© boardworks ltd 2001 ks4: useful products from organic sources organic chemistry

© Boardworks Ltd 2001

KS4: Useful Products from Organic Sources

ORGANIC CHEMISTRY


Crude Oil

Crude oil is a mixture; it contains hundreds of different compounds, some small and some extremely large. Nearly all these compounds contain carbon and hydrogen only and are called hydrocarbons.

Hydrocarbons are molecules that contain carbon and hydrogen only.

The compounds present in crude oil are separated, processed and purified in an oil refinery. These processes are covered in the following slides.


Fuel gas

Petrol

Naphtha

Kerosine

Diesel

Fuel Oil andbitumen

Composition of a typical crude oil

Crude oils from different parts of the world have different composition


Boiling range

20-70C 70-120C 120-170C

170-240C

Name Petrol Naphtha Kerosine Diesel

Colour Pale yellow Yellow Dark yellow

Light brown

Viscosity Very runny

runny Less runny

Less runny

Combustion Extremely flammable

Very flammable

Less flammable

Less flammable

It is difficult to obtain pure compounds from crude oil so instead, crude oil is separated intogroups of compounds called fractions. Each fraction contains compounds of similar boiling point. A description of each fraction is shown below:


Fractional distillation

Crude oil is split into fractions using fractional distillation.

Here crude oil is heated and pumped into the bottom of a tall tower containing trays. Compounds with low boiling points rise towards the top of the tower where they condense. Compounds with higher boiling point condense lower in the column. Thus each tray contains liquids (fractions) of different boiling point. The liquid on each tray is continuously pumped away.

A fractional distillation tower works continuously, unlike distillation in the laboratory.


Crude oil

preheater

Medium ones condense

Short ones condense

Small ones condense

Very small molecules do not condense and come out of the top

Very largemolecules do notvaporise and fallto the bottom

How fractional distillationworks

40°C

70°C

180°C

250°C

340°C

Long ones condense

Liq

uid

ou

t

Get t

ing

coole

r

© Boardworks Ltd 2001preheater

Crude oil

Fuel gas

Petrol

Naphtha

Kerosine

Diesel fuel

Fuel and lubricating oil

Bitumen

What fractional distillation produces

Fractional distillationseparates crude oil according to boiling point.

Use

s of

each

fra

ctio

n o

n n

ext

slid

e

Get t

ing

more

vola

t ile


Fuel gas

Petrol / gasoline

Naphtha

Paraffin / Kerosine

Diesel fuel


Bitumen

Burned in the refinery to fuel the distillation process, sold as LPG, purified and sold as bottled camping gas

Fuel for cars and motorcycles, also used to make chemicals.

Used to make chemicals.

Fuel for greenhouse heaters and jet engines, manufacture of chemicals.

Fuel for lorries, trains.

Fuel for the heating systems of large buildings, fuel for ships, lubricating oil.

Roofing, and road surfaces.

Uses of each fraction


Fuel gas

Petrol / gasoline

Naphtha

Paraffin / Kerosine

Diesel fuel


Bitumen

1-4, mostly 1.

5-10

8-12.

10-16

14-22

20-70

More than 65

Fraction Number of carbon atoms


Burning Hydrocarbons

The apparatus below is used to test the products of combustion of a hydrocarbon.

any hydrocarbon + oxygen water + carbon dioxide

Suctionpump

Candle wax is thehydrocarbon here

ice-water

Cloudiness-indicates carbon dioxideLiquid collected

can be tested withanhydrous cobalt chloridepaper (bluepink).


Alkanes

Carbon atoms are unusual in that they are able to form strong covalent bonds to each other. Therefore, carbon is able to form chains of atoms of different length and so make molecules of different size. There are many thousands of molecules containing C-C bonds. In this course you learn about two groups of these compounds, alkanes and alkenes.

Alkanes are hydrocarbons (molecules containing carbon and hydrogen only) in which each carbon atom is covalently bonded to four other atoms via 4 single covalent bonds. The other atoms can be either carbon or hydrogen.


Alkanes: Methane, CH4

C

H H

HH

C

H

H

H H

‘Dot and cross’ diagramDisplayed formulaor graphical formula

It is common in organic chemistry to show covalent bonds as single lines. This makes it easier to show how the atoms are connected.


C

H

HC

H

H

H C

C H

H

H

H

H

H

H

Alkanes: Ethane, C2H6

Ethane is the simplestalkane containing a C-C bond.


C

H

H

H C

H

H

H

C

H

H

H H

C

H

H

H C

H

H

C

H

H

H

methane, CH4

ethane, C2H6

propane, C3H8

Alkanes


butane, C4H10

pentane, C5H12

C

H

H

H C

H

H

C

H

H

C

H

H

H

C

H

H

H C

H

H

C

H

H

C

H

H

C

H

H

H


C

H

H

H C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

H

hexane, C6H14

and so on…………

Notice the carbon chain

is not straight


methane, CH4

ethane, C2H6

propane, C3H8

butane, C4H10

pentane, C5H12

hexane, C6H14....

Alkane series of molecules

General Formula, CnH2n+2

Since all these molecules contain only single covalent bonds, alkanes are called saturated.

This is called a homologous series


Alkane Formula MeltingPoint /C

BoilingPoint /C

State atroomtemperature

Methane CH4 -182 -164 gas

Ethane C2H6 -183 -89 gas

Propane C3H8 -188 -42 gas

Butane C4H10 -138 0 gas

Pentane C5H12 -130 36 liquid

Hexane C6H14 -95 69 liquid

Incr

easi

ng m

elt

ing

an

d

boili

ng

poin

t.

Physical Properties of the Alkanes

The alkanes show a gradual change in melting and boiling points.


CrackingThe demand for some hydrocarbons is high and cannot be supplied from fractional distillation alone. To boost the amount of some smaller hydrocarbons produced some of the high boiling point fractions are converted to more useful hydrocarbons. This process is called cracking.

Cracking involves breaking C-C bonds.

catalyst

smaller alkane

ethene

Heat tovaporise

Long alkane

Distillationtower

pressure


Cracking

On the surface of the catalyst, long chain molecules are split apart or ‘cracked’. The reaction is:

C CH

H

H

H H

C C C C C C

H

H

H

H

H

H

H

H

H

H

H

H

H

C CH

H

H

H H

C C C C

H

H

H

H

H

H

H

H

H C CH

H

H

H+

C8H18 C6H14 + C2H4

ethene

Octane

hexane

Etheneis usedto makeplastics

Heat pressure

catalyst


CHEMICAL REACTIONS OF ALKANES

Alkanes are generally unreactive. This is because they only contain C-H and C-C bonds.

However, the lower members of the series readily combust in air. With plenty of air, the products are water and carbon dioxide only.

Methane + oxygen water + carbon dioxide

CH4 + 2O2 2H2O + CO2

Combustion with limited amounts of air, however, producescarbon monoxide and water. Carbon monoxide is a poisonous gas.


Isomerism

Alkanes of the same molecular formula can have different arrangements of atoms

Extension

C C C C

H

H

H

H

H

H

H

H

H

H C C C

H

H

H

H

H

H

H

C H

H

H

These molecules are isomers of butane


Alkenes

C

H

H

C

H

H

Ethene is the simplestalkene.

C

H

H

C

H

H

C2H4

Carbon atoms can also form double covalent bonds to each other. Molecules with a double bond are called alkenes.

C CH

H

H

Hor


Alkenes

Alkenes have the general formula

CnH2n

Alkenes are unsaturated because they contain a double C=C bond. In all alkenes at least two carbon atoms are joined to only 3 atoms.


Other alkenes

C CC

H

H

H

H

H

HPropene, C3H6

C CH

H H

CCH

H

H

H

HButene, C4H8


CHEMICAL REACTIONS OF ALKENES

The double bond allows alkenes to undergo reactions where atoms are added to the molecule. Thus, alkenes are more reactive than alkanes.

C CH

H

H

H+ Br2

C C

BrBr

H

HH

H

An addition reaction:

If ethene is bubbled through bromine water the solution becomes colourless - this is used as the test for an alkene. Explanation: bromine colours the solution orange, however, this is used up in the reaction and the product is colourless.


Pressurehightemperature

catalyst

Polymerisation

Alkenes can undergo addition reactions to other alkene molecules. The result is very long chains of carbon atoms called polymers.

C CH

H

H

HC C

HH

H H

C C C C

H

H

H

H

H

H

H

H

This is written as:

Andlotsmore..

Pressure

high temperaturecatalyst

C CH

H

H

HC C

HH

H H

n

n

This is called addition polymerisation

ethene

poly(ethene)


Examples of addition polymers

Poly(propene)C CHH

H CH3

n

npropene

n

tetrafluoroethane Poly(tetrafluoroethane)or PTFE

n

C C

F F

F F

C CCH

H H

H

H

H

C CF

F

F

F


Three very useful plastics

Poly(ethene) Poly(propene) Poly(styrene)

Shopping bagsBottlesBucketsWashing-up bowls

Milk cratesRopeFibres to makecarpets

BoxesFoam packing –insulation and toprotect f romshock.


Poly(ethene) Bottles(HDPE)

Poly(propene) boxes(PP)


REVIEW QUESTIONS


1. i) How is crude oil split into fractionsii) Name the seven fractions produced by fractionaldistillation of crude oil.iii) Give three differences between fractions.

2. i) What is meant by cracking?ii) Complete the formula below:

C8H18 + C2H4

iii) The starting hydrocarbon is octane, name the two products of this reaction.iv) Describe how you could test for the presence of ethene.


3. i) Draw the bonding diagram for methane (show only outer shell electrons, carbon has an atomic number of 6).ii) Draw the graphical formula for methane.iii) Methane is saturated, explain what this means.iv) Draw the graphical formula for another alkane containing five carbon atoms.

4. Propane is used as ‘Calor Gas’. Complete the word and equation for its combustion in the plenty of airPropane + Oxygen _________ + _________

C3H8 + O2 _____ + ___

Why is it necessary to combust hydrocarbons in plenty ofair?


5.

i) In a homologous series the physical properties change gradually in the same direction. Plot a graph of boiling point against number of carbons for the alkanes. Use this graph to predict the boiling point of decane, C10H22.

6.

i) Write down the graphical formula for propene.ii) Write down the equation for the reaction of propenewith bromine.iii) Predict the reaction of propene with hydrogen and write down the equation. Iv) What type of reaction is this?


7.

Using the polymerisation of ethene as an example, explain the term addition polymerisation.


Harder Questions

8. Draw graphical formula for the three isomers of pentane.

9. Draw graphical formula for three isomers of butene.

10. Write down the equation for the combustion ofpropene in plenty of air and with a restricted amount of air.


ANSWERS TO REVIEW

QUESTIONS


1. i) Crude oil is split into ‘fractions’ of different boiling point by fractional distillation. Here crude oil is vaporised and passed into a fractionating tower. Towards the top of the tower, where it is cooler, compounds of low boiling point condense and are collected on trays from which they are removed. Lower in the tower, higher boiling point fractions are collected. ii) Refinery gas / Petrol or gasoline / Naphtha / Kerosine or paraffin / Diesel / Oil / Bitumeniii) boiling point / viscosity / colour / size of hydrocarbon molecule


2. i) Cracking is the process used to convert long hydrocarbon molecules into smaller ones. The hydrocarbon liquid is vaporised and passed over a catalyst. On the surface of the catalyst the carbon chain is split to produce ethene and a smaller alkane.ii)

C8H18 C6H14 + C2H4

iii) C6H14 is hexane, C2H4 is ethene

iv) You could pass the gas through bromine water. The bromine water, which is orange, will become colourless.


C

H H

HH

C

H

H

H H

3i) ii)

iii) Methane is saturated because it contains no double bonds.

iv)C

H

H

H C

H

H

C

H

H

C

H

H

C

H

H

H Pentane, C5H12


Propane + Oxygen carbon dioxide + water

C3H8 + 5O2 3CO2 + 4H2O

4.

In insufficient air/oxygen, carbon monoxide and waterare produced. Carbon monoxide is a poisonous gas.


Boiling Point Trend in the alkanes

-200-150-100-50

050

100150200

0 1 2 3 4 5 6 7 8 9 10 11

number of carbon atoms

Boili

ng P

oint

/ C

174°C

5.


C3H6 + Br2 C3H6Br2

C CCH

H H

H

H

H

+ Br2 C C C

Br

H

H H

H

H

H

Br6. propene

C CCH

H H

H

H

H

+ H2 C C CH

H H

H

H

H

H Hpropane

i and ii)

iii)

iv) Addition reaction


Pressure

high temperaturecatalyst

C CH

H

H

HC C

HH

H H

n

n

This is called addition polymerisation because the molecules of ethene add together without producingany other products.

ethene

poly(ethene)

7.


C C CH

H H

H

H

H H

C C

H H

H

HH

C C CH

H H

HH H

C

H

H

C HH

H

H

C CH

H

C

H

H

C HH

H

HH

C HH

H

8. Isomers of pentane


C CC

C

H

H H

HH

H

H

H

C CC

CH

H

H

H

H

H

H

H

C CC

C

H

H

H

H

HH

HH

C CC

H

H

H

H

CH

H

H

H

9. Isomers of butene


10.

(i) 2C3H6 + 9O2 6CO2 + 6H2O

(ii) C3H6 + 3O2 3CO + 3H2O

© boardworks ltd 2001 ks4: useful products from organic sources organic chemistry

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