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Organic chemistry and Biological chemistry for Health Sciences
59-191
Lecture 8
Geometric isomers:
Isomers that have identical constitution, including the position of the double bond but
differ only in the geometry of the bond are called geometric isomers, and the
phenomenon is called geometric isomerism.
Alkenes and cycloalkenes can exhibit this isomerism because there is no free rotation at
the double bond or in a ring.
Two designated substituents on the same side of the double bond are said to be cis to
each other. When they are on opposite sides, they are trans to each other.
If one end of a double bond has two identical groups, like two H or two methyls, then
there is nothing for a group at the other end to be uniquely cis or trans to. So geometric
isomers are not possible when there are two identical groups at one end of a double bond.
Geometric isomerism also occurs when the atoms involved at the ends of the double bond
are halogen atoms or other groups.
Since ring structures also have restricted rotation they can also have geometric isomers.
F.example
Two geometric isomers of 1,2-dimethylcyclopropane
Neither can be twisted into the other without breaking the ring open.
ADDITION REACTION OF THE DOUBLE BOND:
The carbon-carbon double bond adds H2,Cl2,Br2, HX, H2SO4 and H2O.
Pieces of adding molecule become attached to the carbon atoms at opposite ends of the
double bond, which then becomes a single bond.
Using X-Y as the adding molecule, all additions to alkene group can be represented as
follows:
Hydrogenation:
In the presence of a powdered metal catalyst, like powdered nickel or platinum, hydrogen
adds to a double bond. The reaction, called hydrogenation, converts an alkene to an
alkane.
Cells have molecular carriers that deliver the pieces of H2 to alkene groups.
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One piece of H2 is the hydride ion, H:-, and its carrier is an enzyme that transfers H:- to
one end of the double bond. The other piece of H2 is H+, which is donated to the other
carbon of the double bond by the same enzyme carrier or is plucked from a proton donor
of the surrounding fluid.
Chlorine and bromine rapidly add to the carbon-carbon double bond without any need for
a special catalyst. Iodine does not add, flourine reacts explosively with almost any
organic compound to give a mixture of products.
Hydrogen chloride, Hydrogen bromide and sulfuric acid add easily to the double bonds.
The pattern is same in all the additions. If we represent any of these reactants as H-G,
where G stands for any electron rich group like Cl, Br, or OSO3 H.
Unsymmetrical reactants add selectively to unsymmetrical double bond
Unsymmetrical double bond is one whose two carbon atoms hold unequal numbers of
hydrogen atoms.
F. example
1-butene CH2
CHCH2CH3
Propene CH3CH
CH2
Symmetrical double bond:
2-butene CH3CH
CHCH3
MARKOVNIKOV’S RULE:
When an unsymmetrical reactant of the type H-G adds to an unsymmetrical alkene, the
carbon with the greater number of hydrogens gets one more hydrogen.
The following examples illustrate the rule:
Concentrated sulfuric acid can dissolve sulfuric acid. It reacts with carbon-carbon double
bond by an addition reaction to form an alkyl hydrogen sulfate. Heat is also evolved in
this reaction. Molecules of alkyl hydrogen sulfate are very polar.
Markonikov’s rule does not apply to symmetrical double bond. Reactants like H-G add in
both of the two possible directions, and a mixture of isomers can form.
Water adds to the carbon-carbon double bond to give alcohol in the presence of an acid
catalyst. Water alone or aqueous bases have no effect on alkenes. This kind of reaction is
common in molecular level of life.
Oxidizing agents attack the double bond of alkenes because double bond (two pairs of
electrons) is more electron rich than single bonds.
F.example.
A hot solution of potassium permanganate (KMnO4) vigorously oxidizes molecules at
carbon-carbon double bonds.
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BENGENE RING AND AROMATIC PROPERTIES:
The benzene ring has three double bonds. So it is considerably unsaturated. Even though
benzene ring is unsaturated it does not give addition reaction as readily as alkenes.
Benzene adds hydrogen to give cyclohexene but rigorous conditions of pressure and
temperature are necessary for this reaction.
Benzene ring undergoes substitution reactions instead of addition reaction despite a
high degree of unsaturation.
Benzene reacts with chlorine and bromine in the presence of a catalyst by substitution
reaction. The catalyst is generally iron halide.
Benzene also reacts, by substitution, with sulfur trioxide dissolved in concentrated
sulfuric acid.
Benzene reacts with warm, concentrated nitric acid when it is dissolved in concentrated
sulfuric acid.
Alkenes are readily oxidized by permanganate or dichromate ion, but benzene is utterly
unaffected by these strong oxidizing agent even when boiled with them.
Chemical properties of benzene are completely different from alkene or alkyne. So
benzene is not an alkene or alkyne.
When benzene is used to make chlorobenzene only one monosubstituted compound
forms. Only one C6H5Cl exists.
So the six hydrogen atoms of benzene is equivalent.
Aromatic compounds:
Any substances whose molecules have benzene rings and whose rings give substitution
reactions instead of addition reactions are called aromatic compounds.
1-phenylpropane is an example of aromatic with an aliphatic side chain.
In IUPAC nomenclature the group C6H5, derived from benzene by removing one H is
called the phenyl group.
NAMING COMPOUNDS OF BENZENE:
The names of several monosubstituted benzene is straightforward. The substituent is
indicated by a prefix to the word benzene.
Other aromatic compounds have common names that are always used.
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When two groups are attached to the benzene ring both what they are and where they are
must be specified. One common way to indicate the relative locations of two groups in
disubstituted benzenes is by the prefixes ortho-, meta-, and para-, which usually are
abbreviated o-, m-, and p-, respectively. Two groups that are in 1,2 relationship are ortho
to each other, as in 1,2-dichlorobenzene, commonly called o-dichlorobenzene.
A disubstituted benzene is usually named as a derivative not of benzene but of a
monosubstituted benzene when the latter has a common name, like toluene or aniline.
Then the o-, m-, or p- designations are used to specify relative positions of the two
groups.
When we have trisubstituted benzenes (or higher), numbers are assigned to ring positions
in such a way as to use the lowest numbers possible.