reaction_mechanism of alkanes
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Alkanes
One reason that organic chemistry is so
important in our modern world is the
versatility of carbon.
Many diverse reactions are possible with
this element.
One of the most important reactions is
oxidation reduction (redox)
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Reduction-Oxidation Reaction
When carbon atoms are oxidized, they often
form additional bonds to oxygen.
In the following example carbon in methane is
oxidized to carbon dioxide.
C
H
H
H H + 2O2 O C O + 2H2O
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Reduction-Oxidation Reaction
When carbon atoms are reduced, they often
form additional bonds to hydrogen.
During this reaction, the carbon may decrease
the number of bonds to oxygen and increasethe number of bonds to hydrogen.
The following reaction shows the reduction of
a carbon compound:
C
H
H
H OH +O C O + 2H2 H2O
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Reaction Mechanism
Each class of organic compounds can
undergo important oxidation-reduction
reactions.
Although countless different carbon-
containing molecules exist, there are
relatively few types of organic reactions,
and only a few carbon atoms, at most, areinvolved in any common reaction.
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Substitution Reaction
A reaction where one atom in a molecule is
exchanged by another atom or group of atoms
C
H
H
H
H + Br2 C
H
H
H
B r + BrH
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Elimination Reaction
A reaction in which a single reactant is split into
two products, and one of the products is
eliminated
C C
H
H
H
H
H
Br C C
H
H
H
H
+ BrH
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Addition Reaction
A reaction where two reactants adding together
to form a single product.
It can be thought of as the reverse of an
elimination reaction.
C C
H
H
H
H
H
BrC C
H
H
H
H
+ BrH
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Reaction of Alkane
One type of alkane reaction has inspired
people to make great exploration because
alkanes, as well as other hydrocarbons,
undergo combustion reaction with oxygenwith the evolution of large amounts of heat
energy.
CH4(g) + 2O2(g)CO2(g) + 2H2O(g) +802.5kJ
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Reaction of Alkane
When carbon dioxide is formed, the
alkane has undergone complete oxidation.
The resulting thermal energy can be
converted to mechanical and electrical
energy.
Combustion reactions overshadow all
other reactions of alkanes in economic
importance.
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Reaction of Alkane
But combustion reactions are not usually
of great importance to organic chemists
because carbon dioxide and water are the
only chemical products of completecombustion.
Aside from their combustibility, alkanes
are relatively sluggish and limited inreactivity.
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Combustion
The combustion of carbon compounds,
especially hydrocarbons, has been the
most important source of heat energy for
human civilizations throughout recordedhistory.
The massive and uncontrolled chemical
changes that take place in combustionmake it difficult to deduce mechanistic
paths
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CH3-CH2-CH3 + 5 O2 > 3 CO2 + 4 H2O + heat
Every covalent bond in the reactants has been broken
and an entirely new set of covalent bonds have formed in
the products
Two points concerning this reaction are important:
1. Since all the covalent bonds in the reactant molecules are
broken, the quantity of heat evolved in this reaction is
related to the strength of these bonds (and, of course, the
strength of the bonds formed in the products)
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CH3-CH2-CH3 + 4 O2 > CO2 + 2 CO + 4 H2O + heat
Two points concerning this reaction are important:
2. The stoichiometry of the reactants is important. If
insufficient oxygen is supplied some of the products will
consist of carbon monoxide, a highly toxic gas.
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Reaction of Alkane
But with proper activation, such as withhigh temperature or catalysts, alkanes canbe made to react in a variety of ways.
Non-combustion reactions includes thefollowing: Halogenation
Dehydroggenation Cracking
Isomerization
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Halogenation Reaction
It is a substitution reaction.
A halogen is substituted for a hydrogen atom.
CH4 + Cl2 + energy > CH3Cl + HCl
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Halogenation Reaction
However, one complication is that all thehydrogen atoms of an alkane may undergosubstitution, resulting in a mixture of products,as shown in the following unbalanced equation.
CH4 + Cl2 + energy >
CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl
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Halogenation Reaction
C C
H
H
H
H
H
H + Cl2 C C
H
H
H
H
H
Cl + ClH
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Halogenation Reaction
When a specific halogen such as chlorine isused, the reaction is called chlorination.
RH is an alkane that reacts with the halogen.
The reaction yields alkyl halides, RX, which areuseful as intermediates for the manufacture ofother substances.
Heat or light is usually needed to initiate this
halogenation Fluorine > Bromine > Iodine
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Halogenation Reaction
The following facts must be accomodated by any
reasonable mechanism for the halogenation
reaction.
1. The reactivity of the halogens decreases inthe following order: F2 > Cl2 > Br2 > I2.
2. We shall confine our attention to chlorine and
bromine, since fluorine is so explosively reactive
it is difficult to control, and iodine is generally
unreactive.
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Halogenation Reaction
3. Chlorinations and brominations are normally
exothermic.
4. Energy input in the form of heat or light isnecessary to initiate these halogenations.
5. If light is used to initiate halogenation,
thousands of molecules react for each photon of
light absorbed.6. Halogenation reactions may be conducted in
either the gaseous or liquid phase.
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Halogenation Reaction
7. In gas phase chlorination the presence of
oxygen (a radical trap) inhibits the reaction.8. In liquid phase halogenations radical initiators
such as peroxides facilitate the reaction.
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Halogenation Reaction
7. In gas phase chlorination the presence of
oxygen (a radical trap) inhibits the reaction.8. In liquid phase halogenations radical initiators
such as peroxides facilitate the reaction.
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Halogenation Reaction
All the hydrogens in a complex
alkane do not exhibit equal
reactivity.
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Halogenation Reaction
CH3-CH2-CH3 + Cl2 >
45% CH3-CH2-CH2Cl + 55% CH3-CHCl-CH3
Light-induced gas phase chlorination at 25
C gives 45% 1-chloropropane and 55% 2-
chloropropane.
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Halogenation Reaction
CH3-CH2-CH3 + Br2 >
3% CH3
-CH2
-CH2
Br +
97% CH3-CHBr-CH3
The results of bromination ( light-induced at
25 C ) are even more suprising, with 2-bromopropane accounting for 97% of the
mono-bromo product.
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Bromination of alkanes occurs by a similar mechanism,
but is slower and more selective because a bromine
atom is a less reactive hydrogen abstraction agent thana chlorine atom, as reflected by the higher bond energy
of H-Cl than H-Br.
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Dehydrogenation Reaction
It is an elimination reaction.
Hydrogen is lost from an organic compound
during dehydrogenation.
This reaction yields alkenes, which, like alkylhalides, are useful chemical intermediates.
Hydrogen is a valuable by-product.
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Dehydration Reaction
C C
H
H
H
H
HC
H
H
H C C C
H
H
H
H H
H + H2
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Cracking
Breaking up of large molecules to form smaller
ones.
C16H34 C8H18 + C8H16(alkane) (alkane) (alkene)
The reaction will produce many possible
products.
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Isomerization
Rearrangement of molecular structures
CH2
CH2CH2
CH2CH2 CH2
CH2 CH
CH2
CH2
catalyst
heat, pressure
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Isomers
The properties of an organic substance are
dependent on its molecular structure.
When two or more compounds have the same
molecular formula but different structuralarrangements of their atoms, their properties
changes.
This phenomenon is called isomerism.
The various individual compounds are called
isomers.
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COMMON NAMES
If all alkanes had unbranched
(straight-chain) structures, their
nomenclature would be very
simple.
Most alkanes have structuralisomers, however, and we need
a way of naming all the different
isomers.
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ISOMERS
-different compounds with the samemolecular formula.
Constitutional isomers (structural
isomers)Are connected differently; they differ in
their bonding sequence
Stereoisomers differ only in how their
atoms are oriented in space.
Geometric isomers are stereoisomers
that differ in their cis-trans arrangement on
a ring or a double bond
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For example:
There are two isomers of formula
C4H10.
The unbranched isomer is simplycalled butane (or n-butane, meaning
normal butane), and the branched
isomer is called isobutane, meaningan isomer of butane.
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Cis-Trans Isomerism in
Cycloalkanes
Open-chain alkanes undergo rotations
about their carbon-carbon single bonds,
and they are free to assume any of an
infinite number of conformations.
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Cis-Trans Isomerism in
Cycloalkanes
A cycloalkanes has two distinct faces.
If two substituents point toward the
same face, they are cis.If they point toward opposite faces
they are trans.
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