Reaction of Alkenes and Alkynes
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Transcript of Reaction of Alkenes and Alkynes
8/6/2019 Reaction of Alkenes and Alkynes
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Reactions of Alkenes
and Alkynes
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Alkenes Addition to carbon-carbon double bond
MECHANISM Double bond is broken and in its place
single bonds form to two new atoms or
groups of atoms.
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Addition Reactions Hydrohalogenation
Hydration
Bromination
Hydrogenation (reduction)
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Addition Reaction
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Reason for reactivity of C=C One double bond and one single bond (H-
H bond of H2) are replaced by three single
bonds
Net conversion of one bond of the double
bond to two single bonds
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Properties of Alkene Reactions
Exothermic
Products are more stable (lower energy)
than the reactants
Rate of reactions depends on the
activation energy (some needs catalyst)
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Hydrogenation
The reaction is carried out under pressure
at a temperature of 200 °C in the presence
of a metallic catalyst.
Common industrial catalysts are based
on platinum, nickel or palladium. For
laboratory syntheses, Raneynickel (an alloy of nickel and aluminium) is
often employed.
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Hydrogenation
CH2=CH
2+ H
2 CH
3-CH
3
Ethylene Ethane
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Halogenation
CH2=CH2 + Br 2 BrCH2
-CH2Br
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Hydrohalogenation Reactions of hydrogen halides (HX) with
alkenes produces haloalkanes (alkyl
halide)
Some reactions proceeds to
regioselectivity
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Regioselectivity Was first noted by Vladimir Markovnikov
Regioselective reaction takes place when
one direction of bond forming or breaking
occurs in preference to all other directions.
The Markovnikov¶s Rule
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Markovnikov¶s Rule In the addition of HX to an alkene,
hydrogen adds to the doubly bonded
carbon that has greater number of hydrogens already bonded to it; halogen
adds to the other carbon.
The rich get
richer
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Hydration Addition of Water: Acid-catalyzed
hydration
In the presence of an acid catalyst, most
commonly conc. H2SO4, water adds to the
carbon-carbon double bond to give an
alcohol.
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Hydration In simple alkenes, hydration follows the
Markovnikov¶s rule.
That is, H adds to the carbon of the double
bond with the greater number of
hydrogens and OH adds to the carbon
with the smaller number of hydrogens.
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Halogenation Addition of bromine and chlorine
Br 2 and Cl2 react with alkenes at room
temperature by addition of halogen atoms
to the carbon atoms of the double bonds
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Halogenation Generally carried out either by using pure
reagents or by mixing them in an inert
solvent, such as dichloromethane, CH2Cl2.
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Importance Useful qualitative test for the presence of
an alkene.
How?: Br 2 in CCl4 (red to brown) is mixed
with a suspected alkene, the
disappearance of the red color means that
the bromine was added to the doublebond.
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Hydrogenation Addition of Hydrogen
All alkenes react quantitatively with H2 in
the presence of a transition metal catalyst
to give alkanes.
Catalysts used are platinum, palladium,
ruthenium and nickel.
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Hydrogenation It involves reduction by hydrogen in the
presence of a catalyst, thus; it is also
called as the catalytic reduction or catalytichydrogenation.
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Hydrogenation Catalyst ± finely powdered solid
Carried out by dissolving alkene in ethanol
or other non-reactive organic solvent,
adding the catalyst, and exposing the
mixture to hydrogen gas at pressures
ranging from 1 to 150 atm.
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P
olymerization Formation of chain-growth polymers
Most important reaction of alkenes
With the presence of initiators, many
alkenes form polymers
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P
olymerization Low-density Polyethylene (LDPE)
Transparent polymer from the ethylene
polymerization with peroxide as initiator
Highly branched
Do not pack well together
London dispersion forces are weak
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P
olymerization High-density Polyethylene (HDPE)
Does not rely on peroxide initiators
Developed by Karl Ziegler and Guilio Natta
Little chain branching
Chains are pack together more closely Stronger london dispersion forces
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Sample 2-methylpropene with HI
2-methylcyclopentene with HCl
Propene with HBr
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Sample Hydration of ethene with conc. H
2SO
4
Hydration of propene with conc. H2SO
4
Hydration of 2-methylpropene with conc. H2SO4
Acid-catalyzed hydration of 1-
methylcyclohexene
Acid-catalyzed hydration of 2-methyl-2-butene Acid-catalyzed hydration of 2-methyl-1-butene
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Sample Bromination of cyclopentene with CH2Cl2
Chlorination of 2-methylcyclohexene with
CH2Cl2.
Bromination of 3,3-dimethylbutene with
CH2Cl2
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Sample Catalytic reduction of trans-2-butene with
Pd
Catalytic reduction of ci s-2-butene with Pd
Catalytic hydrogenation of cyclohexene
with Pd
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Benzene and Its
Derivatives
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Benzene: Its Properties
Benzene is an organic chemical
compound with the molecular formula
C6H6.
It is sometimes abbreviated Ph±H.
Benzene is a colorless and highly
flammable liquid with a sweet smell and arelatively high melting point.
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Benzene: Its Properties
Because it is a known carcinogen, its useas an additive in gasoline is now limited,
but it is an important industrial solvent andprecursor in the production of drugs,plastics, synthetic rubber, and dyes.
Benzene is a natural constituent of crudeoil, and may be synthesized from other compounds present in petroleum.
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Benzene: Its Properties
Benzene is an aromatic hydrocarbon
It is also related to the functional group
arene which is a generalized structure of
benzene.
Arene is use to describe aromatic
hydrocarbon.
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Benzene: Its Properties
A group derived from the removal of an H
from a arene is called an aryl group (Ar-).
Discovered by Michael Faraday
Conflict arose between chemists because
of the molecular formula
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Benzene: Its Properties
Has few hydrogens for its six carbons
(compare to hexane); chemists argued it
to be unsaturated.
However, benzene does not behave like
alkenes
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Benzene: Its Structure
First structure was proposed by Friedrich
August Kekulé
Consist of six-membered ring with
alternating single and double bonds, with
one hydrogen bonded to each carbon
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Benzene: Its Structure
Kekulé proposal was consistent with many
chemical properties of benzene
But was objected because it does not
undergo reactions similar to alkenes.
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Resonance Structure of Benzene
Experimentally, the length of the carbon-
carbon bond in benzene is not as long as
a C-C nor as short as a C=C, but rather midway between the two.
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Resonance Structure of Benzene
The closed loop of six electrons ( two from
the second bond of each double bond)
characteristic of a benzene ring issometimes called an aromatic sextet.
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Resonance Structure of Benzene
The benzene ring is greatly stabilized by
resonance, which explains why it does not
undergo the addition reactions of typicalalkenes.
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Nomenclature of Aromatic
Hydrocarbons Aromatic hydrocarbon is evident with the
presence of a benzene ring.
Benzene has six carbon atoms are
arranged in a ring with alternating double
bonds.
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Sample
Ethylbenzene
Methylbenzene (toluene)
Phenylethene (styrene) Hydroxybenzene (phenol)
Anisole
Aniline
Benzaldehyde
Benzoic acid
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Sample
1-phenylcyclohexene
4-phenylbutene
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Nomenclature of Aromatic
Hydrocarbons Monosubstituted benzenes are named by
combining the substituents name with the
word benzene.
For disubstituted benzenes the 3 possible
isomers are named using the prefixes
ortho, metha and para (o, m, p) todesignate the 1,2- , 1,3- and 1,4
relationships of substituents on the
benzene ring.
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Sample
4-bromobenzoic acid (p-bromobenzoic
acid)
3-chloroaniline (m-aniline)
1,3-dimetyhlbenzene (m-xylene or m-
dimethylbenzene)
1-chloro-4-ethylbenzene (p-chloroethylbenzene)
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Nomenclature of Aromatic
Hydrocarbons For three or more substituents, specify
their locations by numbers (smallest set).
If one of the substituents imparts a special
name, then name the molecule as a
derivative of that parent molecule.
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Sample
4-chloro-1-methyl-2-nitro benzene (4-
chloro-2-nitrotoluene)
2,4,6-tribromo-1-hydroxybenzene (2,4,6-
tribromophenol)
2-bromo-1-ethyl-4-nitrobenzene
1-methyl-2,4,6-trinitrobenzene (2,4,6-trinitrotoluene or TNT)
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Sample
m-iodotoluene
3,5-dibromobenzoic acid
P-chloroaniline
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Polynuclear Aromatic
Hydrocarbons (PAH¶s) Molecules containing two or more
benzene rings, with each pair of rings
sharing two adjacent carbon atoms.
Naphthalene, anthracene, phenanthrene
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Characteristic Reactions of
Benzene The most characteristic reaction of
aromatic compounds is substitution at a
ring carbon (aromatic substitution).
Halogenation
Nitration
Sulfonation
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Halogenation
Cl2 and Br 2 noramlly do not react with
benzene but with an Fe catalyst, halogens
reacts rapidly to give halobenzene andHCl.
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Sample
Chlorination of benzene in FeCl3
Bromination of benzene in FeCl3
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Nitration
Heating benzene or its derivative with
conc. nitric acid, one of the hydrogen
atoms bonded to the ring is replaced by anitro (-NO2) group.
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Sample
Nitration of benzene
Nitration of toluene
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PHENOLS
The functional group of phenols is a
hydroxy group bonded to a benzene ring
Subtituted phenols are named either asderivatives of phenols or by common
names.
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Sample
Phenol
3-methylphenol (m-cresol)
1,2-benzenediol
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Sources
Widely distributed in nature
Phenol and the isomeric cresol (o-, m-,
and p-) are found in coal tar.
Thymol and vanillin are important
constituents of thyme and vanilla beans.
Urushiol is the main component of poisonivy ± an irritating oil
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Acidity and Properties
Weak acids
Insoluble in water
Reacts with strong bases like NaOh and
KOH forming water-soluble salts