Hydrocarbons2_Cracking_Isomers_Topic6_120206.ppt

7/28/2019 Hydrocarbons2_Cracking_Isomers_Topic6_120206.ppt

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Hydrocarbons

Topic 6

Cracking and Isomers



Catalytic cracking

Thermal cracking – high temperature

Catalytic cracking – catalyst used

Large hydrocarbon molecules are brokendown into smaller useful hydrocarbonmolecules:

C10H22 C8H18 C2H4+

decane octane ethene



Catalytic cracking

http://www.cybered.net/commerce.asp?CatId=311&ProdId

=X5005108

Catalysts used are:

•Silica (silicon dioxide)

• Alumina (aluminium oxide)

•Zeolite (silicatecompounds)

SiO2

Al2O3

Molecular formula

http://www.cybered.net/commerce.asp?CatId=311&ProdId=X5005108








Catalytic Cracking of Decane

C

H

H

CH C

H

H

H

H

C

H

H

CC C

H

H

H

H

H

H

C

H

H

C C

H

H

H

H

H

C10H22

decane Catalyst at high temperature: 650°C

Heat energy used to break carbon to

carbon bonds

C

H

H

CH C

H

H

H

H

C

H

H

CC C

H

H

H

H

H

H

C

H

H

C C

H

H

H

Bond often breaks here

H +

OctaneA saturated hydrocarbon EtheneAn unsaturated hydrocarbon

H



Catalytic Cracking of Decane – showing bonds breaking

C

H

H

CH CH

H

H

H

C

H

H

CC C

H

H

H

H

H

H

C

H

H

C CH

H

H

H

H

C10H22

decane Catalyst at high temperature: 650°C

Heat energy used to break carbon to

carbon bonds

C

H

H

CH C

H

H

H

H

C

H

H

CC C

H

H

H

H

H

H

C

H

H

C C

H

H

H

H

H+

Bond often breaks here

C H bond breaks

Single electron available to form a new bond

H

This hydrogen atom can move here

to form a new C H bond to produce

octane

H +

Octane

A saturated hydrocarbon

Two electrons join to form a carbon

to carbon double bond

Ethene

An unsaturated hydrocarbon



Isomers

Isomers are chemical substances which have the same

molecular formula but have different structural formulae

Hydrocarbons such as members of the alkane series

which have four or more carbon atoms form isomers

The number of possible isomers increase as thenumber of carbon atoms increase

Isomers may have different functional groups e.g.

cyclobutane and butene are isomers

A structural formula shows the arrangement of

different atoms or groups of a molecule in space



Drawing the isomers of butane1. Draw carbon backbones of different isomers

C

H

HH C

H

C

H

C C H

H

C

H

C

C

2. Draw in four bonds for each carbon atom

HH H

HH

HH

H

HHH

H3. Add a hydrogen atom to each of the remaining bonds

Melting point = -138°C Melting point = -159°C

Boiling point = 0 °C Boiling point = -12 °C

Density = 0.58 g cm-3 Density = 0.56 g cm-3

Differences in physical properties between the two isomers:

n-butane iso-butane

A straight chain hydrocarbon A branched hydrocarbon



The isomers of pentane

C

H

CH C

H

C

H

C C H

H

CH

C

C

HH H

HH

HH

C

HH

H

H H

H

H

C C H

H

C

HC

H

HC

HHH

H

H

H

H H

H

H

1 2 3 4 5

Carbon atom 2 is joined to carbon

atoms 1 and 3 by two carbon tocarbon single bonds

1234

5


atoms 1, 3 and 5 by three carbon

to carbon single bonds

1 2 3

4

5


atoms 1, 3, 4 and 5 by four carbonto carbon single bonds



Which pair of hydrocarbon molecules are not isomers?

CH

H

H

1 2 3 4 5

C C

H

H

C

H

H

H

H

C

H

H

C H

H

H

6

CH

H

H

1234C C C

H

H

H

H

C

H

HC

H

H

H

HH5

CH

H

H

1

2 3 4C C C

H

H

H

H

C

H

HC

H

H

H

HH

5

6

CH

H

H

1 2 3 4C C C

H

H

H

H

C

H

HC

H

H

H

HH

CH

H

H

1 2 3 4C C C

H

H

HCH

H

H

H

HC HH

H

Hydrocarbon A

Hydrocarbon B

Hydrocarbon D Hydrocarbon E

Hydrocarbon C

The correct answer is A and E as they are both the same straight chain molecule All the other molecules are different branched isomers



Which pair of hydrocarbon molecules are not isomers?

C1 2 3 4 5

C C C C C6

C1234

C C C

C

C

5

C

1

2 3 4C C C

C

C

5

6

C1 2 3 4

C C C

C

C

C1 2 3 4

C C C

C

C

Hydrocarbon A

Hydrocarbon B

Hydrocarbon D Hydrocarbon E

Hydrocarbon C

The correct answer is A and E as they are both the same straight chain molecule All the other molecules are different branched isomers

It is much easier to identify isomers if you look at the carbon backbone only:



Number of Alkane Isomers

Name Formula Number of

carbon atoms

Number of

IsomersButane C4H10 4 2

Pentane C5H12 5 3

Hexane C6H14 6 5

Heptane C7H16 7 9Octane C8H18 8 18

Nonane C9H20 9 35

Decane C10H22 10 75

Dodecane C12H26 12 355 Pentadecane C15H32 15 4,347

Icosane C20H42 20 366,319

Triacontane C30H62 30 4,111,846,763



Isomers with different functional groupsIsomers exist for hydrocarbon compounds which have different

functional groups e.g. cyclopropane and propene are isomers

Cyclopropane

HH

CH

CH

H

C3H6

H

C

Name of alkane Full Structural formula Homologous series

C3H6C

H

H

HC H

H

CH

Propene

Cycloalkanes

Alkenes

Cyclopropane and propene are isomers with different functionalgroups as only propene has a carbon to carbon double bond

Same molecular formula



Key Websites

http://www.chemguide.co.uk/physical/catalysis/petrochem.html

1. Catalytic cracking:

http://en.wikipedia.org/wiki/Cracking_(chemistry)

http://science.howstuffworks.com/oil-refining5.htm

http://www.uyseg.org/catalysis/petrol/petrol2.htm

http://bbc.net.uk/schools/gcsebitesize/chemistry/usefulproductsoil/oil_and_oilprod

uctsrev7.shtml





http://bbc.net.uk/schools/gcsebitesize/chemistry/usefulproductsoil/oil_and_oilproductsrev7.shtml












Mr D. Shand

Work address:

Chemistry Department

Harris Academy

Dundee

DD5 4HE

Tel No. 01382 435700

2006

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Hydrocarbons2_Cracking_Isomers_Topic6_120206.ppt