upon heating, the hydrogen atoms at the α-carbon atom are replaced by halogen atoms:
In the case of a higher halogen/acid ratio, deeper chlorination may occur:
Reactions of destruction of the carboxyl group (decarboxylation)
Special chemical properties of formic acid
The formic acid molecule, despite its small size, contains two functional groups:
In this regard, it exhibits not only the properties of acids, but also the properties of aldehydes:
When exposed to concentrated sulfuric acid, formic acid decomposes into water and carbon monoxide.
