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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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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C C

H

H

H

H

H

H + Cl2 C C

H

H

H

H

H

Cl + ClH


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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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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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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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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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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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All the hydrogens in a complex

alkane do not exhibit equal

reactivity.


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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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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
http://var/www/apps/conversion/current/tmp/Isomers%20of%20Hexane.mp4http://var/www/apps/conversion/current/tmp/Isomers%20of%20Hexane.mp4


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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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Reaction_Mechanism of Alkanes