(for the simplest aldehyde R=H)
Limit; for example:
Unlimited; for example:
Aromatic; for example:
Alicyclic; for example:
for example:
Aldehydes and ketones contain a carbonyl group ˃C=O, therefore they are called carbonyl compounds.
The carbon atom of the aldehyde group is in the state of sp 2 hybridization, so all σ-bonds in this group are located in the same plane. Clouds of p-electrons forming a π-bond are perpendicular to this plane and easily shift to the more electronegative oxygen atom. Therefore, the C=O double bond (unlike the C=C double bond in alkenes) is highly polarized.
Aldehydes are reactive compounds that enter into numerous reactions. Most characteristic of aldehydes:
a) addition reactions at the carbonyl group; HX-type reagents are added as follows:
b) oxidation reactions C-H bonds aldehyde group, as a result of which carboxylic acids are formed:
1. Hydrogenation (primary alcohols are formed
2. Addition of alcohols (hemiacetals and acetals are formed)
In an excess of alcohol in the presence of HCl, hemiacetals are converted to acetals:
1. Silver mirror reaction
Simplified:
This reaction is a qualitative reaction to the aldehyde group (a mirror coating of metallic silver is formed on the walls of the reaction vessel).
2. Reaction with copper (II) hydroxide
This reaction is also a qualitative reaction to the aldehyde group y (a red precipitate of Cu 2 O precipitates).
Formaldehyde is oxidized by various O-containing oxidants first to formic acid and further - to H 2 CO 3 (CO 2 + H 2 O):
1. Aldol condensation
2. Trimerization of acetaldehyde
At long-term storage formalin (40% aqueous formaldehyde solution), polymerization occurs in it with the formation of a white precipitate paraform:
Aldehydes are called organic substances related to carbonyl compounds containing the functional group -CH, which is called the carbonyl group.
Depending on the nature of the hydrocarbon skeleton of the molecule, aldehydes are saturated, unsaturated and aromatic. Their molecules may also include halogen atoms or additional functional groups. General formula saturated aldehydes has the form C n H 2 n O. In accordance with the IUPAC nomenclature, their names end with the suffix -al.
The oxidation of aldehydes is important in industry because they are quite easily converted to carboxylic acids. In this case, copper hydroxide, silver oxide, or even atmospheric oxygen can serve as oxidizing agents.
Electronic structure a double bond in the C=O group is characterized by the formation of one σ-bond and another π-bond. The C atom is in the state of sp 2 hybridization, the molecule is of a planar structure with bond angles between bonds of about 120 0 . The difference between the double bond in this functional group lies in the fact that it is located between the carbon atom and the highly electronegative oxygen atom. As a result, the electrons are attracted to the O atom, which means that this bond is very strongly polarized.
The content of such a polarized double bond in the aldehyde group can be called main reason high reactivity of aldehydes. For aldehydes, the reactions of addition of atoms or their groups to the C=O bond are most characteristic. And nucleophilic addition reactions are the easiest to proceed. Also typical for aldehydes are reactions involving H atoms from the functional group of aldehydes. Due to the electron-withdrawing effect of the C=O group, the polarity of the bond increases. This in turn is the reason for the relatively easy oxidation of aldehydes.
Formaldehyde (formaldehyde or methanal) CH 2 O is gaseous substance with a very pungent odor, which is usually obtained by passing a mixture of methanol vapor with air through a red-hot mesh of copper or silver mesh. Its 40% aqueous solution is called formalin. Formaldehyde easily enters into reactions, many of which underlie industrial synthesis a number of important substances. It is also used to obtain pentaerythritol, many medicinal substances, various dyes, for tanning leather, as a disinfectant and deodorant. Formaldehyde is quite toxic, its MPC in the air is 0.001 mg/l.
Acetaldehyde (acetic aldehyde, ethanal) CH 3 SON is a colorless liquid with a suffocating odor, which, when diluted with water, acquires a fruity aroma. Acetaldehyde has all the basic properties of aldehydes. Oxidation of acetaldehyde produces huge volumes acetic acid and acetic anhydride, a variety of pharmaceuticals.
Acrolein (propenal) CH 2 \u003d CH-SON, the simplest unsaturated aldehyde, is a colorless, volatile liquid. Its vapors strongly irritate the mucous membranes of the eyes and upper respiratory tract. It is very poisonous, its MPC in the air is 0.7 mg/m 3 . Propenal is an intermediate product in the synthesis of some polymers, it is necessary in the production of certain medicines.
Benzaldehyde (benzoic aldehyde) С 6 Н 5 СОН is a colorless liquid with an aroma that turns yellow during storage. It is rather quickly oxidized by air to benzoic acid. Contained in essential oils plants (neroli, patchouli), and in the form of glucoside - in the kernels of the seeds of bitter almonds, cherries, apricots and peaches. As a fragrant substance, it is used in perfumery, as a component of food essences, as a raw material for the synthesis of other fragrant substances (cinnamaldehyde, jasminaldehyde).
Oxidation of aldehydes with silver oxide is the most significant qualitative reaction to the corresponding form of the functional group. This reaction got its name due to the thin silver coating on the walls of the test tube, which is formed during this reaction.
Its essence lies in the interaction of the aldehyde R-COH with an ammonia solution of silver (I) oxide, which is a soluble OH complex compound and is called the Tollens reagent. The reaction is carried out at temperatures close to the boiling point of water (80-100 °C). In this case, the aldehydes are oxidized to the corresponding carboxylic acids, and the oxidizing agent is reduced to metallic silver, which precipitates.
For a qualitative determination of the -OH group in aldehydes, a silver complex compound is first prepared. To do this, pour a little solution of ammonia (ammonium hydroxide) in water into a test tube and then a small amount of silver nitrate. In this case, the resulting precipitate of silver oxide immediately disappears:
2AgNO 3 + 2NH 3 + H 2 O -> Ag 2 O↓ + 2NH 4 NO 3
Ag 2 O + 4NH 3 + H 2 O -> 2OH
More reliable results are obtained with Tollens' reagent prepared with the addition of alkali. To do this, 1 g of AgNO 3 is dissolved in 10 g of distilled water and an equal volume of concentrated sodium hydroxide is added. The result is a precipitate of Ag 2 O, which disappears when a concentrated solution of ammonium hydroxide is added. Only freshly prepared reagent should be used for the reaction.
The reaction of the silver mirror corresponds to the equation:
2OΗ + HCOΗ -> 2Ag↓ + ΗCOONΗ 4 + 3NΗ 3 + H 2 O
It should be noted that for aldehydes this interaction has not been sufficiently studied. The mechanism of this reaction is unknown, but a radical or ionic oxidation variant is assumed. The hydroxide of diamminesilver most likely undergoes addition with the formation of a silver salt of the diol, from which silver is then cleaved off to form a carboxylic acid.
For successful experience, the cleanliness of the utensils used is extremely important. This is due to the fact that the colloidal silver particles formed during the experiment must cling to the surface of the glass, creating a mirror surface. In the presence of the slightest impurities, it will precipitate as a gray flocculent precipitate.
Alkali solutions should be used to clean the container. So, for these purposes, you can take a solution of NaOH, which must be washed off with a large volume of distilled water. The glass surface must be free of grease and mechanical particles.
The oxidation reaction of aldehydes with copper (II) hydroxide is also quite effective and effective in determining the type of functional group. It proceeds at a temperature corresponding to the boiling of the reaction mixture. In this case, aldehydes reduce divalent copper in the composition of Fehling's reagent (freshly prepared ammonia solution of Cu (OH) 2) to monovalent. They themselves are oxidized due to the introduction of an oxygen atom through the C-H bond (the oxidation state of C changes from +1 to +3).
Visually, the course of the reaction can be traced by changing the color of the mixture of solutions. The bluish precipitate of copper hydroxide gradually turns yellow, corresponding to monovalent copper hydroxide and the further appearance of a bright red precipitate of Cu 2 O.
This process corresponds to the reaction equation:
R-SON + Cu 2+ + NaOH + H 2 O -> R-COONa + Cu 2 O + 4H +
It should be noted that such a reagent acts in the best way on aldehydes. In this case, the oxidation does not require heating and is carried out at a temperature of 0-20 °C for a fairly short period of time, and the yield of products is more than 80%. The main disadvantage of the Jones reagent is the lack of high selectivity for other functional groups, and besides, an acidic environment sometimes leads to isomerization or destruction.
Jones' reagent is a solution of chromium(VI) oxide in dilute acetone. It can also be obtained from sodium dichromate. When aldehydes are oxidized, carboxylic acids are formed under the action of this reagent.
Oxidation of acetaldehyde in industry is carried out by the action of oxygen in the presence of catalysts - cobalt or manganese ions. First, peracetic acid is formed:
CH 3 -SON + O 2 -> CH 3 -COOH
It, in turn, interacts with the second molecule of acetaldehyde and, through a peroxide compound, gives two molecules of acetic acid:
CH 3 -COOH + CH 3 -SON -> 2CH 3 -COOH
Oxidation is carried out at a temperature of 60-70 °C and a pressure of 2·10 5 Pa.
For the oxidation of aldehyde groups, an iodine solution in the presence of alkali is sometimes used. This reagent is of particular importance in the process of carbohydrate oxidation, since it acts very selectively. Thus, under its influence, D-glucose is converted into D-gluconic acid.
Iodine in the presence of alkalis forms hypoiodide (a very strong oxidizing agent): I 2 + 2NaOΗ -> NaIO + NaI + H 2 O.
Under the action of hypoiodide, formaldehyde is converted into methane acid: ΗСОΗ + NaIO + NaOΗ -> ΗCOONa + NaI + H 2 O.
Oxidation of aldehydes with iodine is used in analytical chemistry to determine their quantitative content in solutions.
Unlike previous reagents, under the action of selenium dioxide, aldehydes are converted into dicarbonyl compounds, and glyoxal is formed from formaldehyde. If methylene or methyl groups are located next to the carbonyl, then they can turn into carbonyl. Dioxane, ethanol or xylene are usually used as a solvent for SeO2.
According to one method, the reaction is carried out in a three-necked flask connected to a stirrer, thermometer and reflux condenser. To the starting material, taken in an amount of 0.25 mol, a solution of 0.25 mol of selenium dioxide in 180 ml of dioxane, as well as 12 ml of H 2 O, is added dropwise. The temperature should not exceed 20 ° C (if necessary, the flask is cooled). After that, with constant stirring, the solution is boiled for 6 hours. Next, the hot solution is filtered to separate selenium and the precipitate is washed with dioxane. After vacuum distillation of the solvent, the residue is fractionated. The main fraction is taken in a wide temperature range (20-30 °C) and re-rectified.
Under the influence of oxygen in the air room temperature aldehyde oxidation is very slow. The main products of these reactions are the corresponding carboxylic acids. The auto-oxidation mechanism is related to the industrial oxidation of ethanal to acetic acid. One of the intermediates is a peracid that reacts with another aldehyde molecule.
Due to the fact that this type of reaction is accelerated by light, peroxides, and traces of heavy metals, it can be concluded that it is a radical mechanism. Formaldehyde in aqueous solutions is oxidized by air much worse than its counterparts, due to the fact that it exists in them in the form of hydrated methylene glycol.
This reaction occurs most successfully in a visual assessment of its passage by the loss of intensity and the complete discoloration of the pink color of the potassium permanganate solution. The reaction takes place at room temperature and normal pressure, so it does not require special conditions. It is enough to pour 2 ml of formaldehyde and 1 ml of formaldehyde acidified with sulfuric acid into the test tube. Shake the test tube with the solution gently to mix the reagents:
5CH 3 -SON + 2KMnO 4 + 3H 2 SO 4 \u003d 5CH 3 -COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O
If the same reaction is carried out at elevated temperatures, then methanal is easily oxidized to carbon dioxide:
5CH 3 -SON + 4KMnO 4 + 6H 2 SO 4 \u003d 5CO 2 + 4MnSO 4 + 2K 2 SO 4 + 11H 2 O
Organic drugs
We study drugs divided into groups according to the chemical classification. The advantage of this classification is the ability to identify and study general patterns in the development of methods for obtaining drugs that make up a group, methods of pharmaceutical analysis based on the physical and chemical properties of substances, establishing a relationship between the chemical structure and pharmacological action.
All drugs are divided into inorganic and organic. Inorganic, in turn, are classified according to the position of the elements in the PS. And organic ones are divided into derivatives of the aliphatic, alicyclic, aromatic and heterocyclic series, each of which is divided into classes: hydrocarbons, halogen derivatives of hydrocarbons, alcohols, aldehydes, ketones, acids, ethers, simple and complex, etc.
ALIPHATIC COMPOUNDS AS HP.
Preparations of aldehydes and their derivatives. Carbohydrates
This group of compounds includes organic medicinal substances containing an aldehyde group, or their functional derivatives.
General formula:
The introduction of an aldehyde group into the structure of an organic compound gives it a narcotic and antiseptic effect. In this, the action of aldehydes is similar to the action of alcohols. But unlike alcohol, the aldehyde group enhances the toxicity of the compound.
Factors influencing the structure on the pharmacological action :
elongation of the alkyl radical increases activity, but at the same time toxicity increases;
the introduction of an unsaturated bond and halogens has the same effect;
the formation of the hydrated form of the aldehyde leads to a decrease in toxicity. But the ability to form a stable hydrated form is manifested only in chlorine derivatives of aldehydes. So, formaldehyde is a protoplasmic poison, it is used for disinfection, acetaldehyde and chloral are not used in medicine due to their high toxicity, and chloral hydrate is a drug used as a hypnotic, sedative.
The strength of the narcotic (pharmacological) action and toxicity grew from formaldehyde to acetaldehyde and chloral. The formation of a hydrated form (chloral hydrate) can dramatically reduce toxicity, while maintaining the pharmacological effect.
By physical condition aldehydes can be gaseous (low molecular weight), liquids and solids. Low molecular weight have a sharp bad smell, high molecular weight - pleasant floral.
In chemical terms, these are highly reactive substances, due to the presence of a carbonyl group in their molecule.
The high reactivity of aldehydes is explained by:
a) the presence of a polarized double bond
b) dipole moment of carbonyl
c) the presence of a partial positive charge on the carbon atom of the carbonyl
σ -
σ + H
The double bond between C and O, unlike the double bond between two carbons, is strongly polarized, since oxygen has a much higher electronegativity than carbon, and the electron density of the π bond shifts towards oxygen. Such a high polarization determines the electrophilic properties of the carbonyl group carbon and its ability to react with nucleophilic compounds (to enter into nucleophilic addition reactions). The oxygen of the group has nucleophilic properties.
Oxidation and nucleophilic addition reactions are characteristic
Aldehydeseasily are oxidized. Oxidation of aldehydes to acids takes place under the influence how strongand weak oxidizing agents .
Many metals - silver, mercury, bismuth, copper - are reduced from solutions of their salts, especially in the presence of alkali. This distinguishes aldehydes from other organic compounds capable of oxidation - alcohols, unsaturated compounds, the oxidation of which requires stronger oxidizing agents. Therefore, the oxidation reactions of aldehydes with complex cations of mercury, copper, silver in an alkaline medium can be used to prove the authenticity of aldehydes.
I. 1 .Reactionwith ammonia solution of silver nitrate (silver mirror reaction) is recommended by FS to confirm the authenticity of substances with an aldehyde group. The basis is the oxidation of an aldehyde to an acid and the reduction of Ag + to Ag↓.
AgNO 3 + 2NH 4 OH → NO 3 + 2H 2 O
NSON+ 2NO 3 + H 2 O → HCOONH 4 + 2Ag↓+ 2NH 4 NO 3 + NH 3
Formaldehyde, oxidized to the ammonium salt of formic acid, reduces to metallic silver, which is depositedon the walls of the test tube brilliant plaque "mirrors" or gray sediment.
I. 2. Reactionwith Fehling's reagent (complex compound of copper (II) with potassium-sodium salt of tartaric acid). Aldehydes reduce the copper (II) compound to copper (I) oxide, a brick-red precipitate is formed. prepared before use).
Felling reagent 1 - CuSO 4 solution
Felling's reagent 2 - an alkaline solution of potassium-sodium salt of tartaric acid
When mixing 1:1 Felling reagents 1 and 2 a blue copper complex compound is formed (II) with potassium-sodium salt of tartaric acid:
blue staining
When aldehyde is added and heated, the blue color of the reagent disappears, an intermediate product is formed - a yellow precipitate of copper (I) hydroxide, which immediately decomposes into a red precipitate of copper (I) oxide and water.
2KNa+ R- COH+2NaOH+ 2KOH→ R- COONa+4KNaC4H4O6 + 2 CuOH ↓ +H2O
2 CuOH ↓ →Cu 2 O ↓ + H2O
yellow precipitate brick red precipitate
Textbooks have a different general reaction scheme
I. 3. Reactionwith Nessler's reagent (an alkaline solution of tetraiodomercurat (II) potassium). Formaldehyde reduces the mercury ion to metallic mercury - a dark gray precipitate.
R-COH + K 2 +3KOH → R-COOK + 4KI + hg↓ + 2H2O
Which are characterized by a double bond between carbon and oxygen atoms and two single bonds of the same carbon atom with a hydrocarbon radical, denoted by the letter R, and a hydrogen atom. The group of atoms >C=O is called the carbonyl group, it is characteristic of all aldehydes. Many aldehydes have a pleasant smell. They can be obtained from alcohols by dehydrogenation (removal of hydrogen), due to which they received the common name - aldehydes. The properties of aldehydes are determined by the presence of a carbonyl group, its location in the molecule, as well as the length and spatial branching of the hydrocarbon radical. That is, knowing the name of the substance reflecting it, one can expect certain chemical, as well as physical properties aldehydes.
There are two main ways of naming aldehydes. The first method is based on the system used by the International Union (IUPAC) and is often referred to as systematic nomenclature. It is based on the fact that the longest chain, in which a carbonyl group is attached to a carbon atom, serves as the basis for the name of an aldehyde, that is, its name comes from the name of a related alkane due to the replacement of the suffix -an by the suffix -al (methane - matanal, ethane - ethanal , propane - propanal, butane - butanal and so on). Another method of forming the name of aldehydes uses the name of the corresponding one into which it will turn as a result of oxidation (methanal - formic aldehyde, ethanal - acetic aldehyde, propanal - propionic aldehyde, butanal - butyric aldehyde, and so on).
It is the polarity of the >C=O group that affects the physical properties of aldehydes: boiling point, solubility, dipole moment. Hydrocarbon compounds, consisting only of hydrogen and carbon atoms, melt and boil at low temperatures. In substances with a carbonyl group, they are much higher. For example, butane (CH3CH2CH2CH3), propanal (CH3CH2CHO), and acetone (CH3COCH3) have the same molecular weight, equal to 58, and the boiling point of butane is 0 °C, while for propanal it is 49 °C, and for acetone it is 56 °C. The reason for the large difference is that polar molecules have more ability to attract each other than non-polar molecules, so more energy is needed to break them and therefore more heat so that these compounds melt or boil.
With growth, the physical properties of aldehydes change. Formaldehyde (HCHO) is a gaseous substance under normal conditions, acetaldehyde (CH3CHO) boils at room temperature. Other aldehydes (with the exception of representatives with a high molecular weight) are liquids under normal conditions. Polar molecules do not mix easily with non-polar molecules because polar molecules are attracted to each other, and non-polar molecules are not able to squeeze between them. Therefore, hydrocarbons do not dissolve in water, since water molecules are polar. Aldehydes, in whose molecules the number of carbon atoms is less than 5, dissolve in water, but if the number of carbon atoms is more than 5, dissolution does not occur. The good solubility of low molecular weight aldehydes is due to the formation of hydrogen bonds between the hydrogen atom of the water molecule and the oxygen atom of the carbonyl group.
The polarity of molecules formed by different atoms can be quantified by a number called the dipole moment. Molecules formed by identical atoms are not polar and do not have a dipole moment. The dipole moment vector is directed towards the element to the right in the periodic table (for one period). If the molecule consists of atoms of one subgroup, then the electron density will shift towards the element with a lower serial number. Most hydrocarbons do not have a dipole moment or its value is extremely small, but for aldehydes it is much higher, which also explains the physical properties of aldehydes.
Aldehydes- organic substances whose molecules contain a carbonyl group C=O, connected to a hydrogen atom and a hydrocarbon radical.
The general formula for aldehydes is:
In the simplest aldehyde, formaldehyde, the role of the hydrocarbon radical is played by another hydrogen atom:
The carbonyl group attached to the hydrogen atom is often referred to as aldehyde:
Ketones- 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:
Depending on the structure of the hydrocarbon radical associated with the aldehyde group, limiting, unsaturated, aromatic, heterocyclic and other aldehydes are distinguished:
In accordance with the IUPAC nomenclature, the names of aldehydes are formed from the name of an alkane with the same number of carbon atoms in the molecule using the suffix -al. For example:
The numbering of carbon atoms of the main chain starts 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 name of ketones according to the systematic nomenclature, the keto group is denoted by the suffix -he and a number that indicates the carbon atom number 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 - the isomerism of the carbon skeleton, which is possible from butanal, and for ketones also the isomerism of the position of the carbonyl group. In addition, they are also characterized by interclass isomerism (propanal and propanone).
In an aldehyde or ketone molecule, due to the greater electronegativity of the oxygen atom compared to the carbon atom, the bond C=O strongly polarized due to electron density shift π -bonds to oxygen:
Aldehydes and ketones are 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 characteristic properties aldehydes.
1. Recovery reactions.
The addition of hydrogen to aldehyde molecules occurs via a 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.
Hydrogenation of aldehydes- reduction reaction, in which the degree of oxidation of the carbon atom included in the carbonyl group decreases.
2. Oxidation reactions. Aldehydes are able not only to recover, but also oxidize. When oxidized, aldehydes form carboxylic acids.
Air oxygen oxidation. For example, propionic acid is formed from propionaldehyde (propanal):
Oxidation with weak oxidizing agents(ammonia solution of silver oxide).
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 a thin, even film. It turns out a wonderful silver mirror. Therefore, this reaction is called the "silver mirror" reaction. It is widely used for making mirrors, silvering decorations and Christmas decorations.
3. Polymerization reaction:
n CH 2 \u003d O → (-CH 2 -O-) n paraforms n \u003d 8-12
Formaldehyde(methanal, formic aldehyde) H 2 C=O:
a) to obtain phenol-formaldehyde resins;
b) obtaining urea-formaldehyde (urea) resins;
c) polyoxymethylene polymers;
d) synthesis medicines(urotropin);
e) disinfectant;
e) preservative biological preparations(due to the ability to fold the protein).
Acetic aldehyde(ethanal, acetaldehyde) CH 3 CH \u003d O:
a) production of acetic acid;
b) organic synthesis.
Acetone CH 3 -CO-CH 3:
a) solvent for varnishes, paints, cellulose acetates;
b) raw materials for the synthesis of various organic substances.
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