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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.
Ass. Medvid I.I., ass. Burmas N.I.Ass. Medvid I.I., ass. Burmas N.I.
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 halogenoalkanes.The methods of extraction of 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.
1. The nomenclature of halogenderivatives of 1. 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
2. The isomery of halogenderivatives of 2. 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
3. The medico-biological importance of 3. The medico-biological importance of halogenderivatives of hydrocarbonshalogenderivatives of hydrocarbons
Because of the atom of halogen is present in the molecule, many Because of the atom of halogen is present in the molecule, many halogenderivatives of hydrocarbons are physiologically active. For halogenderivatives of hydrocarbons are physiologically active. For example: example:
CC22HH55ClCl – – ethyl chlorideethyl chloride – – is the means for the local anaesthetization when is the means for the local anaesthetization when there are neuralgia, large superficial cuts, wounds. Because of the fast there are neuralgia, large superficial cuts, wounds. Because of the fast evaporation from the skin ethyl chloride causes the strong cooling and loss evaporation from the skin ethyl chloride causes the strong cooling and loss of painful sensitivityof painful sensitivity; ;
CHClCHCl33 – – chloroformchloroform – – is the means for inhalative narcosis. It is relatively toxicis the means for inhalative narcosis. It is relatively toxic.. In the presence of light it can oxidize with forming of HCl and phosgene In the presence of light it can oxidize with forming of HCl and phosgene () – () – which is very toxic compound; which is very toxic compound;
CHJCHJ33 – – iodoformiodoform – – is the antiseptic means. It is crystal compound, it has is the antiseptic means. It is crystal compound, it has yellow colour. It is used as powder and ointment;yellow colour. It is used as powder and ointment;
ССFF33–CHBrCl–CHBrCl – fluorotane – fluorotane – (2- – (2-bromobromo-1,1,1--1,1,1-trifluorotrifluoro-2--2-chloroethanechloroethane) – ) – is one is one of the best means of general narcosis;of the best means of general narcosis;
CClCCl22=CHCl =CHCl – trichloroethylene– trichloroethylene – – is the strong narcotic means, especially for is the strong narcotic means, especially for short-term narcosis.short-term narcosis.
Because of the presence of halogen atom in the benzene ring the compound Because of the presence of halogen atom in the benzene ring the compound is more toxic. Because of the presence of halogen atom in the side carbon is more toxic. Because of the presence of halogen atom in the side carbon chain of the benzene ring the compound is more lachrymatorychain of the benzene ring the compound is more lachrymatory. .
4. Physical properties of 4. Physical properties of halogenderivatives of halogenderivatives of hydrocarbonshydrocarbons
Physical state and smellPhysical state and smellHaloalkanes are colorless, sweet-smelling liquids. The lower members like Haloalkanes are colorless, sweet-smelling liquids. The lower members like
methyl chloride, methyl bromide and ethyl chloride are colorless gases methyl chloride, methyl bromide and ethyl chloride are colorless gases while members having very high molecular masses are solids.while members having very high molecular masses are solids.
SolubilitySolubilityHaloalkanes are not able to form hydrogen bonds with water and, even Haloalkanes are not able to form hydrogen bonds with water and, even
though they are polar in nature, they are practically insoluble in water. though they are polar in nature, they are practically insoluble in water. However, they are soluble in organic solvents like alcohol, ether, However, they are soluble in organic solvents like alcohol, ether, benzene, etc.benzene, etc.
DensityDensityChloroalkanes are lighter than water while bromides and alkyl iodides are Chloroalkanes are lighter than water while bromides and alkyl iodides are
heavier. With the increase in the size of the alkyl group, the densities go heavier. With the increase in the size of the alkyl group, the densities go on decreasing in the order of :on decreasing in the order of :
fluoride > chloride > bromide > iodide.fluoride > chloride > bromide > iodide.Boiling pointsBoiling points
The boiling points of alkyl chlorides, bromides and iodides follow the order The boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl where R is an alkyl group. With the increase in the size RI > RBr > RCl where R is an alkyl group. With the increase in the size of halogen, the magnitude of Van der Waals forces increases and, of halogen, the magnitude of Van der Waals forces increases and, consequently, the boiling points increase. Also, for the same halogen consequently, the boiling points increase. Also, for the same halogen atom, the boiling points of haloalkanes increase with increase in the atom, the boiling points of haloalkanes increase with increase in the size of alkyl groups.size of alkyl groups.
The tables below show some physical data for a selection of The tables below show some physical data for a selection of
haloalkanes.haloalkanes.
5. The methods of extraction of 5. The methods of extraction of halogenoalkaneshalogenoalkanes
1. 1. Chlorinating and brominating of the saturated hydrocarbons (the reactions of Chlorinating and brominating of the saturated hydrocarbons (the reactions of radical substitution radical substitution ((SR)SR)..
2. 2. The Finkelshtain reaction.The Finkelshtain reaction.RR––ClCl + + NaJ NaJ → → RR––JJ + + 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. This reaction runs by Markovnikov rulehydrocarbons. This reaction runs by Markovnikov rule. .
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, , HJ or mixture NaClHJ or mixture NaCl + + HH22SOSO44((concentratedconcentrated), ), KBrKBr + + HH22SOSO44((concentratedconcentrated));;bb) ) PClPCl33, , PClPCl55, , PBrPBr33, , PBrPBr55 or mixture Por mixture P + + JJ22;;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
6. Chemical properties of halogenoalkanes6. Chemical 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, butanenitrile is formed.this example, butanenitrile 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
CH2-SH
CH3
CH2-Cl
CH2-NO2
CH2-NH2
CH2-CNR
hydrocarbons
Cl2; h -HCl
R
halogenderivativeshydrocarbons
+NaNO2
-NaClR
Nitrocompounds
+NH3
-HClR amines
[H]
+NaOH, H2O
-NaCl
+NaSH, H2O
-NaCl
Rtioalcohols (mercaptans)
+NaCN
Rnitriles
CH2-SH
CH2-S-R1
CH2-CNCH3
CH2-Cl
CH2-NO2
CH2-NH2
CH2-OH
Br
ONa
ONa
O
O-R
OH
O
OH
O
R
hydrocarbons
Cl2; h -HCl
R
halogenderivativeshydrocarbons
+NaNO2
-NaClR
nitrocompounds
+NH3
-HClR amines
[H
+NaOH, H2O
-NaClR
alcohols
HNO2
+NaSH, H2O
-NaCl
R
tioalcohols+R1
R
tioeters (sulphides)
+NaCN
Rnitriles
+R/
R/-O-CH2-R
ethers
+ R/-Br
+R/-C
R/-C
esters
[O
R -C
aldehydes
[O
R -C
carboxylicacids
+ R-OH
-H2O
+ NaOHalcoholic solution
R-CH=CH 2
alkens
C H 3
C H 2-C l
C H 2-NO 2
C H 2-NH 2
C H 2-OH
C H 2-S H
Br
C H 2-S -R 1
C H 2-C N
O Na
ONa
O
O -R
OH
O
OH
O
OH
O
ONa
O
R
hydrocarbons
Cl 2 ; h -HCl
R
halogenderivativeshydrocarbons
+NaNO 2
-NaClR
nitrocompounds
+NH 3
-HClR amines
[H]
+NaOH, H 2O
-NaClR
alcohols
HNO 2
+NaSH, H 2O
-NaCl
R
tioalcohols+R 1
R
tioeters (sulphides)
+NaCN
Rnitriles
+R /
R /-O-CH 2-R
ethers
+ R /-Br
+R /-C
R /-C
esters
[O]
R -C
aldehydes
[O]
R -C
carboxylicacids
+ R-OH
-H 2O
+2H 2O
R -CH 2-C+ NaOH R -CH 2-C
+ NaOHalloying
+ NaOHalcoholic solution
R-CH=CH 2
alkens
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 Structures A, B, and C represent different conformations of hydrogen peroxide. Conformations are conformations of hydrogen peroxide. Conformations are different spatial arrangements of a molecule that are different spatial arrangements of a molecule that are generated by rotation about single bonds. Although we generated by rotation about single bonds. Although we can't tell from simply looking at these structures, we can't tell from simply looking at these structures, we now know from experimental studies that C is the most now know from experimental studies that C is the most stable conformation.stable 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.
For characteristic of For characteristic of optical isomeryoptical isomery the optical activity the optical activity and chirality are very important. and chirality are very important. Everything has a mirror image, but not all things are Everything has a mirror image, but not all things are superimposable on their mirror images. Mirror-image superimposable on their mirror images. Mirror-image superimposability characterizes many objects we use superimposability characterizes many objects we use every day. Cups and saucers, forks and spoons, chairs every day. Cups and saucers, forks and spoons, chairs and beds are all identical with their mirror images. and beds are all identical with their mirror images. Many other objects though — and this is the more Many other objects though — and this is the more interesting case — are not. Your left hand and your interesting case — are not. Your left hand and your right hand, for example, are mirror images of each right hand, for example, are mirror images of each other but can't be made to coincide point for point, other but can't be made to coincide point for point, palm to palm, knuckle to knuckle, in three dimensions.palm to palm, knuckle to knuckle, in three dimensions.
In 1894, William Thomson (Lord Kelvin) coined a In 1894, William Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if word for this property. He defined an object as chiral if it is not superimposable on its mirror image. Applying it is not superimposable on its mirror image. Applying Thomson's term to chemistry, we say that a molecule is Thomson's term to chemistry, we say that a molecule is chiralchiral if its two mirror-image forms are not if its two mirror-image forms are not superimposable in three dimensions. The word superimposable in three dimensions. The word chiralchiral is is derived from the Greek word derived from the Greek word cheircheir, meaning "hand," , meaning "hand," and it is entirely appropriate to speak of the and it is entirely appropriate to speak of the "handedness" of molecules. The opposite of chiral is "handedness" of molecules. The opposite of chiral is achiral. A molecule that is superimposable on its mirror achiral. A molecule that is superimposable on its mirror image is image is achiral.achiral.
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.
The mirror images of bromochlorofluoromethane The mirror images of bromochlorofluoromethane have the same constitution. That is, the atoms are have the same constitution. That is, the atoms are connected in the same order. But they differ in the connected in the same order. But they differ in the arrangement of their atoms in space; they are arrangement of their atoms in space; they are stereoisomersstereoisomers. Stereoisomers that are related as an . Stereoisomers that are related as an object and its nonsuperimposable mirror image are object and its nonsuperimposable mirror image are classified as classified as enantiomersenantiomers. The word . The word enantiomerenantiomer describes a particular relationship between two describes a particular relationship between two objects. Just as an object has one, and only one, objects. Just as an object has one, and only one, mirror image, a chiral molecule can have one, and mirror image, a chiral molecule can have one, and only one, enantiomer.only one, enantiomer.
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.
Carbons that are part of a double bond or a triple bond can't be chirality centers.A carbon atom in a ring can be a chirality center if it bears two different substituents and the path traced around the ring from that carbon in one direction is different from that traced in the other. The carbon atom that bears the methyl group in 1,2-epoxypropane, for example, is a chirality center. The sequence of groups is O—CH2 as one proceeds clockwise around the ring from that atom, but is CH2—O in the counter clockwise direction. Similarly, C-4 is a chirality center in limonene.
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.
Stereoisomers that are not related as an object and Stereoisomers that are not related as an object and its mirror image are called its mirror image are called diastereomersdiastereomers; ; diastereorners are stereoisomers that are not diastereorners are stereoisomers that are not enantiomers. enantiomers. To convert a molecule with two chirality centers to To convert a molecule with two chirality centers to its enantiomer, the configuration at both centers its enantiomer, the configuration at both centers must be changed. Reversing the configuration at must be changed. Reversing the configuration at only one chirality center converts it to a only one chirality center converts it to a diastereomeric structure. Enantiomers must have diastereomeric structure. Enantiomers must have equal and opposite specific rotations. equal and opposite specific rotations. Diastereomers can have different rotations, with Diastereomers can have different rotations, with respect to both sign and magnitude. respect to both sign and magnitude.
Thus, as figure shows, the (2R,3R) and (2S,3S) Thus, as figure shows, the (2R,3R) and (2S,3S) enantiomers (I and II) have specific rotations that are enantiomers (I and II) have specific rotations that are equal in magnitude but opposite in sign. The (2R,3S) equal in magnitude but opposite in sign. The (2R,3S) and (2S,3R) enantiomers (III and IV) likewise have and (2S,3R) enantiomers (III and IV) likewise have specific rotations that are equal to each other but specific rotations that are equal to each other but opposite in sign. The magnitudes of rotation of I and II opposite in sign. The magnitudes of rotation of I and II are different, however, from those of their diastereomers are different, however, from those of their diastereomers III and IV. III and IV. In writing Fischer projections of molecules with two In writing Fischer projections of molecules with two chirality centers, the molecule is arranged in an eclipsed chirality centers, the molecule is arranged in an eclipsed conformation for projection onto the page. Horizontal conformation for projection onto the page. Horizontal lines in the projection represent bonds coming toward lines in the projection represent bonds coming toward you; vertical bonds point away. you; vertical bonds point away.
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..
The light used to measure optical activity has two The light used to measure optical activity has two properties: it consists of a single wavelength and it is properties: it consists of a single wavelength and it is plane-polarized. The wavelength used most often is plane-polarized. The wavelength used most often is 589 nm (called the D line), which corresponds to the 589 nm (called the D line), which corresponds to the yellow light produced by a sodium lamp. Except for yellow light produced by a sodium lamp. Except for giving off light of a single wavelength, a sodium giving off light of a single wavelength, a sodium lamp is like any other lamp in that its light is lamp is like any other lamp in that its light is unpolarized, meaning that the plane of its electric unpolarized, meaning that the plane of its electric field vector can have any orientation along the line field vector can have any orientation along the line of travel. of travel.
A beam of unpolarized light is transformed to plane-A beam of unpolarized light is transformed to plane-polarized light by passing it through a polarizing filter, polarized light by passing it through a polarizing filter, which removes all the waves except those that have which removes all the waves except those that have their electric field vector in the same plane. This their electric field vector in the same plane. This planepolarized light now passes through the sample planepolarized light now passes through the sample tube containing the substance to be examined, either in tube containing the substance to be examined, either in the liquid phase or as a solution in a suitable solvent the liquid phase or as a solution in a suitable solvent (usually water, ethanol, or chloroform). The sample is (usually water, ethanol, or chloroform). The sample is "optically active" if it rotates the plane of polarized "optically active" if it rotates the plane of polarized light. The direction and magnitude of rotation are light. The direction and magnitude of rotation are measured using a second polarizing filter (the measured using a second polarizing filter (the "analyzer") and cited as a, the observed rotation."analyzer") and cited as a, the observed rotation.
To be optically active, the sample must contain a chiral To be optically active, the sample must contain a chiral substance and one enantiomer must be present in excess of substance and one enantiomer must be present in excess of the other. A substance that does not rotate the plane of the other. A substance that does not rotate the plane of polarized light is said to be optically inactive. All achiral polarized light is said to be optically inactive. All achiral substances are optically inactive. substances are optically inactive. What causes optical rotation? The plane of polarization of a What causes optical rotation? The plane of polarization of a light wave undergoes a minute rotation when it encounters a light wave undergoes a minute rotation when it encounters a chiral molecule. Enantiomeric forms of a chiral molecule chiral molecule. Enantiomeric forms of a chiral molecule cause a rotation of the plane of polarization in exactly equal cause a rotation of the plane of polarization in exactly equal amounts but in opposite directions. amounts but in opposite directions.
A solution containing equal quantities of A solution containing equal quantities of enantiomers therefore exhibits no net rotation enantiomers therefore exhibits no net rotation because all the tiny increments of clockwise because all the tiny increments of clockwise rotation produced by molecules of one rotation produced by molecules of one "handedness" are canceled by an equal number of "handedness" are canceled by an equal number of increments of counterclockwise rotation produced increments of counterclockwise rotation produced by molecules of the opposite handedness.by molecules of the opposite handedness.Mixtures containing equal quantities of enantiomers Mixtures containing equal quantities of enantiomers are called racemic mixtures.Racemic mixtures are are called racemic mixtures.Racemic mixtures are optically inactive. optically inactive.
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.
At one time, the symbols At one time, the symbols d d and and l l were used to were used to distinguish between enantiomeric forms of a distinguish between enantiomeric forms of a substance. Thus the dextrorotatory enantiomer of substance. Thus the dextrorotatory enantiomer of 2-butanol was called d-2-butanol, and the 2-butanol was called d-2-butanol, and the levorotatory form levorotatory form ll-2-butanol; a racemic mixture -2-butanol; a racemic mixture of the two was referred to as of the two was referred to as dldl-2-butanol. -2-butanol. Current custom favors using algebraic signs Current custom favors using algebraic signs instead, as in (+)-2-butanol, (-)-2-butanol, and instead, as in (+)-2-butanol, (-)-2-butanol, and (±)-2-butanol, respectively. (±)-2-butanol, 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 solution twice as concentrated. To account for the effects of path length and concentration, chemists have defined the of 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.
With the lowest ranked group (hydrogen) directed away With the lowest ranked group (hydrogen) directed away from us, the order of decreasing sequence rule from us, the order of decreasing sequence rule precedence is clockwise. The absolute configuration is R.precedence is clockwise. The absolute configuration is R.Geometrical isomers are compounds that have identical Geometrical isomers are compounds that have identical structure and sequence of their atoms but they have structure and sequence of their atoms but they have different localization of substituents in space relatively different localization of substituents in space relatively the plane of the double bond or the plane of the cycle.the plane of the double bond or the plane of the cycle.For denotation the configuration of geometrical isomers it For denotation the configuration of geometrical isomers it is used is used cys-trans-systemcys-trans-system and and E,Z-system.E,Z-system. Cys-trans- Cys-trans-system is not used widely because its usage is possible system is not used widely because its usage is possible then two atoms, connected by double bond, have equal then two atoms, connected by double bond, have equal substituents. 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
Then carbon atoms, connected by double bond, have all Then carbon atoms, connected by double bond, have all different substituents the usage of cys-trans-system is different substituents the usage of cys-trans-system is not possible. In this case E,Z-system is used. This not possible. In this case E,Z-system is used. This system was developed by Cahn, Ingold, and Prelog. system was developed by Cahn, Ingold, and Prelog. Then the highest ranked substituents of every pair of Then the highest ranked substituents of every pair of substituents are situated on the same side relatively the substituents are situated on the same side relatively the plane of double bond, this configuration is denoted plane of double bond, this configuration is denoted ZZ (German zusammen – “together”). Then the highest (German zusammen – “together”). Then the highest ranked substituents of every pair of substituents are ranked substituents of every pair of substituents are situated on the opposite sides relatively the plane of situated on the opposite sides relatively the plane of double bond, this configuration is denoted double bond, this configuration is denoted EE (German (German entgegen – “opposite”). entgegen – “opposite”).
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!