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AN INTRODUCTION TO THE CHEMISTRY OF ALKANES. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. KNOCKHARDY PUBLISHING. THE CHEMISTRY OF ALKANES. INTRODUCTION

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AN INTRODUCTION TO

THE CHEMISTRY

OF ALKANES

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING


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KNOCKHARDY PUBLISHING

THE CHEMISTRY OF ALKANES

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.

Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.

Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...

www.knockhardy.org.uk/sci.htm

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard


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THE CHEMISTRY OF ALKANES

  • CONTENTS

  • Structure of alkanes

  • Physical properties of alkanes

  • Chemical properties of alkanes

  • Breaking covalent bonds

  • Chlorination via free radical substitution

  • Cracking

  • Revision check list


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THE CHEMISTRY OF ALKANES

  • Before you start it would be helpful to…

  • Recall the definition of a covalent bond

  • Be able to balance simple equations

  • Be able to write out structures for hydrocarbons


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ALKANES

General members of a homologous series

general formula is CnH2n+2 - for non-cyclic alkanes

saturated hydrocarbons - all carbon-carbon bonding is single

bonds are spaced tetrahedrally about carbon atoms.

Isomerism the first example of structural isomerism occurs with C4H10

BUTANE 2-METHYLPROPANE

Structural isomers have the SAME MOLECULAR FORMULA BUT

DIFFERENT STRUCTURAL FORMULA

They possess different physical properties such as boiling point,

melting point and density


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2p

2

2s

1

1s

HYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2


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2p

2p

2

2

2s

2s

1

1

1s

1s

HYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3

The process is favourable because the of arrangement of electrons; four unpaired and with less repulsion is more stable


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HYBRIDISATION OF ORBITALS IN ALKANES

The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.

Because one s and three p orbitals are used, it is called sp3 hybridisation

2s22p2

2s12p3

4 x sp3


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THE STRUCTURE OF ALKANES

In ALKANES, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º.

Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond.

109.5º


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PHYSICAL PROPERTIES OF ALKANES

Boiling pointincreases as they get more carbon atoms in their formula

more atoms = greater intermolecular Van der Waals’ forces

greater intermolecular force = more energy to separate the molecules

greater energy required = higher boiling point

CH4(-161°C) C2H6(-88°C) C3H8(-42°C) C4H10(-0.5°C)

difference gets less - mass increases by a smaller percentage


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PHYSICAL PROPERTIES OF ALKANES

Boiling pointincreases as they get more carbon atoms in their formula

more atoms = greater intermolecular Van der Waals’ forces

greater intermolecular force = more energy to separate the molecules

greater energy required = higher boiling point

CH4 (-161°C) C2H6 (-88°C) C3H8 (-42°C) C4H10 (-0.5°C)

difference gets less - mass increases by a smaller percentage

Straight chains molecules have greater interaction than branched

“The greater the branching, the lower the boiling point”

STRUCTURAL ISOMERS OF C5H12

HIGHEST BOILING POINT LOWESTBOILINGPOINT


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PHYSICAL PROPERTIES OF ALKANES

Melting pointgeneral increase with molecular mass

the trend is not as regular as that for boiling point.

Solubility alkanes are non-polar so are immiscible with water

they are soluble in most organic solvents.


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CHEMICAL PROPERTIES OF ALKANES

Introduction-fairly unreactive; (old family name, paraffin, meant little reactivity)

- have relatively strong, almost NON-POLAR, SINGLE covalent bonds

- they have no real sites that will encourage substances to attack them

Combustion- make useful fuels - especially the lower members of the series

- react with oxygen in an exothermic reaction

complete CH4(g) + 2O2(g) ——> CO2(g) + 2H2O(l)

combustion

incompleteCH4(g) + 1½O2(g) ——> CO(g) + 2H2O(l)

combustion

the greater the number of carbon atoms, the more energy produced

BUT the greater the amount of oxygen needed for complete combustion.

Handy tipWhen balancing equations involving complete combustion, remember...

every carbon in the original hydrocarbon gives one carbon dioxide and

every two hydrogen atoms gives a water molecule.

Put the numbers into the equation, count up the O’s and H’s on the RHS

of the equation then balance the oxygen molecules on the LHS.


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POLLUTION

Processes involving combustion give rise to a variety of pollutants...

power stationsSO2emissions produce acid rain

internal combustion enginesCO, NOxandunburnt hydrocarbons

Removal

SO2 react effluent gases with a suitable compound (e.g. CaO)

CO and NOx pass exhaust gases through a catalytic converter

Catalytic converters

In the catalytic converter ... CO is converted to CO2

NOx are converted to N2

Unburnt hydrocarbons are converted to CO2 and H2O

e.g. 2NO + 2CO ———> N2 + 2CO2

• catalysts are made of finely divided rare metals Rh, Pd, Pt

• leaded petrol must not pass through the catalyst as the lead

deposits on the catalyst’s surface and “poisons” it, thus blocking

sites for reactions to take place.


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BREAKING COVALENT BONDS

There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond.

UNEQUAL SPLITTING

produces IONS

known as HETEROLYSIS or

HETEROLYTICFISSION

EQUAL SPLITTING

produces RADICALS

known as HOMOLYSIS or

HOMOLYTICFISSION

• If several bonds are present the weakest bond is usually broken first

• Energy to break bonds can come from a variety of energy sources - heat / light

• In the reaction between methane and chlorine either can be used, however...

• In the laboratory a source of UV light (or sunlight) is favoured.


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FREE RADICALS

TYPICAL PROPERTIES

• reactive species (atoms or groups) which possess an unpaired electron

• their reactivity is due to them wanting to pair up the single electron


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FREE RADICALS

TYPICAL PROPERTIES

• reactive species (atoms or groups) which possess an unpaired electron

• their reactivity is due to them wanting to pair up the single electron

•formed by homolytic fission (homolysis) of covalent bonds

• formed during the reaction between chlorine and methane

• formed during thermal cracking

• involved in the reactions taking place in the ozone layer


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CHLORINATION OF METHANE

Reagentschlorine and methane

ConditionsUV light or sunlight - heat is an alternative energy source

Equation(s) CH4(g) + Cl2(g) ——> HCl(g) + CH3Cl(g) chloromethane

CH3Cl(g) + Cl2(g) ——> HCl(g) + CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) + CHCl3(l) trichloromethane CHCl3(l) + Cl2(g) ——> HCl(g) + CCl4(l) tetrachloromethane

Mixturesfree radicals are very reactive - they are trying to pair their electron

with sufficient chlorine, every hydrogen will eventually be replaced.


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CHLORINATION OF METHANE

Reagentschlorine and methane

ConditionsUV light or sunlight - heat is an alternative energy source

Equation(s) CH4(g) + Cl2(g) ——> HCl(g) + CH3Cl(g) chloromethane

CH3Cl(g) + Cl2(g) ——> HCl(g) + CH2Cl2(l) dichloromethane CH2Cl2(l) + Cl2(g) ——> HCl(g) + CHCl3(l) trichloromethane CHCl3(l) + Cl2(g) ——> HCl(g) + CCl4(l) tetrachloromethane

Mixturesfree radicals are very reactive - they are trying to pair their electron

with sufficient chlorine, every hydrogen will eventually be replaced.

MechanismMechanisms portray what chemists think is going on in the reaction,

whereas an equation tells you the ratio of products and reactants.

Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting in

hydrogen atoms being substituted by chlorine atoms.

The process is a chain reaction.

In the propagation step, one radical is produced for each one used


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CHLORINATION OF METHANE

InitiationCl2 ——> 2Cl•RADICALS CREATED

The single dots represent UNPAIRED ELECTRONS

During initiation, the WEAKEST BOND IS BROKEN as it requires less energy.

There are three possible bonds in a mixture of alkanes and chlorine.

412 348 242

Average bond enthalpy kJ mol-1

The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.


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CHLORINATION OF METHANE

PropagationCl• + CH4 ——> CH3• + HClRADICALS USEDand

Cl2 + CH3• ——> CH3Cl + Cl• then RE-GENERATED

Free radicals are very reactive because they want to pair up their single electron.

They do this by abstracting a hydrogen atom from methane; a methyl radical is formed

The methyl radical is also very reactive and attacks a chlorine molecule

A chlorine radical is produced and the whole process can start over again


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CHLORINATION OF METHANE

TerminationCl• + Cl• ——> Cl2RADICALS REMOVED

Cl• + CH3• ——> CH3Cl

CH3• + CH3• ——> C2H6

Removing the reactive free radicals brings an end to the reaction.

This is not very likely at the start of the reaction because of their low concentration.


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CHLORINATION OF METHANE

OVERVIEW

InitiationCl2 ——> 2Cl• radicals created

PropagationCl• + CH4 ——> CH3• + HCl radicals used and

Cl2 + CH3• ——> CH3Cl + Cl• then re-generated

TerminationCl• + Cl• ——> Cl2 radicals removed

Cl• + CH3• ——> CH3Cl

CH3• + CH3• ——> C2H6

Summary

Due to lack of reactivity, alkanes need a very reactive species to persuade them to react

Free radicals need to be formed by homolytic fission of covalent bonds

This is done by shining UV light on the mixture (heat could be used)

Chlorine radicals are produced because the Cl-Cl bond is the weakest

You only need one chlorine radical to start things off

With excess chlorine you get further substitution and a mixture of chlorinated products


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CHLORINATION OF METHANE

RADICALS

PRODUCED

Initiation

Propagation

Termination

RADICALSUSED

ANDREGENERATED

RADICALS

REMOVED


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CHLORINATION OF METHANE

Further

propagationIf excess chlorine is present, further substitution takes place

The equations show the propagation steps for the formation of...

dichloromethaneCl• + CH3Cl ——> CH2Cl• + HCl

Cl2 + CH2Cl• ——> CH2Cl2 + Cl•

trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl

Cl2 + CHCl2• ——> CHCl3 + Cl•

tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl

Cl2 + CCl3• ——> CCl4 + Cl•

Mixtures Because of the many possible reactions there will be a mixture of products.

Individual haloalkanes can be separated by fractional distillation.


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CRACKING

Involves the breaking of C-C bonds in alkanes

Converts heavy fractions into higher value products

THERMALproceeds via a free radical mechanism

CATALYTICproceeds via a carbocation (carbonium ion) mechanism

THERMAL

HIGH PRESSURE ... 7000 kPa

HIGH TEMPERATURE ... 400°C to 900°C

FREE RADICAL MECHANISM

HOMOLYTIC FISSION

PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making polymers and ethanol

PRODUCES HYDROGEN ... used in the Haber Process and in margarine manufacture

Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission


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CRACKING

Involves the breaking of C-C bonds in alkanes

Converts heavy fractions into higher value products

THERMALproceeds via a free radical mechanism

CATALYTICproceeds via a carbocation (carbonium ion) mechanism

CATALYTIC

SLIGHT PRESSURE

HIGH TEMPERATURE ... 450°C

ZEOLITE CATALYST

CARBOCATION (IONIC) MECHANISM

HETEROLYTIC FISSION

PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS

USED FOR MOTOR FUELS

ZEOLITES are crystalline aluminosilicates; clay like substances


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REVISION CHECK

What should you be able to do?

Recallandexplain the physical properties of alkanes

Recall the use of alkanes as fuels

Recallandexplain the different ways to break a covalent bond

Write balanced equations representing combustion and chlorination

Understand the conditions and mechanism of free radical substitution

Recall the conditions and products from thermal and catalytic cracking

CAN YOU DO ALL OF THESE? YES NO


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You need to go over the relevant topic(s) again

Click on the button to

return to the menu


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WELL DONE!

Try some past paper questions


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AN INTRODUCTION TO

THE CHEMISTRY

OF ALKANES

THE END

© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING