Halogenderivatives of the
hydrocarbons.
Isomery of the organic compounds.
Spatial construction of the
molecules.
Ass. Medvid I.I., ass. Burmas N.I.
1.
2.
3.
4.
5.
6.
7.
8.
Outline
The nomenclature of halogenderivatives of
hydrocarbons.
The isomery of halogenderivatives of
hydrocarbons.
The medico-biological importance of
halogenderivatives of hydrocarbons.
Physical properties of halogenderivatives of
hydrocarbons.
The methods of extraction of halogenoalkanes.
Chemical properties of halogenoalkanes.
Structural isomery of organic compounds.
Spatial isomery of organic compounds.
1. The nomenclature of halogenderivatives of
hydrocarbons
Halogenderivatives of hydrocarbons are the products
of substitution one or several atoms of hydrogen to
atoms of halogens in the hydrocarbon molecules.
The names of halogenderivatives of hydrocarbons
are the names of the same hydrocarbons with added
prefix which means the halogen radical. i.e.
Cl
H3C
Br
CH2 CH CH3
Br
2-bromobutane
chlorocyclohexane
bromobenzene
If there are several halogen radicals in the
molecule of halogenderivatives of hydrocarbons,
then all substutients are called in alphabetical
order.
CH3 CH CH2 CH CH3
Br
CH3
2-bromo-4-methylpentane
Some halogenderivatives of hydrocarbons
have trivial names:
H
Cl
C
H
Cl
Cl
chloroform
I
C
I
iodoform
I
2. The isomery of halogenderivatives of
hydrocarbons
Halogenderivatives of hydrocarbons are characterized by
structural, geometrical and optical isomery. Structural
isomery is formed by different structure of carbon chain
and different location of halogen atoms in the molecule of
organic compound.
CH2
CH2CH2CH3
H3C
Cl
CH
CH2CH3
Cl
1-chlorobutane
2-chlorobutane
CH3
H3C
CH
CH2
CH3 Cl
2-methyl-1-chloropropane
H3C
C
CH3
Cl
2-methyl-2-chloropropane
Geometrical isomery is possible for molecules of
halogenderivatives which contain the carbon atoms connected
with different substutients.
H
H
C
H 3C
H
Cl
C
C
C
H 3C
Cl
H
trans-1-chlorpropene
cys-1-chlorpropene
Optical isomery is possible for molecules of halogenderivatives
which contain asymmetric carbon atom.
CH3
H
CH3
Cl
C2H5
D-2-chlorobutane
or
S-2-chlorobutane
Cl
H
C2H5
L-2-chlorobutane
or
R-2-chlorobutane
3. The medico-biological importance of
halogenderivatives of hydrocarbons
Because of the atom of halogen is present in the molecule, many
halogenderivatives of hydrocarbons are physiologically active. For
example:
C2H5Cl – ethyl chloride – is the means for the local anaesthetization when
there are neuralgia, large superficial cuts, wounds. Because of the fast
evaporation from the skin ethyl chloride causes the strong cooling and loss
of painful sensitivity;
CHCl3 – chloroform – is the means for inhalative narcosis. It is relatively toxic.
In the presence of light it can oxidize with forming of HCl and phosgene () –
which is very toxic compound;
CHJ3 – iodoform – is the antiseptic means. It is crystal compound, it has
yellow colour. It is used as powder and ointment;
СF3–CHBrCl – fluorotane – (2-bromo-1,1,1-trifluoro-2-chloroethane) – is one
of the best means of general narcosis;
CCl2=CHCl – trichloroethylene – is the strong narcotic means, especially for
short-term narcosis.
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
chain of the benzene ring the compound is more lachrymatory.
4. Physical properties of halogenderivatives of
hydrocarbons
Physical state and smell
Haloalkanes are colorless, sweet-smelling liquids. The lower members like
methyl chloride, methyl bromide and ethyl chloride are colorless gases
while members having very high molecular masses are solids.
Solubility
Haloalkanes are not able to form hydrogen bonds with water and, even
though they are polar in nature, they are practically insoluble in water.
However, they are soluble in organic solvents like alcohol, ether,
benzene, etc.
Density
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
on decreasing in the order of :
fluoride > chloride > bromide > iodide.
Boiling points
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
of halogen, the magnitude of Van der Waals forces increases and,
consequently, the boiling points increase. Also, for the same halogen
atom, the boiling points of haloalkanes increase with increase in the size
of alkyl groups.
The tables below show some physical data for a selection of
haloalkanes.
5. The methods of extraction of
halogenoalkanes
1. Chlorinating and brominating of the saturated hydrocarbons (the reactions of
radical substitution (SR).
HCl + H3C
CH4 + Cl2
Cl
chlormethane
2. The Finkelshtain reaction.
R–Cl + NaJ → R–J + NaCl 
3. Hydrohalogenation is the joining HCl, HBr or HJ to ethylene and acethylene
hydrocarbons. This reaction runs by Markovnikov rule.
CH2 CH2 + HBr
CH3
CH2
Br
bromomethane
4. The substitution of the functional groups (for example, –ОН) to atom of any
halogen by the action of the following reagents:
a) HCl, HBr, HJ or mixture NaCl + H2SO4(concentrated),
KBr + H2SO4(concentrated);
b) PCl3, PCl5, PBr3, PBr5 or mixture P + J2;
c) SOCl2, SO2Cl2.
H3C
CH2 OH + HCl
t
H3C
CH2 Cl + H2O
6. Chemical properties of halogenoalkanes
1.Halogenalkanes react with water
C2H5Br + H2O ↔ C2H5OH + HBr
2. Halogenalkanes react with NaOH or KOH
C2H5Br + NaOH ↔ C2H5OH + NaBr
3. Williamson reaction
C2H5Br + NaOC2O5 → C2H5−O−C2H5 + NaBr
4. Reaction with salts of carboxylic acids
C2H5
Br + Na
O
C
O
CH3
C2H5
O
C
O
CH3 + NaBr
5. Reaction with ammonium
C2H5Br + NH3 → [C2H5NH3]+Br− C2H5NH2
6. Halogenalkanes react with NaCN or KCN
For example, using 1-bromopropane as a typical primary
halogenoalkane:
You could write the full equation rather than the ionic one, but it
slightly obscures what's going on:
The bromine (or other halogen) in the halogenoalkane is simply
replaced by a -CN group - hence a substitution reaction. In
this example, butanenitrile is formed.
C2H5Br + NaCN → C2H5−C≡N + NaBr
7. Reaction with salts of HNO2
C2H5Br + NaNO2 → C2H5NO2 + NaBr
8. Finkelshtain reaction (catalyst is acetone):
C2H5Cl + NaI → C2H5I + NaCl
9. Reaction with NaSN (thioalkohols form) or
Na2S (thioethers form):
C2H5I + NaSN → C2H5SN + NaI
2C2H5I + Na2S → C2H5−S−C2H5 + 2NaI
10. Reaction with metals:
C2H5I + Mg → C2H5MgI
11. Reduction (the reaction runs in the
presence of catalysts):
C2H5Cl + H2 → C2H6 + HCl
R CH2-CN
nitriles
+NaCN
hydrocarbons
R CH3
Cl2; hn -HCl
+NaNO2
-NaCl
+NaSH, 2HO
-NaCl
halogenderivatives
hydrocarbons
-HCl
+NaOH, 2HO
-NaCl
R CH2-SH
tioalcohols (mercaptans)
R CH2-NO2
[H]
+NH3
R CH2-Cl
Nitrocompounds
R CH2-NH2 amine
R
CH 2-CN
nitriles
+NaCN
hydrocarbons
R
Cl2; hn
+NaNO
CH 3
2
-NaCl
-HCl
nitrocompounds
CH 2-NO 2
R
[H
+NH3
+NaSH, H 2O
-NaCl
R
R
CH 2-Cl
halogenderivatives
hydrocarbons
-HCl
+NaOH, H2O
CH 2-SH
R/-O-CH 2-R
/
+R -C
CH 2-S-R 1
ONa
ethers
O
R -C
aldehydes H
tioeters (sulphides)
O
R/-C
alcoholic solution
[O
+ R/-Br
O
+R1 Br
+ NaOH
CH 2-OH
alcohols
+R/ ONa
ioalcohols
R
HNO2
R
-NaCl
R
CH 2-NH 2 amines
O-R
esters
[O
+ R-OH
O
-H2 O
R -C
OH
R-CH=CH 2
alkens
carboxylic
acids
O
O
+ NaOH
R -CH 2 -C
R -CH 2 -C
ONa
+ NaOH
alloying
OH
+2H 2 O
R
hydrocarbons
C H 2-C N
nitriles
R
Cl 2 ; h n
+NaCN
+NaNO
CH3
-HCl
R
2O
[H]
3
R
C H 2-C l
+NaOH, H
R
HNO
2O
R
-NaCl
R C H 2-S H
tioalcohols
+R 1 Br
C H 2-N H 2 amines
-HCl
halogenderivatives
hydrocarbons
-NaCl
C H 2-N O 2
R
-NaCl
+NH
+NaSH, H
nitrocompounds
2
C H 2-O H
alcohols
+R / O N a
R / -O-CH 2 -R
+R -C
C H 2-S -R 1
ONa
tioeters (sulphides)
[O]
+ R / -Br
O
/
ethers
2
O
R -C
aldehydes
H
O
/
R -C
O -R
esters
+ NaOH
alcoholic solution
[O]
+ R-OH
O
-H 2 O
R-CH=CH
alkens
2
R -C
OH
carboxylic
acids
7. Isomery 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
Isomery of
chain
Isomery of
functional
group
Isomery of
location of
functional
group
Spatial
Configurative
Conformative
Geometrical
Optical
Isomery of Carbon chain is formed by different sequence
of atoms in the molecule of the organic compound.
H3C
C4H10
CH2 CH2 CH3
butane
CH3
CH3 CH CH3
isobutane
For cyclic compounds the isomery can change the Carbon
cycle in the molecule of the isomer.
H3C
CH3
C6H12
cyclohexane
1,2-dimethylcyclobutane
CH3
CH3
1-methylcyclopentane
H3C
CH3
1,2,3-trimethylcyclopropane
Isomery of the location of the functional group is formed by
different locations of identical functional groups and double
or triple bonds.
H3C
C3H7Cl
CH2 CH2 Cl
1-chlorpropane
Cl
H3C CH CH3
2-chlorpropane
Cl
C6H10Cl2
Cl
Cl
1,2-dichlorcyclohexane
Cl
Cl
1,4-dichlorcyclohexane
Cl
1,3-dichlorcyclohexane
C4H8
H2C
CH
CH2
CH3
butene-1
H 3C
C CH
H
butene-2
CH3
Isomery of the functional group is formed by different
functional groups in the molecules.
C2H6O
H3C
H3C
O
CH2 OH
ethanol
CH3
dimethylether
Conformation is the different spatial localization
of atoms or atom groups in the molecule as a
result of its rotation around -bonds. Hydrogen
peroxide is formed in the cells of plants and
animals but is toxic to them. Consequently,
living systems have developed mechanisms to
rid themselves of hydrogen peroxide, usually by
enzyme-catalyzed reduction to water. An
understanding of how reactions take place, be
they reactions in living systems or reactions in
test-tubes, begins with a thorough knowledge of
the structure of the reactants, products, and
catalysts.
Even a simple molecule such as hydrogen
peroxide may be structurally more complicated
than you think. Suppose we wanted to write the
structural formula for H202 in enough detail to
show the positions of the atoms relative to one
another. We could write two different planar
geometries A and B that differ by a 180 rotation
about the O—O bond. We could also write an
infinite number of nonplanar structures, of which
C is but one example, that differ from one
another by tiny increments of rotation about the
O—O bond.
Structures A, B, and C represent different
conformations of hydrogen peroxide. Conformations are
different spatial arrangements of a molecule that are
generated by rotation about single bonds. Although we
can't tell from simply looking at these structures, we
now know from experimental studies that C is the most
stable conformation.
There is also the conformation in the structure of molecules
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
of one carbon bisects an H—C—H angle of the
other carbon. In the eclipsed conformation, each
C—H bond of one carbon is aligned with a C—
H bond of the other carbon.
The staggered and eclipsed conformations
interconvert by rotation around the C—C bond,
and do so very rapidly. Among the various ways
in which the staggered and eclipsed forms are
portrayed, wedge-and-dash, sawhorse, and
Newman projection drawings are especially
useful.
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
by a torsion angle or dihedral angle, which is the angle
between the H—C—C plane and the C—C—H plane. The
torsion angle is easily seen in a Newman projection of ethane
as the angle between C—H bonds of adjacent carbons.
Eclipsed bonds are characterized by a torsion angle of 0.
When the torsion angle is approximately 60, it means that
the spatial relationship is gauche; and when it is 180 it is
called anti. Staggered conformations have only gauche or anti
relationships between bonds on adjacent atoms.
For characteristic of optical isomery the optical activity
and chirality are very important.
Everything has a mirror image, but not all things are
superimposable on their mirror images. Mirror-image
superimposability characterizes many objects we use
every day. Cups and saucers, forks and spoons, chairs
and beds are all identical with their mirror images.
Many other objects though — and this is the more
interesting case — are not. Your left hand and your
right hand, for example, are mirror images of each
other but can't be made to coincide point for point,
palm to palm, knuckle to knuckle, in three dimensions.
In 1894, William Thomson (Lord Kelvin) coined a word
for this property. He defined an object as chiral if it is
not superimposable on its mirror image. Applying
Thomson's term to chemistry, we say that a molecule is
chiral if its two mirror-image forms are not
superimposable in three dimensions. The word chiral is
derived from the Greek word cheir, meaning "hand,"
and it is entirely appropriate to speak of the
"handedness" of molecules. The opposite of chiral is
achiral. A molecule that is superimposable on its mirror
image is achiral.
In organic chemistry, chirality most often occurs in
molecules that contain a carbon that is attached to four
different groups. An example is
bromochlorofluoromethane (BrClFCH).
As shown in figure, the two mirror images of
bromochlorofluoromethane cannot be superimposed on
each other. Because the two mirror images of
bromochlorofiuoromethane are not superimposable,
BrClFCH is chiral.
The mirror images of bromochlorofluoromethane
have the same constitution. That is, the atoms are
connected in the same order. But they differ in the
arrangement of their atoms in space; they are
stereoisomers. Stereoisomers that are related as an
object and its nonsuperimposable mirror image are
classified as enantiomers. The word enantiomer
describes a particular relationship between two
objects. Just as an object has one, and only one,
mirror image, a chiral molecule can have one, and
only one, enantiomer.
A molecule of chlorodifluoromethane (ClF2CH), in which two
of the atoms attached to carbon are not chiral. Figure shows
two molecular models of ClF2CH drawn so as to be mirror
images. As is evident from these drawings, it is a simple
matter to merge the two models so that all the atoms match.
Because mirror-image representations of
chlorodifluoromethane are superimposable on each other,
ClF2CH 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
its mirror image are called diastereomers;
diastereorners are stereoisomers that are not
enantiomers.
To convert a molecule with two chirality centers to
its enantiomer, the configuration at both centers
must be changed. Reversing the configuration at
only one chirality center converts it to a
diastereomeric structure. Enantiomers must have
equal and opposite specific rotations.
Diastereomers can have different rotations, with
respect to both sign and magnitude.
Thus, as figure shows, the (2R,3R) and (2S,3S)
enantiomers (I and II) have specific rotations that are
equal in magnitude but opposite in sign. The (2R,3S)
and (2S,3R) enantiomers (III and IV) likewise have
specific rotations that are equal to each other but
opposite in sign. The magnitudes of rotation of I and II
are different, however, from those of their diastereomers
III and IV.
In writing Fischer projections of molecules with two
chirality centers, the molecule is arranged in an eclipsed
conformation for projection onto the page. Horizontal
lines in the projection represent bonds coming toward
you; vertical bonds point away.
Organic chemists use an informal nomenclature system based on Fischer
projections to distinguish between diastereomers. When the carbon chain
is vertical and like substituents are on the same side of the Fischer
projection, the molecule is described as the erythro diastereomer. When
like substituents are on opposite sides of the Fischer projection, the
molecule is described as the threo diastereomer. Thus, as seen in the
Fischer projections of the stereoisomeric 2,3-dihydroxybutanoic acids,
compounds I and II are erythro stereoisomers and III and IV are threo.
Because diastereomers are not mirror images of each other,
they can have quite different physical and chemical
properties. For example, the (2R,3R) stereoisomer of 3amino-2-butanol is a liquid, but the (2R,3S) diastereomer is
a crystalline solid.
The experimental facts that led van't Hoff and Le Bel to propose that
molecules having the same constitution could differ in the arrangement
of their atoms in space concerned the physical property of optical
activity. Optical activity is the ability of a chiral substance to rotate the
plane of plane-polarized light and is measured using an instrument called
a polarimeter.
The light used to measure optical activity has two
properties: it consists of a single wavelength and it is
plane-polarized. The wavelength used most often is
589 nm (called the D line), which corresponds to the
yellow light produced by a sodium lamp. Except for
giving off light of a single wavelength, a sodium
lamp is like any other lamp in that its light is
unpolarized, meaning that the plane of its electric
field vector can have any orientation along the line of
travel.
A beam of unpolarized light is transformed to planepolarized light by passing it through a polarizing filter,
which removes all the waves except those that have
their electric field vector in the same plane. This
planepolarized light now passes through the sample tube
containing the substance to be examined, either in the
liquid phase or as a solution in a suitable solvent
(usually water, ethanol, or chloroform). The sample is
"optically active" if it rotates the plane of polarized
light. The direction and magnitude of rotation are
measured using a second polarizing filter (the
"analyzer") and cited as a, the observed rotation.
To be optically active, the sample must contain a chiral
substance and one enantiomer must be present in excess of
the other. A substance that does not rotate the plane of
polarized light is said to be optically inactive. All achiral
substances are optically inactive.
What causes optical rotation? The plane of polarization of a
light wave undergoes a minute rotation when it encounters a
chiral molecule. Enantiomeric forms of a chiral molecule
cause a rotation of the plane of polarization in exactly equal
amounts but in opposite directions.
A solution containing equal quantities of
enantiomers therefore exhibits no net rotation
because all the tiny increments of clockwise
rotation produced by molecules of one
"handedness" are canceled by an equal number of
increments of counterclockwise rotation produced
by molecules of the opposite handedness.
Mixtures containing equal quantities of enantiomers
are called racemic mixtures.Racemic mixtures are
optically inactive.
Conversely, when one enantiomer is present in excess, a
net rotation of the plane of polarization is observed. At
the limit, where all the molecules are of the same
handedness, we say the substance is optically pure.
Optical purity, or percent enantiomeric excess, is defined
as:
Rotation of the plane of polarized light in the clockwise
sense is taken as positive (+), and rotation in the
counterclockwise sense is taken as a negative (-) rotation.
Older terms for positive and negative rotations were
dextrorotatory and levorotatory, from the Latin prefixes
dextro- ("to the right") and levo- ("to the left"), respectively.
At one time, the symbols d and l were used to
distinguish between enantiomeric forms of a
substance. Thus the dextrorotatory enantiomer of
2-butanol was called d-2-butanol, and the
levorotatory form l-2-butanol; a racemic mixture
of the two was referred to as dl-2-butanol.
Current custom favors using algebraic signs
instead, as in (+)-2-butanol, (-)-2-butanol, and
(±)-2-butanol, respectively.
The observed rotation  of an optically pure substance
depends on how many molecules the light beam
encounters. A filled polarimeter tube twice the length of
another produces twice the observed rotation, as does a
solution twice as concentrated. To account for the effects of
path length and concentration, chemists have defined the
term specific rotation, given the symbol []. Specific
rotation is calculated from the observed rotation according
to 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
prefixing the sign of rotation to the name of the substance.
For example, optically pure (+)-2-butanol has a specific
rotation []27D of +13.5; optically pure (-)-2-butanol has
an exactly opposite specific rotation []27D of –13.5.
Cahn, Ingold, and Prelog first developed their ranking
system to deal with the problem of the absolute
configuration at a chirality center, and this is the system's
major application. The Cahn-Ingold-Prelog system is called
the sequence rules; it is used to specify the absolute
configuration at the chirality center in (+)-2-butanol.
(+)-2-butanol has the S configuration. Its mirror image is
(-)-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 absolute
configuration of (+)-2-butanol
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
from the point of attachment at the chirality center.
2. Orient the molecule so that the lowest ranked substituent
points away from you.
3. Draw the three highest ranked substituents as they appear to
you when the molecule is oriented so that the lowest ranked
group points away from you. 4. If the order of decreasing
precedence of the three highest ranked substituents appears in
a clockwise sense, the absolute configuration is R (Latin
rectus, "right," "correct"). If the order of decreasing
precedence is counterclockwise, the absolute configuration is
S (Latin sinister, "left"). In order of decreasing precedence, the
four substituents attached to the chirality center of 2-butanol
are
As represented in the wedge-and-dash drawing at the
top of this table, the molecule is already appropriately
oriented. Hydrogen is the lowest ranked atom attached
to the chirality center and points away from us.
The order of decreasing precedence is
counterclockwise. The configuration at the chirality
center is S.
Compounds in which a chirality center is part of a ring
are handled in an analogous fashion. To determine, for
example, whether the configuration of (+)-4methylcyclohexene is R or S, it is necessary treat the
right- and left-hand paths around the ring as if they were
independent substituents.
With the lowest ranked group (hydrogen) directed away
from us, the order of decreasing sequence rule
precedence is clockwise. The absolute configuration is R.
Geometrical isomers are compounds that have identical
structure and sequence of their atoms but they have
different localization of substituents in space relatively
the plane of the double bond or the plane of the cycle.
For denotation the configuration of geometrical isomers it
is used cys-trans-system and E,Z-system. Cys-transsystem is not used widely because its usage is possible
then two atoms, connected by double bond, have equal
substituents.
Then equal substituents are situated on the same side
relatively the plane of double bond, this configuration is
denoted cys-. Then equal substituents are situated on
the opposite sides relatively the plane of double bond,
this configuration is denoted trans-.
H
H
C
Cl
H
C
Cl
C
Cl
cys-1,2-dichlorethane
Cl
C
H
trans-1,2-dichlorethane
Then carbon atoms, connected by double bond, have all
different substituents the usage of cys-trans-system is
not possible. In this case E,Z-system is used. This
system was developed by Cahn, Ingold, and Prelog.
Then the highest ranked substituents of every pair of
substituents are situated on the same side relatively the
plane of double bond, this configuration is denoted Z
(German zusammen – “together”). Then the highest
ranked substituents of every pair of substituents are
situated on the opposite sides relatively the plane of
double bond, this configuration is denoted E (German
entgegen – “opposite”).
There is no connection between these two systems. In
one case cys-isomer is E-isomer, but in another case
cys-isomer can be Z-isomer.
H3C
Cl
Cl
C
Br
C
H
cys-1-brom-1,2-dichlorethene
E-1-brom-1,2-dichlorethene
CH3
C
C
H
H
cys-butene-2
Z-butene-2
Geometrical isomery can exist for atoms which formed
only 3 bonds. In this case the “absent” substituent is
changed by the pair of electrons.
H3C
C6H5 H3C
C
N
H
C
N
C6H5
H
Z-isomer
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!