(for the simplest aldehyde R=H)
Limit; For example:
Unlimited; For example:
Aromatic; For example:
Alicyclic; For example:
For example:
Aldehydes and ketones contain a carbonyl group ˃C=O and are therefore called carbonyl compounds.
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. 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.
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:
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:
1. The “silver mirror” reaction
Simplified:
This reaction is a qualitative reaction to 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 oxidizing agents, first to formic acid and further - to H 2 CO 3 (CO 2 + H 2 O):
1. Aldol condensation
2. Trimerization of acetaldehyde
At long-term storage formalin (40% aqueous solution of formaldehyde) polymerization occurs in it with the formation of a white paraform precipitate:
ALDEHYDES AND KETONES
Aldehydes and ketones are hydrocarbon derivatives containing the carbonyl group C=O. In an aldehyde molecule, at least one valency of the carbonyl group is spent on combining with a hydrogen atom, and the other with a radical (of the limiting series in saturated aldehydes and the unsaturated one in unsaturated aldehydes). General formula of aldehydes:
and R may be equal to H.
In the case of ketones, both valences of the carbonyl group are spent on combining with radicals. General formula of ketones:
Isomerism. Nomenclature.
General formula saturated aldehydes and ketones C n H 2 n O.
The isomerism of aldehydes is associated with the structure of radicals. For example, four aldehydes are known with the formula
(see below).
Aldehydes are named either by the acids into which they transform upon oxidation (with the same number of carbon atoms), or by saturated hydrocarbons with the addition of the suffix -al(systematic nomenclature).
| formic aldehyde (formaldehyde), methanal (Fig. 1 A) | |
| acetaldehyde, ethanal (Fig. 1 b) | |
 | propionaldehyde, propanal |
| CH 3 -CH 2 -CH 2 -CHO | butyraldehyde, butanal |
 | isobutyraldehyde, 2-methylpropanal |
| CH 3 -CH 2 -CH 2 -CH 2 -CHO | valeraldehyde, pentanal |
 | isovalernanaldehyde, 3-methylbutanal |
 | methylethylacetic aldehyde, 2-methylbutanal |
 | trimethylacetic aldehyde, 2,2-dimethylpropanal |
The isomerism of ketones is associated with the structure of the radicals and the position of the carbonyl group in the carbon chain. Ketones are named after the radicals attached to the carbonyl group. According to systematic nomenclature, the suffix -one is added to the name of the saturated hydrocarbon and the number of the carbon atom associated with the carbonyl oxygen is indicated:
Methods of obtaining
Aldehydes and ketones are prepared by a number of common methods.
1. By oxidation or catalytic dehydrogenation of primary alcohols, aldehydes are obtained, and secondary alcohols - ketones. These reactions have already been given when considering the chemical properties of alcohols.
2. It is also convenient to obtain aldehydes and ketones by pyrolysis of acids and their mixtures in the form of vapors over oxides of some metals (ThO 2, MnO 2, CaO, ZnO) at 400-450 °C:
R - COOH + H-COOH → R-CHO + CO 2 + H 2 0
2R-COOH→R -CO -R + C0 2 + H 2 0
R-COOH + R" - COOH → R - CO-R’+C0 2 + H 2 0
Many textbooks indicate that aldehydes and ketones can be obtained by pyrolysis of Ca and Ba salts carboxylic acids. In fact, this reaction gives very low yields. However, some methyl ketones can still be obtained by pyrolysis of mixtures of barium or iron salts of acetic acid and some other acid. All these reactions have a radical mechanism.
3. Hydrolysis of geminal dihalogen derivatives leads to aldehydes if both halogens are located on one of the outermost carbon atoms, and ketones if the halogen atoms are located on one of the middle carbon atoms. These reactions have already been mentioned in the study of the chemical properties of dihalogenated hydrocarbons.
4. Hydration of acetylene and its homologues under the conditions of the Kucherov reaction leads to acetaldehyde or ketones, respectively:
HC≡CH + H 2 O→ CH 3 -CHO
5. Carbonyl compounds in high yields (about 80%) are formed during the oxidation of the corresponding alcohols with mixtures of dpmethyl sulfoxide with acetic anhydride or anhydrous phosphoric acid.
RCH 2 OH + (CH 3) 2 SO → RCH = O + (CH 3) 2 S
6. The transformation of haloalkyl into aldehydes with chain extension by one carbon atom is achieved by treating them with sodium tetracarbonyl ferrate in the presence of triphenylphosphine, and then with acetic acid:
R - Hlg + Na 2 Fe(CO) 4 RCOFe(CO 3)P(C 6 H 5) 3 R–CH = O
There are several modifications of this method.
7. Ketones are obtained in good yields by reacting acid chlorides with lithium dialkyl cuprate and cadmium alkyls:
R 2 CuLi + R"COCI→R - CO - R"+LiCI + R - Cu
8. In technology, aldehydes are obtained by direct addition of CO and H 2 to olefins (oxo synthesis) at 100-200 ° C under a pressure of 10-20 MPa (100-200 atm) in the presence of cobalt or nickel catalysts (for example, Co + ThO 2 + MgO, supported for kieselguhr):
The reaction with ethylene and propylene is carried out in the gas phase, and with more complex olefins (C 4 -C 20) - in the liquid phase. As can be seen from the diagram above, oxosynthesis produces aldehydes containing one more carbon atom than the original olefins. This synthesis is important for the preparation of higher primary alcohols (by catalytic reduction of aldehydes). The mechanism of oxosynthesis can be represented as follows:
2Co + 8CO→ Co 2 (CO) 8
Co 2 (CO)8 + H 2 → 2HCo(CO) 4
R -CH = CH 2 + HCo(CO) 4 → R - CH 2 -CH 2 - Co(CO) 4
R - CH 2 -CH 2 -Co(CO) 4 +CO→ R-CH 2 -CH 2 -CO - Co(CO) 4
R-CH 2 -CH 2 -CO-Co(CO) 4 + HCo(CO) 4 →R-CH 2 -CH 2 -CHO + Co(CO ) 8
Physical properties
Formic aldehyde is a gas with a very pungent odor. Other lower aldehydes and ketones are liquids that are easily soluble in water; lower aldehydes have a suffocating odor, which when strongly diluted becomes pleasant (reminiscent of the smell of fruits). Ketones smell quite nice.
With the same composition and carbon chain structure, ketones boil at slightly more high temperatures than aldehydes. The boiling points of aldehydes and ketones with a normal chain structure are higher than those of compounds with an isostructure. For example, valeraldehyde boils at 103.4 °C, and isovaleric at 92.5 °C. Aldehydes and ketones boil at a temperature much lower than alcohols with the same number of carbon atoms, for example, propionaldehyde bp. 48.8 °C, for acetone 65.1 °C, for n-propyl alcohol 97.8 °C. This shows that aldehydes and ketones, unlike alcohols, are not strongly associated liquids. At the same time, the boiling points of carbonyl compounds are significantly higher than the boiling points of hydrocarbons with the same molecular weight, which is due to their high polarity. The density of aldehydes and ketones is below unity.
In the IR spectra, the CO group is characterized by intense absorption at 1720 cm -1. In the NMR spectrum, the hydrogen signal of the aldehyde group is in a very low field.
Aldehydes and ketones are highly reactive. Most of their reactions are due to the presence of an active carbonyl group. The double bond of the carbonyl group is similar in physical nature with a double bond between two carbon atoms (σ bond + π bond). However, while E c=c<2Е с-с, энергия связи С=О (749,4 кДж/моль) больше, чем энергия двух простых С-О-связей (2х358 кДж/моль). С другой стороны, кислород является более электроотрицательным элементом, чем углерод, и потому электронная плотность вблизи атома кислорода больше, чем вблизи атома углерода. Дипольный момент карбонильной группы - около 9 10 -30 Кл/м (2,7 D). Благодаря такой поляризации углеродный атом карбонильной группы обладает электрофильными свойствами и способен реагировать с нуклеофильными реагентами. Соответственно атом кислорода является нуклеофильным. В реакциях присоединения отрицательно поляризованная часть присоединяющейся молекулы всегда направляется к углеродному атому карбонильной группы, в то время как ее положительно поляризованная часть направляется к кислородному атому.
The reaction of addition of nucleophilic reagents at the site of a carbonyl bond is a stepwise process. Schematically, the addition reaction of, for example, sodium hydrosulfite to acetaldehyde can be depicted as follows:
Radicals capable of increasing the positive charge on the carbon atom of the carbonyl group greatly increase the reactivity of aldehydes and ketones; radicals or atoms that reduce the positive charge on that carbon atom have the opposite effect.
In addition to addition reactions at the carbonyl group, aldehydes and ketones are also characterized by reactions involving carbon radicals adjacent to the carbonyl group, due to the electron-withdrawing effect of the carbonyl group on them. These include oxidation, halogenation, and condensation reactions.
A. Hydrogenation. The addition of hydrogen to aldehydes and ketones occurs in the presence of hydrogenation catalysts (Ni, Co, Cu, Pt, Pd, etc.). In this case, aldehydes turn into primary alcohols, and ketones into secondary alcohols. One of the methods for producing alcohols is based on this.
Recently, aluminum hydride LiA1H 4 is often used as a reducing agent. The reaction involves the transfer of a hydride ion:
The advantage of reduction with LiAlH 4 is that this reagent does not reduce carbon-carbon double bonds.
When aldehydes or ketones are reduced with hydrogen at the time of separation (with the help of alkali metals or amalgamated magnesium), glycols are also formed along with the corresponding alcohols:
pinacon
The ratio between the resulting alcohol and glycol depends on the nature of the carbonyl compound and the reduction conditions. When ketones are reduced in reaction products in aprotic solvents, pinacones predominate; In the case of aliphatic saturated aldehydes, glycols are formed in small quantities.
The reaction occurs with the intermediate formation of free radicals:
B. Nucleophilic addition reactions.
1. The addition of magnesium haloalkyl is discussed in detail when describing methods for the preparation of alcohols.
2. The addition of hydrocyanic acid leads to the formation of α-hydroxynitriles, the saponification of which produces α-hydroxy acids:
α-hydroxypropionic acid nitrile
This reaction begins with a nucleophilic attack on the carbon atom by a CN - ion. Hydrogen cyanide adds very slowly. Adding a drop of potassium cyanide solution greatly speeds up the reaction, while adding mineral acid reduces the reaction rate to almost zero. This shows that the active reagent in the formation of cyanohydrin is the CN - ion:
3. The addition of sodium hydrosulfite gives crystalline substances, usually called hydrosulfite derivatives of aldehydes or ketones:
When heated with a solution of soda or mineral acids, hydrosulfite derivatives decompose to release free aldehyde or ketone, for example:
The reaction with sodium hydrosulfite is used for the qualitative determination of aldehydes and ketones, as well as for their isolation and purification. It should, however, be noted that only methyl ketones having a CH 3 -CO- group react with sodium hydrosulfite in the fatty series.
4. Interaction with ammonia allows one to distinguish between aldehydes and ketones. Aldehydes release water to form aldimines:
acetaldimine, ethanimi n
which easily polymerize (cyclize into crystalline trimers - aldehyde ammonias:
aldehydammia To
During cyclization, the C=N double bond is broken and three imine molecules are combined into a six-membered ring with alternating carbon and nitrogen atoms.
Ketones do not form such compounds with ammonia. They react very slowly and more complexly, for example like this:
5. With hydroxylamine, aldehydes and ketones, releasing water, form oximes (aldoximes and ketoximes):
acetaldoxime
acetone oxime
These reactions are used for the quantitative determination of carbonyl compounds.
Reaction mechanism (R=H or Alk):
6. Of particular interest are the reactions of carbonyl compounds with hydrazine and its substitutes. Depending on the conditions, hydrazine reacts with aldehydes and ketones in a ratio of 1:1 or 1:2. In the first case, hydrazones are formed, and in the second, azines (aldazines and ketazines):
hydrazone
aldazine
ketazine
Hydrazones of ketones and aldehydes, when heated with solid KOH, release nitrogen and give saturated hydrocarbons (Kizhner reaction):
Currently, this reaction is carried out by heating the carbonyl compound with hydrazine in high-boiling polar solvents (di- and triethylene glycols) in the presence of alkali. The reaction can also be carried out with room temperature under the action of tert-butylpotassium in dimethyl sulfoxide.
Aldehydes and ketones with substituted hydrazines - with phenylhydrazine C 6 H 5 -NH-NH 2 and semicarbazide 
form phenylhydrazones and semicarbazones, respectively. These are crystalline substances. They serve for the qualitative and quantitative determination of carbonyl compounds, as well as for their isolation and purification.
Formation of phenylhydrazones:
Semicarbazones are formed according to the following scheme:
The reactions of aldehydes and ketones with hydrazine derivatives are similar in mechanism to their reactions with ammonia and hydroxylamine. For example, for acetaldehyde and phenylhydrazine:
These reactions are characterized by acid catalysis.
7. Aldehydes and ketones are capable of adding water at the carbonyl group to form hydrates - geminal glycols. These compounds in many cases exist only in solutions. The equilibrium position depends on the structure of the carbonyl-containing compound:
Thus, formaldehyde at 20 °C exists in an aqueous solution 99.99% in the form of hydrate, acetaldehyde - 58%; in the case of acetone, the hydrate content is insignificant, and chloral and trichloroacetone form stable crystalline hydrates.
Aldehydes with a higher molecular weight form solid hemihydrates with water that are stable at low temperatures:
8.
In the presence of traces of mineral acid, acetals are formed:
Acetals are liquids with a pleasant ethereal odor. When heated with dilute mineral acids (but not alkalis), they undergo hydrolysis with the formation of alcohols and the release of aldehydes:
Acetal, derived from butyraldehyde and polyvinyl alcohol, is used as an adhesive in the manufacture of safety glass.
Ketone acetals are obtained more complexly - by the action of ethyl esters of orthoformic HC(OC2H 5)3 or orthosilicic acid on ketones:
9. When aldehydes react with alcohols, hemiacetals are formed:
Aldehydes and ketones, when interacting with PCI 5, exchange an oxygen atom for two chlorine atoms, which is used to obtain geminal dichloroalkanes:
This reaction, in the stage that determines the nature of the final product, is also a nucleophilic addition reaction:
B. Oxidation reactions. The oxidation of aldehydes is much easier than that of ketones. Additionally, oxidation of aldehydes produces acids without changing the carbon skeleton, while ketones are oxidized to form two simpler acids, or an acid and a ketone.
Aldehydes are oxidized by atmospheric oxygen to carboxylic acids. The intermediate products are hydroperoxides:
An ammonia solution of silver hydroxide OH, when slightly heated with aldehydes (but not ketones), oxidizes them into acids to form free metallic silver. If the test tube in which the reaction takes place was previously degreased from the inside, then the silver lies in a thin layer on its inner surface - a silver mirror is formed:
This reaction, known as the silver mirror reaction, is used for the qualitative determination of aldehydes.
Aldehydes are also characterized by a reaction with the so-called feling liquid. The latter is an aqueous-alkaline solution of a complex salt formed from copper hydroxide and sodium potassium salt of tartaric acid. When aldehydes are heated with fehling liquid, copper (II) is reduced to copper (I), and the aldehyde is oxidized to acid:
Red copper oxide Cu 2 O precipitates almost quantitatively. This reaction does not work with ketones.
Aldehydes can be oxidized to carboxylic acids using many common oxidizing agents, such as potassium dichromate, potassium permanganate, by an ionic mechanism, the first stage of the process usually being the addition of an oxidizing agent at the CO group.
The oxidation of ketones occurs with the rupture of the carbon chain in different directions, depending on the structure of the ketones.
Based on the oxidation products, one can judge the structure of ketones, and since ketones are formed during the oxidation of secondary alcohols, then, consequently, the structure of these alcohols.
D. Polymerization reactions. These reactions are typical only for aldehydes. When aldehydes are exposed to acids, they undergo trimerization (partially tetramerization):
The polymerization mechanism can be presented as follows:
D. Halogenation. Aldehydes and ketones react with bromine and iodine at the same rate regardless of the halogen concentration. Reactions are accelerated by both acids and bases.
A detailed study of these reactions led to the conclusion that they proceed with the preliminary conversion of the carbonyl compound to enol:
E. Condensation reactions.
1. Aldehydes in a weakly basic environment (in the presence of acetate, carbonate or potassium sulfite) undergo aldol condensation (A.P. Borodin) with the formation of aldehyde siyrtes (hydroxyaldehydes), abbreviated as aldols. Aldols are formed by the addition of an aldehyde to the carbonyl group of another aldehyde molecule, breaking the C-H bond at the α-position to the carbonyl, as shown in the example acetaldehyde:
aldol
In the case of aldolizatsin other aldehydes, such as propionic aldehydes, only the group located in the a-position to the carbonyl reacts, since only the hydrogen atoms of this group are sufficiently activated by the carbonyl group:
3-hydroxy-2-methylpentanal
If there is a quaternary carbon atom next to the carbonyl, aldolization is not possible. For example, trimethylacetic aldehyde (CH3)3C-CHO does not give an aldol.
The mechanism of the base-catalyzed aldol condensation reaction is as follows. Aldehyde exhibits the properties of a CH acid. The hydroxyl ion (catalyst) reversibly abstracts a proton from the α-carbon atom:
When heated, aldol (without water-removing substances) splits off water to form unsaturated crotonaldehyde:
Therefore, the transition from a saturated aldehyde to an unsaturated aldehyde through an aldol is called croton condensation. Dehydration occurs due to the very high mobility of hydrogen atoms in the α-position relative to the carbonyl group (superconjugation), and, as in many other cases, the p-bond relative to the carbonyl group is broken.
When aldehydes capable of aldol condensation are exposed to strong bases (alkalis), tarring occurs as a result of deep aldol (or croton) polycondensation. Aldehydes that are not capable of aldol condensation undergo the Cannizzaro reaction under these conditions:
2(CH 3) 3 C-CHO + KOH→(CH 3) 3 C-COOK +(CH 3) 3 C-CH 2 OH.
Aldol condensation of ketones occurs under more severe conditions - in the presence of bases, for example Ba(OH) 2. In this case, P-ketone alcohols are formed, which easily lose a water molecule:
Under even more severe conditions, for example when heated with concentrated sulfuric acid, ketones undergo intermolecular dehydration to form unsaturated ketones:
mesityl oxide
Mesityl oxide can react with a new acetone molecule:
foron
Condensation between aldehydes and ketones is also possible, for example:
3-penten-2-one
In all these reactions, aldol condensation occurs first, followed by dehydration of the resulting hydroxyketone.
2. Ester condensation of aldehydes occurs under the action of aluminum alkoxides in a non-aqueous medium (V.E. Tishchenko).
ethyl acetate
AND. Decarbonylation. Aldehydes, when heated with tris(triphenylphosphine)rhodium chloride, undergo decarbonylation to form hydrocarbons:
R-CHO + [(C 6 H 5) 3 P] 3 PhCl → R-H + [(C 6 H 5) 3 P] 3 RhCOCl.
When studying the chemical transformations of aldehydes and ketones, it is necessary to pay attention to the significant differences between them. Aldehydes are easily oxidized without changing the carbon chain (silver mirror reaction), ketones are difficult to oxidize with chain breaking. Aldehydes polymerize under the influence of acids, form aldehyde ammonias, form acetals with alcohols in the presence of acids, enter into ester condensation, and give color with fuchsulfurous acid. Ketones are not capable of such transformations.
Individual representatives. Application
Formic aldehyde (formaldehyde) is a colorless gas with a sharp, specific odor, bp. -21 °C. It is poisonous and irritates the mucous membranes of the eyes and respiratory tract. Highly soluble in water, a 40% aqueous solution of formaldehyde is called formalin. In industry, formaldehyde is produced by two methods - incomplete oxidation of methane and some of its homologues and catalytic oxidation or dehydrogenation of methanol (at 650-700 °C over a silver catalyst):
CH 3 OH → H 2 + H 2 CO.
Due to the absence of an alkyl radical, formaldehyde has some special properties.
1. In an alkaline environment, it undergoes an oxidation-reduction reaction (Cannizzaro reaction):
2. When formaldehyde (formalin) is slightly heated with ammonia, hexamethylenetetramine (urotropine), synthesized for the first time by A. M. Butlerov, is obtained:
6H 2 C=O + 4NH 3 → 6H 2 0 + (CH 2) 6 N 4
urotropin
Hexogen is used in large quantities in the production of phenol-formaldehyde resins and explosives (RDX obtained by nitration of hexogen)
hexagen
in medicine (as a diuretic, as component anti-influenza drug Kaltsex, in the treatment of kidney diseases, etc.).
3. In an alkaline environment, for example in the presence of milk of lime, as was first shown by A.M. Butlerov, formaldehyde undergoes aldolization with the formation of oxyaldehydes up to hexoses and even more complex sugars, for example:
hexose
In the presence of alkalis, formaldehyde can condense with other aldehydes, forming polyhydric alcohols. Thus, by condensation of formaldehyde with acetaldehyde, tetrahydric alcohol is obtained - pentaerythritol C (CH 2 OH) 4
CH 3 CHO + 3H 2 CO → (HOCH 2) 3 CCHO
(HOCH 2) 3 CCNO + H 2 CO → (HOCH 2) 4 C + HCOO -
Pentaerythritol is used to produce resins and a very strong explosive - tetranitropentaerythritol (PETN) C(CH 2 ONO 2) 4.
4. Formaldehyde is capable of polymerization to form cyclic and linear polymers.
5. Formaldehyde is capable of entering into various condensation reactions to form synthetic resins, widely used in industry. Thus, by polycondensation of formaldehyde with phenol, phenol-formaldehyde resins are obtained, and with urea or melamine, urea resins are obtained.
6. The condensation product of formaldehyde with isobutylene (in the presence of H 2 SO 4) is 4,4-dimethyl-1,3-dioxane, which decomposes when heated to 200-240 ° C in the presence of catalysts (SiO 2 + H 4 P 2 O 7) with the formation of isoprene.
Formalin is widely used as a disinfectant for disinfecting grain and vegetable stores, greenhouses, greenhouses, for treating seeds, etc.
Acetaldehyde, acetaldehyde CH 3 CHO is a liquid with a pungent unpleasant odor. Kip. 21 °C. Acetaldehyde vapor causes irritation of mucous membranes, suffocation, and headaches. Acetaldehyde is highly soluble in water and many organic solvents.
Industrial methods for producing acetaldehyde have already been discussed: hydration of acetylene, dehydrogenation of ethyl alcohol, isomerization of ethylene oxide, catalytic oxidation of saturated hydrocarbons with air.
Recently, acetaldehyde has been produced by the oxidation of ethylene with atmospheric oxygen in the presence of a catalyst according to the following scheme:
CH 2 =CH 2 +H 2 O +PdCl 2 →CH 3 -CHO + 2HCl + Pd
Pd + 2CuC1 2 → 2CuCl + PdCl 2
2CuCl + 2HCI + 1/2 O 2 → 2CuCI 2 + H 2 O
2CH 2 = CH 2 + O 2 →2CH 3 CHO
Other 1-alkenes form methyl ketones in this reaction.
From acetaldehyde, acetic acid, acetic anhydride, ethyl alcohol, aldol, butyl alcohol, acetals, ethyl acetate, pentaerythritol and a number of other substances are produced on an industrial scale.
Like formaldehyde, it condenses with phenol, amines and other substances to form synthetic resins that are used in the production of various polymer materials.
Under the influence of a small amount of sulfuric acid, acetaldehyde polymerizes into paraldehyde (C 2 H 4 O 3) 3 and metaldehyde (C 2 H 4 O 3) 4; the amounts of the latter increase with decreasing temperature (up to -10 °C):
Paraldehyde is a liquid with boiling point. 124.5 °C, metaldehyde is a crystalline substance. When heated with traces of acid, both of these substances depolymerize, forming acetaldehyde. From paraldehyde and ammonia, 2-methyl-5-vinylpyridine is obtained, which is used in the synthesis of copolymers - synthetic rubbers.
Trichloroacetic aldehyde, chloral CCI 3 CHO, is obtained by chlorinating ethyl alcohol.
Chloral is a colorless liquid with a pungent odor; with water it forms a crystalline hydrate - chloral hydrate. The stability of chloral hydrate is explained by the enhancement of the electron-withdrawing properties of carbonyl carbon under the influence of the strong inductive effect of chlorine:
Has a hypnotic effect. Insecticides are produced on an industrial scale by condensing chloral with chlorobenzene.
When chloral is exposed to alkalis, chloroform is formed:
Acetone CH 3 SOCH 3 is a colorless liquid with a characteristic odor; Bp = 56.1 °C, Melt = 0.798. It is highly soluble in water and many organic solvents.
Acetone is obtained:
1) from isopropyl alcohol - by oxidation or dehydrogenation;
2) oxidation of isopropylbenzene obtained by alkylation of benzene, along with phenol;
3) acetone-butanol fermentation of carbohydrates.
Acetone as a solvent is used in large quantities in the paint and varnish industry, in the production of acetate silk, film, smokeless powder (pyroxylin), for dissolving acetylene (in cylinders), etc. It serves as the starting product in the production of unbreakable organic glass, ketene, etc. d.
Aldehydes are organic substances that belong to carbonyl compounds containing the functional group -SON, which is called a carbonyl group.
Depending on the nature of the hydrocarbon skeleton, aldehyde molecules are saturated, unsaturated and aromatic. Their molecules may also include halogen atoms or additional functional groups. The general formula of saturated aldehydes is C n H 2 n O. In accordance with the IUPAC nomenclature, their names end with the suffix -al.
The oxidation of aldehydes is important in industry because they are quite easily converted into carboxylic acids. In this case, copper hydroxide, silver oxide, or even air oxygen can serve as oxidizing agents.
The electronic structure of the double bond in the C=O group is characterized by the formation of one σ-bond and another π-bond. The C atom is in a state of sp 2 hybridization, the molecule has a flat structure with bond angles between bonds of about 120 0. The difference between the double bond in this functional group is that it is located between a carbon atom and a very electronegative oxygen atom. As a result, electrons are attracted to the O atom, which means that this bond is very highly polarized.
The content of such a polarized double bond in the aldehyde group can be called the main reason for the high reactivity of aldehydes. For aldehydes, the most typical reactions are the addition of atoms or their groups to the C=O bond. And the easiest reactions to occur are nucleophilic addition. Also typical for aldehydes are reactions involving H atoms from the functional group of aldehydes. Due to the electron-withdrawing effect of the C=O group, the polarity of the bond increases. This in turn is the reason for the relatively easy oxidation of aldehydes.
Formaldehyde (formaldehyde or methanal) CH 2 O is a gaseous substance with a very pungent odor, which is usually obtained by passing a mixture of methanol vapor with air through a hot copper or silver mesh. Its 40% aqueous solution is called formalin. Formaldehyde readily reacts, many of which form the basis for the industrial synthesis of a number of important substances. It is also used to produce pentaerythritol, many medicinal substances, various dyes, for tanning leather, and as a disinfectant and deodorant. Formaldehyde is quite toxic; its maximum permissible concentration in the air is 0.001 mg/l.
Acetaldehyde (acetaldehyde, ethanal) CH 3 COH is a colorless liquid with a suffocating odor, which when diluted with water acquires a fruity aroma. Acetaldehyde has all the basic properties of aldehydes. The oxidation of acetaldehyde produces huge volumes acetic acid and acetic anhydride, a variety of pharmaceuticals.
Acrolein (propenal) CH 2 =CH-SON, the simplest unsaturated aldehyde is a colorless, highly volatile liquid. Its vapors strongly irritate the mucous membranes of the eyes and upper respiratory tract. It is very toxic, the maximum permissible concentration for its content in the air is 0.7 mg/m 3. Propenal is an intermediate product in the synthesis of some polymers; it is necessary in the production of certain medicines.
Benzaldehyde (benzoaldehyde) C 6 H 5 COH is a colorless liquid with an aroma that turns yellow during storage. It is quickly oxidized by air to benzoic acid. Contained in essential oils plants (neroli, patchouli), and in the form of a glucoside - in the kernels of bitter almond, cherry, apricot and peach seeds. As an aromatic substance, it is used in perfumery, as a component of food essences, and as a raw material for the synthesis of other aromatic substances (cinnamaldehyde, jasminaldehyde).
The oxidation of aldehydes by silver oxide is the most indicative qualitative reaction to the corresponding form of the functional group. This reaction got its name due to the thin silver coating on the walls of the test tube that forms during this reaction.
Its essence lies in the interaction of the aldehyde R-СОН with an ammonia solution of silver(I) oxide, which is a soluble OH complex compound and is called Tollens’ reagent. The reaction is carried out at temperatures close to the boiling point of water (80-100 °C). In this case, the aldehydes are oxidized to their corresponding carboxylic acids, and the oxidizing agent is reduced to metallic silver, which precipitates.
To qualitatively determine the -SON group in aldehydes, a silver complex compound is first prepared. To do this, pour a little solution of ammonia (ammonium hydroxide) in water into a test tube, followed by a small amount of silver nitrate. In this case, the resulting silver oxide precipitate immediately disappears:
2AgNO 3 + 2NH 3 + H 2 O -> Ag 2 O↓ + 2NH 4 NO 3
Ag 2 O + 4NΗ 3 + Η 2 O -> 2ОΗ
More reliable results are obtained by Tollens' reagent prepared with the addition of alkali. To do this, 1 g of AgNO 3 is dissolved in 10 g of distilled water and an equal volume of concentrated sodium hydroxide is added. As a result, a precipitate of Ag 2 O is formed, which disappears when a concentrated solution of ammonium hydroxide is added. Only freshly prepared reagent should be used for the reaction.
The reaction of a silver mirror corresponds to the equation:
2OΗ + HCOΗ -> 2Ag↓ + ΗCOONΗ 4 + 3NΗ 3 + H 2 O
It is worth noting that for aldehydes this interaction has not been sufficiently studied. The mechanism of this reaction is unknown, but a radical or ionic version of oxidation is assumed. With diammine silver hydroxide, the addition most likely occurs to form a silver diol salt, from which silver is then cleaved to form a carboxylic acid.
For successful implementation experience, the cleanliness of the utensils used is extremely important. This is due to the fact that the colloidal silver particles formed during the experiment must adhere to the surface of the glass, creating a mirror surface. In the presence of the slightest contaminants, it will fall out in the form of a gray flaky sediment.
Alkaline solutions should be used to clean the container. So, for these purposes, you can take a NaOH solution, which needs to be washed off with a large volume of distilled water. There should be no grease or mechanical particles on the surface of the glass.
The oxidation reaction of aldehydes with copper(II) hydroxide is also quite spectacular and effective in determining the type of functional group. It proceeds at a temperature corresponding to the boiling of the reaction mixture. In this case, aldehydes reduce divalent copper in Fehling’s reagent (freshly prepared ammonia solution of Cu(OH) 2) to monovalent copper. They themselves are oxidized due to the introduction of an oxygen atom into the C-H bond (the oxidation state of C changes from +1 to +3).
The progress of the reaction can be visually monitored by the change in color of the solution mixture. The bluish precipitate of copper hydroxide gradually turns yellow, corresponding to cuprous hydroxide and further appearance bright red precipitate Cu 2 O.
This process corresponds to the reaction equation:
R-SON + Cu 2+ + NaOH + H 2 O -> R-COONa + Cu 2 O + 4H +
It is worth noting that this reagent works best on aldehydes. In this case, oxidation does not require heating and is carried out at a temperature of 0-20 ° C for a fairly short period of time, and the yield of products is more than 80%. The main disadvantage of the Jones reagent is the lack of high selectivity for other functional groups, and besides, the acidic environment sometimes leads to isomerization or destruction.
Jones' reagent is a solution of chromium(VI) oxide in dilute acetone. It can also be obtained from sodium dichromate. When aldehydes are oxidized, carboxylic acids are formed under the influence of this reagent.
Oxidation of acetaldehyde in industry is carried out by exposure to oxygen in the presence of catalysts - cobalt or manganese ions. First, peracetic acid is formed:
CH 3 -SON + O 2 -> CH 3 -COOON
It, in turn, interacts with the second molecule of acetaldehyde and, through a peroxide compound, produces two molecules of acetic acid:
CH 3 -COOON + CH 3 -SON -> 2CH 3 -COOH
Oxidation is carried out at a temperature of 60-70 °C and a pressure of 2·10 5 Pa.
To oxidize aldehyde groups, a solution of iodine in the presence of alkali is sometimes used. This reagent is of particular importance in the process of carbohydrate oxidation, since it acts very selectively. So, under its influence, D-glucose is converted into D-gluconic acid.
Iodine in the presence of alkalis forms hypoiodide (a very strong oxidizing agent): I 2 + 2NaOΗ -> NaIO + NaI + H 2 O.
Under the influence of hypoiodide, formaldehyde is converted into methane acid: ΗСОΗ + NaIO + NaOΗ -> ΗCOONa + NaI + H 2 O.
Oxidation of aldehydes with iodine is used in analytical chemistry to determine their quantitative content in solutions.
Unlike previous reagents, under the influence of selenium dioxide, aldehydes are converted into dicarbonyl compounds, and glyoxal is formed from formaldehyde. If methylene or methyl groups are located next to the carbonyl, they can be converted into carbonyl groups. Dioxane, ethanol or xylene are usually used as a solvent for SeO2.
According to one of the methods, the reaction is carried out in a three-neck flask connected to a stirrer, thermometer and reflux condenser. To the starting substance, taken in an amount of 0.25 mol, a solution of 0.25 mol of selenium dioxide in 180 ml of dioxane, as well as 12 ml of H 2 O, is added dropwise. The temperature should not exceed 20 ° C (if necessary, cool the flask). After this, with constant stirring, the solution is boiled for 6 hours. Next, the hot solution is filtered to separate selenium and the precipitate is washed with dioxane. After vacuum distillation of the solvent, the residue is fractionated. The main fraction is selected in a wide temperature range (20-30 °C) and rectified again.
Under the influence of atmospheric oxygen at room temperature, the oxidation of aldehydes occurs very slowly. The main products of these reactions are the corresponding carboxylic acids. The autoxidation mechanism is similar to the industrial oxidation of ethanal to acetic acid. One of the intermediate products is a peracid, which reacts with another aldehyde molecule.
Due to the fact that this type of reaction is accelerated by light, peroxides, and traces of heavy metals, it can be concluded that it has a radical mechanism. Formaldehyde in aqueous solutions is much worse than its counterparts in being oxidized by air, due to the fact that it exists in them in the form of hydrated methylene glycol.
This reaction occurs most successfully in Visually, its progress can be assessed by the loss of intensity and complete discoloration of the pink color of the potassium permanganate solution. The reaction takes place at room temperature and normal pressure, so it does not require special conditions. It is enough to pour 2 ml of formaldehyde and 1 ml of acidified with sulfuric acid into the test tube. The test tube with the solution must be shaken carefully to mix the reagents:
5CH 3 -SON + 2KMnO 4 + 3H 2 SO 4 = 5CH 3 -COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O
If the same reaction is carried out at elevated temperatures, then methanal is easily oxidized to carbon dioxide:
5CH 3 -SON + 4KMnO 4 + 6H 2 SO 4 = 5CO 2 + 4MnSO 4 + 2K 2 SO 4 + 11H 2 O
Aldehydes are those organic substances whose molecules contain a carbonyl group bonded to at least one hydrogen atom and a hydrocarbon radical.
The chemical properties of aldehydes are determined by the presence of a carbonyl group in their molecule. In this regard, addition reactions can be observed in the carbonyl group molecule.
So, for example, if you take formaldehyde vapor and pass it along with hydrogen over a heated nickel catalyst, then hydrogen will join and formaldehyde will be reduced to methyl alcohol. In addition, the polar nature of this bond also gives rise to reactions of aldehydes such as the addition of water.
Now let's look at all the features of reactions from the addition of water. It should be noted that a hydroxyl group is added to the carbon atom of the carbonyl group, which carries a partial positive charge due to the electron pair of the oxygen atom.
The following reactions are typical for this addition:
First, hydrogenation occurs and primary alcohols RCH2OH are formed.
Secondly, alcohols are added and hemiacetals R-CH (OH) – OR are formed. And in the presence of hydrogen chloride HCl, acting as a catalyst, and with an excess of alcohol, we observe the formation of acetal RCH (OR)2;
Thirdly, sodium hydrosulfite NaHSO3 is added and derivatives of hydrosulfite aldehydes are formed. During the oxidation of aldehydes, one can observe such special reactions as interaction with an ammonia solution of silver (I) oxide and with copper (II) hydroxide and the formation of carboxylic acids.
The polymerization of aldehydes is characterized by such special reactions as linear and cyclic polymerization.
If we talk about the chemical properties of aldehydes, the oxidation reaction should also be mentioned. Such reactions include the “silver mirror” reaction and the traffic light reaction.
You can observe the unusual reaction of the “silver mirror” by conducting interesting experience. To do this, you will need a cleanly washed test tube, into which you should pour a few milliliters of an ammonia solution of silver oxide, and then add four or five drops of formaldehyde to it. The next step in carrying out this experiment is to place the test tube in a glass with hot water and then you can see how a shiny layer appears on the walls of the test tube. This resulting coating is a deposit of metallic silver.
And here is the so-called “traffic light” reaction:
Now let's begin to consider the physical properties of aldehydes. What properties do these substances have? It should be noted that a number of simple aldehydes are colorless gases, more complex ones are presented in the form of a liquid, but higher aldehydes are solids. The more molecular mass aldehydes, the higher the boiling point. For example, propionaldehyde reaches its boiling point at 48.8 degrees, but propyl alcohol boils at 97.8 0C.
If we talk about the density of aldehydes, then it is less than unity. For example, acetaldehyde and formic aldehyde tend to dissolve well in water, while more complex aldehydes have a weaker ability to dissolve.
Aldehydes, which belong to the lowest category, have a sharp and bad smell, and solid and insoluble in water, on the contrary, are characterized by a pleasant floral smell.
In nature, representatives are found everywhere various groups aldehydes. They are present in the green parts of plants. This is one of the simplest groups of aldehydes, which includes formic aldehyde CH2O.
Aldehydes with more complex composition. These types include vanillin or grape sugar.
But since aldehydes have the ability to easily enter into all sorts of interactions and have a tendency to oxidation and reduction, we can say with confidence that aldehydes are very capable of various reactions and therefore pure form they are extremely rare. But their derivatives can be found everywhere, both in plant and animal environments.
The aldehyde group is present in a number of natural substances. Their distinctive feature, at least many of them, is the smell. For example, representatives of higher aldehydes have various aromas and are part of essential oils. Well, as you already know, such oils are present in floral, spicy and fragrant plants, fruits and fruits. They have found large-scale use in the production of industrial goods and in the production of perfumes.
The aliphatic aldehyde CH3(CH2)7C(H)=O can be found in citrus essential oils. Such aldehydes have an orange odor and are used in Food Industry, as a flavoring agent, as well as in cosmetics, perfumes and household chemicals, as a fragrance.
Formic aldehyde is a colorless gas that has a sharp, specific odor and easily dissolves in water. This aqueous solution of formaldehyde is also called formalin. Formaldehyde is very poisonous, but in medicine it is used in diluted form as a disinfectant. It is used to disinfect instruments, and its weak solution is used to wash the skin when sweating heavily.
In addition, formaldehyde is used in tanning leather, as it has the ability to combine with protein substances that are present in the leather.
IN agriculture formaldehyde has proven itself to be effective in treating grain before sowing. It is used to produce plastics, which are so necessary for equipment and household needs.
Acetaldehyde is a colorless liquid that has the smell of rotten apples and easily dissolves in water. It is used to produce acetic acid and other substances. But since it is a toxic substance, it can cause poisoning of the body or inflammation of the mucous membranes of the eyes and respiratory tract.
Among oxygen-containing organic compounds, two classes of substances are of great importance, which are always studied together for their similarity in structure and properties. These are aldehydes and ketones. It is these molecules that underlie many chemical syntheses, and their structure is interesting enough to become the subject of study. Let's take a closer look at what these classes of compounds are.
From a chemical point of view, the class of aldehydes should include organic molecules containing oxygen as part of the functional group -SON, called carbonyl. The general formula in this case will look like this: R-COH. By their nature, these can be both limiting and non-limiting compounds. Also among them there are aromatic representatives, along with aliphatic ones. The number of carbon atoms in the radical chain varies quite widely, from one (formaldehyde or methanal) to several dozen.
Ketones also contain a carbonyl group -CO, but it is not connected to a hydrogen cation, but to another radical, different or identical to the one in the chain. The general formula looks like this: R-CO-R, . It is obvious that aldehydes and ketones are similar in the presence of a functional group of this composition.
Ketones can also be saturated and unsaturated, and the properties exhibited are similar to those of a closely related class. Several examples can be given to illustrate the composition of molecules and reflect the accepted designations for the formulas of the substances in question.
Obviously, the name of these compounds is formed in two ways:
If you bring general formula for both classes of substances, it will become clear that they are isomers of each other: C n H 2n O. They themselves are characterized by the following types isomerism:
To distinguish between representatives of both classes, qualitative reactions are used, most of which allow the identification of the aldehyde. Since the chemical activity of these substances is slightly higher, due to the presence of a hydrogen cation.
Let's look at what aldehydes and ketones look like in space. The structure of their molecules can be reflected in several points.
As a result, it turns out that such a structure of molecules allows the compounds in question to be both oxidized and reduced. The formula of an aldehyde and a ketone with a redistributed electron density makes it possible to predict the products of reactions in which these substances participate.
Like many organic compounds, people succeeded in isolating and studying aldehydes and ketones only in the 19th century, when vitalistic views completely collapsed and it became clear that these compounds could be formed synthetically, artificially, without the participation of living beings.
However, back in 1661, R. Boyle managed to obtain acetone (dimethyl ketone) when he exposed calcium acetate to heat. But he could not study this substance in detail and name it, determine its systematic position among others. It was only in 1852 that Williamson was able to bring this matter to completion, and then the history of the detailed development and accumulation of knowledge about carbonyl compounds began.
Let's consider what physical properties aldehydes and ketones. Let's start with the first ones.
If we denote the properties of ketones, we can also highlight the main ones.
These are the main properties of the compounds under consideration, which belong to the group of physical ones.
The most important thing is what aldehydes and ketones react with and the chemical properties of these compounds. Therefore, we will definitely consider them. First, let's deal with aldehydes.
A qualitative reaction to this class of substances is the “silver mirror” reaction. As a result, metallic reduced silver and the corresponding carboxylic acid are formed. It requires an ammonia solution of silver oxide or Tollins reagent.
Alcohols, aldehydes, and ketones are compounds with similar properties, since they are all oxygen-containing. However, already at the oxidation stage it becomes clear that alcohols are the most active and easily affected compounds. Ketones are the most difficult to oxidize.
Obviously, the reactions of such interactions are very complex, especially those that are specific. These are all the main features that aldehydes and ketones exhibit. Chemical properties underlie many syntheses of important compounds. Therefore, knowing the nature of molecules and their character during interactions is extremely necessary in industrial processes.
We have already examined these reactions, but did not give them such a name. All interactions as a result of which the carbonyl group exhibited activity can be classified as addition. Or rather, a mobile hydrogen atom. That is why in this matter preference is given to aldehydes, due to their better reactivity.
With what substances are reactions of aldehydes and ketones possible by nucleophilic substitution? This:
These reactions are of great industrial importance, since the products are used in various areas of human activity.
There are several main methods by which aldehydes and ketones are synthesized. Production in laboratory and industry can be expressed in the following ways.
Aromatic aldehydes and ketones are prepared by other methods, since these compounds have an aromatic radical (phenyl, for example).
Naturally, industry tries to use those methods in which the feedstock is as cheap as possible and the catalysts are less toxic. For the synthesis of aldehydes, this is the oxidation of alkenes with oxygen.
The use of aldehydes and ketones is carried out in such industries as:
It is possible to identify more than one area, because approximately 6 million tons of formaldehyde alone are synthesized annually! Its 40% solution is called formalin and is used for storing anatomical objects. It is also used for the production of medicines, antiseptics and polymers.
Acetaldehyde, or ethanal, is also a mass-produced product. The amount of annual consumption in the world is about 4 million tons. It is the basis of many chemical syntheses in which important products are formed. For example:
Aromatic aldehydes and ketones are components of many flavorings, both food and perfume. Most of them have very pleasant floral, citrus, herbal aromas. This makes it possible to produce on their basis:
Some of them are aromatic food additives approved for consumption. Their natural content in essential oils, fruits and resins proves the possibility of such use.
An aldehyde such as citral is a liquid with high viscosity and a strong lemon aroma. It is found in nature in essential oils of the latter. Also contains eucalyptus, sorghum, kebab.
Its areas of application are well known:
