Hydrocarbons contain only carbon and hydrogen and can be subdivided into the two classes:

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Hydrocarbons contain only carbon and hydrogen and
can be subdivided into the two classes
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Alkenes are unsaturated hydrocarbons containing a
carboncarbon double bondthat is, two adjacent
carbon atoms joined together with two bonds. The
general formula for an alkene is CnH2n , where n
is an integer greater than 1. The alkenes are
nonpolar compounds with physical properties
similar to those of the alkanes. They are
insoluble in water and have boiling points close
to those of the alkanes.
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Although the two bonds in the structural formula
of an alkene are drawn having equal weight, their
bond strengths are quite different. One strong
bond 355 kJ/mol sigma (?) bond One weaker
bond 250 kJ/mol pi (?) bond The weaker ? bond is
responsible for the increased reactivity of
alkenes over the alkanes. In the alkenes the
hybridization is sp2. The 2s and two of the 2p
orbitals about the carbon atom hybridize to form
three equivalent sp2 orbitals, leaving a single
2p orbital in an unhybridized state.The is sp2
hybrids form a trigoanl planar arrangement and
the p-orbital is perpendicular.
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C2H4, ethene, is formed from two sp2 hybridized
carbon atoms and four hydrogen atoms
Lateral overlap of the unhybridized p orbitals,
above and below the triangular planes, results in
the ? bond of the double bond.
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The two carbons and all four hydrogens all lie
within the same plane. The bond angles are all
120o.
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Alkenes possess two possibilities for the
formation of constitutional isomers Different
carbon skeletons (connectivity of atoms, also
possible for alkanes and cycloalkanes) Different
placements of the double bond within a carbon
skeleton. There are two C4H10 alkanes, however,
there are three C4H8 alkenes
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A CC double bond has restricted rotation, as a
result of restricted rotation, cis-trans
isomerism is possible for compounds containing a
double bond.
The cis- isomer contains two similar substituents
on the same side of the double bond while the
trans- isomer has two similar substituents on
opposite sides of the double bond.
MP. -139o C MP. -106o C
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The use of the cis-trans nomenclature system is
restricted to 1,2-disubstituted alkenesthat is,
alkenes in which each carbon of the double bond
has one hydrogen and one substituent other than
hydrogen. This restriction is analogous to that
of the cis-trans nomenclature system for
cycloalkanes. So, none of the following types of
alkenes possesses cis-trans isomers
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The ? bond of an alkene is sufficiently weak to
undergo reaction with Hydrogen (H2) Halogen (F2,
Cl2, Br2) Hydrogen halide (HCl, HBr,
HI) Water Each of the above reagents undergoes an
addition reaction with an alkene
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Addition of Symmetrical Reagents (AB) such as
hydrogen and halogens
Hydrogen adds to a double bond and hydrogenation
is an example of a reduction reaction.
At room temperature a metal catalyst is required
for this reaction. This is indicated by writing
the catalyst above the arrow connecting the
reactants and the products. A catalyst provides
an alternate, lower activation energy, pathway
for a chemical reaction.
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Bromine and other halogens add to a double bond
as hydrogen does
This proceeds readily at room temperature but
alkanes react with halogens only in the presence
of UV radiation or at high temperatures (200o C),
and then the reaction is of a substitution type,
not an addition type as for the alkenes.
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The ready addition of bromine to alkenes provides
a simple chemical diagnostic test for alkenes
versus alkanes. Bromine has a deep red color
which disappears shortly after addition to an
alkene
Add some bromine to an unknown compound and wait
a few minutes If the red color disappears,
addition has taken place and the unknown is an
alkene. If the red color does not disappear,
addition has not taken place, and the unknown is
an alkane.
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Addition of Asymmetrical Reagents (A?B)
Hydrogen halides (HF, HCl, HI) and water (H2O)
are asymmetrical reagents. In each case above, AB
represents two fragments, one of which is H, and
the other is either a halogen or OH. The
addition of a hydrogen halide is called a
hydrohalogenation
The addition of water is called a hydration
Hydration requires the presence of a strong acid
catalyst such as H2SO4.
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Alkanes do not react with either hydrogen halides
or water. Alkenes can be separated into two
classes Symmetrical alkenes CH2CH2, RCHRCH,
R2CCR2 Unsymmetrical alkenes CH2CHR, CH2CR2,
RCHCR2 The addition of water or a hydrogen
halide to a symmetrical alkene results in only
one possible product. The addition of water or
a hydrogen halide to an unsymmetrical alkene can
result in two different possible products
This is an example of a reaction that can proceed
by two competing pathways.
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When two products are possible Some reactions
are nonselective Both products are formed in
more or less equal amounts. Some reactions are
selective The two products are formed in
unequal amounts. Major and minor products are
formed.
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Additions of asymmetrical reagents to alkenes
follows the Markovnikov rule The major product
formed places the incoming hydrogen atom on the
carbon atom of the double bond that carried the
most hydrogen atoms prior to the reaction. The
rich get richer.
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Alkenes, as well as some other families of
organic compounds, can add to one another to form
a covalently linked chain of molecules called a
polymer. The individually linked molecules in the
polymer are called monomers. Examples of
synthetic polymers include plastics, fibers, and
rubbers. Naturally occurring polymers include
carbohydrates, proteins, and nucleic acids.
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Addition polymerizaton of ethylene produces a
long polymeric molecule called polyethylene
In this polymerization process ? bonds within
each monomer are broken and remade as ? bonds
between the monomers. The process can be
abbreviated as
where n is usually in the 1000s. This type of
polymerization requires the presence of a
catalyst, and does not occur in the presence of
additional reagents which might react
preferentially with the double bonds.
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Alkynes are hydrocarbons containing a
carbon-carbon triple bond. The general formula
for an alkyne is CnH2n-2. The first member of the
alkyne series is ethyne (acetylene)
Because of the triple bond, alkynes are more
reactive than alkenes.
Alkynes react with the same reagents as alkenes,
but with twice as much reagent, since the double
bond left after the first addition is also
reactive.
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Aromatic compounds are unsaturated hydrocarbons
that do not behave like other unsaturated
hydrocarbons. They also have exceptional
stability, far beyond what would be expected.

• The simplest aromatic hydrocarbon is benzene
  (C6H6) and the structure is represented as
• The C-C bond lengths in benzene are all equal
  (139 pm or picometers) while C-C is 154 pm and
  CC is 133 pm.
• Halogens, hydrogen halides, water, and sulfuric
  acid do not add to the multiple bonds of benzene
  under any conditions. However, these reagents
  readily add to the double bonds of alkenes.

or better as
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All carbon and hydrogen atoms in benzene lie in
one plane with C-C bond angles of 120. The
pi-bonding electrons around the ring of benzene
can be written resonance structures or are better
described as delocalized. Any organic compounds
whose structures contain a benzene ring can be
called aromatic compounds.
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Benzene compounds were aromatic because many
had districtive odors.
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Several heterocyclic aromatic rings are of
importance biologically such as the purine and
pyrimidine ring systems found in nucleic acids
such as DNA and RNA.
Other common examples of biological aromatic
systems can be found in the amino acids and the
heme group of hemoglobin.
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The IUPAC system for naming monosubstituted
benzenes places the substituent name in front of
the parent name benzene
Several monosubstituted benzenes have common
names that have been adopted by IUPAC as the
preferred names
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Disubstituted benzenes have three constitution
isomers numbered 1,2 (ortho, or o), 1,3 (meta, or
m), and 1,4 (para, or p)
Trisubstituted benzenes are named similarly,
however, numbers must be used since the o-, m-,
p- system is not applicable
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Certain disubstituted benzenes are named as
derivatives of the monosubstituted benzene, if it
has a preferred common name
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When the aliphatic part of a molecule is more
complex than the aromatic component, the name of
the compound may be derived from the aliphatic
part using the prefix, phenyl, to indicate a
benzene substituent
The phenyl substituent may be indicated by C6H5-,
Ph-, or ?-. A phenyl or substituted phenyl group
is called an aryl group (Ar).
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The CC double bonds in benzene are highly
resistant to addition, however, the C-H bonds in
benzene will undergo substitution
reactions. Halogenation, sulfonation, nitration,
and alkylation reactions all result in the
replacement of a hydrogen atom extending from the
benzene ring with another atom or group.
Halogenation and alkylation both require a metal
halide catalyst, such as FeCl3 or AlCl3.
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Nitration, alkylation, and sulfonation all also
occur with alkenes, but do so as addition
reactions, not substitution reactions. Similar to
other organic compounds, aromatic compounds
undergo oxidation to carbon dioxide and
water. Selective oxidations of alkylbenzenes
using moderately strong oxidizing conditions (hot
acidic KMnO4 or K2Cr2O7), however, the aromatic
ring remains intact
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