an introduction to the chemistry of alkanes knockhardy publishing

AN INTRODUCTION TOAN INTRODUCTION TO

THE CHEMISTRYTHE CHEMISTRYOF ALKANESOF ALKANES

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

CONTENTS

• Structure of alkanes

• Physical properties of alkanes

• Chemical properties of alkanes

• Breaking covalent bonds

• Chlorination via free radical substitution

• Cracking

• Revision check list

THE CHEMISTRY OF ALKANESTHE CHEMISTRY OF ALKANES

General members of a homologous seriesgeneral 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 BUTDIFFERENT STRUCTURAL FORMULA

They possess different physical properties such as boiling point,melting point and density

ALKANESALKANES

HYBRIDISATION OF ORBITALSHYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

22s

2p

HYBRIDISATION OF ORBITALSHYBRIDISATION OF ORBITALS

The electronic configuration of a carbon atom is 1s22s22p2

1 1s

22s

2p

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

1 1s

22s

2p

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

stable

HYBRIDISATION OF ORBITALS IN ALKANESHYBRIDISATION 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

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.

THE STRUCTURE OF ALKANESTHE STRUCTURE OF ALKANES

109.5º

Alkanes - Nomenclature

The names of alkanes are composed of two parts –(i) a prefix which comes from the number of carbon atoms in the longest straightchain in the molecule.

Apart from the first four, which have trivial names, the number of carbons atoms is indicated by a prefix derived from the Greek numbering system.

Prefix C atoms Alkane

meth- 1 methaneeth- 2 ethaneprop- 3 propanebut- 4 butanepent- 5 pentanehex- 6 hexanehept- 7 heptaneoct- 8 octanenon- 9 nonanedec- 10 decane

(ii) a suffix or ending which shows which type of organic compound it is. With alkanes thissuffix is ___ane. Examples -ethane - octane -

CH2CH3 CH2 CH2 CH3CH2 CH2CH2

CH3

CH3

CH3

CH2 CH2CH2

CH3

CH2CH2

CH2CH3 CH3

I.U.P.A.C. NOMENCLATUREI.U.P.A.C. NOMENCLATURE

How long is a chain?

Because organic molecules are three dimensional and paper is two dimensional it can confusing when comparing molecules. This is because...

1. It is too complicated to draw molecules with the correct bond angles

2. Single covalent bonds are free to rotate

All the following written structures are of the same molecule - PENTANE C5H12

A simple way to check is to run a finger along the chain and see how many carbon atoms can be covered without reversing direction or taking the finger off the page. In all the above there are... FIVE CARBON ATOMS IN A LINE.

CH2CH3 CH2 CH2 CH CH3

CH3

CH2CH3 CH3CH

CH2

CH3


How long is the longest chain?

Look at the structures and work out how many carbon atoms are in the longest chain.

CH3

CH3CH

CH2

CH2CH3 CH

CH3

THE ANSWERS AREON THE NEXT SLIDE


CH3

CH2CH3 CH3CH

CH2

CH3


How long is the longest chain?

Look at the structures and work out how many carbon atoms are in the longest chain.

CH3

CH3CH

CH2

CH2CH3 CH

CH3

LONGEST CHAIN = 5

LONGEST CHAIN = 6

LONGEST CHAIN = 6

NOMENCLATURENOMENCLATURE

Ideally a naming system should tell you everything about a structure without ambiguity. There are two types of naming system commonly found in organic chemistry;

Trivial : based on some property or historical aspect;the name tells you little about the structure

Systematic : based on an agreed set of rules (I.U.P.A.C);exact structure can be found from the name (and vice-versa).

HOMOLOGOUS SERIEStrivial name systematic name example(s)paraffin alkane methane, butaneolefin alkene ethene, butenefatty acid alkanoic (carboxylic) acid ethanoic acid

INDIVIDUAL COMPOUNDStrivial name derivation systematic namemethane methu = wine (Gk.) methane (CH4)

butane butyrum = butter (Lat.) butane (C4H10)

acetic acid acetum = vinegar (Lat.) ethanoic acid (CH3COOH)

SIDE-CHAIN carbon based substituents are named before the chain name. they have the prefix -yl added to the basic stem (e.g. CH3 is methyl).

Number the principal chain from one end to give the lowest numbers.

Side-chain names appear in alphabetical order butyl, ethyl, methyl, propyl

Each side-chain is given its own number.

If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa

Numbers are separated from names by a HYPHEN e.g. 2-methylheptane

Numbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane

Alkyl radicals methyl CH3 - CH3

ethyl CH3- CH2- C2H5

propyl CH3- CH2- CH2- C3H7


SIDE-CHAIN carbon based substituents are named before the chain name. they have the prefix -yl added to the basic stem (e.g. CH3 is methyl).

Number the principal chain from one end to give the lowest numbers.

Side-chain names appear in alphabetical order butyl, ethyl, methyl, propyl

Each side-chain is given its own number.

If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa

Numbers are separated from names by a HYPHEN e.g. 2-methylheptane

Numbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane

Example longest chain 8 (it is an octane)3,4,5 are the numbers NOT 4,5,6order is ethyl, methyl, propyl

3-ethyl-5-methyl-4-propyloctane

Alkyl radicals methyl CH3 - CH3

ethyl CH3- CH2- C2H5

propyl CH3- CH2- CH2- C3H7

CH3

CH2 CH3CH

CH2

CH2CH3 CH

CH

CH2

CH2CH3 CH2

CH3



CH3

CH2CH3 CH3CH

CH2

CH3


Apply the rules and name these alkanes

CH3

CH3CH

CH2

CH2CH3 CH

CH3

THE ANSWERS ARE ON THE NEXT SLIDE


CH3

CH2CH3 CH3CH

CH2

CH3


CH3

CH3CH

CH2

CH2CH3 CH

CH3




CH3

CH2CH3 CH3CH

CH2

CH3


CH3

CH3CH

CH2

CH2CH3 CH

CH3

Longest chain = 5 so it is a pentane

A CH3, methyl, group is attached to the third carbon from one end...

3-methylpentane




CH3

CH2CH3 CH3CH

CH2

CH3


CH3

CH3CH

CH2

CH2CH3 CH

CH3



3-methylpentane



Longest chain = 6 so it is a hexane

A CH3, methyl, group is attached to the second carbon from one end...

2-methylhexane


CH3

CH2CH3 CH3CH

CH2

CH3


CH3

CH3CH

CH2

CH2CH3 CH

CH3



3-methylpentane




A CH3, methyl, group is attached to the second carbon from one end...

2-methylhexane


CH3, methyl, groups are attached to the third and fourth carbon atoms (whichever end you count from).

3,4-dimethylhexane

Boiling point increases 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”

Melting point general increase with molecular massthe trend is not as regular as that for boiling point.

Solubilityalkanes are non-polar so are immiscible with waterthey are soluble in most organic solvents.

PHYSICAL PROPERTIES OF ALKANESPHYSICAL 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

incomplete CH4(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 tip When balancing equations involving complete combustion, remember...every carbon in the original hydrocarbon gives one carbon dioxide andevery two hydrogen atoms gives a water molecule.

Put the numbers into the equation, count up the O’s and H’s on the RHSof the equation then balance the oxygen molecules on the LHS.

CHEMICAL PROPERTIES OF ALKANESCHEMICAL PROPERTIES OF ALKANES

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

power stations SO2 emissions produce acid rain

internal combustion engines CO, NOx and unburnt hydrocarbons

RemovalSO2 react effluent gases with a suitable compound (e.g. CaO)

CO and NOx pass exhaust gases through a catalytic converter

Catalytic convertersIn 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.

POLLUTIONPOLLUTION

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

UNEQUAL SPLITTINGproduces IONSknown as HETEROLYSIS or

HETEROLYTIC FISSION

EQUAL SPLITTINGproduces RADICALSknown as HOMOLYSIS or

HOMOLYTIC FISSION

• 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.

BREAKING COVALENT BONDSBREAKING COVALENT BONDS

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

FREE RADICALSFREE RADICALS

Reagents chlorine and methane

Conditions UV 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

Mixtures free radicals are very reactive - they are trying to pair their electronwith sufficient chlorine, every hydrogen will eventually be replaced.

CHLORINATION OF METHANECHLORINATION OF METHANE

Reagents chlorine and methane

Conditions UV 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

Mixtures free radicals are very reactive - they are trying to pair their electronwith sufficient chlorine, every hydrogen will eventually be replaced.

Mechanism Mechanisms 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



Initiation Cl2 ——> 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.


Propagation Cl• + CH4 ——> CH3• + HCl RADICALS USED and

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 formedThe methyl radical is also very reactive and attacks a chlorine moleculeA chlorine radical is produced and the whole process can start over again


Termination Cl• + Cl• ——> Cl2 RADICALS 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.


Initiation Cl2 ——> 2Cl• radicals created

Propagation Cl• + CH4 ——> CH3• + HCl radicals used and

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

Termination Cl• + Cl• ——> Cl2 radicals removed

Cl• + CH3• ——> CH3Cl

CH3• + CH3• ——> C2H6

OVERVIEW

SummaryDue to lack of reactivity, alkanes need a very reactive species to persuade them to reactFree radicals need to be formed by homolytic fission of covalent bondsThis is done by shining UV light on the mixture (heat could be used)Chlorine radicals are produced because the Cl-Cl bond is the weakestYou only need one chlorine radical to start things offWith excess chlorine you get further substitution and a mixture of chlorinated products

Initiation

Propagation

Termination


RADICALSPRODUCED

RADICALS USEDAND REGENERATED

RADICALSREMOVED

Furtherpropagation If excess chlorine is present, further substitution takes place

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

dichloromethane Cl• + 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.


Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

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

Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

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

REVISION CHECKREVISION CHECK

What should you be able to do?

Recall and explain the physical properties of alkanes

Recall the use of alkanes as fuels

Recall and explain 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? CAN YOU DO ALL OF THESE? YES YES NONO

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

Click on the button toClick on the button toreturn to the menureturn to the menu

WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions

© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE THE END

AN INTRODUCTION TOAN INTRODUCTION TO

THE CHEMISTRYTHE CHEMISTRYOF ALKANESOF ALKANES

an introduction to the chemistry of alkanes knockhardy publishing

Documents

carboncarbon bonding

longest chain

number of carbon atoms

sp3 orbitals of carbon

carbon overlaps

sp3in alkanes

alkanes thissuffix

longest straightchain