Do the optical isomers have the following compounds. Spatial isomerism. Optically active substances

Spatial isomers (stereoisomers) have the same qualitative and quantitative composition and the same order of bonding of atoms (chemical structure), but different spatial arrangement of atoms in the molecule.

There are two types of spatial isomerism: optical and geometric.

Optical isomerism

In optical isomerism, different fragments of molecules are located differently relative to a certain atom, i.e. have different configuration. For instance:

Such molecules are not identical, they refer to each other as an object and its mirror image and are called enantiomers.

Enantiomers have chirality properties. The simplest case of chirality is due to the presence in the molecule center of chirality(chiral center), which can be an atom containing four different substituents. Such an atom lacks symmetry elements. For this reason, it is also called asymmetric.

To establish whether a molecule is chiral, it is necessary to build its model, a model of its mirror image (Fig. 3.1 , a) and find out if they fit together in space. If they do not match, the molecule is chiral (Fig. 3.1, b), if they match, it is achiral.

Rice. 3.1.

All chemical properties of enantiomers are identical. Their physical properties are also the same, with the exception of optical activity: one shape rotates the plane of polarization of light to the left, the other by the same angle to the right.

A mixture of equal amounts of optical antipodes behaves like an individual chemical compound, devoid of optical activity and very different in physical properties from each of the antipodes. Such a substance is called racemic mixture, or racemate.

In all chemical transformations in which new asymmetric carbon atoms are formed, racemates are always obtained. There are special methods for the separation of racemates into optically active antipodes.

In the case of the presence of several asymmetric atoms in a molecule, a situation is possible when the spatial isomers are not optical antipodes. For instance:

Spatial isomers that are not enantiomers with respect to each other are called diastereomers.

A special case of diastereomers is geometric (cis- trais-) isomers.

Geometric isomerism

Geometric (cis-trans) isomerism is characteristic of compounds containing double bonds (C = C, C = N, etc.), as well as non-aromatic cyclic compounds and is due to the impossibility of free rotation of atoms around a double bond or in a cycle. Substituents in geometric isomers can be located on one side of the plane of the double bond or the cycle - ^ wc-position, or on opposite sides - thirsch / c-position (Fig. 3.2).

Rice. 3.2. Dis isomer (a) andtrance-isomer(b)

Geometric isomers usually differ significantly in physical properties (boiling and melting points, solubility, dipole moments, thermodynamic stability, etc.)

The term "chirality" means that two objects are in such a relationship to each other, as the left and right hands (from the Greek. Chair - hand), ie. are mirror images that do not match when trying to combine them in space.

The content of the article

OPTICAL ISOMERY."When a Molecule Looks in a Mirror" was the unusual title of an article published in the June 1996 issue of the American Journal of Chemical Education. And on the first page of the cover of this issue there was also an unusual drawing. On the side of the dog, good-naturedly wagging its tail, was the structural formula of penicillamine. The dog looked in the mirror, and from there a terrible beast with a bared fanged mouth and hair standing on end looked at him. On the side of the beast, the same structural formula was depicted as a mirror image of the first. Why, then, does one and the same substance have such different appearances? This is explained by the special property of some chemical compounds, which is closely related to their optical activity.

Light polarization and optical activity.

At the beginning of the 19th century. English physicist, astronomer and physician Thomas Jung showed that light can be viewed as a wave. The French physicist Augustin Fresnel established that light waves are transverse: vibrations in them occur perpendicular to the direction of movement (like waves on the surface of the water: the wave runs forward, and the float on the water vibrates up and down). Already in the 20th century. it was found that light is an electromagnetic wave, like a radio wave, only the wavelength of light is much shorter. The term "electromagnetic" means that light has electric and magnetic fields that oscillate periodically, like waves on the surface of the sea. We are now only interested in the oscillations of the electric field. It turns out that these vibrations do not occur randomly, but only perpendicular to the direction of the light beam. In ordinary light (it is emitted, for example, by the sun, incandescent lamps) vibrations occur randomly, in all directions. But, having passed through some crystals, for example, tourmaline or Icelandic spar (a transparent variety of CaCO 3 calcite), light acquires special properties: the crystal, as it were, “cuts off” all oscillations of the electric field, except for one located in a certain plane. Figuratively speaking, a beam of such light is like a woolen thread, which was pulled through a narrow gap between two sharp razor blades.

French physicist Etienne Louis Malus believed that light consists of particles with two poles - "north" and "south", and in the light that passed through the Icelandic spar, all the poles are turned in the same direction. Therefore, he called this light polarized. It was found that light is partially polarized, reflecting at some angles from shiny surfaces of dielectrics, for example, from glass, or refraction in them. Malus's theory was not confirmed, but the name remained. The human eye cannot distinguish between ordinary light and polarized light, but this is easy to do with the help of the simplest optical devices - polarimeters; They are used, for example, by photographers: polarizing filters help to get rid of the glare in the photograph, which occurs when light reflects off the surface of the water.

It turned out that when polarized light passes through some substances, an interesting phenomenon occurs: the plane in which the "arrows" of the oscillating electric field are located gradually rotates around the axis along which the beam travels. This phenomenon was first discovered in 1811 by the French physicist François Dominique Arago in quartz crystals. Natural quartz crystals have an irregular, asymmetric structure, and they are of two types, which differ in their shape, like an object from its mirror image. These crystals rotate the plane of polarization of light in opposite directions; they were called right- and levogyrate.

In 1815, another French physicist Jean Baptiste Biot and German physicist Thomas Seebeck established that some organic substances (for example, sugar or turpentine) also have this property, and not only in a crystalline state, but also in a liquid, dissolved and even gaseous state. So it was proved that optical activity can be associated not only with the asymmetry of crystals, but also with some unknown property of the molecules themselves. It turned out that, as in the case of crystals, some chemical compounds can exist in the form of both right- and levogyrate varieties, and the most careful chemical analysis does not reveal any differences between them! In fact, this was a new type of isomerism, which was called optical isomerism. It turned out that apart from right- and levogyrate, there is a third type of isomers - optically inactive. This was discovered in 1830 by the famous German chemist Jones Jakob Berzelius using the example of grape (dihydroxy succinic) acid HOOC – CH (OH) –CH (OH) –COOH: this acid is optically inactive, and tartaric acid of exactly the same composition has right rotation in solution. Later was discovered and not found in nature "left" tartaric acid - the antipode of dextrorotatory.

It is possible to distinguish between optical isomers using a polarimeter, a device that measures the angle of rotation of the plane of polarization. For solutions, this angle linearly depends on the layer thickness and the concentration of the optically active substance (Biot's law). For different substances, optical activity can vary over a very wide range. So, in the case of aqueous solutions of different amino acids at 25 ° C, the specific activity (it is denoted as D and is measured for light with a wavelength of 589 nm at a concentration of 1 g / ml and a layer thickness of 10 cm) is –232 ° for cystine, –86, 2 ° for proline, –11.0 ° for leucine, + 1.8 ° for alanine, + 13.5 ° for lysine and + 33.2 ° for asparagine. Modern polarimeters make it possible to measure optical rotation with very high accuracy (up to 0.001 °). Such measurements make it possible to quickly and accurately determine the concentration of optically active substances, for example, the sugar content in solutions at all stages of its production - from raw products to concentrated solution and molasses.

Discovery of Pasteur.

Physicists associated the optical activity of crystals with their asymmetry; completely symmetrical crystals, for example, cubic crystals of sodium chloride, are optically inactive. The reason for the optical activity of molecules remained completely mysterious for a long time. The first discovery that shed light on this phenomenon was made in 1848 by an then unknown Louis Pasteur. While still a student, Pasteur was interested in chemistry and crystallography, working under the guidance of the physicist J.B. Bio and the prominent French chemist Jean Baptiste Dumas. After graduating from the Higher Normal School in Paris, young (he was only 26 years old) Pasteur worked as a laboratory assistant for Antoine Balard. Balar was already a famous chemist who, 22 years earlier, had become famous for the discovery of a new element - bromine. He gave his assistant a topic in crystallography, without suggesting that this would lead to an outstanding discovery.

In the course of his research, Pasteur obtained the acidic sodium salt of tartaric acid C 4 H 5 O 6 Na, saturated the solution with ammonia and by slow evaporation of water obtained beautiful prismatic crystals of the sodium-ammonium salt C 4 H 3 O 6 NaNH 4. These crystals turned out to be asymmetric, some of them were, as it were, a mirror image of others: half of the crystals had one characteristic face on the right, and for others, on the left. Armed with a magnifying glass and tweezers, Pasteur divided the crystals into two piles. Their solutions, as expected, had opposite optical rotation. Pasteur did not stop there. From each solution, he isolated the original acid (which was inactive). Imagine his surprise when it turned out that one solution is the well-known dextrorotatory tartaric acid, and the other is the same acid, but rotating to the left!

The memories of eyewitnesses testify to the incredible nervous excitement of the young scientist, which seized him at that moment; realizing what he had managed to do, Pasteur ran out of the laboratory and, meeting a laboratory assistant in the physics room, rushed to him and, embracing him, exclaimed: "I have just made a great discovery!" And it consisted in the fact that the long-known inactive tartaric acid is just a mixture of equal amounts of the also known "right" tartaric acid and the previously unknown "left" one. That is why the mixture is not optically active. For such a mixture, they began to use the name racemate (from the Latin racemus - grapes). And the two antipodes of tartaric acid obtained by Pasteur were called enantiomers (from the Greek enantios - the opposite). Pasteur introduced the L- and D-isomers for them (from the Latin words laevus - left and dexter - right). Later, the German chemist Emil Fischer associated these designations with the structure of two enantiomers of one of the simplest optically active substances - glycerolic aldehyde OHCH 2 –CH (OH) –CHO. In 1956, at the suggestion of the English chemists Robert Kahn and Christopher Ingold and the Swiss chemist Vladimir Prelog, the designations S (from Latin sinister - left) and R (Latin rectus - right) were introduced for optical isomers; the racemate is designated by the RS symbol. However, traditionally, old designations are also widely used (for example, for carbohydrates, amino acids). It should be noted that these letters indicate only the structure of the molecule ("right" or "left" arrangement of certain chemical groups) and are not associated with the direction of optical rotation; the latter is indicated by plus and minus signs, for example, D (-) - fructose, D (+) - glucose.

In addition to the "manual method", Pasteur discovered two more methods for separating the racemate into two antipodes. The biochemical method is based on the selective ability of some microorganisms to assimilate only one of the isomers. For example, fungal mold Penicillum glaucum growing on dilute solutions of tartaric acid or its salts, "eats" only the right isomer, leaving the left unchanged.

The third method for the separation of racemates was purely chemical. But for him it was required to have an optically active substance in advance, which, when interacting with a racemic mixture, would "select" only one enantiomer from it. For example, an optically active organic base formed with tartaric acid an optically active salt from which the corresponding enantiomer of tartaric acid could be isolated.

Optical isomerism theory.

Pasteur's work, proving the possibility of the "splitting" of an optically inactive compound into antipodes - enantiomers, initially aroused disbelief in many chemists. Even Bio himself did not believe his assistant, until he repeated his experience with his own hands and became convinced of Pasteur's rightness. This and subsequent works by Pasteur attracted the close attention of chemists. Soon, Joseph Le Bel, using the third Pasteur method, split several alcohols into optically active antipodes. Johann Wislicenus established that there are two lactic acids: optically inactive, formed in sour milk (fermentation lactic acid), and dextrorotatory, which appears in a working muscle (lactic acid). There were more and more such examples, and a theory was required to explain how the antipode molecules differ from each other. This theory was created by the young Dutch scientist Van't Hoff. According to this theory, molecules, like crystals, can be "right-handed" and "left-handed," being mirror images of each other. The simplest example was this. The carbon atom in organic compounds is tetravalent; four chemical bonds are directed from it at equal angles to the vertices of the tetrahedron. If all the atoms or groups of atoms located at the vertices of the tetrahedron and associated with the central carbon atom are different, then two different structures are possible that are not aligned with each other by rotation in space. If at least two of the four substituents are the same, the molecules will become completely identical (this can be easily verified using a model of matches and colored plasticine). Such structures, which differ from each other as the right hand from the left, are called chiral (from the Greek heir - hand). Thus, optical activity is a consequence of the spatial isomerism (stereoisomerism) of molecules.

A carbon atom bonded to four different substituents is called asymmetric. Atoms of other elements - silicon, nitrogen, phosphorus, sulfur - can also be asymmetric. However, compounds without asymmetric carbon atoms can also be optically active if they can exist in the form of two mirror isomers. A molecule will be asymmetric if there is not a single element of symmetry in it - no center, no axes, no plane of symmetry. An example is the allene molecule H 2 C = C = CH 2, in which there are two different substituents: R 1 R 2 C = C = CR 1 R 2. The point is that these substituents are not in the same plane (as, for example, in alkenes), but in two mutually perpendicular planes. Therefore, the existence of two mirror isomers is possible, which cannot be combined with each other by any displacements and rotations.

More complex relationships are found in the case of molecules with several asymmetric carbon atoms. For example, in tartaric acid, two hydroxyl groups at two adjacent carbon atoms can be arranged so that the molecule is symmetric and has no mirror isomers. This leads to the formation of another, optically inactive, isomer, which is called meso tartaric (or anti-tartaric) acid. Thus, dihydroxy succinic acid can be in the form of four isomers: dextrorotatory (D-tartaric acid, which in medicine is called tartaric acid), levorotatory (L-tartaric acid), optically inactive (meso-tartaric acid), and also as a mixture of L- and R -isomers, that is, the racemate (i-tartaric, or tartaric acid). Optically active tartaric acids, upon prolonged heating of their aqueous solutions, racemize, turning into a mixture of antipodes.

The situation is even more complicated when the molecule has many asymmetric centers. For example, there are four of them in a glucose molecule. Therefore, it is theoretically possible for it to exist 16 stereoisomers, which form 8 pairs of mirror antipodes. They have long been known to chemists; it is glucose itself, as well as allose, altrose, mannose, gulose, idose, galactose and talose. Many of these are naturally occurring, such as D-glucose (but not L-glucose, which has been produced synthetically).

If there are equal parts of "right" and "left" molecules in a substance, it will be optically inactive. It is these substances that are obtained in a flask as a result of conventional chemical synthesis. And only in living organisms, with the participation of asymmetric agents (for example, enzymes), optically active compounds are formed. Of course, the question immediately arose of how such compounds appeared on Earth, for example, the same natural dextrorotatory tartaric acid, or "asymmetric" microorganisms feeding on only one of the enantiomers. Indeed, in the absence of a person, there was no one to carry out the directed synthesis of optically active substances, there was no one to divide the crystals into right and left! However, such questions turned out to be so complex that there is still no answer to them. For example, no one knows why almost all natural amino acids from which proteins are built belong to the L-series (S-configuration), and their antipodes are only occasionally found in some antibiotics.

Van't Hoff's theory did not immediately gain recognition. Thus, the outstanding German experimental chemist Adolf Kolbe (several organic reactions were named after him) published a stinging article in May 1877, in which he sharply criticized the new theory. Fortunately, Kolbe was in a clear minority, and Van't Hoff's theory, which laid the foundations of modern stereochemistry, gained general recognition, and its creator in 1901 became the first Nobel Prize winner in chemistry.

This theory has made it possible to explain many chemical phenomena. For example, in reactions of substitution of halogen atoms for hydroxyl groups: in optically active alkyl halides R – X + OH - ® R – OH + X - (X is a halogen atom), in some cases the optical activity disappears, in others it is preserved, but changes sign. It turned out that this reaction can go in different ways. The first mechanism involves the dissociation of the halide with the formation of intermediate R + ions, which quickly combine with OH - anions, giving the reaction product, alcohol. If the initial R – X halide had optical activity, it is lost as a result of this reaction, since the hydroxyl can approach the intermediate planar cation from either side, so that a mixture of enantiomers is formed. If the reaction proceeds according to the second mechanism, the OH– anion approaches the carbon atom from the side opposite to the C – X bond and “displaces” the halogen atom in the form of an anion. If the initial halide R 1 R 2 R 3 C – X had optical activity, it is retained as a result of this reaction, but the sign of the optical rotation is reversed. This happens because three substituents at the asymmetric carbon atom R 1, R 2 and R 3, which, like the halogen atom, are at the vertices of the tetrahedron, upon the approach of the attacking agent - hydroxyl, change their configuration relative to the fourth substituent; this change in configuration is analogous to turning an umbrella in a strong wind.

Optical isomerism and life.

Chemists often refer to enantiomers as a single compound because their chemical properties are identical. However, their biological activity can be completely different. This became apparent after the tragic story of thalidomide, a drug that in the 60s of the 20th century. doctors in many countries have prescribed pregnant women as an effective sleeping pill and sedative. However, over time, its terrible side effect manifested itself: the substance turned out to be teratogenic (damaging the embryo, from the Greek teratos - monster, freak), and a lot of babies with congenital deformities were born. Only in the late 1980s did it become clear that only one of the enantiomers of thalidomide, its dextrorotatory form, was the cause of the misfortunes. Unfortunately, such a difference in the action of dosage forms was not previously known, and thalidomide was a racemic mixture of both antipodes.

Currently, many drugs are available in the form of optically pure compounds. Thus, of the 25 most common drugs in the United States, only six are nonchiral compounds, three are racemates, and the rest are pure enantiomers. The latter are obtained by three methods: separation of racemic mixtures, modification of natural optically active compounds (these include carbohydrates, amino acids, terpenes, lactic and tartaric acids, etc.) and direct synthesis. For example, the well-known chemical company Merck has developed a method for the production of the antihypertensive drug methyldopa, which involves spontaneous crystallization of only the desired enantiomer by introducing a small seed of this isomer into the solution. Direct synthesis also requires chiral sources, since any other traditional synthesis methods give both enantiomers in equal proportions - racemate. This, incidentally, is one of the reasons for the very high cost of some drugs, since the targeted synthesis of only one of them is a very difficult task. Therefore, it is not surprising that out of more than 500 synthetic chiral preparations produced worldwide, only about one tenth are optically pure. At the same time, out of 517 preparations obtained from natural raw materials, only eight are racemates.

The need for optically pure enantiomers is explained by the fact that often only one of them has the desired therapeutic effect, while the second antipode can cause unwanted side effects or even be toxic. It also happens that each enantiomer has its own specific action. So, S (-) - thyroxine ("levotroid") is a natural thyroid hormone. And dextrorotatory R (+) - thyroxine ("dextroid") lowers blood cholesterol. Some manufacturers come up with trade names for such cases, palindromes, for example, Darvon and Novrad.

What explains the different action of enantiomers? Man is a chiral being. Both his body and the molecules of biologically active substances that make up it are asymmetric. Chiral drug molecules interacting with certain chiral centers in the body, such as enzymes, can act differently depending on which enantiomer the drug is. The "right" medicine approaches its receptor like a key to a lock and triggers the desired biochemical reaction. The action of the "wrong" antipode can be likened to an attempt to shake the right hand of his guest with his right hand.

If the drug is a racemate, then one of the enantiomers may at best be indifferent, at worst - cause a completely undesirable effect. Here are some examples. So, the antiarrhythmic agent S (-) - anaprilin acts 100 times stronger than the R (+) - form! In the case of verapamil, both enantiomers have a similar effect, but its R (+) - form has a significantly less potent cardiodepressant side effect. Used for anesthesia, ketamine can cause side effects in 50% of patients in the form of agitation, delirium, etc., and this is mainly inherent only in the R (-) isomer, as well as in the racemate. In the anthelmintic drug, levamisole is active mainly in S ( -) is an isomer, while its R (+) - antipode causes nausea, so at one time racemic levamisole was replaced by one of the enantiomers. But it turns out that it does not always make economic sense to synthesize pure isomers. For example, for the widely used analgesic ibuprofen under the action of enzymes, it is possible to isomerize the therapeutically inactive R (-) - form into the active S (+) - isomer, therefore, in this case, a much cheaper racemate can be used.

Different biological action of "right" and "left" isomers is manifested not only among drugs, but in all cases when a chiral compound interacts with living organisms. A striking example is the amino acid isoleucine: its dextrorotatory isomer is sweet, and the levorotatory isomer is bitter. Another example. Carvone is a substance with a very strong aroma (the human nose can smell it when it contains only 17 millionths of a milligram per liter in the air). Carvone is isolated from caraway seeds, the oil of which contains about 60% of it. However, exactly the same compound with the same structure is found in spearmint oil - there its content reaches 70%. Everyone agrees that the smell of mint and caraway is not at all the same. It turned out that in fact there are two carvons - "right" and "left". The difference in odor of these compounds indicates that the receptor cells in the nose responsible for smelling must also be chiral.

Let's return now to the formula depicted on the dog and the wolf. Penicillamine (3,3-dimethylcysteine) is a fairly simple derivative of the amino acid cysteine. This substance is used for acute and chronic poisoning with copper, mercury, lead, and other heavy metals, since it has the ability to give strong complexes with the ions of these metals; the resulting complexes are removed by the kidneys. Penicillamine is also used for various forms of rheumatoid arthritis, for systemic scleroderma, and in a number of other cases. In this case, only the S-form of the drug is used, since the R-isomer is toxic and can lead to blindness.

Ilya Leenson

The optical activity of a substance is understood as its ability to deflect the plane of a polarized light beam to the right or left at a certain angle.

The phenomenon of optical activity was discovered in 1815 by the physicist J.B. Bio (France).

In 1848, Louis Pasteur, while studying crystals of tartaric acid, noticed that optically inactive Na-ammonium tartrate existed in the form of two types of crystals, which were mirror images of each other. Pasteur divided right-handed and left-handed crystals. Their aqueous solutions were found to be optically active. The specific rotation of the two solutions was the same in magnitude, but different in sign. Since different optical rotations were observed for solutions, Pasteur concluded that this property characterizes molecules, not crystals, and suggested that the molecules of these substances are mirror images of each other. This assumption formed the basis of stereochemistry, which studies the spatial structure of molecules and its effect on the chemical and physical properties of substances.

The first stereochemical theory explaining the reasons for the optical activity of substances was created in 1874 by two scientists simultaneously - the Dutch chemist J.H. Van't Hoff and the Frenchman J. Le Bel. The basis of this theory was the concept of a tetrahedral model of the carbon atom, i.e. all four valences of a carbon atom do not lie in the same plane, but are directed to the corners of the tetrahedron.

It was found that most often optical activity is due to the presence in the molecule asymmetric carbon atom, i.e. C-atom, all the valencies of which, directed to the corners of the tetrahedron, are filled with different atoms or groups of atoms (radicals or substituents). Asymmetric C-atoms in chemistry denote *. For instance:

glyceraldehyde malic acid

The phenomenon of optical activity is associated with the presence of optical isomers - substances that have the same order of bonds between atoms in a molecule, but their different spatial arrangement. In terms of spatial structure, optical isomers are like mirror images of each other, i.e. mirror antipodes or enantiomers. Enantiomers refer to each other as right and left hand. All constants of enantiomers, except for specific rotation (α), are the same.

Two forms of a substance with mirror-opposite conformations rotate a polarized light beam in opposite directions: (+) - to the right, (-) - to the left by the same angle, are called optical antipodes or enantiomers.

The currently generally accepted conventional designation method was first proposed by E. Fisher (1891), then somewhat modified by M.A. Rozanov (1906) and discussed in detail by Hudson (1949). Glyceric aldehyde is used as a standard:

D (+) - glycerin L (-) - glycerin

aldehyde aldehyde

However, it turned out that belonging to the D (d) –or L (l) –type of configuration does not always mean that the direction of rotation goes (+) to the right or (-) to the left. It is possible that D is a conformation and rotates the plane of the polarized beam to the left (-), or L is a conformation and rotates to the right (+). Therefore, the letter designations D (d) or L (l) determine the spatial orientation of atoms or atomic groups around an asymmetric C-atom, and the signs (+) - right rotation, (-) - left rotation.

A mixture of (+) and (-) forms (and in most cases it is a mixture of D- and L- forms) in a 1: 1 ratio is called a racemate or racemic mixture. It is optically inactive (±). If there are several asymmetric C-atoms in an organic compound, the number of optical isomers is determined by the formula:

where N is the number of optical isomers;

n is the number of asymmetric C-atoms.

Isomerism of lactic acid

D (-) - lactic acid L (+) - lactic acid

(Formed in the muscles during intense work) (Formed when milk sour)

Isomerism of tartaric acid

Mesotartaric acid L (-) - tartaric D (+) - tartaric acid

In meso-forms, one half of the molecule has a (+) configuration, the other (-) configuration (for example, in meso-tartaric acid). As a result of "internal compensation" of the sign of rotation, the meso-forms are optically inactive and, unlike racemates, they cannot be divided into enantiomers.

The value of optical isomerism

Each optically active substance, when examined under certain conditions, rotates the plane of polarization by a certain angle, the value of which is constant and characteristic for the given substance, i.e. the same constant as the melting point, boiling point of a substance, density, etc. The constant characterizing the optical activity of a substance is called specific rotation. Thus, by determining the specific rotation, the authenticity of the substance can be determined.

Optical isomerism is of great biological importance. Enzymes that catalyze biochemical reactions in living organisms have optical specificity, i.e. they act only on certain optical isomers (for example, D-monosaccharides, L-amino acids, etc.). The enzymes do not act on the optical antipodes of these substances; do not involve them in metabolism. Accumulating in tissues, such isomers can cause pathological processes.

It manifests itself in those cases when the isomers of the same compound, in connection with different arrangement of substituents at certain center, not compatible in space... For derivatives of the aliphatic series, isomerism is associated with the stereochemical features of the sp 3 hybrid carbon atom.

Even Le-Bel at the end of the 18th century suggested the tetrahedral structure of the carbon atom. In the event that a carbon atom is connected with four different substituents, there is a possibility of the existence of 2 isomers, which are mirror images of each other.

A carbon atom having all the different substituents is called asymmetric or chiral center ("hiros" - hand).

Consider the example of promising formulas:

Stereoisomers I and II are incompatible in space, are antipodes or optical isomers ( enantiomers, stereomers).

Fisher projection formulas

Consider promising formulas in a different plane.

Place the asymmetric center (carbon atom) in the plane of the sheet; alternates a and b behind the plane of the sheet ( from observer); alternates f and d above the plane of the sheet ( closer to observer) - in accordance with the arrows indicating the direction of the observer's gaze. We obtain a mutually perpendicular direction of bonds with the chiral center. Such a construction of isomers is called Fischer's projection formulas.

Thus, in Fischer's projection formulas, the substituents located horizontally are directed towards the observer, and vertically - beyond the plane of the sheet.

When constructing projection formulas, the most voluminous substituents are placed vertically. If the substituents are atoms or small groups that are not related to the main chain, then they are arranged horizontally. For 2-bromobutane

there are two antipode:

Enantiomers, antipodes, stereomers are practically indistinguishable in properties (boiling point, melting point, etc.), and also have similar thermodynamic constants. At the same time, they have differences:

4) - solid antipodes crystallize with the formation of crystals mirror-like to each other, but incompatible in space.

5) - the antipodes rotate the plane of polarized light at the same angle, but in different directions. If the angle of rotation of the light is positive (clockwise), then the antipode is called dextrorotatory; if it is negative (counterclockwise), then it is called levorotatory.

The angle of optical rotation of plane-polarized light is designated [ α D]. If [ α D] = -31.2 °, the levorotatory antipode was studied.

Polarimeter device

Substances that can rotate the plane of polarized light are called optically active or optically active.

A mixture of two enantiomers in a 1: 1 ratio does not rotate the plane of polarized light and is called a racemic mixture, a racemate.

If one antipode prevails in the mixture over another, then one speaks of its optical purity (ee). It is calculated from the difference in the content of enantiomers in the mixture.

II - 30%, ee = 70 - 30 = 40 (%)

Secondary and tertiary amines can also be optically active. The fourth substituent is the lone pair of electrons on the nitrogen atom.

5.4.1 Diastereomers

Diastereometry is a phenomenon that has a more significant effect on the properties of substances and is observed in cases where there are two or more asymmetric centers in the compound. For instance:

4-chloropentanol-2

Let's depict all possible antipodes (I-IV) for connection:

Optical isomers (stereoisomers) of the same compound, which are not antipodes, are called diastereomers. That is, pairs of isomers I and III, I and IV, II and III, II and IV are diastereomeric pairs. The number of isomers is calculated by the formula: q = 2 n, where

q is the total number of stereoisomers,

n is the number of asymmetric centers (C *).

For example, glucose has 4 chiral centers, then q = 2 4 = 16 (D-glucose - 8 isomers, L-glucose - 8 isomers).

D-glucose

In nature, there are cases when asymmetric atoms in a compound have the same environment. This leads to the fact that half of the antipodes are not optically active.

wine acid

å α =0 å α =0 å α =2α å α =-2α

mesoform

Mesoform is an optically inactive form resulting from internal symmetry in an optically active substance.

Unlike antipodes, diastereomers differ in boiling point, density (d 4 20), refractive index (n 4 20), etc.

Introduced the term ISOMERIA and suggested that the differences arise from "the different distribution of simple atoms in a complex atom" (ie, a molecule). Isomerism received a true explanation only in the second half of the 19th century. based on the theory of chemical structure of A.M.Butlerov (structural isomerism) and the stereochemistry of Ya. G. Van't Hoff (spatial isomerism).

Structural isomerism

Structural isomerism is the result of differences in chemical structure. This type includes:

Isomerism of the hydrocarbon chain (carbon skeleton)

Isomerism of the carbon skeleton, due to the different bond order of carbon atoms. The simplest example is butane CH 3 -CH 2 -CH 2 -CH 3 and isobutane (CH 3) 3 CH. Dr. examples: anthracene and phenanthrene (formulas I and II, respectively), cyclobutane and methylcyclopropane (III and IV).

Valence isomerism

Valence isomerism (a special type of structural isomerism), in which isomers can be transformed into each other only due to the redistribution of bonds. For example, the valence isomers of benzene (V) are bicyclohexa-2,5-diene (VI, "Dewar's benzene"), Prisman (VII, "Ladenburg's benzene"), benzovalene (VIII).

Functional group isomerism

Differs in the nature of the functional group. Example: Ethanol (CH 3 -CH 2 -OH) and Dimethyl ether (CH 3 -O-CH 3)

Isomerism of position

A type of structural isomerism characterized by a difference in the position of the same functional groups or double bonds with the same carbon skeleton. Example: 2-chlorobutanoic acid and 4-chlorobutanoic acid.

Spatial isomerism (stereoisomerism)

Enantiomerism (optical isomerism)

Spatial isomerism (stereoisomerism) results from differences in the spatial configuration of molecules with the same chemical structure. This type of isomer is subdivided into enantiomerism(optical isomerism) and diastereomerism.

Enantiomers (optical isomers, mirror isomers) are pairs of optical antipodes of substances characterized by opposite in sign and equal in magnitude rotations of the plane of polarization of light with the identity of all other physical and chemical properties (except for reactions with other optically active substances and physical properties in a chiral medium ). A necessary and sufficient reason for the appearance of optical antipodes is the assignment of a molecule and one of the following point symmetry groups C n, D n, T, O, I (Chirality). Most often we are talking about an asymmetric carbon atom, that is, an atom bound to four different substituents, for example:

Other atoms can also be asymmetric, for example, atoms of silicon, nitrogen, phosphorus, sulfur. The presence of an asymmetric atom is not the only reason for enantiomerism. Thus, there are optical antipodes derivatives of adamantane (IX), ferrocene (X), 1,3-diphenylallene (XI), 6,6 "-dinitro-2,2" -diphenic acid (XII). The reason for the optical activity of the last compound is atropisomerism, that is, spatial isomerism caused by the absence of rotation around a simple bond. Enantiomerism is also manifested in the helical conformations of proteins, nucleic acids, hexagelicene (XIII).

(R) -, (S) - nomenclature of optical isomers (naming rule)

The four groups attached to the asymmetric carbon atom C abcd are assigned different seniorities corresponding to the sequence: a> b> c> d. In the simplest case, the priority is established by the ordinal number of the atom attached to the asymmetric carbon atom: Br (35), Cl (17), S (16), O (8), N (7), C (6), H (1) ...

For example, in bromochloroacetic acid:

The precedence of the substituents at the asymmetric carbon atom is as follows: Br (a), Cl (b), C of the COOH (c), H (d) group.

In butanol-2, oxygen is the senior substituent (a), hydrogen is the junior (d):

It is required to resolve the issue of substituents CH 3 and CH 2 CH 3. In this case, the seniority is determined by the ordinal number or the numbers of other atoms in the group. Leadership remains with the ethyl group, since in it the first C atom is bonded to another C (6) atom and to other H (1) atoms, while in the methyl group, carbon is bonded to three H atoms with serial number 1. In more complex cases continue to compare all the atoms until they come to atoms with different serial numbers. If there are double or triple bonds, then the atoms that are with them are counted as two and three atoms, respectively. Thus, the -COH group is considered as C (O, O, H), and the -COOH group - as C (O, O, OH); the carboxyl group is older than the aldehyde group, since it contains three atoms with the atomic number 8.

In D-glyceraldehyde, the OH (a) group is the oldest, followed by CHO (b), CH 2 OH (c) and H (d):

The next step is to determine whether the arrangement of the groups is right, R (Latin rectus), or left, S (Latin sinister). Passing to the corresponding model, it is oriented so that the junior group (d) in the perspective formula is at the bottom, and then viewed from above along the axis passing through the shaded face of the tetrahedron and group (d). In D-glycyrrhine aldehyde groups

are located in the direction of clockwise rotation, and therefore, it has an R-configuration:

(R) -glyceric aldehyde

In contrast to the D, L nomenclature, the designations of the (R) - and (S) - isomers are enclosed in parentheses.

Diastereomerism

σ-diastereomerism

Any combination of spatial isomers that do not constitute a pair of optical antipodes are considered diastereomeric. Distinguish between σ and π-diastereomers. σ-diastereomers differ from each other in the configuration of some of the chirality elements present in them. So, diastereomers are (+) - tartaric acid and meso-tartaric acid, D-glucose and D-mannose, for example:

For some types of diastereomerism, special designations have been introduced, for example, threo- and erythro-isomers - this is diastereomerism with two asymmetric carbon atoms and spaces, the arrangement of substituents on these atoms, reminiscent of the corresponding threose (related substituents are on opposite sides in Fisher's projection formulas) and erythrose ( deputies - on one side):

Erythro-isomers, which asymmetric atoms are linked with the same substituents, are called meso-forms. They, unlike other σ-diastereomers, are optically inactive because of the intramolecular compensation of contributions to the rotation of the plane of polarization of light from two identical asymmetric centers of opposite configuration. Pairs of diastereomers that differ in the configuration of one of several asymmetric atoms are called epimers, for example:

The term "anomers" refers to a pair of diastereomeric monosaccharides differing in the configuration of the glycosidic atom in the cyclic form, for example anomeric α-D- and β-D-glucose.

π-diastereomerism (geometric isomerism)

π-diastereomers, also called geometric isomers, differ from each other by the different spatial arrangement of substituents relative to the plane of the double bond (most often C = C and C = N) or the ring. These include, for example, maleic and fumaric acids (formulas XIV and XV, respectively), (E) - and (Z) -benzaldoximes (XVI and XVII), cis- and trans-1,2-dimethylcyclopentanes (XVIII and XIX).

Conformers. Tautomers

The phenomenon is inextricably linked with the temperature conditions of its observation. So, for example, chlorocyclohexane at room temperature exists in the form of an equilibrium mixture of two conformers - with equatorial and axial orientations of the chlorine atom:

However, at minus 150 ° C, an individual a-form can be distinguished, which behaves under these conditions as a stable isomer.

On the other hand, compounds that are isomers under normal conditions may turn out to be tautomers in equilibrium with increasing temperature. For example, 1-bromopropane and 2-bromopropane are structural isomers; however, as the temperature rises to 250 ° C, an equilibrium characteristic of tautomers is established between them.

Isomers that transform into each other at temperatures below room temperature can be considered as non-rigid molecules.

The existence of conformers is sometimes referred to as "rotational isomerism". Among the dienes, s-cis and s-trans isomers are distinguished, which, in essence, are conformers resulting from rotation around a simple (s-single) bond:

Isomerism is also characteristic of coordination compounds. So, isomeric compounds that differ in the method of coordination of ligands (ionization isomerism), for example, isomeric:

SO 4 - and + Br -

Here, in essence, there is an analogy with the structural isomerism of organic compounds.

Chemical transformations, as a result of which structural isomers are converted into each other, is called isomerization. Such processes are essential in industry. For example, isomerization of normal alkanes into isoalkanes is carried out to increase the octane number of motor fuels; pentane is isomerized to isopentane for subsequent dehydrogenation to isoprene. Intramolecular rearrangements are also isomerization, of which, for example, the conversion of cyclohexanone oxime into caprolactam, a raw material for the production of nylon, is of great importance.

The process of interconversion of enantiomers is called racemization: it leads to the disappearance of optical activity as a result of the formation of an equimolar mixture of (-) - and (+) - forms, that is, a racemate. The interconversion of diastereomers leads to the formation of a mixture in which a thermodynamically more stable form prevails. In the case of π-diastereomers, usually the trans form. The interconversion of conformational isomers is called conformational equilibrium.

The phenomenon of isomerism greatly contributes to an increase in the number of known (and even more so in the number of potentially possible) compounds. So, the possible number of structurally isomeric decyl alcohols is more than 500 (about 70 of them are known), there are more than 1500 spaces, isomers.

In theoretical consideration of the problems of isomerism, topological methods are becoming more and more widespread; mathematical formulas are derived to calculate the number of isomers. To designate spaces, isomers of different types, a stereochemical nomenclature has been developed, collected in section E of the IUPAC Nomenclature Rules for Chemistry.

Literature

Fizer L., Fizer M., Organic chemistry. Advanced course. vol. 1. lane from English, Ed. Doctor of Chemical Sciences N.S. Wolfson. Ed. "Chemistry". M., 1969.
Palm VA, Introduction to theoretical organic chemistry, M., 1974;
Sokolov V I., Introduction to theoretical stereochemistry, M., 1979;
Slanina 3., Theoretical aspects of the phenomenon of isomerism in chemistry, trans. from Czech., M., 1984;
Potapov V.M., Stereochemistry M., 1988.