Isomerism of organics. Isomerism and its types – Knowledge Hypermarket

Isomerism –the existence of different substances with the same molecular formula. This phenomenon is due to the fact that the same atoms can connect to each other in different ways. All isomers are divided into two large classes - structural isomers And spatial isomers (stereoisomers).

Structural are isomers that correspond to different structural formulas of organic compounds (with different order of connection of atoms).

Stereoisomers are compounds that have the same composition and the same order of connection of atoms, but differ in the arrangement of atoms in space.

Structural isomers. In accordance with the above classification of organic compounds by type, three groups are distinguished among structural isomers:

1) compounds containing various functional groups and belonging to different classes of organic compounds, for example:

2) compounds that differ in carbon skeletons:

3) compounds that differ in the position of the substituent or multiple bond in the molecule:

Spatial isomers (stereoisomers). Stereoisomers can be divided into two types: geometric isomers and optical isomers.

Geometric isomerism characteristic of compounds containing a double bond or ring. In such molecules it is often possible to draw a conventional plane in such a way that the substituents on different carbon atoms can be on the same side (cis-) or on different sides (trance-) from this plane. If a change in the orientation of these substituents relative to the plane is possible only due to the breaking of one of the chemical bonds, then they speak of the presence of geometric isomers.

Geometric isomers can differ significantly in their physical and chemical properties.

Optical isomers are molecules whose mirror images are incompatible with each other. They can be divided into two types: enantiomers And diastereomers.

Stereoisomers having a mirror configuration of asymmetric (chiral) centers are called enantiomers or optical antipodes.

Enantiomerism is characteristic of molecules that have one asymmetric (chiral) atom carbon, i.e. an atom bonded to four different atoms or groups of atoms. Molecules of enantiomers relate to each other as an object and an incompatible mirror image. Enantiomers have the same physical and chemical properties, but differ in the sign of rotation of polarized light.

For example, lactic acid exists in the form of enantiomers. CH 3 -CH(OH)-COOH:

An equimolar mixture of (+) and (–) enantiomers is optically inactive and is called racemic mixture or racemate.

Diastereomers– spatial isomers, the molecules of which are not mirror images of each other. Diastereomers differ from each other in physical and chemical properties.

Every year at the CT and Unified State Exams in chemistry there are certain tasks on the concepts of “isomerism” and “isomers”. In this article I will give you examples of interclass isomerism and show you how to use this knowledge to complete CT and Unified State Exam tasks.

Isomers- these are substances that have the same qualitative and quantitative composition, but different structures and, therefore, different properties. In other words, isomers are substances that have same formula, but different structure. Or Isomers – substances that have the same molecular but different structural formulas.

The phenomenon of the existence of isomers is called isomerism .

But we will talk about interclass isomerism.

Isomerism of various classes of organic compounds (interclass isomerism) is caused by different positions and combinations of atoms in molecules of substances that have the same molecular formula, but belong to different classes.

So, examples of isomeric classes of organic substances:

1. Alkenes and cycloalkanes, for example 1-hexene and cyclohexane:

2. Alkynes and alkadienes (dienes), for example, butyne-1 and butaene-1,3:

3. Ethers and monohydric alcohols, for example, diethyl ether and 1-butanol:

4. Esters and monocarboxylic acids, such as methyl acetate and propanoic acid:

5. Aldehydes and ketones, for example, propanal and propanone-2:

6. Amino acids (containing one nitrogen atom and one carboxyl group) and nitroalkanes, for example, glycine and nitroethane:

These are examples of interclass isomers that were encountered in various years of the Central Television and the Unified State Examination.

And now I’ll give an example of tasks from the CT and the Unified State Exam, in which it was necessary to use knowledge about isomers, including interclass ones:

Demo Unified State Exam 2018:

Task 12. From the proposed list, select two substances that are structural isomers of pentene-2.

1) pentane;

2) cyclopentane;

3) pentin-1;

4) pentadiene-1,3;

5) 2-methylbutene-2.

Write down the numbers of the selected substances in the answer field.

See the answer and video solution below:

Task B2 from CT 2015:

AT 2. Match an organic substance with its isomer. Write the answer as a combination of letters and numbers, for example: A1B2B3G4D5E6Zh7

Substance	Isomer
A. Pentanal B. Methyl acetate

During the lesson, you will get a general idea of ​​the types of isomerism and learn what an isomer is. Learn about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). Using the structural formulas of substances, consider the subtypes of structural isomerism (skeletal and positional isomerism), learn about the types of spatial isomerism: geometric and optical.

Topic: Introduction to organic chemistry

Lesson: Isomerism. Types of isomerism. Structural isomerism, geometric, optical

The types of formulas describing organic substances that we examined earlier show that several different structural formulas can correspond to one molecular formula.

For example, the molecular formula C 2H 6O correspond two substances with different structural formulas - ethyl alcohol and dimethyl ether. Rice. 1.

Ethyl alcohol, a liquid that reacts with sodium metal to release hydrogen, boils at +78.5 0 C. Under the same conditions, dimethyl ether, a gas that does not react with sodium, boils at -23 0 C.

These substances differ in their structure - different substances have the same molecular formula.

Rice. 1. Interclass isomerism

The phenomenon of the existence of substances that have the same composition, but different structures and therefore different properties is called isomerism (from the Greek words “isos” - “equal” and “meros” - “part”, “share”).

Types of isomerism

There are different types of isomerism.

Structural isomerism is associated with different orders of atoms in a molecule.

Ethanol and dimethyl ether are structural isomers. Since they belong to different classes of organic compounds, this type of structural isomerism is called also interclass . Rice. 1.

Structural isomers can also exist within the same class of compounds, for example, the formula C 5 H 12 corresponds to three different hydrocarbons. This carbon skeleton isomerism. Rice. 2.

Rice. 2 Examples of substances - structural isomers

There are structural isomers with the same carbon skeleton, which differ in the position of multiple bonds (double and triple) or atoms replacing hydrogen. This type of structural isomerism is called positional isomerism.

Rice. 3. Structural position isomerism

In molecules containing only single bonds, almost free rotation of molecular fragments around the bonds is possible at room temperature, and, for example, all images of the formulas of 1,2-dichloroethane are equivalent. Rice. 4

Rice. 4. Position of chlorine atoms around a single bond

If rotation is hindered, for example, in a cyclic molecule or with a double bond, then geometric or cis-trans isomerism. In cis-isomers, the substituents are located on one side of the plane of the ring or double bond, in trans-isomers - on opposite sides.

Cis-trans isomers exist when they are bonded to a carbon atom. two different deputy Rice. 5.

Rice. 5. Cis and trans isomers

Another type of isomerism arises due to the fact that a carbon atom with four single bonds forms a spatial structure with its substituents - a tetrahedron. If a molecule has at least one carbon atom bonded to four different substituents, optical isomerism. Such molecules do not match their mirror image. This property is called chirality - from the Greek Withhier- "hand". Rice. 6. Optical isomerism is characteristic of many molecules that make up living organisms.

Rice. 6. Examples of optical isomers

Optical isomerism is also called enantiomerism (from Greek enantios- “opposite” and meros- “part”), and optical isomers - enantiomers . Enantiomers are optically active; they rotate the plane of polarization of light by the same angle, but in opposite directions: d- , or (+)-isomer, - to the right, l- , or (-)-isomer, - to the left. A mixture of equal amounts of enantiomers called racemate, is optically inactive and is indicated by the symbol d,l- or (±).

Summing up the lesson

During the lesson, you received a general understanding of the types of isomerism and what an isomer is. We learned about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). Using the structural formulas of substances, we examined the subtypes of structural isomerism (skeletal and positional isomerism), and became acquainted with the types of spatial isomerism: geometric and optical.

Bibliography

1. Rudzitis G.E. Chemistry. Fundamentals of general chemistry. 10th grade: textbook for general education institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. Grade 10. Profile level: academic. for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2008. - 463 p.

3. Chemistry. Grade 11. Profile level: academic. for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. Nos. 1,2 (p.39) Rudzitis G.E. Chemistry. Fundamentals of general chemistry. 10th grade: textbook for general education institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Why is the number of isomers in hydrocarbons of the ethylene series greater than that of saturated hydrocarbons?

3. Which hydrocarbons have spatial isomers?

Organic substances are capable of forming isomers. These are compounds with the same number of atoms, but different in structure or position in space. The structure and position of the molecule influence the physical and chemical properties of organic compounds.

Classification and nomenclature

An explanation of isomerism was obtained in the second half of the 19th century thanks to the theory of the chemical structure of organic substances by Alexander Butlerov. The chemist showed that the properties of substances depend not only on the number of atoms, but also on their position in the molecule and space.

Rice. 1. Alexander Butlerov.

In this regard, there are two types of isomerism:

• structural- is associated with the position of atoms or groups of atoms in a molecule of a substance, as well as the position of multiple bonds;
• spatial- reflects the position of the molecule in space relative to the conventional plane.

The number of isomers of one substance depends on the number of carbon atoms in the molecule. The longer the chain, the more options for isomerism.

Structural

The position of the substituent, double bonds, and functional group in the molecule can change. In this regard, the following types of structural isomerism are distinguished:

• carbon skeleton;
• provisions.

Isomerism of the carbon skeleton involves the transfer of the methyl group -CH 2 to any carbon atom of the molecule. For example, one CH 2 group can detach from pentane (CH 3 -CH 2 -CH 2 -CH 2 -CH 3) and attach to a second atom, forming 2-methylbutane.

Positional isomerism is of three types:

• multiple bonds- isomers are formed due to the movement of multiple bonds in the molecule: CH 2 =C=CH-CH 3 (butadiene-1,2) and CH 2 =CH-CH=CH 2 (butadiene-1,3);
• functional group- change in the position of the functional radical: CH 3 -CH 2 -CH 2 -CH 2 OH (butanol-1) and CH 3 -CH 2 -CHOH-CH 3 (butanol-2);
• deputy- addition of a radical to another carbon atom in the molecule: CH 3 -CHCl-CH 2 -CH 3 (2-chlorobutane) and CH 2 Cl-CH 2 -CH 2 -CH 3 (1-chlorobutane).

  Separately, interclass isomerism is distinguished, which essentially depends on the position of the functional group. In some cases, when an atom is transferred, for example, from the end to the middle of a molecule, a substance of a different class is formed. At the same time, the molecular formula of the substances remains the same. For example, CH 3 -CH 2 -OH is ethanol, and CH 3 -O-CH 3 is dimethyl ether. The molecular formula of both substances is C 2 H 6 O. Another example: propylene and cyclopropane with the formula C 3 H 6.

  

  Rice. 2. Structural formulas of propylene and cyclopropane.

  The name of structural isomers consists of the names of the radicals and the carbon chain. At the beginning of the name there are numbers indicating the number of the atom to which the radical is attached (counting starts from the branched end). Numbers may also be placed at the end of the name, indicating the number of the atom with a double or triple bond.

  Spatial

  This type is classified into two groups:

  • optical or mirror isomerism;
  • geometric isomerism.

  The essence of optical isomerism is the mirror reflection of molecules. The isomers seem to reflect each other.

  Geometric isomerism is divided into two types:

  • cis isomerism- radicals are located on one side of the conventional plane dividing the molecule in half;
  • trans isomerism- radicals lie on different sides of the conventional plane.

  

  Rice. 3. Optical and geometric isomerism.

  Isomers of spatial isomerism are called stereoisomers or spatial isomers. Mirror molecules are called enantiomers. If the molecules do not reflect each other, they are called diastereomers or geometric isomers.

  What have we learned?

  Isomerism is the phenomenon of the occurrence of isomers. These are substances that are identical in composition, but different in structure and position in space. There are two types - structural and spatial isomerism. Structural isomerism reflects the structure of molecules. It can be manifested by the carbon skeleton, the position of the functional group, multiple bonds, and substituent. Interclass structural isomerism is also distinguished. Spatial isomerism can be optical or geometric. It is due to the peculiarities of the position of the molecule in space.

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Isomers are compounds that have the same qualitative and quantitative composition (molecular formula), but differ from each other in the sequence of bonding of atoms or their arrangement in space. Since the structure of these compounds is different, the chemical or physical properties of the isomers are different.

Types of isomerism: structural (structural isomers) and stereoisomerism (spatial).

Structural isomerism can be of three types:

– isomerism of the carbon skeleton (isomers according to the structure of the carbon chain), for example butane (a compound with an unbranched or normal structure) and 2-methylpropane (a compound with a branched structure);

– isomers of the position of functional groups (or multiple bonds), for example 1-butanol (hydroxyl group is connected to the 1st carbon atom in the chain) and 2-butanol (hydroxyl group is bonded to the 2nd carbon atom in the chain);

– functional group isomers (or interclass isomerism), for example, 1-butanol (alcohol) and diethyl ether (ether).

Stereoisomerism is divided into conformational and configurational.

The conformations of a molecule are its various geometric shapes, resulting from rotation around simple -bonds.

Configuration is the arrangement of atoms in space without taking into account differences arising due to rotation around simple -bonds.

Conformations of organic molecules. Rotation around the -bond C – C is done relatively easily, the hydrocarbon chain can take different forms. Conformational forms easily transform into each other and therefore are not different compounds - they are different unstable dynamic forms of the same molecule. The energy difference between conformers is of the same order as the energy of thermal motion (several kJ/mol). Therefore, at ordinary temperatures, individual conformers cannot be isolated.

A distinction is made between eclipsed and inhibited conformations (Fig. 2).

Rice. 2. Conformations of pentane: a – eclipsed; b – inhibited

In Fig. Figure 2 shows the conformations of pentane based on the bond between the second and third carbon atoms of the chain. It can be seen that in the eclipsed conformation, the hydrogen or carbon atoms seem to obscure each other. The inhibited conformation occurs as a result of a rotation of one of the atoms by 60 and the distance between unbonded atoms increases slightly, the repulsive forces of the electron orbitals of the atoms decrease, and this configuration is energetically more favorable. The molecules of many organic compounds are mixtures of conformers; as a result of thermal movement, the molecules undergo continuous conformational transformations.

Newman's projection formulas. To depict conformations, Newman’s projection formulas are used, which are obtained by projecting the molecule C onto the plane – C – connections. As an example in Fig. Figure 3 shows the conformations of pentane relative to the C 2 –C 3 bond.

The carbon atom closest to the observer (C 2) is designated by a dot in the center of the circle; the circle symbolizes the removed carbon atom (C 3). Three bonds from an atom are depicted as lines diverging from the center of the circle - for a nearby atom (C 2) or “protruding” from behind the circle - for a distant atom (C 3). If the atoms and groups associated with the carbon atoms in question as if they obscure each other, the conformation is called eclipsed (Fig. 3. a), when one of the atoms rotates relative to the other by 60°, we obtain an energetically more favorable inhibited conformation (Fig. 3. b).

Rice. 3. Newman’s projection formula for a: eclipsed conformation of pentane and b: inhibited conformation of pentane.

Conformations of cyclic compounds. Cyclic non-aromatic compounds are generally not planar. To reduce the angular and torsional stresses that may arise due to the difference in the values ​​of bond angles and polygon angles, one or more ring atoms can be located in a different plane with respect to the remaining atoms. Thus, five-membered cycles can have the shape of an envelope in space (Fig. 4), and six-membered ones can have the shape of a bathtub or chair (Fig. 5).

Rice. 4. Conformation of cyclopentane

In the envelope conformation, one of the carbon atoms moves out of the plane in which the other four atoms are located. Any of the five atoms can emerge from the plane, and the cycle therefore seems to be in constant wave-like motion.

Rice. 5. Conformations of cyclohexane: a – chair and b – bath.

In the chair and bath conformations, 2 carbon atoms are located outside the plane in which 4 more atoms are located.

In the chair conformation of cyclohexane there are no occluded positions of hydrogen and carbon atoms: the arrangement of hydrogen atoms on all carbon atoms is the same as in the inhibited conformation of ethane.

Six C bonds – H, parallel to the symmetry axes of the chair-shaped form of cyclohexane, directed alternately up and down, are called axial (symbol A).The remaining six C – The H bonds are located at an angle of 109.5° to this axis and are also alternately directed up and down. These connections are called equatorial (symbol e). Thus, each carbon atom has one bond with a hydrogen atom located axially and one bond equatorially. The chair conformation is energetically more favorable.

Configuration isomers. Optical isomerism. Configurational are stereoisomers with different arrangements around certain atoms of other atoms, radicals or functional groups in space relative to each other. These primarily include enantiomers - optically active substances that are mirror images of each other.

What substances are called optically active? These are compounds capable of changing the angle of inclination of the plane of polarization of plane-polarized light. Recall that ordinary light (from the sun or a lamp) is an electromagnetic wave in which particles vibrate in all directions in mutually perpendicular planes and perpendicular to the direction of propagation of the wave. In plane-polarized light, particle vibrations lie in the same plane. If a beam passes through a transparent substance that is capable of rotating the plane of electric field oscillations by a certain angle and giving them a new direction, then such a substance is said to have optical activity.

Two signs of optical activity of organic compounds can be formulated: the presence of an asymmetric carbon atom and the absence of symmetry elements in the molecule.

An asymmetric carbon atom is an atom bonded to four different atoms or groups, usually denoted by an asterisk: *C.

Let's consider the molecule of alanine (2-aminopropanoic acid), an amino acid that is part of the protein (Fig. 6). The molecule has one asymmetric carbon atom (the second, associated with four different substituents: an amino group, a carboxyl group, a hydrogen atom and a methyl group -CH 3). The carbon atom of the carboxyl group is not asymmetric, because he has not 4, but only 3 substituents. The third carbon atom (the methyl carbon) is also not asymmetric. It has 4 substituents, but 3 of them are the same (hydrogen atoms). The molecule of this compound is asymmetrical; therefore, alanine is an optically active compound and can exist in the form of two enantiomers. Enantiomers are named by D,L nomenclature, which describes the relative configuration of the isomers (relative to the configuration of glyceraldehyde).

In order to depict and name an enantiomer, it is convenient to position the carbon chain of the molecule vertically, then the substituents on the asymmetric carbon atom appear to the right and left of it. If the senior substituent (in our case, the amino group) is located on the left, it is an L-isomer, if on the right, it is a D-isomer (Fig. 6).

Rice. 6. Enantiomers of alanine.

Enantiomers, unlike isomers, have the same physical and chemical properties; they differ only in that they rotate the plane of polarization of plane-polarized light by the same angle, but in opposite directions (one to the left, the other to the right). A mixture consisting of equal molar amounts of enantiomers is called a racemic mixture or racemate. The racemate is not optically active.

Despite this seemingly minor difference in properties, the biological activity of the enantiomers is very different. For example, proteins contain only L-enantiomers of amino acids, this explains the peculiarities of the spatial structure of proteins and determines the selectivity of the catalytic action of enzymes. D-isomers of amino acids when entering the body can cause various negative processes, therefore the spatial configuration of amino acids must be taken into account in the production of medicines and various food additives.

An isomer of this compound, -alanine (3-aminopropanoic acid), is also present in our body. This compound is not part of proteins and is not optically active, because there are no asymmetric atoms in it. The first carbon atom of the carboxyl group has only 3 substituents, the second and third have 2 identical substituents (hydrogen atoms).

Let us continue our consideration of the phenomenon of optical isomerism. Substances characterized by this phenomenon are often called stereoisomers. Stereoisomers are identical in physical and physicochemical properties, but differ in two respects:

1. Crystallize in forms that do not have plane (plane) symmetry, but relate to each other as an object to its mirror image, for example, two types of tartaric acid crystals, highlighting stereoisomeric tartaric acids.

2. Stereoisomers, as noted above, polarize light differently. P The reason for optical stereoisomerism is due precisely to the arrangement of substituent groups on the carbon atom in the sp 3 -hybridization state, that is, with the saturated carbon at the vertices of the tetrahedron (the arrangement of atoms in space, leading to the presence of stereoisomerism, is called configuration).

And is denoted as follows:

For example,

L-alcohol,= –5.9

For these substances, left- and right-handed configurations are possible.

A racemate is a mixture of equal amounts of L- and D-isomers, optically inactive.

What configurations rotate the plane of polarization of light to the right and left is a special question. It is not considered here.

Point to carbon - bonds located above the drawing plane, point to substituent - below this plane.

Classic examples of a stereoisomer are:

H The number of asymmetric atoms can be several, in the general case.

* – asymmetric atom. The number of stereoisomers is equal to 2 n, where, as the attentive reader has already understood, n is the number of asymmetric optically active atoms.

Geometric (cis- and trans-) isomers. These include configurational isomers containing a  bond. This type of diastereomerism is characteristic, in particular, of alkenes. Relative to the plane of the  bond, identical substituents on two carbon atoms can be located one at a time (cis-) or different (trans-) sides (Fig. 7). The main reason for the existence of cis- and trans isomers is the impossibility of rotation around the  bond without breaking it.

Rice. 7. Geometric isomers of 2-butane.

Cis- and trans isomers have the same atomic binding sequence, but differ from each other in the spatial arrangement of substituents and therefore are stereoisomers. On the other hand, their molecules do not contain asymmetric carbon atoms and are not optically active.

Cis- and trans isomers have different physical properties and can undergo reactions (for example, addition, at different rates).

Geometric isomers are often found among natural compounds; in particular, the retinol isomer (vitamin A), in which all 4 double bonds are in the trans configuration, is especially important for ensuring visual acuity. Hydrocarbon radicals of unsaturated acids that make up liquid fats are in the cis configuration relative to double bonds.