T Omsk State University
department organic chemistry
Aldehydes and ketones

Aldehydes and ketones differ in the presence of a carbonyl group >C=O.

The carbonyl group is bond polarized C-O:

Aldehydes and ketones can be considered as derivatives alkanes, which have one of methyl (-CH 3) or methylene groups ( -CH 2 - ) is replaced by a carbonyl group:

Ketones have two alkyl radicals as substituents on the carbonyl group, while aldehydes have one substituent. b- an alkyl group, the other is a hydrogen. This difference leads to significant differences in chemical properties (cm. below).

Nomenclature

NomenclatureIUPAC

When naming aldehydes and ketones, according to the rules of IUPAC nomenclature, the longest carbon chain is selected, which includes a carbonyl group. The numbering of carbon atoms in this chain is carried out from the edge where the carbonyl group is closer, and when forming the name to the name of the hydrocarbon corresponding to the number of carbon atoms in the main chain (1-methane, 2-ethane, 3-propane, 4-butane, 5 - pentane, etc.) the ending is added -a eh (for aldehydes) or -he for ketones.

The position of the carbonyl group on ketones is indicated by a dash if multiple isomers are possible. The position of the carbonyl group of aldehydes is not indicated by a number, since in all cases it is at the first number:

Rational nomenclature

Ketones are often named after radicals connected through a carbonyl group, with the addition of the word ketone. For example, hexanone-3 or methylethyl ketone , acetone or dimethyl keto n.

Aldehydes can be named as derivatives ethanal or acetaldehyde:

Other name e - trimethylethanal.

Chemical properties of carbonyl compounds

All reactions of carbonyl compounds can be divided into groups:

Reactions at the carbonyl group (addition)

Reactions on the carbon skeleton

Oxidation reactions

Recovery reactions

Addition reactions at the carbonyl group (addition of nucleophilic reagents)

1. water connection

The emerging gem-diols unstable and the equilibrium in this reaction is strongly shifted to the left. The exceptions are aldehydes and ketones with electron withdrawing groups, for example, chloral or hexafluoroacetone, which exist in the aquatic environment in the form gem-diols:

2. addition of bisulfite

Attachment goes through the more nucleophilic sulfur atom, not oxygen, although it has a negative charge. Derivatives are formed alkanesulfonic acids(salts alkoxysulfonic acid).

The emerging adducts insoluble in saturated sodium bisulfite solution or in alcohols and precipitate as crystals. So it is possible to separate carbonyl compounds from a mixture with alcohols. The carbonyl compound is isolated in free form from adduct when treated with acid.

When reacting with ketones, bisulfites add only to methyl ketones CH 3 -CO-R.

3. addition of cyanides

The reaction is catalyzed by potassium cyanide or sodium. The emerging oxynitriles(or cyanohydrins) can be hydrolyzed before oxycarboxylic acids:

4. addition of alcohols

When the first molecule of alcohol is added, hemiacetals. The reaction is catalyzed by acids or bases:

The addition of a second alcohol molecule leads to the formation acetals. Education acetals catalyzed only in an acidic environment:

Acetalsstable in neutral and alkaline environments, so they can be used for temporary protection of aldehyde groups. Acetals wide common in nature.

5. connection of reagents Grignard

Interaction of organometallic compounds of the type R-Mg-X(reagents Grignard), where X \u003d halogen, with carbonyl groups (nucleophilic addition at a multiple bond FROM=O):

Interaction formaldehyde, aldehydes, ketones and - leads to primary, secondary and tertiary alcohols, respectively.

Tertiary alcohols are obtained from ketones. Yes, from methyl ethyl ketone(butanone-2) is obtained 2-methylbutanol-2. Aldehydes in a similar reaction give secondary alcohols. From propionic aldehyde ( propanal) turns out butanol-2:

Primary alcohols are formed from formaldehyde. When interacting reagents Grignard With acid halides carboxylic acids and esters form tertiary alcohols, which have two identical alkyl substituents. This consumes two moles of the reagent Grignard:

6. Addition of ammonia and amines

Primary amines add to aldehydes and ketones to formimines (grounds Schiff :

A similar reaction of secondary amines with carbonyl compounds gives enamines :

Hydrazine and its derivatives can also interact with carbonyl compounds to form hydrazones:

Hydroxylamines add to aldehydes and ketones to form aldoximes and ketoximes:

7. Aldol-crotonic condensation

Condensation can occur in both acidic and alkaline environments.

Acid catalyzed condensation

enter into condensation enol and protonated carbonyl group of the second molecule of the compound:

base catalyzed condensation

Education enolate ion, generating carbanion, proceeds according to the scheme:

Further carbanion attaches to the carbonyl group of the second molecule, and proceeds C-alkylation, Unlike thermodynamically disadvantageous O- alkylation:

The emerging aldehyde alcohol (aldol) easily loses water in the presence of catalytic amounts of bases or acids, as well as with slight heating, with the formation of a, b - unsaturated carbonyl compound, this completes the condensation reaction (R, X \u003d alkyl or H):

Thus, in the aldoln reaction about- croton condensation (including self-condensation) can enter both aldehydes and ketones having alpha carbon hydrogen atoms. In the case of ketones, the equilibrium position is unfavorable for the formation of products, however, carrying out the reaction in special conditions(eg by avoiding contact of the product with a basic catalyst) substantial yields can be achieved. Cross-reactions between aldehydes and ketones are not of laboratory use because they form difficult to separate a mixture of four products and unreacted original compounds. More often, for synthetic purposes, a reaction is carried out between two carbonyl compounds, one of which is a source of carbanions ( methylene component ), and the other serves carbonyl component (not having alpha carbon hydrogen atoms). Usually, formaldehyde, aromatic aldehydes, esters of carbonic, oxalic and formic acids are used as the carbonyl component. As a methylene component, they are used, among other things, C-H acids and even derivatives of acetylenic hydrocarbons with a terminal triple bond.

8. Cannizzaro reaction

Aldehydes that do not have alpha carbon hydrogen atoms, when heated with strong bases, enter into an oxidation-reduction reaction, when one of the molecules is reduced to alcohol due to the oxidation of the second molecule to a carboxylic acid. Such reactions are called Cannizzaro reactions, and proceed according to the scheme:

Intramolecular oxidation-reduction reactions are also known:

With a peculiar kind of intramolecular oxidation-reduction is benzyl rearrangement :

Reactions on the carbon skeleton of aldehydes and ketones

Reactions affecting the carbon skeleton include:

Keto-enol tautomerism of aldehydes and ketones;

Halogenation (haloform reaction and substitution of a - carbon hydrogen atoms)

1. Keto-enol tautomerism

Carbonyl compounds can coexist in two forms - ketone and enol:

The transformation of aldehydes and ketones into enols (unsaturated alcohols) proceeds both spontaneously and with catalysis by acids and bases. Although enol forms are present in aldehydes and ketones in insignificant concentrations, they play a significant role in their reactivity. Through the formation of enols, a number of important reactions of aldehydes and ketones take place. Let us consider the mechanisms of the transition of ketone forms to enols occurring under the catalytic action of acids and bases.

Enolization acid catalyzed

Enol formation can be acid catalyzed according to the scheme below (R"=alkyl or H):

The reaction begins with the protonation of the oxygen atom of the carbonyl group and ends with the elimination of a proton already from alpha carbon atom. Thus, formally, the proton plays the role of a catalyst.

Enolization , catalyzed basis

The formation of the enolate ion proceeds according to the scheme:

In the formation of enols during base catalysis, the acidity of alpha-carbon hydrogen atoms plays an important role. Their increased acidity is associated with the close proximity to the carbonyl group and its negative inductive effect, withdrawing electrons. S-N connections and thus facilitating the elimination of a proton. In other words, proton elimination is facilitated because the resulting carbanion is stabilized by the delocalization of the negative charge onto the carbonyl group.

Halogens are added to the formed enols at the C=C multiple bond. Only unlike alkenes, where such addition is completed by the complete binding of the halogen, in aldehydes and ketones only one halogen atom is added (to the carbon adjacent to the carbonyl group). The second halogen atom (on the carbonyl group) does not attach, and the reaction ends with the elimination of a proton and the regeneration of the carbonyl group:

In an acidic environment, the reaction stops there. Substitution of the second hydrogen atom by a halogen does not occur. But in an alkaline medium, a fast substitution reaction of the second occurs, and an even faster reaction of substitution of the third carbon atom for a halogen (an increase in the number of halogen atoms at carbon sharply increases the acidity of its hydrogens):

Ultimately, all three hydrogen atoms are replaced by halogens, followed by the elimination of the group CX 3 as an anion, followed by immediate proton exchange:

As a result, a trihalomethane, called haloform (iodoform CHJ 3, bromoform CHBr 3, chloroform CHCl 3) and an anion of a carboxylic acid. And the process itself is called the haloform reaction. Any methyl ketones are subject to the haloform reaction. The haloforms precipitate as a colored precipitate (yellow iodoform), have a specific odor, and can serve as a qualitative reaction to the presence of methyl ketones. The haloform reaction is also given by alcohols, the oxidation of which can form methyl ketones (for example, isopropanol). Oxidation is carried out by an excess amount of halogen.

Oxidation of aldehydes and ketones

Aldehydes are easily oxidized to the corresponding acids:

Ketones are oxidized with difficulty, under harsh conditions. Oxidation is accompanied by the breaking of the C-C bond in the vicinity of the carbonyl group. The result is a set of oxidation products - carboxylic acids with different carbon chain lengths:

Methods receiving

1. Oxidation primary alcohols aldehydes are obtained, and secondary alcohols give ketones:

Oxidation can be carried out by "dry" and "wet" methods. The first is to pass alcohol vapor through a heated to 300-350 FROM copper oxide CuO. The “wet” method is the oxidation of alcohols with an acidified solution of potassium or sodium bichromate:

When oxidized by the "wet" method, the resulting aldehyde should be distilled off from the reaction sphere, otherwise it is easily oxidized further, to a carboxylic acid:

2. Aldehydes and ketones obtained with hydrolysis gem-dihaloalkanes

First, two halogen atoms are replaced by hydroxyl groups. But unstable gem-diols quickly rearrange into carbonyl compounds with the elimination of a water molecule:

3. Ozonolysis alkenes

leads to the formation of mixtures of aldehydes and ketones, depending on the structure of the initial alkene:

At the first stage of ozonation, ozonide is obtained, upon decomposition of which with water, carbonyl compounds and hydrogen peroxide are formed. To prevent peroxide from provoking further oxidation of aldehydes, zinc dust is added to the water during the decomposition of ozonides. Alkene ozonation is aimed not so much at the synthesis of aldehydes and ketones as at determining the location of the multiple bond:

4. Addition of water to alkynes

The addition of water to a triple bond in the presence of mercury salts leads, in the case of acetylene, to acetaldehyde, and in the case of substituted acetylenes, to ketones. Waterjoins according to Markovnikov's rule:

The structure of aldehydes and ketones

Aldehydes- organic substances whose molecules contain carbonyl group:

bonded to a hydrogen atom and a hydrocarbon radical. The general formula for aldehydes is:

In the simplest aldehyde, the role of the hydrocarbon radical is played by another hydrogen atom:

Formaldehyde

The carbonyl group attached to the hydrogen atom is often referred to as aldehyde:

Ketones are organic substances in the molecules of which the carbonyl group is bonded to two hydrocarbon radicals. Obviously, the general formula for ketones is:

The carbonyl group of ketones is called keto group.

In the simplest ketone, acetone, the carbonyl group is bonded to two methyl radicals:

Nomenclature and isomerism of aldehydes and ketones

Depending on the structure of the hydrocarbon radical associated with the aldehyde group, there are saturated, unsaturated, aromatic, heterocyclic and other aldehydes:

In accordance with the IUPAC nomenclature, the names of saturated aldehydes are formed from the name of an alkane with the same number of carbon atoms from the molecule using the suffix -al. For example:

Numbering carbon atoms of the main chain start from the carbon atom of the aldehyde group. Therefore, the aldehyde group is always located at the first carbon atom, and it is not necessary to indicate its position.

Along with the systematic nomenclature, trivial names of widely used aldehydes are also used. These names are usually derived from the names of carboxylic acids corresponding to aldehydes.

For the title ketones according to the systematic nomenclature, the keto group is denoted by the suffix -he and a number that indicates the number of the carbon atom of the carbonyl group (numbering should start from the end of the chain closest to the keto group).

For example:

For aldehydes only one type of structural isomerism is characteristic - isomerism of the carbon skeleton, which is possible with butanal, and for ketones- also carbonyl position isomerism. In addition, they are also characterized interclass isomerism(propanal and propanone).

Physical properties of aldehydes and ketones

In an aldehyde or ketone molecule, due to the greater electronegativity of the oxygen atom compared to the carbon atom, the bond C=O is highly polarized due to the shift of the electron density of the π-bond to oxygen:

Aldehydes and ketones polar substances with excess electron density on the oxygen atom. The lower members of the series of aldehydes and ketones (formaldehyde, acetaldehyde, acetone) are infinitely soluble in water. Their boiling points are lower than those of the corresponding alcohols. This is due to the fact that in the molecules of aldehydes and ketones, unlike alcohols, there are no mobile hydrogen atoms and they do not form associates due to hydrogen bonds.

Lower aldehydes have a pungent odor; aldehydes containing from four to six carbon atoms in the chain have an unpleasant odor; higher aldehydes and ketones have floral odors and are used in perfumery.

The presence of an aldehyde group in a molecule determines the characteristic properties of aldehydes.

recovery reactions.

1. Addition of hydrogen to aldehyde molecules occurs at the double bond in the carbonyl group:

The product of hydrogenation of aldehydes are primary alcohols, ketones are secondary alcohols.

So, when acetaldehyde is hydrogenated on a nickel catalyst, ethyl alcohol is formed, and when acetone is hydrogenated, propanol-2 is formed.

2. Hydrogenation of aldehydes- reduction reaction, in which the degree of oxidation of the carbon atom included in the carbonyl group decreases.

Oxidation reactions.

Aldehydes can not only be reduced, but also oxidized. When oxidized, aldehydes form carboxylic acids. Schematically, this process can be represented as follows:

1. Oxidation by atmospheric oxygen. For example, propionic acid is formed from propionaldehyde (propanal):

2. Oxidation with weak oxidizing agents(ammonia solution of silver oxide). In a simplified form, this process can be expressed by the reaction equation:

For example:

More precisely, this process is reflected by the equations:

If the surface of the vessel in which the reaction is carried out was previously degreased, then the silver formed during the reaction covers it with an even thin film. Therefore, this reaction is called the "silver mirror" reaction. It is widely used for making mirrors, silvering decorations and Christmas decorations.

3. Oxidation with freshly precipitated copper (II) hydroxide. Oxidizing the aldehyde, Cu 2+ is reduced to Cu + . The copper (I) hydroxide CuOH formed during the reaction immediately decomposes into red copper (I) oxide and water.

This reaction, like the reaction silver mirror”, is used to detect aldehydes.

Ketones are not oxidized either by atmospheric oxygen or by such a weak oxidizing agent as an ammonia solution of silver oxide.

Chemical properties of aldehydes and acids - abstract

Individual representatives of aldehydes and their meaning

Formaldehyde(methanal, formic aldehyde HCHO) is a colorless gas with a pungent odor and a boiling point of -21 ° C, we will readily dissolve in water. Formaldehyde is poisonous! A solution of formaldehyde in water (40%) is called formalin and is used for formaldehyde and acetic disinfection. AT agriculture formalin is used for dressing seeds, in the leather industry - for processing leather. Formaldehyde is used to make urotropin- medicinal substance. Sometimes compressed in the form of briquettes, urotropin is used as a fuel (dry alcohol). A large amount of formaldehyde is consumed in the production of phenol-formaldehyde resins and some other substances.

Acetic aldehyde(ethanal, acetaldehyde CH 3 CHO) - a liquid with a sharp, unpleasant odor and a boiling point of 21 ° C, we will dissolve well in water. Acetic acid and a number of other substances are obtained from acetaldehyde on an industrial scale, it is used for the production of various plastics and acetate fibers. Acetic aldehyde is poisonous!

A group of atoms -

called carboxyl group, or carboxyl.

Organic acids containing one carboxyl group in the molecule are monobasic.

The general formula for these acids is RCOOH, for example:

Carboxylic acids containing two carboxyl groups are called dibasic. These include, for example, oxalic and succinic acids:

There are also polybasic carboxylic acids containing more than two carboxyl groups. These include, for example, tribasic citric acid:

Depending on the nature of the hydrocarbon radical, carboxylic acids are divided into marginal, unsaturated, aromatic.

limiting, or saturated, carboxylic acids are, for example, propanoic (propionic) acid:

or already familiar to us succinic acid.

Obviously, saturated carboxylic acids do not contain π-bonds in the hydrocarbon radical.

In molecules of unsaturated carboxylic acids, the carboxyl group is bonded to an unsaturated, unsaturated hydrocarbon radical, for example, in acrylic (propene) molecules

CH 2 \u003d CH-COOH

or oleic

CH 3 -(CH 2) 7 -CH \u003d CH-(CH 2) 7 -COOH

and other acids.

As can be seen from the formula of benzoic acid, it is aromatic, since it contains an aromatic (benzene) ring in the molecule:

The name of a carboxylic acid is formed from the name of the corresponding alkane (an alkane with the same number of carbon atoms in the molecule) with the addition of the suffix -ov— , ending -and I and words acid. Numbering of carbon atoms starts with a carboxyl group. For example:

The number of carboxyl groups is indicated in the name by prefixes di-, tri-, tetra-:

Many acids also have historically developed, or trivial, names.

The composition of limiting monobasic carboxylic acids will be expressed by the general formula C n H 2n O 2, or C n H 2n+1 COOH, or RCOOH.

Physical properties of carboxylic acids

Lower acids, i.e., acids with a relatively small molecular weight, containing up to four carbon atoms in a molecule, are liquids with a characteristic pungent odor (for example, the smell of acetic acid). Acids containing from 4 to 9 carbon atoms are viscous oily liquids with an unpleasant odor; containing more than 9 carbon atoms in a molecule - solids that do not dissolve in water. The boiling points of limiting monobasic carboxylic acids increase with an increase in the number of carbon atoms in the molecule and, consequently, with an increase in the relative molecular weight. So, the boiling point of formic acid is 100.8 °C, acetic acid - 118 °C, propionic acid - 141 °C.

The simplest carboxylic acid, formic HCOOH, having a small relative molecular weight (M r (HCOOH) = 46), under normal conditions is a liquid with a boiling point of 100.8 °C. At the same time, butane (M r (C 4 H 10) \u003d 58) is gaseous under the same conditions and has a boiling point of -0.5 ° C. This discrepancy between boiling points and relative molecular weights is explained by formation of dimers of carboxylic acids in which two acid molecules are bonded by two hydrogen bonds:

The occurrence of hydrogen bonds becomes clear when considering the structure of carboxylic acid molecules.

Molecules of saturated monobasic carboxylic acids contain a polar group of atoms - carboxyl

And practically non-polar hydrocarbon radical. The carboxyl group is attracted to water molecules, forming hydrogen bonds with them:

Formic and acetic acids are infinitely soluble in water. Obviously, with an increase in the number of atoms in the hydrocarbon radical, the solubility of carboxylic acids decreases.

Chemical properties of carboxylic acids

The general properties characteristic of the class of acids (both organic and inorganic) are due to the presence in the molecules of a hydroxyl group containing a strong polar bond between hydrogen and oxygen atoms. Let us consider these properties using the example of water-soluble organic acids.

1. Dissociation with the formation of hydrogen cations and anions of the acid residue:

More precisely, this process is described by an equation that takes into account the participation of water molecules in it:

The equilibrium of dissociation of carboxylic acids is shifted to the left; the vast majority of them are weak electrolytes. However, the sour taste of, for example, acetic and formic acids is due to the dissociation into hydrogen cations and anions of acidic residues.

Obviously, the presence of “acidic” hydrogen, i.e., the hydrogen of the carboxyl group, in the molecules of carboxylic acids also determines other characteristic properties.

2. Interaction with metals standing in the electrochemical series of voltages up to hydrogen:

So, iron reduces hydrogen from acetic acid:

3. Interaction with basic oxides with the formation of salt and water:

4. Interaction with metal hydroxides with the formation of salt and water (neutralization reaction):

5. Interaction with salts of weaker acids with the formation of the latter. Thus, acetic acid displaces stearic acid from sodium stearate and carbonic acid from potassium carbonate:

6. Interaction of carboxylic acids with alcohols with the formation of esters - the esterification reaction (one of the most important reactions characteristic of carboxylic acids):

The interaction of carboxylic acids with alcohols is catalyzed by hydrogen cations.

The esterification reaction is reversible. The equilibrium shifts towards ester formation in the presence of dewatering agents and when the ester is removed from the reaction mixture.

In the reverse esterification reaction, which is called ester hydrolysis (reaction of an ester with water), an acid and an alcohol are formed:

Obviously, polyhydric alcohols, for example, glycerol, can also react with carboxylic acids, i.e., enter into an esterification reaction:

All carboxylic acids (except formic), along with a carboxyl group, contain a hydrocarbon residue in their molecules. Of course, this cannot but affect the properties of acids, which are determined by the nature of the hydrocarbon residue.

7. Multiple bond addition reactions- unsaturated carboxylic acids enter into them. For example, the hydrogen addition reaction is hydrogenation. For an acid containing one n-bond in the radical, the equation can be written in general form:

So, when oleic acid is hydrogenated, saturated stearic acid is formed:

Unsaturated carboxylic acids, like other unsaturated compounds, add halogens to the double bond. For example, acrylic acid decolorizes bromine water:

8. Substitution reactions (with halogens)- saturated carboxylic acids are able to enter into them. For example, by reacting acetic acid with chlorine, various chlorine derivatives of acids can be obtained:

Chemical properties of carboxylic acids - compendium

Individual representatives of carboxylic acids and their significance

Formic (methane) acid HCOOH- liquid with a pungent odor and a boiling point of 100.8 ° C, highly soluble in water.

Formic acid is poisonous and causes burns if it comes into contact with the skin! The stinging fluid secreted by ants contains this acid.

Formic acid has a disinfectant property and therefore finds its application in the food, leather and pharmaceutical industries, and medicine. It is used in dyeing textiles and paper.

Acetic (ethanoic) acid CH 3 COOH- a colorless liquid with a characteristic pungent odor, miscible with water in any ratio. Aqueous solutions of acetic acid go on sale under the name of vinegar (3-5% solution) and vinegar essence (70-80% solution) and are widely used in the food industry. Acetic acid is a good solvent for many organic matter and therefore is used in dyeing, in the leather industry, in the paint and varnish industry. In addition, acetic acid is a raw material for the production of many technically important organic compounds: for example, it is used to obtain substances used to control weeds - herbicides. Acetic acid is the main component of wine vinegar, the characteristic smell of which is due to it. It is a product of the oxidation of ethanol and is formed from it when wine is stored in air.

The most important representatives of the highest limiting monobasic acids are palmitic C 15 H 31 COOH and stearic C 17 H 35 COOH acids. Unlike lower acids, these substances are solid, poorly soluble in water.

However, their salts - stearates and palmitates - are highly soluble and have a detergent effect, which is why they are also called soaps. It is clear that these substances are produced on a large scale.

From unsaturated higher carboxylic acids highest value It has oleic acid C 17 H 33 COOH, or CH 3 - (CH 2) 7 - CH \u003d CH - (CH 2) 7 COOH. It is an oil-like liquid without taste or smell. Its salts are widely used in technology.

The simplest representative of dibasic carboxylic acids is oxalic (ethanedioic) acid HOOC-COOH, salts of which are found in many plants, such as sorrel and oxalis. Oxalic acid is a colorless crystalline substance, highly soluble in water. It is used in the polishing of metals, in the woodworking and leather industries.

Reference material for passing the test:

periodic table

Solubility table

The first group of properties is addition reactions. In the carbonyl group, between carbon and oxygen, there is a double bond, which, as you remember, consists of a sigma bond and a pi bond. In addition reactions, the pi bond breaks and two sigma bonds are formed, one with carbon and the other with oxygen. Carbon has a partial positive charge, and oxygen has a partial negative charge. Therefore, a negatively charged particle of the reagent, an anion, is attached to carbon, and a positively charged part of the molecule is attached to oxygen.

First property hydrogenation, addition of hydrogen.

The reaction takes place when heated. The hydrogenation catalyst already known to you, nickel, is used. Primary alcohols are obtained from aldehydes, secondary alcohols from ketones.

In secondary alcohols, the hydroxo group is bonded to a secondary carbon atom.

Second property hydration, water addition. This reaction is possible only for formaldehyde and acetaldehyde. Ketones do not react with water at all.

All addition reactions proceed in such a way that plus goes to minus, and minus to plus.

As you remember from the video about alcohols, the presence of two hydroxo groups on one atom is an almost impossible situation, such substances are extremely unstable. So, specifically, these two cases formaldehyde hydrate and acetaldehyde are possible, although they exist only in solution.

It is not necessary to know the reactions themselves. Most likely, the question on the exam may sound like a statement of fact, for example, they react with water and substances are listed. Among their list of which may be methanal or ethanal.

Third property addition of hydrocyanic acid.

Again, plus goes to minus, and minus to plus. Substances called hydroxynitriles are obtained. Again, the reaction itself is not common, but you need to know about this property.

Fourth property addition of alcohols.

Here again, you do not need to know the reaction equation by heart, you just need to understand that such an interaction is possible.

As usual in reactions of addition to a carbonyl group, plus to minus, and minus to plus.

Fifth property reaction with sodium hydrosulfite.

And again, the reaction is quite complicated, it is unlikely to learn it, but this is one of the qualitative reactions for aldehydes, because the resulting sodium salt precipitates. That is, in fact, you should know that aldehydes react with sodium hydrosulfite, this will be enough.

This concludes the first group of reactions. The second group is polymerization and polycondensation reactions.

2. Polymerization and polycondensation of aldehydes

You are familiar with polymerization: polyethylene, butadiene and isoprene rubbers, polyvinyl chloride are the products of combining many molecules (monomers) into one large, into a single polymer chain. That is, one product is obtained. During polycondensation, the same thing happens, but in addition to the polymer, low molecular weight products, such as water, are also obtained. That is, there are two products.

So, sixth property polymerization. Ketones do not enter into these reactions; only the polymerization of formaldehyde is of industrial importance.

The pi bond breaks and two sigma bonds are formed with neighboring monomers. It turns out polyformaldehyde, also called paraform. Most likely, the question on the exam may sound like this: substances enter into the polymerization reaction. And a list of substances is given, among which there may be formaldehyde.

The seventh property is polycondensation. Once again: during polycondensation, in addition to the polymer, a low-molecular compound is also obtained, for example, water. Formaldehyde enters into such a reaction with phenol. For clarity, we first write the equation with two phenol molecules.

As a result, such a dimer is obtained and a water molecule is split off. Now we write the reaction equation in general form.

The polycondensation product is phenol-formaldehyde resin. She finds wide application from adhesives and varnishes to plastics and chipboard components.

Now the third group of properties oxidation reactions.

3. Oxidation of aldehydes and ketones

Eighth reaction in general list is a qualitative reaction to the aldehyde group oxidation with an ammonia solution of silver oxide. Silver mirror reaction. I will say right away that ketones do not enter into this reaction, only aldehydes.

The aldehyde group is oxidized to a carboxyl, acidic group, but in the presence of ammonia, which is a base, a neutralization reaction immediately occurs and a salt, ammonium acetate, is obtained. The silver precipitates, coating the inside of the tube and creating a mirror-like surface. This reaction occurs on the exam all the time.

By the way, the same reaction is qualitative for other substances that have an aldehyde group, for example, formic acid and its salts, as well as glucose.

ninth the reaction is also qualitative for the aldehyde group oxidation with freshly precipitated copper hydroxide two. Here, too, I note that ketones do not enter into this reaction.

Visually, the formation of a yellow precipitate will be observed first, which then turns red. In some textbooks, information is found that copper hydroxide alone is first formed, which has a yellow color, which then decomposes into red copper oxide alone and water. So this is not true according to the latest data, in the process of precipitation, the size of copper oxide particles changes, which ultimately reach sizes that are painted exactly in red. The aldehyde is oxidized to the corresponding carboxylic acid. The reaction occurs on the exam very often.

The tenth reaction is the oxidation of aldehydes with an acidified solution of potassium permanganate when heated.

Discoloration of the solution occurs. The aldehyde group is oxidized to a carboxyl group, that is, the aldehyde is oxidized to the corresponding acid. For ketones, this reaction has no practical meaning, since the destruction of the molecule occurs and the result is a mixture of products.

It is important to note that formic aldehyde, formaldehyde, oxidizes to carbon dioxide, because the corresponding formic acid itself is not resistant to strong oxidizing agents.

As a result, carbon goes from oxidation state 0 to oxidation state +4. Let me remind you that methanol, as a rule, under such conditions is oxidized to the maximum to CO 2, skipping the stage of both aldehyde and acid. This feature must be remembered.

Eleventh reaction combustion, complete oxidation. Both aldehydes and ketones burn to carbon dioxide and water.

Let us write the reaction equation in general form.

According to the law of conservation of mass, there should be as many atoms on the left as there are atoms on the right. Because in fact chemical reactions atoms do not go anywhere, but the order of bonds between them simply changes. So there will be as many carbon dioxide molecules as there are carbon atoms in a molecule of a carbonyl compound, since the molecule contains one carbon atom. That is n CO 2 molecules. There will be half as many water molecules as hydrogen atoms, that is, 2n / 2, which means just n.

There are the same number of oxygen atoms on the left and on the right. On the right, there are 2n of them from carbon dioxide, because each molecule has two oxygen atoms, plus n of water, for a total of 3n. On the left, there are the same number of oxygen atoms 3n, but one of the atoms is in the aldehyde molecule, which means it must be subtracted from the total to get the number of atoms per molecular oxygen. It turns out that 3n-1 atoms contain molecular oxygen, which means there are 2 times fewer molecules, because one molecule contains 2 atoms. That is (3n-1)/2 oxygen molecules.

Thus, we have compiled the equation for the combustion of carbonyl compounds in a general form.

And finally twelfth property related to substitution reactions halogenation at the alpha carbon atom. Let us turn once again to the structure of the aldehyde molecule. Oxygen pulls electron density onto itself, creating a partial positive charge on carbon. The methyl group tries to compensate for this positive charge by shifting electrons from hydrogen to it along a chain of sigma bonds. The carbon-hydrogen bond becomes more polar and the hydrogen breaks off more easily when attacked with a reagent. This effect is observed only for the alpha carbon atom, that is, the atom following the aldehyde group, regardless of the length of the hydrocarbon radical.

Thus, it is possible to obtain, for example, 2-chloroacetaldehyde. Further substitution of hydrogen atoms to trichloroethane is possible.

Aldehydes and ketones are derivatives of hydrocarbons that contain a carbonyl group in their molecules. Aldehydes differ in structure from ketones in the position of the carbonyl group. O physical properties aldehydes and ketones, as well as their classification and nomenclature, we are talking 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. So, 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 the 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 floral odors.

Rice. 1. Table classification of aldehydes and ketones.

The general formula for aldehydes and ketones is as follows:

• the formula of aldehydes is R-COH
• the formula of ketones is R-CO-R

Classification and nomenclature

Aldehydes and ketones differ in the type of carbon chain in which the carbonyl group is located. Consider fatty and aromatic compounds:

• acyclic, limiting. The first member of the homologous series of aldehydes is formic aldehyde (formaldehyde, methanal) - CH 2 \u003d O.

Formic aldehyde is used as an antiseptic. With its help disinfection of premises, dressing of seeds is carried out.

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

Rice. 2. Formula acetaldehyde.

• unlimited. It is necessary to mention such an unsaturated aldehyde as acrolein (propenal). This aldehyde is formed during the thermal decomposition of glycerol and fats, of which glycerol is an integral part.
• aromatic. The first member of the homologous series of aromatic aldehydes is benzaldehyde (benzaldehyde). Also noteworthy is an aldehyde of plant origin, such as vanillin (3-methoxy-4-hydroxybenzaldehyde).

Rice. 3. Vanillin formula.

Ketones can be purely aromatic and fat-aromatic. Purely aromatic is, for example, diphenylketone (benzophenone). Fat-aromatic is, for example, methyl-phenylketone (acetophenone)

What have we learned?

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

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5.1. general characteristics

The related classes of aldehydes and ketones contain a functional carbonyl group and are referred to as carbonyl compounds. They also use the common name oxo compounds, since the group = O is called an oxo group.

Aldehydes are compounds in which the carbonyl group is bonded to an organic radical and a hydrogen atom; ketones - carbonyl compounds with two organic radicals.

The group -CH=O, which is part of aldehydes, is called aldehyde, respectively a group in ketones - ketone, or keto group.

Depending on the nature of organic radicals, aldehydes and ketones can belong to aliphatic or aromatic row; ketones are mixed(Table 5.1).

Unlike alcohols, aldehydes and ketones do not have mobile hydrogen atoms bound to oxygen atoms. In this regard, aldehydes and ketones are not associated due to the formation of hydrogen bonds, but tend to form hydrogen bonds with water molecules and therefore dissolve well in it (especially the first members of the homologous series).

Table 5.1.Aldehydes and ketones

5.2. Reaction centers of aldehydes and ketones

sp 2 -Hybridized carbon atom of the carbonyl group forms three σ-bonds lying in the same plane, and a π-bond with the oxygen atom due to the unhybridized p-orbital. Due to the difference in the electronegativity of carbon and oxygen atoms, the π-bond between them is highly polarized (Fig. 5.1). As a result, a partial positive charge δ+ appears on the carbon atom of the carbonyl group, and a partial negative charge δ- appears on the oxygen atom. Since the carbon atom is electron-deficient, it represents a center for nucleophilic attack.

The distribution of electron density in the molecules of aldehydes and ketones, taking into account the transfer of the electronic influence of the electron

Rice. 5.1.Electronic structure of the carbonyl group

deficient carbon atom of the carbonyl group in σ-bonds is presented in Scheme 5.1.

Scheme 5.1.Reaction centers in the molecule of aldehydes and ketones

There are several reaction centers in the molecules of aldehydes and ketones:

The electrophilic center - the carbon atom of the carbonyl group - predetermines the possibility of a nucleophilic attack;

The main center - an oxygen atom - determines the possibility of an attack by a proton;

CH-acid center, the hydrogen atom of which has a weak proton mobility and can, in particular, be attacked by a strong base.

In general, aldehydes and ketones are highly reactive.

5.3. Nucleophilic addition

For aldehydes and ketones, nucleophilic addition reactions are most characteristic A N.

General description of the nucleophilic addition mechanism A N

The ease of nucleophilic attack on the carbon atom of the carbonyl group of an aldehyde or ketone depends on the magnitude of the partial

positive charge on the carbon atom, its spatial availability and acid-base properties of the medium.

Taking into account the electronic effects of the groups associated with the carbonyl carbon atom, the value of the partial positive charge δ+ on it in aldehydes and ketones decreases in the following series:

The spatial availability of the carbonyl carbon atom decreases when hydrogen is replaced by bulkier organic radicals, so aldehydes are more reactive than ketones.

General scheme of nucleophilic addition reactions A N to the carbonyl group involves a nucleophilic attack on the carbonyl carbon followed by the addition of an electrophile to the oxygen atom.

In an acidic environment, the activity of the carbonyl group, as a rule, increases, since due to the protonation of the oxygen atom, a positive charge arises on the carbon atom. Acid catalysis is usually used when the attacking nucleophile has low activity.

According to the above mechanism, a number of important reactions of aldehydes and ketones are carried out.

Many reactions characteristic of aldehydes and ketones occur in the body, these reactions are presented in the subsequent sections of the textbook. This chapter will discuss the most important reactions of aldehydes and ketones, which are summarized in Scheme 5.2.

addition of alcohols. Alcohols, when interacting with aldehydes, easily form hemiacetals. Hemiacetals are not usually isolated due to their instability. With an excess of alcohol in an acidic environment, hemiacetals turn into acetals.

The use of an acid catalyst in the conversion of hemiacetal to acetal is clear from the reaction mechanism below. The central place in it is occupied by the formation of a carbocation (I), stabilized due to the participation of the lone pair of electrons of the neighboring oxygen atom (+M effect of the C 2 H 5 O group).

The reactions of formation of hemiacetals and acetals are reversible; therefore, acetals and hemiacetals are easily hydrolyzed by excess water in an acidic medium. In an alkaline environment, hemiacetals are stable, since the alkoxidion is a more difficult leaving group than the hydroxide ion.

The formation of acetals is often used as a temporary protection of the aldehyde group.

Water connection. Adding water to a carbonyl group - hydration- reversible reaction. The degree of hydration of an aldehyde or ketone in an aqueous solution depends on the structure of the substrate.

The product of hydration, as a rule, cannot be isolated by distillation in a free form, since it decomposes into its original components. Formaldehyde in an aqueous solution is hydrated by more than 99.9%, acetaldehyde is approximately half, and acetone is practically not hydrated.

Formaldehyde (formaldehyde) has the ability to coagulate proteins. His 40% water solution, called formalin, used in medicine as a disinfectant and preservative of anatomical preparations.

Trichloroacetic aldehyde (chloral) is fully hydrated. The electron-withdrawing trichloromethyl group stabilizes chloral hydrate to such an extent that this crystalline substance splits off water only during distillation in the presence of dehydrating substances - sulfuric acid, etc.

At the heart of the pharmacological effect of CC1 chloral hydrate s CH(OH)2 lies the specific effect on the body of the aldehyde group, which determines the disinfectant properties. Halogen atoms enhance its action, and hydration of the carbonyl group reduces the toxicity of the substance as a whole.

Addition of amines and their derivatives. Amines and other nitrogen-containing compounds of the general formula NH 2 X (X = R, NHR) react with aldehydes and ketones in two steps. First, nucleophilic addition products are formed, which then, due to instability, split off water. In this regard, this process is generally classified as a reaction attachment-detachment.

In the case of primary amines, substituted imines(also called Schiff bases).

Imines are intermediates in many enzymatic processes. The preparation of imines proceeds through the formation of amino alcohols, which are relatively stable, for example, in the reaction of formaldehyde with α-amino acids (see 12.1.4).

Imines are intermediates in the production of amines from aldehydes and ketones by reductive amination. This general way consists in restoring a mixture of a carbonyl compound with ammonia (or an amine). The process proceeds according to the addition-cleavage scheme with the formation of an imine, which is then reduced to an amine.

When aldehydes and ketones react with hydrazine derivatives, hydrazones. This reaction can be used to isolate aldehydes and ketones from mixtures and their chromatographic identification.

Schiff's bases and other similar compounds are easily hydrolyzed by aqueous solutions of mineral acids to form the starting products.

In most cases, the reactions of aldehydes and ketones with nitrogenous bases require acid catalysis, which accelerates the dehydration of the addition product. However, if the acidity of the medium is increased too much, then the reaction will slow down as a result of the conversion of the nitrogenous base into the non-reactive conjugate acid XNH 3+.

polymerization reactions. These reactions are characteristic mainly of aldehydes. When heated with mineral acids, aldehyde polymers decompose into the starting products.

The formation of polymers can be viewed as the result of a nucleophilic attack by an oxygen atom of one aldehyde molecule on the carbonyl carbon atom of another molecule. So, when formalin is standing, a polymer of formaldehyde, paraform, precipitates in the form of a white precipitate.

5.4. Condensation reactions

The presence of a CH-acid center in an aldehyde or ketone molecule leads to the fact that the α-hydrogen atoms of these carbonyl compounds have some proton mobility. Under the action of bases, such protons can be split off with the formation of the corresponding carbanions. Carbanions play the role of nucleophiles with respect to the carbonyl substrate. This makes it possible to carry out reactions in which one molecule, as a nucleophile, is added to the carbonyl group of another molecule of a neutral carbonyl compound. Such processes are referred to as condensation reactions.

Condensation is a reaction that leads to the emergence of a new carbon-carbon bond, and from two or more relatively simple molecules a new, more complex molecule is formed.

So, in an alkaline medium, two molecules of acetaldehyde form hydroxyaldehyde with twice the number of carbon atoms.

The reaction product containing hydroxyl and aldehyde groups is called aldol(from words ald aegis and alcohol ol), and the reaction itself is called aldol condensation, or aldol addition.

Aldol condensation mechanism. Under the action of a base in a carbonyl compound, a proton is cleaved from the α-position and a carbanion (I) is formed, in which the negative charge is delocalized with the participation of the carbonyl group.

The anion (I) is a strong nucleophile (shown in color in the next step of the mechanism) that attaches to the second (non-ionized) molecule of the carbonyl compound. As a result of this interaction, a new C-C connection and an intermediate alkoxide ion (II) is formed. In an aqueous medium, this anion is stabilized by splitting off a proton from a water molecule and turns into the final product, the aldol.

The aldol addition reaction is shown using propanal as an example (the molecule that adds to the C=O group of another molecule is highlighted in color); a similar reaction is shown using acetone as an example.

The condensation product, the aldol, is capable of splitting off water to form an α,β-unsaturated carbonyl compound. This usually happens at elevated temperatures. In this case, the reaction as a whole is called croton condensation.

Condensation reactions can also occur in a mixed version, using different carbonyl compounds, and one of them may not contain a CH-acid center, such as formaldehyde and benzaldehyde in the following reactions:

Aldol condensation is a reversible reaction; the reverse process is called aldol splitting(or retroaldol reaction). Both reactions occur in many biochemical processes.

5.5. Recovery and oxidation

Recoveryaldehydes and ketones is carried out using complex metal hydrides LiAlH 4 , NaBH 4 . The reaction involves a nucleophilic attack on the carbonyl carbon by a hydride ion.

Upon subsequent hydrolysis of the resulting alcoholate, primary or secondary alcohol is obtained.

Oxidationaldehydes to carboxylic acids is carried out under the action of most oxidizing agents, including atmospheric oxygen. Ketones do not oxidize under mild conditions.

Silver oxide in the form of an ammonia complex 2 OH (Tollens' reagent) oxidizes aldehydes to carboxylic acids, while metallic silver is released. Hence the name - reaction "Silver Mirror"

Aldehydes are also easily oxidized by copper(II) hydroxide in an alkaline medium.

Both of these reactions are often used as qualitative ones for the detection of the aldehyde group, although they are not specific to aldehydes: for example, polyhydric phenols, aminophenols, aromatic amines, hydroxyketones, and other easily oxidizing compounds are subjected to oxidation with the indicated reagents.