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Unsaturated Hydrocarbons Chapter 20. Larry Emme Chemeketa Community College. Unsaturated hydrocarbons enhance our lives in many ways:. Polyethylene plastic bags and bottles Polystyrene Styrofoam cups Plastic wraps - PowerPoint PPT Presentation
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• Unsaturated hydrocarbons enhance our lives in many ways:
1. Polyethylene plastic bags and bottles
2. Polystyrene Styrofoam cups
3. Plastic wraps
4. Essential oils in plants contain multiple bonds between carbon atoms.
– Cosmetics, medicines, flavorings, perfumes
5. Hydrocarbons also form rings of carbon atoms (aromatics)– Detergents, insecticides, and dyes
Fragrant Alkenes
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Bonding in Unsaturated Bonding in Unsaturated HydrocarbonsHydrocarbons
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• The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms.
• Alkenes contain carbon-carbon double bonds.• Alkynes contain carbon-carbon triple bonds.• Aromatic compounds contain benzene rings.
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Schematic hybridization of 2s22px12py
1 orbitals of carbon to form three sp2 electron orbitals and one p electron orbital
The four orbitals available for bonding in alkenes are three sp2 orbitals and one p orbital.
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(a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp2 orbitals.
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• The carbon-carbon pi () bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma () bond.
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• The formation of a triple bond between carbon atoms, as in acetylene, CHCH, may be visualized as shown below.
• These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance.
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Nomenclature Nomenclature of Alkenesof Alkenes
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The general formula for alkenes is:
CnH2n
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IUPAC Rules for Naming Alkenes
1. Select the longest continuous carbon-carbon chain that contains the double bond.
2. Name this parent compound as you would an alkane, but change the –ane ending to –ene.
CH3CH2CH3 propane CH3CH=CH2 propene
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IUPAC Rules for Naming Alkenes
3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name.
CH3CH=CHCH3 2- butene
CH3CH2CH2CH=CH2 1-pentene
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IUPAC Rules for Naming Alkenes
4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded.
H3C CHH2C
HC CH2
CH3
4-methyl-1-pentene
12345
H2C
H2C CH
HC CH2
12345H3C
H2CH2C CH3
3-propyl-1-hexene
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How would we write the structural formula for 4-methyl-2-pentene?
• The name indicates:– Five carbons in the longest chain
– A double bond between carbons 2 and 3
– A methyl group on carbon 4
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Write a structural formula for: 7-methyl-2-octene
• Octene indicates an 8-carbon chain
• The chain contains a C=C between carbons
2 and 3.
• There is a –CH3 group on carbon 7
H3C
HC
CH
H2C
CH2
H2C
CH
CH3
CH3
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• Longest chain containing C=C is 5 carbons
• 2-ethyl-1-pentene
Name this compound:
H3C
H2C
C
H2C
CH2
CH3
CH2
2
1
3
4
5ethyl
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Geometric Isomerism Geometric Isomerism in Alkenesin Alkenes
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Geometric Isomerism in Alkenes• Compounds containing a carbon-carbon double
bond (pi bond) have restricted rotation about that double bond.
• This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism.
• Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers.– They are also called cis-trans isomers.
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Trans is a Latin noun or prefix, meaning “across”, “beyond” or “on the opposite side”.
Cis is a Latin prefix, meaning “on the same side [as]” , “on this side [of]”, or “near side [of]”.
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Geometric Isomers in Alkenes
Cl
C
H
C
H
Cl H
C
Cl
C
H
Cl
cis-1,2-dichloroethene(bp = 60.1 C)
trans-1,2-dichloroethene(bp = 48.4 C)
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An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it.
a
C
b
C
b
a a
C
b
C
a
b
cis isomer trans isomer
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An alkene does not show cis-trans isomerism if even one carbon of the double bond has two identical groups attached to it.
H
C
H
C
H
H H3C
C
H3C
C
H
CH3
two groupsthe same
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Draw a structure for cis-5-chloro-2-hexene
• The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5.
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Is the compound below the cis or trans isomer?
H
C C
CH3
H2C
H3C
CH3
trans-3-methyl-2-pentene
1
2 3
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C CH H
ter terC C
H
HAshley
Samantha
Other Alkenes
Cisters Transisters
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CycloalkenesCycloalkenesCycloalkenesCycloalkenes
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Cycloalkenes• As the name implies, cycloalkenes are cyclic
compounds that contain a C=C in the ring.
• The carbons of the double bond are assigned numbers 1 and 2.
1
2
1
2
cyclopentene
cyclohexene
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Cycloalkenes
1
2
1
2
CH3
CH3
CH3
3
4
5 4
5
6
3
1-methylcyclopentene
1,3-dimethylcyclohexene
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Preparation and Preparation and Physical Properties of Physical Properties of
AlkenesAlkenes
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Preparation of Alkenes
• Cracking
• Dehydration of Alcohols
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Cracking• Cracking, or pyrolysis, is the process in which saturated
hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina):
Alkane (CnH2n+2) Mixture of alkenes + Alkanes + H2 (g)
2CH3CH2CH3 CH3CH=CH2 + CH2=CH2 + CH4 + H2
Heat
catalyst
~500°C
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Dehydration of Alcohols
• Dehydration involves the elimination of a molecule of water from a reactant molecule.
H3C C
H
H
C
OH
H
CH3
conc. H2SO4
heatH3C C
H
C
H
CH3 + H2O
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Physical Properties of Alkenes
• Alkenes have physical properties very similar to the corresponding alkanes.
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Chemical Properties Chemical Properties of Alkenesof Alkenes
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Addition Reactions of Alkenes
• Addition at the C=C bond is the most common reaction of alkenes.– H2
– Br2 and Cl2
– HBr, HCl
– H2O
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Addition of H2
• Hydrogenation
H3C C
H
C
H
CH3Pt, 25 C
1 atmH3C C
H
C
H
CH3+ H2
H H
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Addition of X2
Bromine changes from a red-orange (flask on the left) to colorless when added to an alkene as shown in the flask on the right.
Bromination (Br2) or Chlorination (Cl2)
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Addition of HX
• Hydrobromination (HBr) or Hydrochlorination (HCl)
H3C C
H
C
H
CH3 H3C C
H
C
H
CH3+ HCl
H Cl
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Addition of H2O
H3C C
H
C
H
CH3 H3C C
H
C
H
CH3+ H2O
H OH
H+
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Addition of HX to an Unsymmetrical Alkene
Why????
H3CHC CH2 + H-Cl
H3CHC CH2
HCl
H3CHC CH2
ClH
(about 100% yield)
(trace)Them that has, gets!
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Markovnikov’s Rule
• When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms.
Vladimir Markovnikov
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Markovnikov’s Rule
H3CHC CH2 + H-Cl
H3CHC CH2
HCl
H3CHC CH2
ClH
(about 100% yield)
(trace)This reaction proceeds via the formation of the most stable carbocation intermediate (2°).
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Write formulas for the organic products formed when
2-methyl-1-butene reacts with:
a) H2, Pt/25°C
b) Cl2
c) HCl
d) H20, H+
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2-methyl-1-butene + H2, Pt/25 °C
H2C C
CH3
CH2CH3 + H2 H2C C
CH3
CH2CH3
H H
Pt
2-methylbutane
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2-methyl-1-butene + Cl2
H2C C
CH3
CH2CH3 + Cl2 H2C C
CH3
CH2CH3
Cl Cl
1,2-dichloro-2-methylbutane
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2-methyl-1-butene + HCl
H2C C
CH3
CH2CH3 + HCl H2C C
CH3
CH2CH3
H Cl
2-chloro-2-methylbutane
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2-methyl-1-butene + H2O
H2C C
CH3
CH2CH3 + H2O H2C C
CH3
CH2CH3
H OH
H+
2-hydroxy-2-methylbutane
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OxidationOxidationOxidationOxidation
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Oxidation at the C=C Bond
• Baeyer Test
H2C CH2 + KMnO4 (aq) + H2O H2C CH2
OH OH
+ MnO2 + KOHethene(ethylene)
(purple)
1,2-ethanediol(ethylene glycol)
(brown)
+ –
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Alkynes: Nomenclature Alkynes: Nomenclature and Preparationand Preparation
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The rules for naming alkynes are the same as those for alkenes, but the ending –yne is used to indicate the presence of a triple bond.
IUPAC Rules for Naming Alkynes
H3CH2C C CH
1-butyne
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Physical and Chemical Physical and Chemical Properties of AlkynesProperties of Alkynes
Physical and Chemical Physical and Chemical Properties of AlkynesProperties of Alkynes
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Physical Properties of Alkynes
• Acetylene is a colorless gas with little odor when pure.
• Acetylene is insoluble in water and is a gas at normal temperature and pressure.
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Chemical Properties of Alkynes• Alkynes undergo addition reactions
rather similar to those of alkenes.
–Cl2 and Br2
–HCl and HBr–Positive reaction with Baeyer’s
test.
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Bromination of Acetylene
HCCH + Br2 CHBr=CHBr
HCCH + 2 Br2 CHBr2-CHBr2
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Bromine Potassium permanganate (Baeyer’s test)
Tests for Unsaturation
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HCl Addition to Unsymmetrical Alkynes
• This addition follows Markovnikov’s rule:
CH3CCH + HCl CH3CCl=CH2
CH3CCH + 2 HCl CH3CCl2-CH3
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Aromatic Hydrocarbons: Aromatic Hydrocarbons: StructureStructure
Aromatic Hydrocarbons: Aromatic Hydrocarbons: StructureStructure
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Benzene, or benzol, is an compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H.
Michael Faraday (1791 –1867 ) first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen.
The empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogen atom for each carbon atom, was challenging to determine. Several in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its very early years, and too little evidence was then available to help chemists decide on any particular structure.
1820
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Friedrich Auguste Kekulé
1829-1896
Friedrich August von Kekule had a dream of whirling snakes, of the structure of benzene - the organic chemical compound made up of a ring of carbon atoms. He reported the dream in the following words many years after it took place, in a speech at a dinner commemorating his discovery.
I turned my chair to the fire (after having worked on the problem for some time) and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen.
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Kekulé, moments before his brilliant insight into the structure of benzene.
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Aromatic Compounds
Benzene is• an aromatic compound.
• a ring of 6 C atoms and 6 H atoms.• a flat ring structure drawn with three double bonds. • represented by two structures because the electrons
are shared among all the C atoms.
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Bonding in Benzene
• The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule.
• This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds.
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Bonding in Benzene
(a) sp2-sp2 orbital overlap to form the carbon ring structure.
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Bonding in Benzene
(b) carbon-hydrogen bonds formed by sp2-s orbital overlap and overlapping p orbitals.
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Bonding in Benzene
(c) pi electron clouds above and below the plane of the carbon ring.
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Naming Aromatic Naming Aromatic CompoundsCompounds
Naming Aromatic Naming Aromatic CompoundsCompounds
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Naming Substituted Benzene Compounds• A substituted benzene is derived by
replacing one or more hydrogen atoms of benzene by another atom or group of atoms.
• Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom.
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Monosubstituted Benzenes
• Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.
O2N
CH2CH3
Cl
Br
nitrobenzene ethylbenzene chlorobenzene bromobenzene
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• Certain monosubstituted benzenes have special names.
CH3
OH
H2N
C
O
HC
O
OH
benzoic acid benzaldehyde
CH=CH2
styrene phenol aniline
toluene
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The word phenyl represents the C6H5- group. It is used
to name benzene derivatives that would otherwise be
difficult to name.
CH
CH3
CHCH2CH3
CH2
diphenylmethane
Cl
3-chloro-2-phenylpentane
1
23 4 5
This is the phenyl group which is a benzene ring minus ahydrogen atom .
These are examples of benzene derivatives that are easier to name using the phenyl group.
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Disubstituted Benzenes• The prefixes ortho-, meta-, and para-
(abbreviated o-, m-, and p-) are used to name disubstituted benzenes.
G
ortho
meta
para
meta
ortho
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Dichlorobenzenes, C6H4Cl2
• The three isomers of dichlorobenzene have different physical properties.
Cl
ortho-dichlorobenzene
Cl Cl
Cl
Cl
Clpara-dichlorobenzene
meta-dichlorobenzene
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Disubstituted Benzenes• When the two substituents are different and
neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene.
Cl
ortho-bromochlorobenzene
CH2CH3
Br
NO2
para-ethylnitrobenzene
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Isomers of Dimethyl Benzene
The dimethylbenzenes have the special name xylene.
CH3
ortho-xylene
CH3
CH3
CH3
para-xylenemeta-xylene
CH3
C
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Isomers of Methylphenol
The methylphenols have the special name cresol.
CH3
ortho-xylene
CH3
CH3
CH3
para-xylenemeta-xylene
CH3
C
OH
OH
OH
ortho-cresol meta-cresol para-cresol
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Disubstituted Benzenes• When one of the substituents corresponds to a
monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound.
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Polysubstituted Benzenes• When there are more than two
substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups.
• Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups.
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Polysubstituted Benzenes
CH3
O2N NO2
NO2
2,4,6-trinitrotoluene (TNT)
12
3
4
5
6
OH
Cl
5-bromo-2-chlorophenol
12
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6
Br4
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Polycyclic Aromatic Polycyclic Aromatic CompoundsCompounds
Polycyclic Aromatic Polycyclic Aromatic CompoundsCompounds
Polycyclic Aromatic Hydrocarbons PAHs
mothballs dyes carcinogen
Product of combustion of coal tar, tobacco smoke, barbecued meats.
Potent carcinogen
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Sources and Physical Sources and Physical Properties of Aromatic Properties of Aromatic
HydrocarbonsHydrocarbons
Sources and Physical Sources and Physical Properties of Aromatic Properties of Aromatic
HydrocarbonsHydrocarbons
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Sources of Aromatic Hydrocarbons• The aromatic hydrocarbons, such as
benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar.
• Coal Coke + Coal gas + Coal tar• Because of the great demand for
aromatic hydrocarbons, processes were devised to obtain them from petroleum.
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Properties of Aromatic Hydrocarbons
• Aromatic hydrocarbons are essentially nonpolar substances, insoluble in water but soluble in many organic solvents.
• They are liquids or solids and usually have densities less than that of water.
• Aromatic hydrocarbons burn readily, usually with smoky (sooty) yellow flames as a result of incomplete carbon combustion.
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Chemical Properties of Chemical Properties of Aromatic HydrocarbonsAromatic HydrocarbonsChemical Properties of Chemical Properties of Aromatic HydrocarbonsAromatic Hydrocarbons
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Substitution Reactions of Aromatic Hydrocarbons
• Halogenation – net addition of -Br or -Cl
• Nitration – net addition of –NO2
• Alkylation– net addition of –R (alkyl group)
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Halogenation of Benzene• When benzene reacts with chlorine or
bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces an H atom to form the products.
+ X2
X
+ HXFeX3
benzene
bromine orchlorine
bromobenzene orchlorobenzene
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Nitration of Benzene• When benzene reacts with a mixture of
concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed.
+ HO-NO2
NO2
+ H2OH2SO4
benzene
nitric acid
nitrobenzene
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Alkylation of Benzene• Alkylation of benzene is known as the
Friedel-Crafts reaction.
• The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring.
+ CH3CH2Cl
CH2CH3
+ HCl
benzene
chloro-ethane
ethylbenzene
AlCl3
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Mr. Ortho Paranoid
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Ms. Molly Meta
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Ortho/Para Directors
• Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the ortho or para positions.
• These groups usually have unshared electron pairs next to the ring.
• Examples include: –OH, –NH2, –OCH3, – CH3, –Br, and –Cl
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Examples of Ring Directors
Br
+ HNO3H2SO4
Br
NO2
+
Br
NO2
OH
+ 3 Cl 2FeCl 3
OH
ClCl
Cl
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Meta Directors
• Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the meta position by deactivating the ortho and para positions.
• These groups have a positive charge next to the ring.
• Examples include: –NO2 and –CN
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Examples of Ring Directors
NO2
+AlCl 3
NO2
CH2CH3
CH3CH2Cl
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Side-Chain Oxidation
• Carbon chains attached to an aromatic ring are fairly easy to oxidize.
CH2CH3
ethylbenzene
K2Cr2O7/H2SO4
heat
COOH
+ CO2
benzoic acid
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