What qualitative reactions allow us to distinguish aldehydes from ketones? Give examples.

How do aldehydes differ from ketones?

Aldehydes and ketones differ in a number of properties, but due to the presence of a carbonyl group in both, their properties have much in common. We will describe these compounds in one chapter, looking at their similarities and differences.

Ketones are not oxidized under these conditions, therefore both reactions are used as qualitative ones, allowing one to distinguish between aldehydes and ketones.

HOW TO DISTINGUISH ALDEHYDES FROM KETONES

Qualitative reactions that distinguish aldehydes from ketones are based on the fact that aldehydes are oxidized much more easily than ketones. Many mild oxidizing agents react readily with aldehydes but are inert towards ketones. Fehling's reagent - a solution of a complex compound containing ions - gives a precipitate of copper(1) oxide with aldehydes. Tollens' reagent containing complex ions gives a silver mirror reaction with aldehydes. This produces metallic silver. Ketones do not reduce either of these reagents.

To distinguish ketones from aldehydes, one can use the easier oxidation of the latter. G

How do ketones differ from aldehydes: a) by structure, b) by methods of preparation, c) by oxidation reactions. Support your answer with reaction equations.

Aldehydes and ketones are highly reactive, which is largely due to the large dipole moment of the carbonyl group.

What substances belong to the class of ketones, what is their functional group and general formula? How do ketones differ from aldehydes in structure and properties?

It is usually possible to distinguish an aldehyde from a ketone using o-dianisidine and benzidine loops. The phosphoric acid loop subtracts epoxy compounds but does not subtract carbonyl compounds (Chapter 5, Section I), and can be used to exclude epoxy compounds. Hydroxyl amine was also used to subtract carbonyl compounds

Ketones differ from aldehydes in their different behavior in oxidation-reduction reactions. For example, keto compounds do not produce a silver mirror and do not reduce Fehling's solution. In general, ketones are very stable against many common oxidizing agents. Only under the influence of very strong oxidizing agents, for example, hot nitric acid, ketones are broken down, forming a mixture of different acids, depending on which direction (I or II) the splitting occurs

In qualitative organic analysis, the bisulfite test is preferred over the silver mirror reaction to distinguish aldehydes from ketones, since methyl ketones can easily be further identified using the iodoform reaction.

Judging by infrared spectrometry data, the molecular structure of kerogen after destruction is characterized by the loss of a significant amount of lipid components, first with the functions of carboxylic acids, ketones and aldehydes, and then by long-chain structures with CH groups. There is an increase in aromatization and polycondensation of the residual part of kerogen, which, in its elemental composition and molecular structure, loses its sapropelic appearance and is almost no different from hydrogen-poor humic organic matter.

For the identification of aldehydes and ketones, crystalline oximes, phenylhydrazones, 2,4-dinitrophenylhydrazones and semicarbazones are used primarily. Aldehydes are distinguished from ketones by the reducing properties of aldehydes (reduction of Tollens reagents, New

Chemical properties. Aldehydes and ketones are highly reactive. Most of their reactions are due to the presence of an active carbonyl group. The double bond is similar in physical nature to the double bond between two carbon atoms (b-bond - b-bond) (Fig. 32). However, unlike a carbon-carbon double bond, the energy of a C-O bond (179 kcal) is greater than the energy of two single C-0 bonds (85.5x2 kcal). On the other hand, oxygen is a more electronegative element, tea

Chemical and spectral identification of aldehydes and ketones is based on the presence of a carbonyl group (C==0) in their molecules. Aldehydes are generally more reactive than ketones. In addition, the C-H bond in the aldehyde group CHO allows, using spectral analysis, to distinguish aldehydes from ketones that do not contain such a bond.

Ketones are not oxidized by those weak oxidizing agents that easily oxidize aldehydes. For example, ketones do not give a silver mirror reaction and are not oxidized by copper hydroxide and fehling solution. However, ketones can be oxidized by such strong oxidizing agents as KMPO4 or chromium mixture. In this case, the carbon chain of the ketone is broken at the carbonyl group to form acids with fewer carbon atoms compared to the original ketone. This also distinguishes ketones from aldehydes.

Unsaturated ketones, aldehydes and other compounds with a double bond activated by electron-withdrawing groups. Unsaturated ketones and aldehydes are a very reactive group of compounds that easily attach various nucleophilic reagents. In unsaturated ketones, the reactivity of the carbonyl group is somewhat reduced due to a decrease in its polarization due to alkyl or aryl groups connected to it. Unlike unsaturated aldehydes, the reaction

This method, commonly called the Angeli reaction, can often prove with certainty whether a given substance is an aldehyde or not. The method is so sensitive that it makes it possible to

Chemical properties. Aldehydes and ketones are highly reactive. Most of their reactions are due to the presence of an active carbonyl group. Double connection skhrd-6

What reactions can be used to distinguish an aldehyde from a ketone?

In which carbonyl is involved. The same connection of the CO group with two alkyl residues causes some difference between ketones and aldehydes. This applies, for example, to their interaction with oxidizing agents.

Ozonides are oily liquids that explode easily. They are not isolated, but are subjected to hydrolytic action of water. The ozonolysis reaction is used to determine the location of the double bond in the carbon chain. This is easy to do after identifying and quantifying the ketones and aldehydes produced during ozonolysis. This method of oxidative cleavage of alkenes is good because, unlike other oxidizing agents, it does not produce by-products.

The skeletal structure of a halogen derivative can be determined by obtaining its parent hydrocarbon from it. The reduction of halogen derivatives to hydrocarbons is carried out by the action of magnesium in ether and subsequent hydrolysis of the organomagnesium compound or (in the case of no-halogen derivatives) by the action of hydrogen iodide when heated in a sealed tube. If a halogen derivative had two halogen atoms located at adjacent carbon atoms, magnesium eliminates both halogen atoms and both carbons are connected by a double bond. Its location is established by the oxidation of the resulting unsaturated compound, accompanied by chain cleavage at the site of the double bond (such reactions will be discussed in the section on olefins). In other cases, the halide derivative is subjected to hydrolysis; the monohalide derivative is converted into an alcohol, the dihalide derivative into a dihydric alcohol (glycol), or, if both halides are located at the same carbon, into an oxo compound (ketone or aldehyde). All of these compounds are easily distinguished by their reactions. The location of the hydroxyl group (OH) in these compounds or carbonyl group (CO) is established by oxidation to acids (these reactions will be considered for alcohols, aldehydes and ketones).

The addition of cerium halides similarly suppresses enolization. In addition, organotitanium reagents are selective in their reactivity towards different types of carbonyl groups; they can distinguish aldehydes from ketones, as in the following example.

Carbonyl compounds are completely subtracted from the gas flow by L1BH4 and L1A1H4. However, alcohols, ethers and epoxy compounds are also subtracted. Attempts have been made to distinguish aldehydes from ketones using NaBN4, but the results, as a rule, were not quantitative.

In addition, the presence of an isolated functional group or a combination of several functional groups in one molecule causes the appearance of characteristic absorption bands, due to which they can be identified in the spectrum of an unknown compound. In this way, using the spectra of model compounds, it is not difficult to distinguish, for example, a ketone from an aldehyde or an amine from an amide. Unfortunately, the infrared spectra of all but the simplest compounds are extremely complex, which usually prevents the assignment of more than 10% of the spectral bands.

Oximation does not distinguish aldehydes from ketones, since both react quantitatively. In addition, for samples containing, along with free carbonyl compounds, acetals, ketals or vinyl ethers, this method is not applicable, since hydroxylamine salts also react with these compounds. Therefore, oximation can also be used for the quantitative determination of acetals, ketals and vinyl ethers (see p. 392).

Ketones and aldehydes react with diazomethane to form homologous products, as shown in schemes (138) and (139). Intermediate (181) undergoes an intramolecular rearrangement, and one can expect that the most electron-donating group I will migrate to the potential cationic center. In fact, the order of migration ability is phenyl > Mb2C=CH > Me n-Pr > iso-Pr > benzyl > tert-Bi it differs from the series found for typical cationic rearrangements see, for example, (182) in equation (140). Other diazoalkanes and diazoacetic acid esters lead to similar rearrangements. Their main use is in the conversion of symmetrical cycloalkanones into their homologues and especially in the expansion of six-membered or large rings. The reaction with α,β-unsaturated ketones requires Lewis acid catalysis and involves the migration of a vinyl α-carbon atom to form a β,7-unsaturated ketone.

Diethyl succinic acid (diethyl succinate) is often used as one of the components in ester condensation. In this case, the reactions proceed according to the usual schemes. However, when diethyl succinate interacts with ketones and aldehydes, which do not enter into self-condensation too easily, in the presence of bases (sodium ethoxide, potassium tributoxide, sodium hydride, etc.), as Stobbe found, specific transformations occur. In this case, unlike other esters, diethyl succinate acts as the methylene component (like diethyl malonate)

An interesting and synthetically important modification of pinacone reduction is the reductive dimerization of ketones or aldehydes, catalyzed by titanium salts of the lowest oxidation state. Such a catalyst is formed by the reduction of titanium chloride (1P) Ti lj with silk metal or zinc-copper pair. In contrast to the classical pinacone reduction, the products of reductive dimerization of carbonyl compounds catalyzed by titanium salts of the lowest oxidation state are alkenes

In contrast, the dehydration of ditertiary, disecondary and even primary-tertiary 1,2-diols, catalyzed by sulfuric acid, p-toluenesulfonic acid, Lewis acids (BP3, etc.), is accompanied by 1,2-migration of an alkyl, aryl group or hydride ion . The rearrangement products are ketones or aldehydes. This rearrangement was discovered by R. Fittig in 1859 during the dehydration of pinacone into pinacoline using concentrated sulfuric acid, so it was called the pinacolta rearrangement

The hydrogen atom of the aldehyde group is very convenient for determining both aliphatic and aromatic aldehydes. From the data of Klink and Stothers, obtained by them in the analysis of a number of aromatic aldehydes, it follows that the chemical shift for the hydrogen atom of the formyl group is in the range of 9.65-10.44 million "relative to the tetramethylsilane line. As a rule, electron-donating groups increase the shielding of the hydrogen atom of the formyl groups, and electron-withdrawing groups reduce it.Aldehydes analyzed by NMR included benzaldehyde, f-toluylaldehyde, /g-toluicaldehyde, p-anisaldehyde, l-fluorobenzaldehyde, o-nitro-benzaldehyde and o- chlorobenzaldehyde.In ortho-substituted aldehydes, the shielding of the formyl hydrogen atom is reduced, possibly due to steric effects.Hypothetically, NMR can distinguish an aldehyde from a ketone without resorting to chemical analysis.

Chemical properties. Aldehydes and ketones are highly reactive. Most of their reactions are due to the presence of an active carbonyl group. The double bond C=0 is similar in physical nature to the double bond between two carbon atoms (a-bond--z-bond). However, oxygen is a more negative element than carbon, and therefore the electron density near the oxygen atom is greater than that near the carbon atom. The dipole moment of the carbonyl group is about 2.7 O. Thanks to this polarization, the carbon atom of the carbonyl group has electrophilic properties

Carbonyl compounds. The weak absorption of the carbonyl group at 260-300 nm can in some cases be a useful addition to infrared spectra as it can distinguish ketones or aldehydes from esters. For example, five-membered cyclic ketones and aliphatic esters absorb in the infrared spectra around 1740 cm->, but only the former have noticeable absorption in the ultraviolet region above 210 nm. The absorption maximum of the carbonyl group can be used for other purposes, however, its position is shifted under the influence of chlorine or bromine atoms in the a-position, and for substituted cyclohexanones the magnitude of this shift depends on whether the halogen atom is in the equatorial or axial position. A similar shift is observed in the spectra of a-hydroxy- and a-acetoxytones. Therefore, this addition to infrared spectra is a way to determine the configuration of substituted cyclic ketones.

A significant amount of oxygen compounds, mainly ketones and aldehydes, contained in the products of the distillation of shale resins, distinguishes them from products of petroleum origin and, of course, affects the specific gravity. Moreover, if the distillates of vacuum and atmospheric distillations of petroleum products differ mainly in specific gravities and boiling points, then the vacuum distillation of shale resins produces products that differ in composition from the products of atmospheric distillation. I. Hysse, for example, found that the resin of Estonian oil shale, when distilled under atmospheric pressure, does not give fractions with specific gravities of more than 0.97, while when distilled under vacuum, the same amount of distillation gives fractions with specific gravities of 1.015-G.02 .

Unlike ketones, the reduction of aldehydes on skeletal nickel is greatly facilitated by the introduction of platinum chloride as an activating additive. The addition of triethylamine, a mixture of triethylamine and platinum chloride, or ready-made chloroplatinate (C.,H-)..K], [H,P1Clc] also increases the activity of the catalyst in this reaction. The best results were achieved in cases where there was an excess of amine in the mixture and the amine was in excess. Below are c.133,

The general formula of ketones is: R 1 -CO-R 2.

According to the IUPAC nomenclature, the names of ketones are formed by adding the suffix “on” to the name of the corresponding hydrocarbons or to the name of the radicals associated with the C=O keto group, the word “ketone”; if an older group is present, the keto group is designated by the prefix "oxo". For example, the compounds CH 3 -CH 2 -CO-CH 2 -CH 2 -CH 3 are called 3-hexanone or ethylpropyl ketone, the compounds CH 3 -CO-CH 2 -CH 2 -COOH are called 4-oxopentanoic acid. Some ketones have common names.

Among other carbonyl compounds, the presence in ketones of precisely two carbon atoms directly bonded to the carbonyl group distinguishes them from carboxylic acids and their derivatives, as well as aldehydes.

A special class of cyclic unsaturated diketones are quinones.

Physical properties

The simplest ketones are colorless, volatile liquids that dissolve in water. Ketones have a pleasant odor. Higher ketones are solid, fusible substances. There are no gaseous ketones, since the simplest of them (acetone) is a liquid. Many chemical properties characteristic of aldehydes are also exhibited by ketones.

Keto-enol tautomerism

Tautomerism is a type of isomerism in which rapid spontaneous reversible interconversion of structural isomers - tautomers - occurs. The process of interconversion of tautomers is called tautomerization.

Ketones that have at least one α-hydrogen atom undergo keto-enol tautomerization.

For oxo compounds having a hydrogen atom in the α-position relative to the carbonyl group, there is an equilibrium between the tautomeric forms. For the vast majority of oxo compounds, this equilibrium is shifted towards the keto form. The process of converting the keto form into the enol form is called enolization. This is the basis for the ability of such ketones to react as C- or O-nucleophiles. The concentration of the enol form depends on the structure of ketones and is (in%): 0.0025 (acetone), 2 (cyclohexanone), 80 (acetylacetone). The rate of enolization increases in the presence of acids and bases.

Chemical properties

In terms of the degree of oxidation, ketones, like aldehydes, occupy an intermediate position between alcohols and acids, which largely determines their chemical properties.
1. Ketones are reduced to secondary alcohols by metal hydrides, for example LiAlH 4 or NaBH 4 , hydrogen (cat. Ni, Pd), isopropanol in the presence of Al alcoholate (Meerwein-Ponndorff-Verley reaction).

R 2 CO + 2H → R 2 CH(OH)

2. When ketones are reduced with sodium or electrochemically (cathodic reduction), pinacones are formed.

2R 2 CO + 2H → R 2 CH(OH)-CR 2 (OH)

3. When ketones interact with amalgamated Zn and concentrated HCl (Clemmensen reaction) or with hydrazine in an alkaline medium (Kizhner-Wolff reaction), the C=O group is reduced to CH 2.

4. Oxidation of ketones

Unlike aldehydes, many ketones are stable to oxygen during storage. Ketones containing an α-methylene group are oxidized by SeO 2 to 1,2-diketones, more energetic oxidizing agents, for example. KMnO 4 - to a mixture of carboxylic acids. Cyclic ketones, when interacting with HNO 3 or KMnO 4, undergo oxidative cleavage of the cycle, for example, adipic acid is formed from cyclohexanone. Linear ketones are oxidized by peracids to esters, cyclic ketones to lactones (Bayer-Villiger reaction).

If, for example, a chromium mixture (a mixture of concentrated sulfuric acid and a saturated solution of potassium dichromate) is used as an oxidizing agent when heated. The oxidation of ketones is always accompanied by the rupture of carbon-carbon bonds, resulting in the formation, depending on the structure of the original ketone, a mixture of acids and ketones with a smaller number of carbon atoms. Oxidation proceeds according to the following scheme:

The carbon in the α-position relative to the carbonyl group, usually the least hydrogenated, is oxidized first. If the ketone is a methyl ketone, then one of its oxidation products will be carbon dioxide. The bond between adjacent carbonyl carbons breaks easily, resulting in:

The oxidation of ketones to carboxylic acids cannot occur without cleavage of the carbon skeleton and requires more stringent conditions than the oxidation of aldehydes. A. N. Popov, who studied the oxidation of ketones, showed that all four possible carboxylic acids can be formed from an asymmetrically constructed ketone during oxidation (Popov’s rule):

If the ketone contains a tertiary carbon atom in the α-position, then as a result of oxidation three carboxylic acids and a new ketone are formed, which, depending on the conditions, can either undergo further oxidation or remain unchanged:

5. Aldol and creton condensation

Ketones form products of substitution of α-H atoms during halogenation by the action of Br 2, N-bromosuccinimide, SO 2 Cl 2, and during thiylation with disulfides. During the alkylation and acylation of ketone enolates, either products of substitution of α-H atoms in ketones or O-derivatives of enols are formed. Aldol and creton condensations are of great importance in organic synthesis, for example:

When condensing with aldehydes, ketones react mainly as CH acids, for example, from ketones and CH 2 O in the presence of a base, α, β-unsaturated ketones are obtained:

RCOCH 3 + CH 2 O → RCOCH=CH 2 + H 2 O

Due to the polarity of the carbonyl group

ketones can react as C-electrophiles, for example during condensation with carboxylic acid derivatives (Stobbe condensation, Darzan reaction, etc.):

(CH 3) 2 CO + (C 2 H 5 OOCCH 2) 2 + (CH 3) 3 COK → (CH 3) 2 =C(COOC 2 H 5)CH 2 COOK + C 2 H 5 OH + (CH 3 ) 3 COH

α,β-unsaturated ketones are especially susceptible to nucleophilic attack, but in this case the double bond is attacked (Michael reaction), for example:

6. Interaction with the Ylides

When interacting with ylides P (alkylidene phosphoranes), ketones exchange the O atom for an alkylidene group (Wittig reaction):

R 2 C=O + Ph 3 P=CHR" → R 2 C=CHR" + Ph 3 PO

7. With cyclopentadiene, ketones form fulvenes, for example:

8. The condensation of ketones with hydroxylamine produces ketoximes R 2 C=NOH, with hydrazine - hydrazones R 2 C=N-NH 2 and azines R 2 C=N-N=CR 2, with primary amines - Schiff bases R 2 C=NR" , with secondary amines - enamines.

9. Addition at the carbonyl group

Ketones are capable of adding water, alcohols, sodium bisulfite, amines and other nucleophiles at the carbonyl group, although these reactions do not proceed as easily as in the case of aldehydes.

Since in alcohol solutions the equilibrium between the ketone and its hemiketal is strongly shifted to the left, it is difficult to obtain ketals from ketones and alcohols:

RCOR" + R"OH ↔ RR"C(OH)OR"

For this purpose, the reaction of ketones with orthoformic acid esters is used. Ketones react with C-nucleophiles, for example with lithium, zinc or organomagnesium compounds, as well as with acetylenes in the presence of bases (Favorsky reaction), forming tertiary alcohols:

In the presence of bases, HCN adds to ketones, giving α-hydroxynitriles (cyanohydrins):

R 2 C=O + HCN → R 2 C(OH)CN

When catalyzed by acids, ketones react as C-electrophiles with aromatic compounds, for example:

Homolytic addition of ketones to olefins leads to α-alkyl-substituted ketones, photocycloaddition to oxetanes, for example:

Getting Ketones

1. Oxidation of alcohols

Ketones can be produced by oxidation of secondary alcohols. The oxidizing agent commonly used for this purpose in laboratories is chromic acid, most often used in the form of a “chromic mixture” (a mixture of potassium or sodium dichromate with sulfuric acid). Sometimes permanganates of various metals or manganese peroxide and sulfuric acid are also used.

2. Dehydrogenation (dehydrogenation) of secondary alcohols

When alcohol vapor is passed through heated tubes with finely crushed copper metal reduced by hydrogen, secondary alcohols break down into ketone and hydrogen. This reaction is somewhat worse in the presence of nickel, iron or zinc.

3. From monobasic carboxylic acids

Ketones can be obtained by dry distillation of calcium and barium salts of monobasic acids. For all acids except formic acid, the reaction proceeds as follows:

More often, it is not the acids themselves that are reduced, but their derivatives, for example, acid chlorides:

CH 3 -CO-Cl + 2H → CH 3 -CHO + HCl

i.e., a ketone with two identical radicals and calcium carbonate are formed.

If you take a mixture of salts of two acids or a mixed salt, then along with the previous reaction, a reaction also occurs between molecules of different salts:

Instead of dry distillation of prepared salts, a contact method is also used, the so-called acid ketonization reaction, which consists of passing acid vapors at elevated temperatures over catalysts, which include calcium or barium carbonate salts, manganese oxide, thorium oxide, aluminum oxide, etc. .

Here, salts of organic acids are first formed, which then decompose, regenerating substances that are catalysts. As a result, the reaction proceeds, for example, for acetic acid according to the following equation:

2CH 3 -COOH → CH 3 -CO-CH 3 + H 2 O + CO 2

4. Effect of water on dihalide compounds

Ketones can be obtained by reacting with water dihalide compounds containing both halogen atoms at the same carbon atom. In this case, one would expect the exchange of halogen atoms for hydroxyls and the formation of dihydric alcohols in which both hydroxyl groups are located at the same carbon atom, for example:

But such dihydric alcohols do not exist under ordinary conditions; they split off a water molecule, forming ketones:

5. The effect of water on acetylene hydrocarbons (Kucherov reaction)

When water acts on acetylene homologues in the presence of mercuric oxide salts, ketones are obtained:

CH 3 -C≡CH + H 2 O → CH 3 -CO-CH 3

6. Preparation using organomagnesium and organozinc compounds

When derivatives of carboxylic acids interact with some organometallic compounds, the addition of one molecule of an organometallic compound to the carbonyl group proceeds according to the following scheme:

If the resulting compounds are exposed to water, they react with it to form ketones:

When an acid amide is exposed to two molecules of an organomagnesium compound and then water, ketones are obtained without the formation of tertiary alcohols:

7. Effect of organocadmium compounds on acid chlorides

Organocadmium compounds interact with acid chlorides differently than organomagnesium or organozinc compounds:

R-CO-Cl + C 2 H 5 CdBr → R-CO-C 2 H 5 + CdClBr

Since organocadmium compounds do not react with ketones, tertiary alcohols cannot be produced here.

Uses of ketones

In industry, ketones are used as solvents, pharmaceuticals, and to make various polymers. The most important ketones are acetone, methylethyl ketone and cyclohexanone.

Physiological action

Toxic. They have an irritating and local effect, penetrate the skin, especially well-unsaturated aliphatic ones. Some substances have carcinogenic and mutagenic effects. Halogenated ketones cause severe irritation of mucous membranes and burns upon contact with skin. Alicyclic ketones have a narcotic effect.

Ketones play an important role in the metabolism of substances in living organisms. Thus, ubiquinone is involved in the redox reactions of tissue respiration. Compounds containing a ketone group include some important monosaccharides (fructose, etc.), terpenes (mentone, carvone), essential oil components (camphor, jasmone), natural dyes (indigo, alizarin, flavones), steroid hormones (cortisone, progesterone ), musk (muscone), antibiotic tetracycline.

In the process of photosynthesis, 1,5-diphosphate-D-erythropentulose (phospholated ketopentose) is a catalyst. Acetoacetic acid is an intermediate product in the Krebbs cycle.

The presence of ketones in a person’s urine and blood indicates hypoglycemia, various metabolic disorders or ketoacidosis.

Aldehydes and ketones.

Aldehydes and ketones have similar chemical structures. Therefore, the story about them is combined in one chapter.

The structure of both compounds contains a divalent carbonyl group:

The difference between aldehydes and ketones is as follows. In aldehydes, the carbonyl group is bonded to one hydrogen atom and a hydrocarbon radical, while in ketones it is bonded to two hydrocarbon radicals.

Chemical properties of aldehydes and ketones.

The presence of a carbonyl group in both aldehydes and ketones determines a certain similarity in their properties. However, there are also differences. This difference is explained by the presence in the aldehyde molecule of a hydrogen atom bonded to the carbonyl group. (There is no such atom in the ketone molecule).

The carbonyl group and the hydrogen atom associated with it are separated into a separate functional group. This group was named aldehyde functional group.

Due to the presence of hydrogen in the aldehyde molecule, the latter are easily oxidized (add oxygen) and converted into carboxylic acids.

For example, the oxidation of acetaldehyde produces acetic acid:

Due to their easy oxidation, aldehydes are energetic reducing agents. This makes them significantly different from ketones, which are much more difficult to oxidize.

Preparation of aldehydes and ketones.

Aldehydes and ketones can be prepared by oxidation of the corresponding alcohols having the same carbon skeleton and hydroxyl at the same carbon atom, which forms a carbonyl group in the resulting aldehyde or ketone.

If a primary alcohol is used as the oxidized alcohol, the oxidation will result in an aldehyde.

Formic aldehyde (formaldehyde).

is the simplest aldehyde with the formula:

Formaldehyde is obtained from methyl alcohol, the simplest of alcohols.

In formaldehyde, the radical is a hydrogen atom.

Properties:

is a gas with a pungent unpleasant odor, highly soluble in water. It has antiseptic and tanning properties.

Receipt:

Receive formaldehyde from methyl alcohol by catalytic oxidation with atmospheric oxygen or by dehydrogenation (elimination of hydrogen).

Application:

An aqueous solution of formaldehyde (usually 40%) is called formaldehyde. Formalin is widely used for disinfection and preservation of anatomical specimens. Significant amounts of formaldehyde are used to produce phenol-formaldehyde resins.

It is one of the most important aldehydes. It matches ethyl alcohol and can be obtained by its oxidation.

Acetaldehyde widely found in nature and produced in large quantities industrially. It is present in coffee, ripe fruits, bread, and is synthesized by plants as a result of their metabolism.

Properties:

Acetaldehyde– lightly boiling colorless liquid (boiling point 21 degrees C). It has a characteristic smell of rotten apples and is highly soluble in water.

Receipt:

In industry acetaldehyde it turns out:

1. ethylene oxidation,
2. adding water to acetylene,
3. oxidation or dehydrogenation of ethyl alcohol.

Application:

Apply acetaldehyde for the production of acetic acid, butadiene, some organic substances, aldehyde polymers.

Dimethyl ketone (acetone).

Dimethyl ketone (acetone) is the simplest ketone. In its molecule, the role of hydrocarbon radicals is played by methyl CH 3(methane residue).

Properties:

Acetone– colorless liquid with a characteristic odor.

Boiling temperature 56,2 degrees WITH.

Acetone mixes with water in all proportions.

It is one of the metabolites produced by the human body.

Receipt:

1. Acetone can be obtained by oxidation of propene,
2. Methods used to obtain acetone from isopropyl alcohol and acetylene,
3. Main part acetone obtained as a co-product in the production of phenol from benzene using the cumene method.

Application:

Acetone- Very good solvent many organic substances. Widely used in the paint and varnish industry, in the production of certain types of artificial fiber, unbreakable organic glass, film, and smokeless powder. Acetone also used as a starting material for the synthesis of a number of organic compounds.

Lecture No. 11

ALDEHYDES AND KETONES

Plan

1. Receipt methods.

2. Chemical properties.

2.1. Nucleophilic reactions
accession.

2.2. Reactions by a -carbon atom.

2.3.

Lecture No. 11

ALDEHYDES AND KETONES

Plan

1. Receipt methods.

2. Chemical properties.

2.1. Nucleophilic reactions
accession.

2.2. Reactions by a -carbon atom.

2.3. Oxidation and reduction reactions.

Aldehydes and ketones contain a carbonyl group
C=O. General formula:

1. Methods of obtaining.

2. Chemical
properties.

Aldehydes and ketones are one of the most reactive classes
organic compounds. Their chemical properties are determined by the presence
carbonyl group. Due to the large difference in electronegativity
carbon and oxygen and high polarizability p -bonds The C=O bond has significant polarity
( m C=O =2.5-2.8 D). Carbonyl carbon atom
group carries an effective positive charge and is an object for attack
nucleophiles. The main type of reactions of aldehydes and ketones is reactions
nucleophilic addition Ad N. In addition, the carbonyl group affects
reactivity of the C-H bond a -position, increasing its acidity.

Thus, molecules of aldehydes and ketones
contain two main reaction centers - the C=O bond and the C-H bond in a-position:

2.1. Nucleophilic reactions
accession.

Aldehydes and ketones easily add nucleophilic reagents to the C=O bond.
The process begins with an attack by a nucleophile on the carbonyl carbon atom. Then
The tetrahedral intermediate formed in the first stage adds a proton and
gives the addition product:

Activity of carbonyl compounds in
Ad N –reactions depend on the magnitude
effective positive charge on the carbonyl carbon atom and volume
substituents on the carbonyl group. Electron-donating and bulky substituents
complicate the reaction, electron-withdrawing substituents increase the reaction
carbonyl compound ability. Therefore, aldehydes in
Ad N -reactions are more active than
ketones.

The activity of carbonyl compounds increases in
presence of acid catalysts, which increase the positive charge by
carbonyl carbon atom:

Aldehydes and ketones add water, alcohols,
thiols, hydrocyanic acid, sodium hydrosulfite, compounds like
N.H. 2 X. All addition reactions
proceed quickly, under mild conditions, but the resulting products, as a rule,
thermodynamically unstable. Therefore, the reactions proceed reversibly, and the content
addition products in the equilibrium mixture may be low.

Connecting water.

Aldehydes and ketones add water to
formation of hydrates. The reaction is reversible. Forming hydrates
thermodynamically unstable. The balance is shifted towards products
addition only in the case of active carbonyl compounds.

Trichloroacetic aldehyde hydration product
chloral hydrate is a stable crystalline compound that is used in
medicine as a sedative and hypnotic.

Addition of alcohols and
thiols.

Aldehydes combine with alcohols to form hemiacetals. In excess of alcohol and in the presence of an acid catalyst
the reaction goes further - until the formation acetals

The reaction of hemiacetal formation proceeds as
nucleophilic addition and is accelerated in the presence of acids or
grounds.

The process of acetal formation goes like this:
nucleophilic substitution of the OH group in the hemiacetal and is possible only under conditions
acid catalysis, when the OH group is converted into a good leaving group
(H 2 O).

The formation of acetals is a reversible process. IN
In an acidic environment, hemiacetals and acetals are easily hydrolyzed. In an alkaline environment
hydrolysis does not occur. The formation and hydrolysis reactions of acetals play an important role in
chemistry of carbohydrates.

Ketones under similar conditions do not
give.

Thiols are stronger nucleophiles than alcohols
form addition products with both aldehydes and ketones.

Joining hydrocyanic
acids

Hydrocyanic acid adds to a carbonyl compound under conditions
basic catalysis to form cyanohydrins.

The reaction has preparative value and
used in synthesis a-hydroxy- and a -amino acids (see lecture No. 14). Fruits of some plants
(eg bitter almonds) contain cyanohydrins. Stands out when they
When broken down, hydrocyanic acid has a poisonous effect.

Bisulfite addition
sodium

Aldehydes and methyl ketones add sodium bisulfite NaHSO 3 with the formation of bisulfite derivatives.

Bisulfite derivatives of carbonyl compounds
– crystalline substances that are insoluble in excess sodium bisulfite solution.
The reaction is used to isolate carbonyl compounds from mixtures. Carbonyl
the compound can be easily regenerated by treating the bisulfite derivative
acid or alkali.

Interaction with common connections
formula NH 2 X.

Reactions proceed according to the general scheme as a process
attachment-elimination. The adduct formed at the first stage is not
stable and easily removes water.

According to the given scheme with carbonyl
compounds react with ammonia, primary amines, hydrazine, substituted hydrazines,
hydroxylamine.

The resulting derivatives are
crystalline substances that are used for isolation and identification
carbonyl compounds.

Imines (Schiff bases) are intermediate
products in many enzymatic processes (transamination under the influence
coenzyme pyridoxal phosphate; reductive amination of keto acids at
participation of the coenzyme NADN). The catalytic hydrogenation of imines produces
amines The process is used to synthesize amines from aldehydes and ketones and
called reductive amination.

Reductive amination occurs in vivo
during the synthesis of amino acids (see lecture No. 16)

2.2. Reactions by a -carbon atom.

Keto-enol tautomerism.

Hydrogen in a -position to the carbonyl group is acidic
properties, since the anion formed during its elimination is stabilized by
resonance account.

The result of the proton mobility of the hydrogen atom
V a -position
is the ability of carbonyl compounds to form enol forms due to
proton migration from a -position to the oxygen atom of the carbonyl group.

Ketone and enol are tautomers.
Tautomers are isomers that can quickly and reversibly convert into each other
due to the migration of a group (in this case, a proton). Equilibrium between
ketone and enol are called keto-enol tautomerism.

The enolization process is catalyzed by acids and
reasons. Enolization under the influence of a base can be represented by
with the following diagram:

Most carbonyl compounds exist
predominantly in ketone form. The content of the enol form increases with
an increase in the acidity of the carbonyl compound, as well as in the case of
additional stabilization of the enol form due to hydrogen bonding or due to
pairing.

Table 8. Content of enol forms and
acidity of carbonyl compounds

For example, in 1,3-dicarbonyl compounds
the mobility of the protons of the methylene group increases sharply due to
electron-withdrawing effect of two carbonyl groups. In addition, enol
the form is stabilized due to the presence in it of a system of conjugate p -bonds and intramolecular
hydrogen bond.

If a compound in enol form is
is a conjugated system with high stabilization energy, then the enol form
prevails. For example, phenol exists only in the enol form.

Enolization and formation of enolate anions are
the first stages of the reactions of carbonyl compounds occurring through a -carbon atom. The most important
of which are halogenation And aldolic-crotonic
condensation .

Halogenation.

Aldehydes and ketones easily react with halogens (Cl2,
Br 2, I 2 ) with education
exclusively a -halogen derivatives.

The reaction is catalyzed by acids or
reasons. The reaction rate does not depend on the concentration and nature of the halogen.
The process proceeds through the formation of the enol form (slow stage), which
then reacts with halogen (fast step). Thus, the halogen is not
involved in speed—defining stage
process.

If a carbonyl compound contains several a -hydrogen
atoms, then the replacement of each subsequent one occurs faster than the previous one,
due to an increase in their acidity under the influence of electron-withdrawing influence
halogen. In an alkaline environment, acetaldehyde and methyl ketones give
trihalogen derivatives, which are then decomposed by excess alkali with
formation of trihalomethanes ( haloform reaction) .

The breakdown of triiodoacetone occurs as a reaction
nucleophilic substitution. CI groups 3 — hydroxide anion, like S N -reactions in the carboxyl group (see lecture No. 12).

Iodoform precipitates from the reaction mixture in the form
pale yellow crystalline sediment with a characteristic odor. Iodoform
the reaction is used for analytical purposes to detect compounds of the type
CH 3 -CO-R, including
clinical laboratories for the diagnosis of diabetes mellitus.

Condensation reactions.

In the presence of catalytic amounts of acids
or alkalis carbonyl compounds containing a -hydrogen atoms,
undergo condensation to form b -hydroxycarbonyl compounds.

Carbonyl is involved in the formation of the C-C bond.
carbon atom of one molecule ( carbonyl component) And a -carbon atom is different
molecules ( methylene component). This reaction is called aldol condensation(by the name of the condensation product of acetaldehyde -
aldol).

When the reaction mixture is heated, the product easily
dehydrates to form a ,b -unsaturated carbonyl
connections.

This type of condensation is called croton(by the name of the condensation product of acetaldehyde - croton
aldehyde).

Let us consider the mechanism of aldol condensation in
alkaline environment. In the first stage, the hydroxide anion abstracts a proton from a -carbonyl position
compounds to form an enolate anion. Then the enolate anion as a nucleophile
attacks the carbonyl carbon atom of another carbonyl compound molecule.
The resulting tetrahedral intermediate (alkoxide anion) is strong
base and further abstracts a proton from the water molecule.

During aldol condensation of two different
carbonyl compounds (cross-aldol condensation) possible
formation of 4 different products. However, this can be avoided if one of the
does not contain carbonyl compounds a -hydrogen atoms (for example, aromatic aldehydes
or formaldehyde) and cannot act as a methylene component.

As a methylene component in reactions
condensation can be not only carbonyl compounds, but also other
C-H-acids. Condensation reactions have preparative value, since they allow
extend the chain of carbon atoms. According to the type of aldol condensation and
retroaldol decomposition (reverse process) many biochemical reactions occur
processes: glycolysis, synthesis of citric acid in the Krebs cycle, synthesis of neuraminic acid
acids.

2.3. Oxidation reactions and
recovery

Recovery

Carbonyl compounds are reduced to
alcohols as a result of catalytic hydrogenation or under the influence
reducing agents that are donors of hydride anions.

[H]: H 2 /cat., cat. – Ni, Pt,
Pd;

LiAlH4; NaBH4.

Reduction of carbonyl compounds
complex metal hydrides involves nucleophilic attack of the carbonyl group
hydride anion. Subsequent hydrolysis produces alcohol.

Recovery occurs in the same way
carbonyl group in vivo under the influence of the coenzyme NADN, which is
donor of hydride ion (see lecture No. 19).

Oxidation

Aldehydes oxidize very easily
any oxidizing agents, even such weak ones as air oxygen and compounds
silver(I) and copper(II).

The last two reactions are used as
qualitative for the aldehyde group.

In the presence of alkalis, aldehydes that do not contain a -hydrogen atoms
disproportionate to form alcohol and acid (Cannizzaro reaction).

2HCHO + NaOH ® HCOONa + CH 3 OH

This is the reason that the aqueous solution
formaldehyde (formalin) during long-term storage becomes acidic
reaction.

Ketones are resistant to oxidizing agents
neutral environment. In acidic and alkaline environments under the influence of strong
oxidizing agents(KMnO 4 ) They
oxidize by breaking the C-C bond. The carbon skeleton is broken down by
carbon-carbon double bond of enol forms of a carbonyl compound, similar to
oxidation of double bonds in alkenes. This produces a mixture of products
containing carboxylic acids or carboxylic acids and ketones.

Aldehydes and ketones are derivatives of hydrocarbons whose molecules contain a carbonyl group. Aldehydes differ in structure from ketones in the position of the carbonyl group. We talk about the physical properties of aldehydes and ketones, as well as their classification and nomenclature in this article.

Physical properties

Unlike alcohols and phenols, aldehydes and ketones are not characterized by the formation of hydrogen bonds, which is why their boiling and melting points are much lower. Thus, formaldehyde is a gas; acetaldehyde boils at a temperature of 20.8 degrees, while methanol boils at a temperature of 64.7 degrees. Similarly, phenol is a crystalline substance, and benzaldehyde is a liquid.

Formaldehyde is a colorless gas with a pungent odor. The remaining members of the aldehyde series are liquids, while higher aldehydes are solids. The lower members of the series (formaldehyde, acetaldehyde) are soluble in water and have a pungent odor. Higher aldehydes are highly soluble in most organic solvents (alcohols, ethers), C 3 -C 8 aldehydes have a very unpleasant odor, and higher aldehydes are used in perfumery because of their floral odors.

Rice. 1. Table classification of aldehydes and ketones.

The general formula of aldehydes and ketones is as follows:

• aldehyde formula – R-COH
• ketone formula – R-CO-R

Classification and nomenclature

Aldehydes and ketones differ in the type of carbon chain that contains the carbonyl group. Let's consider fatty and aromatic compounds:

• acyclic, limit. The first member of the homologous series of aldehydes is formic aldehyde (formaldehyde, methanal) – CH 2 =O.

Formic aldehyde is used as an antiseptic. It is used to disinfect premises and treat seeds.

The second member of the aldehyde series is acetaldehyde (acetaldehyde, ethanal). It is used as an intermediate product in the synthesis of acetic acid and ethyl alcohol from acetylene.

Rice. 2. Formula: acetaldehyde.

• unlimited. It is necessary to mention such unsaturated aldehyde as acrolein (propenal). This aldehyde is formed during the thermal decomposition of glycerin and fats, of which glycerin is an integral part.
• aromatic. The first member of the homologous series of aromatic aldehydes is benzene aldehyde (benzaldehyde). You can also note a plant-derived aldehyde such as vanillin (3-methoxy-4-hydroxybenzaldehyde).

Rice. 3. Vanillin formula.

Ketones can be purely aromatic or fatty-aromatic. For example, diphenyl ketone (benzophenone) is purely aromatic. Fatty aromatic is, for example, methyl phenyl ketone (acetophenone)

What have we learned?

In 10th grade chemistry lessons, the most important task is to study aldehydes and ketones. In aldehydes, the carbon atom of the carbonyl group is primary, and in ketones it is secondary. Therefore, in aldehydes, the carbonyl group is always bonded to a hydrogen atom. The aldehyde group is more chemically active than the ketone group, especially in oxidation reactions.