Halogenderivatives of the Halogenderivatives of the hydrocarbons.hydrocarbons.

IIsomery of somery of the the organic compoundsorganic compounds. . Spatial construction of the Spatial construction of the

molecules.molecules.

LECTURE № 4

associate. prof. Ye. B. Dmukhalska, assistant. I.I. Medvid

Outline1.1. The nomenclature of halogenderivatives of The nomenclature of halogenderivatives of

hydrocarbons.hydrocarbons.2.2. The isomery of halogenderivatives of The isomery of halogenderivatives of

hydrocarbons.hydrocarbons.3.3. The medico-biological importance of The medico-biological importance of

halogenderivatives of hydrocarbons.halogenderivatives of hydrocarbons.4.4. Physical properties of Physical properties of halogenderivatives of halogenderivatives of

hydrocarbons.hydrocarbons.5.5. The methods of extraction of The methods of extraction of

halogenoalkanes.halogenoalkanes.6.6. Chemical properties of halogenoalkanes.Chemical properties of halogenoalkanes.7.7. Structural isomery of organic compounds.Structural isomery of organic compounds.8.8. Spatial isomery of organic compounds.Spatial isomery of organic compounds.

The nomenclature of halogenderivatives of The nomenclature of halogenderivatives of hydrocarbonshydrocarbons

Halogenderivatives of hydrocarbons are the products Halogenderivatives of hydrocarbons are the products of substitution one or several atoms of hydrogen to of substitution one or several atoms of hydrogen to atoms of halogens in the hydrocarbon molecules. atoms of halogens in the hydrocarbon molecules. The names of halogenderivatives of hydrocarbons The names of halogenderivatives of hydrocarbons are the names of the same hydrocarbons with added are the names of the same hydrocarbons with added prefix which means the halogen radical. i.e.prefix which means the halogen radical. i.e.

H3C CH2 CH

Br

CH3

2-bromobutane

Cl

chlorocyclohexane

Br

bromobenzene

If there are several halogen radicals in the If there are several halogen radicals in the molecule of halogenderivatives of hydrocarbons, molecule of halogenderivatives of hydrocarbons, then all substutients are called in alphabetical then all substutients are called in alphabetical order.order.

Some halogenderivatives of hydrocarbons Some halogenderivatives of hydrocarbons have trivial names:have trivial names:

CH3 CH

Br

CH2

2-bromo-4-methylpentane

CH

CH3

CH3

H

C Cl

Cl

Cl

chloroform

H

C I

I

I

iodoform

The isomery of halogenderivatives of The isomery of halogenderivatives of

hydrocarbonshydrocarbons Halogenderivatives of hydrocarbons are characterized by Halogenderivatives of hydrocarbons are characterized by

structural, geometrical and optical isomery. Structural structural, geometrical and optical isomery. Structural isomery is formed by different structure of carbon chain isomery is formed by different structure of carbon chain and different location of halogen atoms in the molecule of and different location of halogen atoms in the molecule of organic compound. organic compound.

CH2

Cl

CH2CH2CH3 H3C CH

Cl

CH2CH3

H3C CH

CH3

CH2

Cl

H3C C

CH3

CH3

Cl

1-chlorobutane 2-chlorobutane

2-methyl-1-chloropropane 2-methyl-2-chloropropane

Geometrical isomery is possible for molecules of Geometrical isomery is possible for molecules of halogenderivatives which contain the carbon atoms connected halogenderivatives which contain the carbon atoms connected with different substutients.with different substutients.

Optical isomery is possible for molecules of halogenderivatives Optical isomery is possible for molecules of halogenderivatives

which contain asymmetric carbon atomwhich contain asymmetric carbon atom. .

C C

Cl

HH

H3C

cys-1-chlorpropene

C C

H

ClH

H3C

trans-1-chlorpropene

CH3

C2H5

H Cl

CH3

C2H5

Cl H

D-2-chlorobutaneor

S-2-chlorobutane

L-2-chlorobutaneor

R-2-chlorobutane

The tables below show some physical data for a selection of The tables below show some physical data for a selection of

haloalkaneshaloalkanes..

The methods of preparation of The methods of preparation of halogenoalkaneshalogenoalkanes

1. 1. Halogenation of the saturated hydrocarbons Halogenation of the saturated hydrocarbons ((SR)SR)..

2. 2. The Finkelshtain reaction.The Finkelshtain reaction.RR––ClCl + + NaI NaI → → RR––II + + NaCl NaCl

3. 3. Hydrohalogenation is the joining HClHydrohalogenation is the joining HCl, , HBr or HJ to ethylene and acethylene HBr or HJ to ethylene and acethylene hydrocarbons. hydrocarbons.

4. 4. The substitution of the functional groups The substitution of the functional groups ((for example, for example, –ОН) –ОН) to atom of any to atom of any halogen by the action of the following reagents: halogen by the action of the following reagents:

a)a) HClHCl, , HBrHBr, , HI or mixture NaClHI or mixture NaCl + + HH22SOSO44((concentratedconcentrated), ), KBrKBr + + HH22SOSO44((concentratedconcentrated));;bb) ) PClPCl33, , PClPCl55, , PBrPBr33, , PBrPBr55 or mixture Por mixture P + + II22;;c) SOClc) SOCl22, SO, SO22ClCl22. .

CH2 CH2 + HBr CH3 CH2 Brbromomethane

H3C CH2 OH + HClt

H3C CH2 Cl + H2O

CH4 Cl2 HCl H3C Cl+ +chlormethane

Chemical properties of halogenoalkanesChemical properties of halogenoalkanes

1.Halogenalkanes react with water1.Halogenalkanes react with water

CC22HH55Br + HBr + H22O ↔ CO ↔ C22HH55OH + HBrOH + HBr

2. Halogenalkanes react with NaOH or KOH2. Halogenalkanes react with NaOH or KOH

CC22HH55Br + NaOH ↔ CBr + NaOH ↔ C22HH55OH + NaBrOH + NaBr

3. Williamson reaction3. Williamson reaction

CC22HH55Br + NaOCBr + NaOC22OO55 → C → C22HH55−O−C−O−C22HH55 + NaBr + NaBr

4. Reaction with salts of carboxylic acids4. Reaction with salts of carboxylic acids

C2H5 Br O C CH3

O

Na O C CH3

O

C2H5 NaBr+ +

5. Reaction with ammonium 5. Reaction with ammonium CC22HH55Br + NHBr + NH33 → [C → [C22HH55NHNH33]+Br− C]+Br− C22HH55NHNH22

6. Halogenalkanes react with NaCN or KCN6. Halogenalkanes react with NaCN or KCNFor example, using 1-bromopropane as a typical primary For example, using 1-bromopropane as a typical primary

halogenoalkane:halogenoalkane:

You could write the full equation rather than the ionic one, but it You could write the full equation rather than the ionic one, but it slightly obscures what's going on:slightly obscures what's going on:

The bromine (or other halogen) in the halogenoalkane is simply The bromine (or other halogen) in the halogenoalkane is simply replaced by a -CN group - hence a substitution reaction. In replaced by a -CN group - hence a substitution reaction. In this example, ethanenitrile is formed.this example, ethanenitrile is formed.

CC22HH55Br + NaCN → CBr + NaCN → C22HH55−C≡N + NaBr−C≡N + NaBr7. Reaction with salts of HNO7. Reaction with salts of HNO22

CC22HH55Br + NaNOBr + NaNO22 → C → C22HH55NONO22 + NaBr + NaBr

8. Finkelshtain reaction (catalyst is acetone):8. Finkelshtain reaction (catalyst is acetone):

CC22HH55Cl + NaI → CCl + NaI → C22HH55I + NaClI + NaCl

9. Reaction with NaSN (thioalkohols form) or 9. Reaction with NaSN (thioalkohols form) or NaNa22S (thioethers form):S (thioethers form):

CC22HH55I + NaSN → CI + NaSN → C22HH55SN + NaISN + NaI

2C2C22HH55I + NaI + Na22S → CS → C22HH55−S−C−S−C22HH55 + 2NaI + 2NaI

10. Reaction with metals:10. Reaction with metals:

CC22HH55I + Mg → CI + Mg → C22HH55MgIMgI

11. Reduction (the reaction runs in the 11. Reduction (the reaction runs in the presence of catalysts):presence of catalysts):

CC22HH55Cl + HCl + H22 → C → C22HH66 + HCl + HCl

7. IIsomery of organic compounds somery of organic compounds Isomery is the phenomenon of existence of compounds which are similar by qualitative and quantitive structures but are different by locations of bonds in molecule. Different compounds that have the same molecular formula are called isomers. If they are different because their atoms are connected in a different order, they are called constitutional isomers. They can have different properties.

Formamide (left) and formaldoxime (right) are constitutional isomers; both have the same molecular formula (CH3NO), but the atoms are connected in a different order.

Isomery

Structural Spatial

Isomery of chain

Isomery of location of functional

group

Isomery of functional

group

Configurative Conformative

Geometrical

Optical

Isomery of Carbon chainIsomery of Carbon chain is formed by different sequence is formed by different sequence of atoms in the molecule of the organic compound. of atoms in the molecule of the organic compound.

C4H10

H3C CH2 CH2 CH3

butane

CH3 CH CH3

isobutane

CH3

For cyclic compounds the isomery can change the Carbon For cyclic compounds the isomery can change the Carbon cycle in the molecule of the isomer.cycle in the molecule of the isomer.

C6H12

cyclohexane

1-methylcyclopentane

CH3

1,2-dimethylcyclobutane

CH3H3C

1,2,3-trimethylcyclopropane

CH3

H3C CH3

Isomery of the location of the functional group is formed by Isomery of the location of the functional group is formed by different locations of identical functional groups and double different locations of identical functional groups and double or triple bonds.or triple bonds.

C3H7Cl

H3C CH2 CH2 Cl1-chlorpropane

H3C CH CH32-chlorpropane

Cl

CC66HH1010ClCl22 Cl

Cl

1,2-dichlorcyclohexane

Cl

1,3-dichlorcyclohexaneCl

Cl

1,4-dichlorcyclohexaneCl

CC44HH88

H2C CH CH2

butene-1

CH3

H3C CH

CH CH3

butene-2

Isomery of the functional group is formed by different Isomery of the functional group is formed by different functional groups in the molecules.functional groups in the molecules.

C2H6O

H3C CH2 OHethanol

H3C O CH3

dimethylether

ConformationConformation is the different spatial localization is the different spatial localization of atoms or atom groups in the molecule as a of atoms or atom groups in the molecule as a result of its rotation around result of its rotation around -bonds. Hydrogen -bonds. Hydrogen peroxide is formed in the cells of plants and peroxide is formed in the cells of plants and animals but is toxic to them. Consequently, animals but is toxic to them. Consequently, living systems have developed mechanisms to living systems have developed mechanisms to rid themselves of hydrogen peroxide, usually by rid themselves of hydrogen peroxide, usually by enzyme-catalyzed reduction to water. An enzyme-catalyzed reduction to water. An understanding of how reactions take place, be understanding of how reactions take place, be they reactions in living systems or reactions in they reactions in living systems or reactions in test-tubes, begins with a thorough knowledge of test-tubes, begins with a thorough knowledge of the structure of the reactants, products, and the structure of the reactants, products, and catalysts. catalysts.

Even a simple molecule such as hydrogen Even a simple molecule such as hydrogen peroxide may be structurally more complicated peroxide may be structurally more complicated than you think. Suppose we wanted to write the than you think. Suppose we wanted to write the structural formula for H202 in enough detail to structural formula for H202 in enough detail to show the positions of the atoms relative to one show the positions of the atoms relative to one another. We could write two different planar another. We could write two different planar geometries A and B that differ by a 180geometries A and B that differ by a 180 rotation rotation about the O—O bond. We could also write an about the O—O bond. We could also write an infinite number of nonplanar structures, of which infinite number of nonplanar structures, of which C is but one example, that differ from one C is but one example, that differ from one another by tiny increments of rotation about the another by tiny increments of rotation about the O—O bond.O—O bond.

Structures A, B, and C represent different conformations Structures A, B, and C represent different conformations of hydrogen peroxide. Conformations are different of hydrogen peroxide. Conformations are different spatial arrangements of a molecule that are generated by spatial arrangements of a molecule that are generated by rotation about single bonds. Although we can't tell from rotation about single bonds. Although we can't tell from simply looking at these structures, we now know from simply looking at these structures, we now know from experimental studies that C is the most stable experimental studies that C is the most stable conformation.conformation.

There is also the conformation in the structure of molecules There is also the conformation in the structure of molecules of organic compounds (alkanes and cycloalkanes). of organic compounds (alkanes and cycloalkanes).

Ethane is the simplest hydrocarbon that can have distinct conformations. Two, the staggered conformation and the eclipsed conformation, deserve special mention and are illustrated with molecular models below.

In the staggered conformation, each C—H bond In the staggered conformation, each C—H bond of one carbon bisects an H—C—H angle of the of one carbon bisects an H—C—H angle of the other carbon. In the eclipsed conformation, each other carbon. In the eclipsed conformation, each C—H bond of one carbon is aligned with a C—C—H bond of one carbon is aligned with a C—H bond of the other carbon. H bond of the other carbon. The staggered and eclipsed conformations The staggered and eclipsed conformations interconvert by rotation around the C—C bond, interconvert by rotation around the C—C bond, and do so very rapidly. Among the various ways and do so very rapidly. Among the various ways in which the staggered and eclipsed forms are in which the staggered and eclipsed forms are portrayed, wedge-and-dash, sawhorse, and portrayed, wedge-and-dash, sawhorse, and Newman projection drawings are especially Newman projection drawings are especially useful.useful.

by a torsion angle or dihedral angle, which is the angle by a torsion angle or dihedral angle, which is the angle between the H—C—C plane and the C—C—H plane. The between the H—C—C plane and the C—C—H plane. The torsion angle is easily seen in a Newman projection of ethane torsion angle is easily seen in a Newman projection of ethane as the angle between C—H bonds of adjacent carbons.as the angle between C—H bonds of adjacent carbons.

Here it is illustrated the structural feature that is the spatial relationship between atoms on adjacent carbons. Each H—C—C—H unit in ethane is characterized

Eclipsed bonds are characterized by a torsion angle of 0Eclipsed bonds are characterized by a torsion angle of 0. . When the torsion angle is approximately 60When the torsion angle is approximately 60, it means that the , it means that the spatial relationship is gauche; and when it is 180spatial relationship is gauche; and when it is 180 it is called it is called anti. Staggered conformations have only gauche or anti anti. Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms.relationships between bonds on adjacent atoms.

In organic chemistry, chirality most often occurs in In organic chemistry, chirality most often occurs in molecules that contain a carbon that is attached to four molecules that contain a carbon that is attached to four different groups. An example is different groups. An example is bromochlorofluoromethane (BrClFCH).bromochlorofluoromethane (BrClFCH).

As shown in figure, the two mirror images of As shown in figure, the two mirror images of bromochlorofluoromethane cannot be superimposed on bromochlorofluoromethane cannot be superimposed on each other. Because the two mirror images of each other. Because the two mirror images of bromochlorofiuoromethane are not superimposable, bromochlorofiuoromethane are not superimposable, BrClFCH is chiral. BrClFCH is chiral.

A molecule of chlorodifluoromethane (ClFA molecule of chlorodifluoromethane (ClF22CH), in which two CH), in which two

of the atoms attached to carbon are not chiral. Figure shows of the atoms attached to carbon are not chiral. Figure shows two molecular models of ClFtwo molecular models of ClF22CH drawn so as to be mirror CH drawn so as to be mirror

images. As is evident from these drawings, it is a simple images. As is evident from these drawings, it is a simple matter to merge the two models so that all the atoms match. matter to merge the two models so that all the atoms match. Because mirror-image representations of Because mirror-image representations of chlorodifluoromethane are superimposable on each other, chlorodifluoromethane are superimposable on each other, ClFClF22CH is achiral. CH is achiral.

Molecules of the general type are chiral when w, x, y, and z are different. In 1996, the IUPAC recommended that a tetrahedral carbon atom that bears four different atoms or groups be called a chirality center, which is the term that

we will use. Several earlier terms, including “asymmetric center”, “asymmetric carbon”, “chiral center”, “stereogenic center” and “stereocenter”, are still widely used.

Noting the presence of one (but not more than one) chirality center is a simple, rapid way to determine if a molecule is chiral. For example, the second atom of carbon C-2 is a chirality center in 2-butanol; it bears a hydrogen atom and methyl, ethyl, and hydroxyl groups as its four different substituents. By way of contrast, none of the carbon atoms bear four different groups in the achiral alcohol 2-propanol.

A molecule may have one or more chirality centers. When a molecule contains two chirality centers, as does 2,3-dihydroxybutanoic acid, there are possible several stereoisomers.

Organic chemists use an informal nomenclature system based on Fischer Organic chemists use an informal nomenclature system based on Fischer projections to distinguish between diastereomers. When the carbon chain projections to distinguish between diastereomers. When the carbon chain is vertical and like substituents are on the same side of the Fischer is vertical and like substituents are on the same side of the Fischer projection, the molecule is described as the projection, the molecule is described as the erythro erythro diastereomer. When diastereomer. When like substituents are on opposite sides of the Fischer projection, the like substituents are on opposite sides of the Fischer projection, the molecule is described as the molecule is described as the threothreo diastereomer. Thus, as seen in the diastereomer. Thus, as seen in the Fischer projections of the stereoisomeric 2,3-dihydroxybutanoic acids, Fischer projections of the stereoisomeric 2,3-dihydroxybutanoic acids, compounds I and II are erythro stereoisomers and III and IV are threo.compounds I and II are erythro stereoisomers and III and IV are threo.

Because diastereomers are not mirror images of each other, Because diastereomers are not mirror images of each other, they can have quite different physical and chemical they can have quite different physical and chemical properties. For example, the (2R,3R) stereoisomer of 3-properties. For example, the (2R,3R) stereoisomer of 3-amino-2-butanol is a liquid, but the (2R,3S) diastereomer is amino-2-butanol is a liquid, but the (2R,3S) diastereomer is a crystalline solid.a crystalline solid.

The experimental facts that led van't Hoff and Le Bel to propose that The experimental facts that led van't Hoff and Le Bel to propose that molecules having the same constitution could differ in the arrangement molecules having the same constitution could differ in the arrangement of their atoms in space concerned the physical property of of their atoms in space concerned the physical property of optical optical activityactivity. Optical activity is the ability of a chiral substance to rotate the . Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light and is measured using an instrument called plane of plane-polarized light and is measured using an instrument called a a polarimeterpolarimeter..

Conversely, when one enantiomer is present in excess, a Conversely, when one enantiomer is present in excess, a net rotation of the plane of polarization is observed. At net rotation of the plane of polarization is observed. At the limit, where all the molecules are of the same the limit, where all the molecules are of the same handedness, we say the substance is optically pure. handedness, we say the substance is optically pure. Optical purity, or percent enantiomeric excess, is defined Optical purity, or percent enantiomeric excess, is defined as:as:

Rotation of the plane of polarized light in the clockwise Rotation of the plane of polarized light in the clockwise sense is taken as positive (+), and rotation in the sense is taken as positive (+), and rotation in the counterclockwise sense is taken as a negative (-) rotation. counterclockwise sense is taken as a negative (-) rotation. Older terms for positive and negative rotations were Older terms for positive and negative rotations were dextrorotatory and levorotatory, from the Latin prefixes dextrorotatory and levorotatory, from the Latin prefixes dextro-dextro- ("to the right") and ("to the right") and levo-levo- ("to the left"), respectively. ("to the left"), respectively.

The observed rotation The observed rotation of an optically pure substance of an optically pure substance depends on how many molecules the light beam depends on how many molecules the light beam encounters. A filled polarimeter tube twice the length of encounters. A filled polarimeter tube twice the length of another produces twice the observed rotation, as does a another produces twice the observed rotation, as does a solution twice as concentrated. To account for the effects of solution twice as concentrated. To account for the effects of path length and concentration, chemists have defined the path length and concentration, chemists have defined the term specific rotation, given the symbol [term specific rotation, given the symbol []. Specific ]. Specific rotation is calculated from the observed rotation according rotation is calculated from the observed rotation according to the expressionto the expression

where c - the concentration of the sample in grams per 100 mL of solution, and l- the length of the polarimeter tube in decimeters.

It is convenient to distinguish between enantiomers by It is convenient to distinguish between enantiomers by prefixing the sign of rotation to the name of the substance. prefixing the sign of rotation to the name of the substance. For example, optically pure (+)-2-butanol has a specific For example, optically pure (+)-2-butanol has a specific rotation [rotation []]2727

DD of +13.5 of +13.5; optically pure (-)-2-butanol has an ; optically pure (-)-2-butanol has an

exactly opposite specific rotation [exactly opposite specific rotation []]2727DD of –13.5 of –13.5..

Cahn, Ingold, and Prelog first developed their ranking Cahn, Ingold, and Prelog first developed their ranking system to deal with the problem of the absolute system to deal with the problem of the absolute configuration at a chirality center, and this is the system's configuration at a chirality center, and this is the system's major application. The Cahn-Ingold-Prelog system is called major application. The Cahn-Ingold-Prelog system is called the sequence rules; it is used to specify the absolute the sequence rules; it is used to specify the absolute configuration at the chirality center in (+)-2-butanol.configuration at the chirality center in (+)-2-butanol.

(+)-2-butanol has the S configuration. Its mirror image is (+)-2-butanol has the S configuration. Its mirror image is (-)-2-butanol, which has the R configuration.(-)-2-butanol, which has the R configuration.

Often, the R or S configuration and the sign of rotation are incorporated into the name of the compound, as in (R)-(-)-2-butanol and (S)-(+)-2-butanol.

Rules of determination of Rules of determination of absolute configuration of (+)-2-absolute configuration of (+)-2-

butanolbutanol

1. Identify the substituents at the chirality center, and rank them in order of decreasing precedence according to the Cahn-Ingold-Prelog priority rules following below.

Precedence is determined by atomic number, working outward Precedence is determined by atomic number, working outward from the point of attachment at the chirality center. from the point of attachment at the chirality center. 2. Orient the molecule so that the lowest ranked substituent 2. Orient the molecule so that the lowest ranked substituent points away from you. points away from you. 3. Draw the three highest ranked substituents as they appear to 3. Draw the three highest ranked substituents as they appear to you when the molecule is oriented so that the lowest ranked you when the molecule is oriented so that the lowest ranked group points away from you. 4. If the order of decreasing group points away from you. 4. If the order of decreasing precedence of the three highest ranked substituents appears in precedence of the three highest ranked substituents appears in a clockwise sense, the absolute configuration is R (Latin a clockwise sense, the absolute configuration is R (Latin rectus, "right," "correct"). If the order of decreasing rectus, "right," "correct"). If the order of decreasing precedence is counterclockwise, the absolute configuration is precedence is counterclockwise, the absolute configuration is S (Latin sinister, "left"). In order of decreasing precedence, the S (Latin sinister, "left"). In order of decreasing precedence, the four substituents attached to the chirality center of 2-butanol four substituents attached to the chirality center of 2-butanol areare

As represented in the wedge-and-dash drawing at the As represented in the wedge-and-dash drawing at the top of this table, the molecule is already appropriately top of this table, the molecule is already appropriately oriented. Hydrogen is the lowest ranked atom attached oriented. Hydrogen is the lowest ranked atom attached to the chirality center and points away from us.to the chirality center and points away from us.

The order of decreasing precedence is The order of decreasing precedence is counterclockwise. The configuration at the chirality counterclockwise. The configuration at the chirality center is S.center is S.

Compounds in which a chirality center is part of a ring Compounds in which a chirality center is part of a ring are handled in an analogous fashion. To determine, for are handled in an analogous fashion. To determine, for example, whether the configuration of (+)-4-example, whether the configuration of (+)-4-methylcyclohexene is R or S, it is necessary treat the methylcyclohexene is R or S, it is necessary treat the right- and left-hand paths around the ring as if they were right- and left-hand paths around the ring as if they were independent substituents.independent substituents.

Then equal substituents are situated on the same side Then equal substituents are situated on the same side relatively the plane of double bond, this configuration is relatively the plane of double bond, this configuration is denoted denoted cys-cys-. Then equal substituents are situated on . Then equal substituents are situated on the opposite sides relatively the plane of double bond, the opposite sides relatively the plane of double bond, this configuration is denoted this configuration is denoted trans-trans-..

C C

Cl

HH

Cl

cys-1,2-dichlorethane

C C

H

ClH

Cl

trans-1,2-dichlorethane

There is no connection between these two systems. In There is no connection between these two systems. In one case cys-isomer is E-isomer, but in another case one case cys-isomer is E-isomer, but in another case cys-isomer can be Z-isomer.cys-isomer can be Z-isomer.

C

Cl

BrC

Cl

H

cys-1-brom-1,2-dichloretheneE-1-brom-1,2-dichlorethene

C

H3C

HC

CH3

H

cys-butene-2Z-butene-2

Geometrical isomery can exist for atoms which formed Geometrical isomery can exist for atoms which formed only 3 bonds. In this case the “absent” substituent is only 3 bonds. In this case the “absent” substituent is changed by the pair of electrons.changed by the pair of electrons.

C

H3C

HN

C6H5

Z-isomer

C

H3C

HN

C6H5

E-isomer

Geometrical isomers have different physical and chemical properties, temperatures of melting and boiling. That is why it is easy to determine the their configurations using physical and chemical physical and chemical methods.

Thank you for attention!