Physical and chemical properties of aldehydes. Physical and chemical properties of aldehydes Chemical properties of aldehydes

To determine the chemical formula of an organic substance, a small mass of it is burned, and then the combustion products are examined. So, for example, when burning 3.75 g received formaldehyde 2.25 g water vapor and 5.5 g carbon monoxide (IV). It was found that the vapor density of formaldehyde in terms of hydrogen 15 . Using these data, find how many grams of carbon and hydrogen are contained in 3.75 g formaldehyde:

M(CO2) = 12 + 32 = 44; M = 44 g/mol
44 g CO 2 contains 12 g
5.5 g CO 2 " x 1

44 ÷ 5.5 = 12 ÷ x 1; x 1 = (5.5 12) / 44 = 1.5; m(C) = 1.5 g
M (H 2 O) = 2 + 16 = 18; M = 18 g/mol
18 g H 2 O contains 2 g
2.25 g H 2 O " x 2

18 ÷ 2.25 = 2 ÷ x 2; x 2 = (2.25 2) / 18 = 0.25; m(H) = 0.25 g

Find the total mass of carbon and hydrogen:

X 1 + x 2 = 1.5 + 0.25 = 1.75

Since 3.75 g of formaldehyde was taken for combustion, the mass of oxygen can be calculated:

3.75 - 1.75 = 2; m(O) = 2 g

Determine the simplest formula:

C: H: O = (1.5 ÷ 12) : (0.25 ÷ 1) : (2 ÷ 16) = 0.125: 0.25: 0.125 = 1: 2: 1

Hence, simplest formula test substance CH2O.
Knowing the hydrogen vapor density of formaldehyde, calculate its molar mass:

M = 2D (H 2) = 2 15 = 30; M = 30 g/mol

Find the molar mass using the simplest formula:

M (CH 2 O) = 12 + 2 + 16 = 30; M(CH2O) = 30 g/mol

Therefore, the molecular formula of formaldehyde is CH2O

In the formaldehyde molecule, there is a σ bond between the carbon and hydrogen atoms, and one σ and one π bond between the carbon and oxygen atoms.

Isomerism and nomenclature

Aldehydes are characterized by isomerism of the hydrocarbon radical. It can have either a straight or branched chain. The names of aldehydes come from the historical names of the corresponding organic acids into which they are converted during oxidation (formaldehyde, acetaldehyde, propionaldehyde, etc.). According to the international nomenclature, the names of aldehydes are derived from the names of the corresponding hydrocarbons with the addition of a suffix -al.

The most important representatives of aldehydes.

Methanal, or formaldehyde*
Ethanal, or acetaldehyde*
Propanal
Butanal
2-Methylpropanal
Pentanal
Hexanal

Receipt

IN laboratories aldehydes are obtained by oxidation of primary alcohols. Used as oxidizing agents copper(II) oxide, hydrogen peroxide and other substances that can release oxygen. IN general view it can be shown like this:

IN industry aldehydes are obtained in various ways. It is most economically profitable to receive methanal direct oxidation of methane with atmospheric oxygen in a special reactor.
To prevent methanal from oxidizing, a mixture of methane and air is passed through the reaction zone at high speed.
Methanal is also obtained by oxidizing methanol, passing its vapor along with air through a reactor with a hot copper or silver mesh. However, this method is less economically profitable.
Ethanal can also be obtained by hydration of acetylene in the presence of mercury salts as a catalyst ( reaction of M. G. Kucherov). Since in this reaction toxic substances - mercury salts - are used as a catalyst, then lately A new method for producing acetaldehyde has been developed: a mixture of ethylene and air is passed through an aqueous solution of copper, iron and palladium salts.

Physical properties

Methanal- colorless gas with a pungent odor. A solution of methanal in water (35 - 40%) is called formalin. The remaining members of the aldehyde series are liquids, while the higher ones are solids.

Chemical properties

The most typical reactions for aldehydes are oxidation and addition.

1. Oxidation reactions

A) The qualitative reaction to aldehydes is the reaction "Silver Mirror". To carry it out, pour into a clean test tube ammonia solution of silver(I) oxide(Ag 2 O is practically insoluble in water, but forms a soluble OH compound with ammonia), an aldehyde solution is added to it and heated.
The reduced silver settles on the walls of the test tube in the form of a shiny coating, and the aldehyde is oxidized into the corresponding organic acid.
b) Another characteristic reaction is the oxidation of aldehydes copper(II) hydroxide. If an aldehyde solution is added to a blue precipitate of copper (II) hydroxide and the mixture is heated, then a yellow precipitate of copper (I) hydroxide first appears, which upon further heating turns into red copper(I) oxide. In this reaction the oxidizing agent is copper with the oxidation number +2 , which is reduced to the oxidation state +1 .

2. Addition reactions

Addition reactions are due to the presence of π bonds, which breaks easily. At the site of its rupture, atoms and atomic groups are attached. For example, when a mixture of methanal and hydrogen is passed over a heated catalyst, it is reduced to methanol.
Hydrogen and other aldehydes are added similarly.

WORKBOOKS

Continuation. See the beginning in № 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 28, 29, 30, 31, 32/2004

Chemical properties. Let's consider the behavior of aldehydes in relation to a standard set of reagents: atmospheric oxygen O2, oxidizing agents [O], as well as H2, H2O, alcohols, Na, HCl.

Aldehydes are slowly oxidized by atmospheric oxygen into carboxylic acids:

A qualitative reaction to aldehydes is the “silver mirror” reaction. The reaction consists of the interaction of the aldehyde RCHO with an aqueous ammonia solution of silver(I) oxide, which is a soluble complex compound OH. The reaction is carried out at a temperature close to the boiling point of water (80–100 °C). As a result, on the walls glass vessel(test tubes, flasks) a precipitate of metallic silver is formed - a “silver mirror”:

The reduction of copper(II) hydroxide to copper(I) oxide is another characteristic reaction of aldehydes. The reaction occurs when the mixture is boiled and consists of the oxidation of the aldehyde. More precisely, the introduction of the [O] atom of the oxidizing agent Cu(OH) 2 into the C–H bond of the aldehyde group occurs. In this case, the oxidation states of the carbonyl carbon (from +1 to +3) and the copper atom (from +2 to +1) change. When the blue precipitate of Cu(OH) 2 is heated in a mixture with an aldehyde, the blue color disappears and a red precipitate of Cu 2 O forms:

Aldehydes add hydrogen H 2 via double bond C=O when heated in the presence of a catalyst (Ni, Pt, Pd). The reaction is accompanied by a rupture
-bond in the carbonyl group C=O and the addition of two H atoms of the hydrogen molecule H–H at the site of its break. Thus, alcohols are obtained from aldehydes:

Aldehydes with electron-withdrawing substituents in-position water is added to the aldehyde group with the formation of aldehyde hydrates (diols-1,1):

In order to hold two electronegative hydroxyl groups, the carbon atom must carry a sufficient positive charge. Creation of additional positive charge on the carbonyl carbon, three electron-withdrawing chlorine atoms at the neighboring -carbon of chloral contribute.

Reaction of aldehydes with alcohols. Synthesis of hemiacetals and acetals. IN favorable conditions(for example: a) when heated with acid or in the presence of water-removing agents; b) during intramolecular condensation with the formation of five- and six-membered rings), aldehydes react with alcohols. In this case, either one alcohol molecule (the product is a hemiacetal) or two alcohol molecules (the product is an acetal) can be added to one aldehyde molecule:

Aldehydes are not added HCl via double bond C=O. Also aldehydes don't react with Na, i.e. The aldehydic hydrogen of the –CHO group does not have noticeable acidic properties.

Application of aldehydes based on their high reactivity. Aldehydes are used as starting and intermediate compounds in the synthesis of substances with beneficial properties other classes.
Formaldehyde HCHO - a colorless gas with a pungent odor - is used for the production polymer materials. Substances with mobile H atoms in the molecule (usually with C–H or N–H bonds, but not O–H) combine with formaldehyde CH 2 O as follows:

If the molecule of the starting substance has two or more mobile protons (phenol C 6 H 5 OH has three such protons), then the reaction with formaldehyde produces a polymer. For example, with phenol - phenol-formaldehyde resin:

Similarly, urea with formaldehyde produces urea-formaldehyde resins:

Formaldehyde serves as a starting material for the production dyes, pharmaceuticals, synthetic rubber, explosives and many other organic compounds.

Formalin (40% aqueous solution of formaldehyde) is used as antiseptic(disinfectant). The ability of formaldehyde to coagulate proteins is used in tanning and to preserve biological products.

Acetaldehyde CH 3 CHO is a colorless liquid ( t kip = 21 °C) with a pungent odor, highly soluble in water. The main use of acetaldehyde is to obtain acetic acid. It is also obtained from synthetic resins, drugs etc.

EXERCISES

1. Describe how chemical reactions The following pairs of substances can be distinguished:
a) benzaldehyde and benzyl alcohol; b) propionaldehyde and propyl alcohol. State what will be observed during each reaction.

2. Give reaction equations that confirm the presence in the molecule
p-hydroxybenzaldehyde of the corresponding functional groups.

3. Write the equations for the reactions of butanal with the following reagents:
A) H 2, t, cat. Pt; b) KMnO 4, H 3 O +, t; V) OH V NH 3 /H 2 O; G) NOCH 2 CH 2 OH, t, cat. HCl.

4. Write down reaction equations for a chain of chemical transformations:

5. As a result of hydrolysis of the acetal aldehyde is formed RCHO and alcohol R'OH in molar ratio 1:2. Write down equations for the hydrolysis reactions of the following acetals:

6. The oxidation of saturated monohydric alcohol with copper(II) oxide produced 11.6 g organic compound with a yield of 50%. When the resulting substance interacted with an excess of an ammonia solution of silver oxide, 43.2 g of precipitate was released. What alcohol was taken and what is its mass?

7. 5-Hydroxyhexanal in an acidified aqueous solution is predominantly in the form of a six-membered cyclic hemiacetal. Write an equation for the corresponding reaction:

1. You can distinguish between two substances using reactions that are characteristic of only one of these substances. For example, aldehydes are oxidized to acids under the action of weak oxidizing agents. Heating of a mixture of benzaldehyde and an ammonia solution of silver oxide occurs with the formation of a “silver mirror” on the walls of the flask:

Benzaldehyde is reduced by catalytic hydrogenation to benzyl alcohol:

Benzyl alcohol reacts with sodium and hydrogen is released in the reaction:

2C 6 H 5 CH 2 OH + 2Na 2C 6 H 5 CH 2 ONa + H 2.

When heated in the presence of a copper catalyst, benzyl alcohol is oxidized by atmospheric oxygen into benzaldehyde, which is detected by the characteristic smell of bitter almonds:

Propionic aldehyde and propyl alcohol can be distinguished in a similar manner.

2. IN n-hydroxybenzaldehyde has three functional groups: 1) aromatic ring; 2) phenolic hydroxyl; 3) aldehyde group. IN special conditions– when protecting the aldehyde group from oxidation (designation – [–CHO]) – chlorination can be carried out n-hydroxybenzaldehyde to benzene ring:

Reaction with alkali on phenolic hydroxyl:

Oxidation of the aldehyde group CHO into carboxyl COOH, for example, when heated with a suspension of freshly prepared Cu(OH) 2:

b) Oxidation scheme n-butanal with potassium permanganate in a neutral environment:

C 3 H 7 CHO + KMnO 4 C 3 H 7 COOK + MnO 2 + H 2 O.

Electron-ion balance:

4. Reaction equations for a chain of chemical transformations:

Alcohol weight: m = M= 0.4 60 = 24 g.

Answer. Propanol-1 alcohol weighing 24 g was taken.

DEFINITION

Aldehydes – organic matter, belonging to the class of carbonyl compounds containing the functional group –CH = O, which is called carbonyl.

The general formula for saturated aldehydes and ketones is C n H 2 n O. The names of aldehydes contain the suffix –al.

The simplest representatives of aldehydes are formaldehyde (formaldehyde) -CH 2 = O, acetaldehyde (acetic aldehyde) - CH 3 -CH = O. There are cyclic aldehydes, for example, cyclohexane-carbaldehyde; aromatic aldehydes have trivial names - benzaldehyde, vanillin.

The carbon atom in the carbonyl group is in a state of sp 2 hybridization and forms 3σ bonds (two C-H bonds and one C-O bond). The π bond is formed by the p electrons of the carbon and oxygen atoms. The C=O double bond is a combination of σ and π bonds. The electron density is shifted towards the oxygen atom.

Aldehydes are characterized by isomerism of the carbon skeleton, as well as interclass isomerism with ketones:

CH 3 -CH 2 -CH 2 -CH = O (butanal);

CH 3 -CH (CH 3) -CH = O (2-methylpentanal);

CH 3 -C (CH 2 -CH 3) = O (methyl ethyl ketone).

Chemical properties of aldehydes

Aldehyde molecules have several reaction centers: an electrophilic center (carbonyl carbon atom), which participates in nucleophilic addition reactions; the main center is an oxygen atom with lone electron pairs; α-CH acid center responsible for condensation reactions; C-H connection, breaking apart in oxidation reactions.

1. Addition reactions:

- water with the formation of heme-diols

R-CH = O + H 2 O ↔ R-CH(OH)-OH;

— alcohols with the formation of hemiacetals

CH 3 -CH = O + C 2 H 5 OH ↔CH 3 -CH(OH)-O-C 2 H 5 ;

— thiols with the formation of dithioacetals (in an acidic environment)

CH 3 -CH = O + C 2 H 5 SH ↔ CH 3 -CH(SC 2 H 5) -SC 2 H 5 + H 2 O;

— sodium hydrosulfite with the formation of sodium α-hydroxysulfonates

C 2 H 5 -CH = O + NaHSO 3 ↔ C 2 H 5 -CH(OH)-SO 3 Na;

- amines with the formation of N-substituted imines (Schiff bases)

C 6 H 5 CH = O + H 2 NC 6 H 5 ↔ C 6 H 5 CH = NC 6 H 5 + H 2 O;

- hydrazines to form hydrazones

CH 3 -CH = O + 2 HN-NH 2 ↔ CH 3 -CH = N-NH 2 + H 2 O;

— hydrocyanic acid with the formation of nitriles

CH 3 -CH = O + HCN ↔ CH 3 -CH(N)-OH;

- recovery. When aldehydes react with hydrogen, primary alcohols are obtained:

R-CH = O + H 2 → R-CH 2 -OH;

2. Oxidation

- “silver mirror” reaction - oxidation of aldehydes with an ammonia solution of silver oxide

R-CH = O + Ag 2 O → R-CO-OH + 2Ag↓;

- oxidation of aldehydes with copper (II) hydroxide, which results in the formation of a red precipitate of copper (I) oxide

CH 3 -CH = O + 2Cu(OH) 2 → CH 3 -COOH + Cu 2 O↓ + 2H 2 O;

These reactions are qualitative reactions to aldehydes.

Physical properties of aldehydes

The first representative of the homologous series of aldehydes is formaldehyde (formaldehyde) - a gaseous substance (n.s.), aldehydes of unbranched structure and composition C 2 -C 12 - liquids, C 13 and longer - solids. The more carbon atoms a straight aldehyde contains, the higher its boiling point. With increase molecular weight aldehydes, the values ​​of their viscosity, density and refractive index increase. Formaldehyde and acetaldehyde are able to mix with water in unlimited quantities, however, with the growth of the hydrocarbon chain, this ability of aldehydes decreases. Lower aldehydes have a pungent odor.

Preparation of aldehydes

The main methods for obtaining aldehydes:

- hydroformylation of alkenes. This reaction consists of the addition of CO and hydrogen to an alkene in the presence of carbonyls of some Group VIII metals, for example, octacarbonyl dicobalt (Co 2 (CO) 8). The reaction is carried out by heating to 130 C and a pressure of 300 atm

CH 3 -CH = CH 2 + CO +H 2 →CH 3 -CH 2 -CH 2 -CH = O + (CH 3) 2 CHCH = O;

- hydration of alkynes. The interaction of alkynes with water occurs in the presence of mercury (II) salts and in an acidic environment:

HC≡CH + H 2 O → CH 3 -CH = O;

- oxidation of primary alcohols (the reaction occurs when heated)

CH 3 -CH 2 -OH + CuO → CH 3 -CH = O + Cu + H 2 O.

Application of aldehydes

Aldehydes are widely used as raw materials for the synthesis of various products. Thus, various resins (phenol-formaldehyde, etc.) are obtained from formaldehyde (large-scale production), medicines(urotropin); acetaldehyde is a raw material for the synthesis of acetic acid, ethanol, various pyridine derivatives, etc. Many aldehydes (butyric, cinnamon, etc.) are used as ingredients in perfumery.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2