Chemical properties of amino acids by carboxyl group. Amino acids. Formation of internal salts of amino acids in aqueous solution

Amino acids contain amino and carboxyl groups and exhibit all the properties characteristic of compounds with such functional groups. When writing amino acid reactions, formulas with non-ionized amino and carboxy groups are used.

1) reactions at the amino group. The amino group in amino acids exhibits the usual properties of amines: amines are bases and act as nucleophiles in reactions.

1. Reaction of amino acids as bases. When amino acids interact with acids, ammonium salts are formed:

glycine hydrochloride, glycine hydrochloride salt

2. Action of nitrous acid. When nitrous acid acts, hydroxy acids are formed and nitrogen and water are released:

This reaction is used for the quantitative determination of free amine groups in amino acids, as well as in proteins.

3. Formation of N - acyl derivatives, acylation reaction.

Amino acids react with anhydrides and acid halides, forming N - acyl derivatives of amino acids:

Benzyl ether sodium salt N carbobenzoxyglycine - chloroformic glycine

Acylation is one of the ways to protect the amino group. N-acyl derivatives are of great importance in the synthesis of peptides, since N-acyl derivatives are easily hydrolyzed to form a free amino group.

4. Formation of Schiff bases. When a-amino acids interact with aldehydes, substituted imines (Schiff bases) are formed through the stage of formation of carbinolamines:

alanine formaldehyde N-methylol derivative of alanine

5. Alkylation reaction. The amino group in the a-amino acid is alkylated to form N-alkyl derivatives:

The reaction with 2,4-dinitrofluorobenzene is of greatest importance. The resulting dinitrophenyl derivatives (DNP derivatives) are used in establishing the amino acid sequence of peptides and proteins. The interaction of a-amino acids with 2,4-dinitrofluorobenzene is an example of a nucleophilic substitution reaction in the benzene ring. Due to the presence of two strong electron-withdrawing groups in the benzene ring, the halogen becomes mobile and undergoes a substitution reaction:

2.4 – dinitro -

fluorobenzene N - 2,4 - dinitrophenyl - a - amino acid

(DNPB) DNP - derivatives of a - amino acids

6.Reaction with phenyl isothiocyanate. This reaction is widely used in determining the structure of peptides. Phenyl isothiocyanate is a derivative of isothiocyanic acid H-N=C=S. The interaction of a-amino acids with phenyl isothiocyanate proceeds through the mechanism of a nucleophilic addition reaction. The resulting product then undergoes an intramolecular substitution reaction, leading to the formation of a cyclic substituted amide: phenylthiohydantoin.

Cyclic compounds are obtained in quantitative yield and are phenyl derivatives of thiohydantoin (PTH - derivatives) - amino acids. PTG derivatives differ in the structure of the R radical.

In addition to ordinary salts, a-amino acids can, under certain conditions, form intracomplex salts with heavy metal cations. All a-amino acids are characterized by beautifully crystallizing, intensely blue-colored intracomplex (chelate) copper salts):
Alanine ethyl ester

The formation of esters is one of the methods for protecting the carboxyl group in peptide synthesis.

3. Formation of acid halides. When acting on a-amino acids with a protected amino group with sulfur oxydichloride (thionyl chloride) or phosphorus oxide trichloride (phosphorus oxychloride), acid chlorides are formed:

The production of acid halides is one of the ways to activate the carboxyl group in peptide synthesis.

4.Obtaining a-amino acid anhydrides. Acid halides are very reactive, which reduces the selectivity of the reaction when used. Therefore, a more commonly used method for activating a carboxyl group in peptide synthesis is to convert it into an anhydride group. Anhydrides are less active than acid halides. When an a-amino acid having a protected amino group interacts with ethyl chloroformic acid (ethyl chloroformate), an anhydride bond is formed:

5. Decarboxylation. a - Amino acids that have two electron-withdrawing groups at the same carbon atom are easily decarboxylated. In laboratory conditions, this is carried out by heating amino acids with barium hydroxide. This reaction occurs in the body with the participation of decarboxylase enzymes with the formation of biogenic amines:

ninhydrin

Relation of amino acids to heat. When a-amino acids are heated, cyclic amides called diketopiperazines are formed:

Diketopiperazine

g - and d - Amino acids easily split off water and cyclize to form internal amides, lactams:

g - lactam (butyrolactam)

In cases where the amino and carboxyl groups are separated by five or more carbon atoms, when heated, polycondensation occurs with the formation of polymer polyamide chains with the elimination of a water molecule.

Properties of amino acids can be divided into two groups: chemical and physical.

Chemical properties of amino acids

Depending on the compounds, amino acids can exhibit different properties.

Amino acid interactions:

Amino acids, as amphoteric compounds, form salts with both acids and alkalis.

As carboxylic acids, amino acids form functional derivatives: salts, esters, amides.

Interaction and properties of amino acids with reasons:
Salts are formed:

NH 2 -CH 2 -COOH + NaOH NH 2 -CH 2 -COONa + H2O

Sodium salt + 2-aminoacetic acid Sodium salt of aminoacetic acid (glycine) + water

Interaction with alcohols:

Amino acids can react with alcohols in the presence of hydrogen chloride gas, turning into ester. Amino acid esters do not have a bipolar structure and are volatile compounds.

NH 2 -CH 2 -COOH + CH 3 OH NH 2 -CH 2 -COOCH 3 + H 2 O.

Methyl ester / 2-aminoacetic acid /

Interaction ammonia:

Amides are formed:

NH 2 -CH(R)-COOH + H-NH 2 = NH 2 -CH(R)-CONH 2 + H 2 O

Interaction of amino acids with strong acids:

We get salts:

HOOC-CH 2 -NH 2 + HCl → Cl (or HOOC-CH 2 -NH 2 *HCl)

These are the basic chemical properties of amino acids.

Physical properties of amino acids

Let us list the physical properties of amino acids:

• Colorless
• Have a crystalline form
• Most amino acids have a sweet taste, but depending on the radical (R), they can be bitter or tasteless
• Easily soluble in water, but poorly soluble in many organic solvents
• Amino acids have the property of optical activity
• Melts with decomposition at temperatures above 200°C
• Non-volatile
• Aqueous solutions of amino acids in acidic and alkaline environments conduct electric current

Based on the nature of hydrocarbon substituents, amines are divided into

General structural features of amines

Just like in the ammonia molecule, in the molecule of any amine the nitrogen atom has a lone electron pair directed to one of the vertices of the distorted tetrahedron:

For this reason, amines, like ammonia, have significantly expressed basic properties.

Thus, amines, similar to ammonia, react reversibly with water, forming weak bases:

The bond between the hydrogen cation and the nitrogen atom in the amine molecule is realized using a donor-acceptor mechanism due to the lone electron pair of the nitrogen atom. Saturated amines are stronger bases compared to ammonia, because in such amines, hydrocarbon substituents have a positive inductive (+I) effect. In this regard, the electron density on the nitrogen atom increases, which facilitates its interaction with the H + cation.

Aromatic amines, if the amino group is directly connected to the aromatic ring, exhibit weaker basic properties compared to ammonia. This is due to the fact that the lone electron pair of the nitrogen atom is shifted towards the aromatic π-system of the benzene ring, as a result of which the electron density on the nitrogen atom decreases. In turn, this leads to a decrease in basic properties, in particular the ability to interact with water. For example, aniline reacts only with strong acids, but practically does not react with water.

Chemical properties of saturated amines

As already mentioned, amines react reversibly with water:

Aqueous solutions of amines have an alkaline reaction due to the dissociation of the resulting bases:

Saturated amines react with water better than ammonia due to their stronger basic properties.

The basic properties of saturated amines increase in the series.

Secondary saturated amines are stronger bases than primary saturated amines, which in turn are stronger bases than ammonia. As for the basic properties of tertiary amines, if we are talking about reactions in aqueous solutions, then the basic properties of tertiary amines are expressed much worse than those of secondary amines, and even slightly worse than those of primary ones. This is due to steric hindrances, which significantly affect the rate of amine protonation. In other words, three substituents “block” the nitrogen atom and interfere with its interaction with H + cations.

Interaction with acids

Both free saturated amines and their aqueous solutions react with acids. In this case, salts are formed:

Since the basic properties of saturated amines are more pronounced than those of ammonia, such amines react even with weak acids, such as carbonic acid:

Amine salts are solids that are highly soluble in water and poorly soluble in non-polar organic solvents. The interaction of amine salts with alkalis leads to the release of free amines, similar to the displacement of ammonia when alkalis act on ammonium salts:

2. Primary saturated amines react with nitrous acid to form the corresponding alcohols, nitrogen N2 and water. For example:

A characteristic feature of this reaction is the formation of nitrogen gas, and therefore it is qualitative for primary amines and is used to distinguish them from secondary and tertiary ones. It should be noted that most often this reaction is carried out by mixing the amine not with a solution of nitrous acid itself, but with a solution of a salt of nitrous acid (nitrite) and then adding a strong mineral acid to this mixture. When nitrites interact with strong mineral acids, nitrous acid is formed, which then reacts with the amine:

Secondary amines under similar conditions give oily liquids, so-called N-nitrosamines, but this reaction does not occur in real USE tests in chemistry. Tertiary amines do not react with nitrous acid.

Complete combustion of any amines leads to the formation of carbon dioxide, water and nitrogen:

Interaction with haloalkanes

It is noteworthy that exactly the same salt is obtained by the action of hydrogen chloride on a more substituted amine. In our case, when hydrogen chloride reacts with dimethylamine:

Preparation of amines:

1) Alkylation of ammonia with haloalkanes:

In case of ammonia deficiency, its salt is obtained instead of amine:

2) Reduction by metals (to hydrogen in the activity series) in an acidic environment:

followed by treatment of the solution with alkali to release the free amine:

3) The reaction of ammonia with alcohols when passing their mixture through heated aluminum oxide. Depending on the alcohol/amine proportions, primary, secondary or tertiary amines are formed:

Chemical properties of aniline

Aniline - the trivial name for aminobenzene, which has the formula:

As can be seen from the illustration, in the aniline molecule the amino group is directly connected to the aromatic ring. Such amines, as already mentioned, have much less pronounced basic properties than ammonia. Thus, in particular, aniline practically does not react with water and weak acids such as carbonic acid.

Reaction of aniline with acids

Aniline reacts with strong and medium strength inorganic acids. In this case, phenylammonium salts are formed:

Reaction of aniline with halogens

As was already said at the very beginning of this chapter, the amino group in aromatic amines is drawn into the aromatic ring, which in turn reduces the electron density on the nitrogen atom, and as a result increases it in the aromatic ring. An increase in electron density in the aromatic ring leads to the fact that electrophilic substitution reactions, in particular reactions with halogens, proceed much more easily, especially in the ortho and para positions relative to the amino group. Thus, aniline easily reacts with bromine water, forming a white precipitate of 2,4,6-tribromoaniline:

This reaction is qualitative for aniline and often allows it to be identified among other organic compounds.

Reaction of aniline with nitrous acid

Aniline reacts with nitrous acid, but due to the specificity and complexity of this reaction, it does not appear in the real Unified State Exam in chemistry.

Aniline alkylation reactions

Using sequential alkylation of aniline at the nitrogen atom with halogenated hydrocarbons, secondary and tertiary amines can be obtained:

Obtaining aniline

1. Reduction of nitrobenzene by metals in the presence of strong non-oxidizing acids:

C 6 H 5 -NO 2 + 3Fe + 7HCl = +Cl- + 3FeCl 2 + 2H 2 O

Cl - + NaOH = C 6 H 5 -NH 2 + NaCl + H 2 O

Any metals located before hydrogen in the activity series can be used as metals.

Reaction of chlorobenzene with ammonia:

C 6 H 5 −Cl + 2NH 3 → C 6 H 5 NH 2 + NH 4 Cl

Chemical properties of amino acids

Amino acids are compounds whose molecules contain two types of functional groups - amino (-NH 2) and carboxy- (-COOH) groups.

In other words, amino acids can be considered as derivatives of carboxylic acids, in the molecules of which one or more hydrogen atoms are replaced by amino groups.

Thus, the general formula of amino acids can be written as (NH 2) x R(COOH) y, where x and y are most often equal to one or two.

Since amino acid molecules contain both an amino group and a carboxyl group, they exhibit chemical properties similar to both amines and carboxylic acids.

Acidic properties of amino acids

Formation of salts with alkalis and alkali metal carbonates

Esterification of amino acids

Amino acids can react with esterification with alcohols:

NH 2 CH 2 COOH + CH 3 OH → NH 2 CH 2 COOCH 3 + H 2 O

Basic properties of amino acids

1. Formation of salts when interacting with acids

NH 2 CH 2 COOH + HCl → + Cl —

2. Interaction with nitrous acid

NH 2 -CH 2 -COOH + HNO 2 → HO-CH 2 -COOH + N 2 + H 2 O

Note: interaction with nitrous acid proceeds in the same way as with primary amines

3. Alkylation

NH 2 CH 2 COOH + CH 3 I → + I —

4. Interaction of amino acids with each other

Amino acids can react with each other to form peptides - compounds containing in their molecules the peptide bond –C(O)-NH-

At the same time, it should be noted that in the case of a reaction between two different amino acids, without observing some specific synthesis conditions, the formation of different dipeptides occurs simultaneously. So, for example, instead of the reaction of glycine with alanine above, leading to glycylananine, the reaction leading to alanylglycine can occur:

In addition, the glycine molecule does not necessarily react with the alanine molecule. Peptization reactions also occur between glycine molecules:

And alanine:

In addition, since the molecules of the resulting peptides, like the original amino acid molecules, contain amino groups and carboxyl groups, the peptides themselves can react with amino acids and other peptides due to the formation of new peptide bonds.

Individual amino acids are used to produce synthetic polypeptides or so-called polyamide fibers. Thus, in particular, using the polycondensation of 6-aminohexane (ε-aminocaproic) acid, nylon is synthesized in industry:

The resulting nylon resin is used to produce textile fibers and plastics.

Formation of internal salts of amino acids in aqueous solution

In aqueous solutions, amino acids exist predominantly in the form of internal salts - bipolar ions (zwitterions):

Obtaining amino acids

1) Reaction of chlorinated carboxylic acids with ammonia:

Cl-CH 2 -COOH + 2NH 3 = NH 2 -CH 2 -COOH + NH 4 Cl

2) Breakdown (hydrolysis) of proteins under the action of solutions of strong mineral acids and alkalis.

>> Chemistry: Amino acids

The general formula of the simplest amino acids can be written as follows:

H2N-CH-COOH
I
R

Because amino acids contain two different functional groups that influence each other, their reactions differ from the characteristic properties of carboxylic acids and amines.

Receipt

Amino acids can be obtained from carboxylic acids by replacing the hydrogen atom in their radical with a halogen, and then with an amino group when reacting with ammonia. A mixture of amino acids is usually obtained by acid hydrolysis of proteins.

Properties

The amino group -NH2 determines the basic properties of amino acids, since it is capable of attaching a hydrogen cation to itself via a donor-acceptor mechanism due to the presence of a free electron pair at the nitrogen atom.

The -COOH group (carboxyl group) determines the acidic properties of these compounds. Therefore, amino acids are amphoteric organic compounds.

They react with alkalis as acids. With strong acids - like amine bases.

In addition, the amino group in the amino acid molecule interacts with the carboxyl group included in its composition, forming an internal salt:

Since amino acids in aqueous solutions behave like typical amphoteric compounds, in living organisms they play the role of buffer substances that maintain a certain concentration of hydrogen ions.

Amino acids are colorless crystalline substances that melt and decompose at temperatures above 200 °C. They are soluble in water and insoluble in ether. Depending on the composition of the R- radical, they can be sweet, bitter or tasteless.

Amino acids are optically active because they contain carbon atoms (asymmetric atoms) linked to four different substituents (the exception is amino-acetic acid - glycine). An asymmetric carbon atom is indicated by an asterisk.

As you already know, optically active substances occur in the form of pairs of antipodal isomers, the physical and chemical properties of which are the same, with the exception of one thing - the ability to rotate the plane of a polarized beam in opposite directions. The direction of rotation of the plane of polarization is indicated by the sign (+) - right rotation, (-) - left rotation.

There are D-amino acids and L-amino acids. The location of the NH2 amino group in the projection formula on the left corresponds to the L-configuration, and on the right - to the D-configuration. The sign of rotation is not related to whether the connection belongs to the L- or D-series. Thus, L-ce-rin has a rotation sign (-), and L-alanine has a rotation sign (+).

Amino acids are divided into natural (found in living organisms) and synthetic. Among natural amino acids (about 150), proteinogenic amino acids (about 20) are distinguished, which are part of proteins. They are L-shapes. About half of these amino acids are considered essential, as they are not synthesized in the human body. Essential amino acids are valine, leucine, isoleucine, phenylalaline, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food (Table 7). If their quantity in food is insufficient, the normal development and functioning of the human body is disrupted. In certain diseases, the body is unable to synthesize some other amino acids. Thus, in phenylketonuria, tyrosine is not synthesized.

The most important property of amino acids is the ability to enter into molecular condensation with the release of water and the formation of an amide group -NH-CO-, for example:

H2N-(CH2)5-COOH + H-NH-(CH2)5-COOH ->
aminocaproic acid

H2N-(CH2)5-CO-NH-(CH2)5-COOH + H20

The high-molecular compounds obtained as a result of this reaction contain a large number of amide fragments and are therefore called polyamides.

These, in addition to the synthetic fiber nylon mentioned above, include, for example, enant, formed during the polycondensation of aminoenanthic acid. Amino acids with amino and carboxyl groups at the ends of the molecules are suitable for producing synthetic fibers (think about why).

Table 7. Daily requirement of the human body for amino acids

Polyamides of a-amino acids are called peptides. Depending on the number of amino acid residues, dipeptides, tripeptides, and polypeptides are distinguished. In such compounds, the -NP-CO- groups are called peptide groups.

Isomerism and nomenclature

Amino acid isomerism is determined by the different structure of the carbon chain and the position of the amino group. The names of amino acids in which the positions of the amino group are designated by letters of the Greek alphabet are also widespread. Thus, 2-aminobutanoic acid can also be called a-aminobutyric acid:

20 amino acids are involved in protein biosynthesis in living organisms, for which historical names are often used. These names and the Russian and Latin letter designations adopted for them are given in Table 8.

1. Write down the equations for the reactions of aminopropionic acid; you with sulfuric acid and sodium hydroxide, as well as methyl alcohol. Give all substances names according to the international nomenclature.

2. Why are amino acids heterofunctional compounds?

3. What structural features should the amino acids used for the synthesis of fibers and the amino acids involved in the biosynthesis of proteins in the cells of living organisms have?

4. How do polycondensation reactions differ from polymerization reactions? What are their similarities?

5. How are amino acids obtained? Write down the reaction equations for producing aminopropionic acid from propane.

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Amino acids are heterofunctional compounds that necessarily contain two functional groups: an amino group - NH 2 and a carboxyl group - COOH, associated with a hydrocarbon radical. The general formula of the simplest amino acids can be written as follows:

Because amino acids contain two different functional groups that influence each other, the characteristic reactions differ from those of carboxylic acids and amines.

Properties of amino acids

The amino group - NH 2 determines the basic properties of amino acids, since it is capable of attaching a hydrogen cation to itself via a donor-acceptor mechanism due to the presence of a free electron pair at the nitrogen atom.

The -COOH group (carboxyl group) determines the acidic properties of these compounds. Therefore, amino acids are amphoteric organic compounds. They react with alkalis as acids:

With strong acids - like bases - amines:

In addition, the amino group in an amino acid interacts with its carboxyl group, forming an internal salt:

The ionization of amino acid molecules depends on the acidic or alkaline nature of the environment:

Since amino acids in aqueous solutions behave like typical amphoteric compounds, in living organisms they play the role of buffer substances that maintain a certain concentration of hydrogen ions.

Amino acids are colorless crystalline substances that melt and decompose at temperatures above 200 °C. They are soluble in water and insoluble in ether. Depending on the R- radical, they can be sweet, bitter or tasteless.

Amino acids are divided into natural (found in living organisms) and synthetic. Among natural amino acids (about 150), proteinogenic amino acids (about 20) are distinguished, which are part of proteins. They are L-shapes. About half of these amino acids are irreplaceable, because they are not synthesized in the human body. Essential acids are valine, leucine, isoleucine, phenylalanine, lysine, threonine, cysteine, methionine, histidine, tryptophan. These substances enter the human body with food. If their quantity in food is insufficient, the normal development and functioning of the human body is disrupted. In certain diseases, the body is unable to synthesize some other amino acids. Thus, in phenylketonuria, tyrosine is not synthesized. The most important property of amino acids is the ability to enter into molecular condensation with the release of water and the formation of the amide group -NH-CO-, for example:

The high-molecular compounds obtained as a result of this reaction contain a large number of amide fragments and are therefore called polyamides.

These, in addition to the synthetic nylon fiber mentioned above, include, for example, enant, formed during the polycondensation of aminoenanthic acid. Amino acids with amino and carboxyl groups at the ends of the molecules are suitable for producing synthetic fibers.

Alpha amino acid polyamides are called peptides. Depending on the number of amino acid residues, they are distinguished dipeptides, tripeptides, polypeptides. In such compounds, the -NH-CO- groups are called peptide groups.