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Molecular and empirical formulae
There are many ways of representing organic compounds
by using different formulae.
The molecular formula of a
compound shows the number
of each type of atom present in
one molecule of the compound.
The empirical formula of a
compound shows the simplest
ratio of the atoms present.
Molecular
formula
C2H6
Empirical
formula
CH3
C6H12O6
CH2O
C2H4O2
CH2O
Neither the molecular nor empirical formula gives information
about the structure of a molecule.
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Displayed formula of organic compounds
The displayed formula of a compound shows the
arrangement of atoms in a molecule, as well as all the bonds.
Single bonds are represented
by a single line, double
bonds with two lines and
triple bonds by three lines.
The displayed formula can show the different structures of
compounds with the same molecular formulae.
ethanol (C2H6O)
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methoxymethane (C2H6O)
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Structural formula of organic compounds
The structural formula of a compound shows how the
atoms are arranged in a molecule and, in particular, shows
which functional groups are present.
Unlike displayed formulae, structural formulae do not show
single bonds, although double/triple bonds may be shown.
CH3CHClCH3
H2C=CH2
CH3C≡N
2-chloropropane
ethene
ethanenitrile
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Displayed and structural formula activity
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Skeletal formula of organic compounds
The skeletal formula of a
compound shows the
bonds between carbon
atoms, but not the atoms
themselves. Hydrogen
atoms are also omitted, but
other atoms are shown.
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Types of formulae
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Functional groups and homologous series
A functional group is an atom or group of atoms
responsible for the typical chemical reactions of a molecule.
A homologous series is a group of molecules with the same
functional group but a different number of –CH2 groups.
methanoic acid
(HCOOH)
ethanoic acid
(CH3COOH)
propanoic acid
(CH3CH2COOH)
Functional groups determine the pattern of reactivity of a
homologous series, whereas the carbon chain length
determines physical properties such as melting/boiling points.
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Functional groups
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Homologous series and general formulae
The general formula of a homologous series can be used
to calculate the molecular formula of any member of the
series by substituting n for the number of carbon atoms.
For example, the general formula of a halogenoalkane is
CnH2n+1X, where X is a halogen.
Example: what is the molecular formula of chloroethane?
1. Write down the general formula:
CnH2n+1X
2. Write down the value of n:
n=2
3. Substitute n into the general formula:
C2H5Cl
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Homologous series
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What is isomerism?
Isomers are molecules with the same molecular formula
(i.e. the same number and type of atoms) but in which the
atoms are arranged in a different way.
There are two main categories of isomerism: structural
isomerism and stereoisomerism.

Structural isomers have different structural formulae.
Three types of structural isomerism are chain isomerism,
positional isomerism and functional group isomerism.

Stereoisomers have the same structural formula, but
the 3D arrangement of atoms is different. Two types
are cis–trans isomerism and optical isomerism.
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Chain isomerism in alkanes
In chain isomers, the carbon chain is arranged differently.
For example, hexane has several chain isomers, all with the
molecular formula C6H14:
hexane
2,3-dimethylbutane
3-methylpentane
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Positional isomerism
In positional isomers, the functional group is attached to
a different carbon atom.
For example,
chloropentane has
several positional
isomers, all with
the molecular
formula C5H11Cl:
1-chloropentane
2-chloropentane
3-chloropentane
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Positional isomerism in alkenes
Positional isomerism also exists in alkenes with four or more
carbon atoms.
hex-1-ene
For example,
hexene has
several positional
isomers, all with
the molecular
formula C6H12:
hex-2-ene
hex-3-ene
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Functional group isomerism
Functional group isomers contain different functional groups
and so are members of different homologous series.
For example, both alcohols and ethers have the general
formula CnH2n+2O so they may be functional group isomers:
propanol (C3H8O)
an alcohol
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methoxyethane (C3H8O)
an ether
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Structural isomers activity
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Rotation around the C=C bond in alkenes
Molecules can rotate freely around single C-C covalent
bonds, but not around C=C double bonds.
This leads to type of stereoisomerism called cis–trans
isomerism, in which isomers differ in the arrangement of
the groups attached to the carbons in the double bonds.
is not the
same as
These isomers cannot be superimposed on each other
because the arrangement of the methyl groups is different.
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Cis–trans isomerism
If an alkyl group or atom other than hydrogen is attached
to each carbon then the isomers can be named either
cis (‘on the same side’) or trans (‘on the opposite side’).
cis-but-2-ene
cis-1,2-dichloroethene
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trans-but-2-ene
trans-1,2-dichloroethene
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Limitations of cis–trans isomerism
In more complex organic compounds, in which multiple
hydrogens have been substituted by different groups,
isomers cannot be defined using the cis–trans notation.
For example, is it possible to identify which of these
halogenoalkanes is the cis isomer and which is the
trans isomer?
Instead, a different system is used for these type of
molecules: E–Z notation.
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E–Z isomerism
The E–Z notation is used to identify stereoisomers that
cannot be called cis or trans.
Isomers are identified as either E or Z depending on what
‘priority’ is given to the groups attached to the carbon
atoms in the double bond. The priority of these groups is
determined by a complex series of rules.

E represents the German word ‘entgegen’, and
corresponds to trans isomers. The highest priority
groups are on the opposite side of the double bond.

Z represents the German word ‘zusammen’, and
corresponds to cis isomers. The highest priority
groups are on the same side of the double bond.
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Optical isomerism
Another form of stereoisomerism is optical isomerism, in
which a molecule can exist as two isomers that are nonsuperimposable, mirror images of each other, just like a left
hand and right hand.
optical isomers of the
amino acid alanine
Optical isomers have the same physical properties, but they
rotate polarized light in opposite directions.
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Stereoisomerism: true or false?
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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What are alkanes?
The alkanes are a homologous series of hydrocarbons
with the general formula CnH2n+2 and names ending –ane.
Alkanes contain only single carbon–carbon bonds and so
are saturated.
No. of
carbon atoms
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Molecular
formula
Name
1
CH4
methane
2
C2H6
ethane
3
C3H8
propane
4
C4H10
butane
5
C5H12
pentane
6
C6H14
hexane
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Alkanes and isomersim
Alkanes with four or more carbon atoms display structural
isomerism because the carbon chain may be either
straight or branched.
pentane:
straight chain
2-methylbutane:
branched chain
The naming of alkanes depends on whether they are
straight or branched.
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Naming branched chain alkanes
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Naming the alkanes activity
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Trends in boiling points
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Trends in boiling points
The boiling point of straight-chain alkanes increases with
chain length due to increasing van der Waals forces
between molecules.
As the length of the chain increases,
so does its surface area, and so the
van der Waals forces are stronger.
Branched-chain alkanes have lower
boiling points because the chains
cannot pack as closely together.
There are fewer points of contact
between molecules so the van der
Waals forces are weaker.
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Crude oil and alkanes
Crude oil is a mixture composed mainly of straight and
branched chain alkanes.
It also includes lesser amounts of cycloalkanes and arenes,
both of which are hydrocarbons containing a ring of carbon
atoms, as well as impurities such as sulfur compounds.
The exact composition of
crude oil depends on the
conditions under which it
formed, so crude oil
extracted at different
locations has different
compositions.
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Fractional distillation
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Uses of fractions
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Fractions and boiling point
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Supply and demand
The demand for lower boiling
point (shorter chain) fractions is
greater than the proportion
found in crude oil.
Crude oil contains more higher
boiling point (longer chain)
fractions, which are in lower
demand and are less
economically valuable.
There is therefore a shortage of
shorter chain fractions and a
surplus of longer chain ones.
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What is cracking?
Cracking is a process that splits long chain alkanes into
shorter chain alkanes, alkenes and hydrogen.
C10H22 → C7H16 + C3H6
Cracking has the following uses:

it increases the amount of gasoline and other
economically important fractions

it increases branching in chains, an important factor
for petrol

it produces alkenes, an important feedstock for chemicals.
There are two main types of cracking: thermal and catalytic.
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Thermal cracking
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Catalytic cracking
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Thermal vs. catalytic cracking
Catalytic cracking has several advantages over thermal
cracking:

it produces a higher proportion of branched alkanes, which
burn more easily than straight-chain alkanes and are
therefore an important component of petrol

the use of a lower temperature and pressure mean it is
cheaper

it produces a higher proportion of arenes, which are
valuable feedstock chemicals.
However, unlike thermal cracking, catalytic cracking
cannot be used on all fractions, such as bitumen, the
supply of which outstrips its demand.
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Other products from cracking
Alkenes such as ethene are always produced in cracking.
They are an important feedstock for use in the chemical
industry, particularly in the production of plastics.
Arenes such as benzene
are also produced during
catalytic cracking.
Benzene is added in small
quantities to petrol as a
replacement for the lead
compounds. It too is now
the subject of health
concerns, and its use is
being reduced.
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Cracking: true or false?
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Complete combustion
In excess oxygen, short chain alkanes can undergo
complete combustion:
alkane + oxygen → carbon dioxide + water
For example:
propane + oxygen → carbon dioxide + water
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
The combustion of alkanes is a highly exothermic process.
This makes them good fuels because they release a
relatively large amount of energy per gram of fuel.
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Incomplete combustion
If oxygen is limited then incomplete combustion will occur:
alkane + oxygen → carbon monoxide + water
alkane + oxygen → carbon + water
For example:
propane + oxygen → carbon monoxide + water
C3H8(g) + 3½O2(g) → 3CO(g) + 4H2O(g)
propane + oxygen → carbon + water
C3H8(g) + 2O2(g) → 3C(s) + 4H2O(g)
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The internal combustion engine: carbon
Alkanes with chain lengths of 5–10 carbon atoms are used
as fuels in internal combustion engines.
This releases carbon dioxide into the atmosphere:
nonane + oxygen → carbon dioxide + water
C9H20(g) + 14O2(g) → 9CO2(g) + 10H2O(g)
Although modern internal combustion engines are more
efficient than in the past, incomplete combustion still occurs:
nonane + oxygen → carbon monoxide + water
2C9H20(g) + 19O2(g) → 18CO(g) + 20H2O(g)
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The internal combustion engine: nitrogen
The temperature in an internal combustion engine can
reach over 2000 °C. Here, nitrogen and oxygen, which at
normal temperatures don’t react, combine to form
nitrogen monoxide:
N2(g) + O2(g) → 2NO(g)
Nitrogen monoxide reacts further forming nitrogen dioxide:
2NO(g) + O2(g) → 2NO2(g)
Nitrogen dioxide gas reacts with rain water and more oxygen
to form nitric acid, which contributes to acid rain:
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
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The catalytic converter
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Sulfur contamination of fossil fuels
Sulfur is found as an impurity in crude oil and other fossil
fuels. It burns in oxygen to form sulfur dioxide:
S(s) + O2(g) → SO2(g)
Sulfur dioxide may be oxidized to sulfur trioxide:
2SO2(g) + O2(g) → 2SO3(g)
Both of these oxides dissolve in water forming acidic
solutions:
SO2(g) + H2O(l) → H2SO3(aq)
SO3(g) + H2O(l) → H2SO4(aq)
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What is acid rain?
Acid rain is caused by acidic non-metal oxides such as
sulfur oxides and nitrogen oxides dissolving in rain water.
Rain water is naturally
acidic because carbon
dioxide dissolves in it,
forming weak carbonic
acid. However, sulfur
and nitrogen oxides
form more acidic
solutions, which can
damage trees and
affect aquatic life in
lakes and rivers.
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Removing sulfur dioxide pollution
Sulfur dioxide emissions from vehicle fuels such as petrol
and diesel are reduced by removing nearly all of the sulfur
impurities from the fuel before it is burnt.
Removing the sulfur from coal before it is burnt is not
practical. Instead, the acidic sulfur oxides are removed
from the waste gases using a base such as calcium oxide.
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Carbon dioxide in the atmosphere
Burning fossil fuels releases carbon dioxide into the
atmosphere.
Fossil fuels are being
burned faster than they
are being formed, which
means that their
combustion leads to a
net increase in the
amount of atmospheric
carbon dioxide.
It has been suggested that increases in the amount of
carbon dioxide and other greenhouse gases may be
responsible for apparent changes to the climate.
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Greenhouse gases
Carbon dioxide, water vapour and
methane have been described as the
main greenhouse gases.
This is because these have been
suggested as the gases responsible
for the majority of the greenhouse
effect.
The greenhouse effect is a theory that has been suggested
to explain apparent rises in the average temperature of
the Earth.
Increasing the amount of any of the greenhouse gases traps
more heat energy from the Sun in the Earth’s atmosphere,
raising the average temperature.
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Pollutant gases
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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What are alkenes?
The alkenes are a homologous series of hydrocarbons
with the general formula CnH2n and names ending –ene.
Alkenes contain a carbon – carbon double bond and so are
unsaturated.
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No. of
carbon atoms
Molecular
formula
2
C2H4
ethene
3
C3H6
propene
4
C4H8
butene
5
C5H10
pentene
6
C6H12
hexene
Name
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Naming alkenes
Alkenes with four or more carbon atoms display positional
isomerism because the carbon–carbon double bond may
appear between different carbon atoms.
If there are two or more possible positions for the double
bond, a number is used before the –ene to indicate the first
carbon involved.
but-1-ene
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but-2-ene
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Structure of alkenes
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Bonding in alkenes
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Isomerism in alkenes
Rotation around the double bond in alkenes is restricted
by the presence of the  bond. This leads to E–Z
stereoisomerism in some alkenes.
E-pent-2-ene
Z-pent-2-ene
The two E-Z stereoisomers have the same structure; the
only difference between them is the arrangement of the
atoms in space.
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Structure of alkenes summary
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Double bonds and electrophiles
The double bond of an alkene is an area of high electron
density, and therefore an area of high negative charge.
The negative charge of the double bond may be attacked
by electron-deficient species, which will accept a lone pair
of electrons.
These species have either a full positive charge or slight
positive charge on one or more of their atoms. They are
called electrophiles, meaning ‘electron loving’.
Alkenes undergo addition reactions when attacked by
electrophiles. This is called electrophilic addition.
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Electrophilic addition mechanism: 1
In the first stage of electrophilic addition,
the positive charge on the electrophile is
attracted to the electron density in the
double bond.
δ+

As the electrophile approaches the
double bond, electrons in the A–B
bond are repelled towards B.
δ-
The pi bond breaks, and A bonds to the
carbon, forming a carbocation – an ion
with a positively-charge carbon atom.
The two electrons in the A–B bond
move to B forming a B- ion.
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Electrophilic addition mechanism: 2
In the second stage of electrophilic
addition, the B- ion acts as a nucleophile
and attacks the carbocation.

The lone pair of electrons on the B- ion
are attracted towards the positivelycharged carbon in the carbocation,
causing B to bond to it.
Because both electrons in the bond that joins B- to the
carbocation ion come from B-, the bond is a co-ordinate bond.
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What is hydrogenation?
Hydrogen can be added to the carbon–carbon double bond
using a nickel catalyst in a process called hydrogenation.
C2H4 + H2  C2H6
Vegetable oils are unsaturated
and may be hydrogenated to
make margarine, which has a
higher melting point.
As well as a nickel catalyst,
this requires a temperature
of 200 °C and a pressure of
1000 kPa.
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Testing for alkenes
The presence of
unsaturation (a carbon–
carbon double bond) can
be detected using
bromine water, a
red/orange coloured
solution of bromine.
A few drops of bromine water are added to the test liquid
and shaken. If a carbon–carbon double bond is present, the
bromine adds across it and the solution turns colourless.
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More on the bromine water test
A simple equation for the bromine water test with ethene is:
CH2=CH2 + Br2 + H2O  CH2BrCH2Br + H2O
However, because water is present in such a large
amount, a water molecule (which has a lone pair) adds to
one of the carbon atoms, followed by the loss of a H+ ion.
CH2=CH2 + Br2 + H2O  CH2BrCH2OH + HBr
The major product of the test is not 1,2-dibromoethane
(CH2BrCH2Br) but 2-bromoethan-1-ol (CH2BrCH2OH).
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Electrophilic addition reactions
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Addition to unsymmetrical alkenes
When an electrophile
(e.g. HBr) attacks an
alkene with three or
more carbon atoms
(e.g. propene), a mix
of products is
formed. This is
because these
alkenes are
unsymmetrical.
minor product:
1-bromopropane
HBr
major product:
2-bromopropane
Unequal amounts of each product are formed due to the
relative stabilities of the carbocation intermediates.
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Structure of carbocations
A chain of carbon atoms can be represented by R when
drawing organic structures. This is an alkyl group (general
formula CnH2n+1).

Primary (1°) carbocations have
one alkyl group attached to the
positively-charged carbon.

Secondary (2°) carbocations
have two alkyl groups attached
to the positively-charged carbon.

Tertiary (3°) carbocations have
three alkyl groups attached to
the positively-charged carbon.
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Stability of carbocations
The stability of carbocations increases as the number of alkyl
groups on the positively-charged carbon atom increases.
primary
secondary
tertiary
increasing stability
The stability increases because alkyl groups contain a
greater electron density than hydrogen atoms. This
density is attracted towards, and reduces, the positive
charge on the carbon atom, which has a stabilizing effect.
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Structure of carbocations
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Electrophiles: true or false?
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Polyalkenes
Alkenes can undergo addition reactions with themselves
to form a long chain polymer molecule. This reaction is
addition polymerization.
The polymer can be represented by showing the repeating
unit with square brackets around it. The n stands for a
unspecified number of monomer units.
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Polymerization of ethene
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LDPE and HDPE
The reaction conditions under which ethene polymerizes
affect the structure and properties of the poly(e)thene.

Low-density polythene (LDPE) is formed under a high
pressure (1400 atm) and a temperature of about 170 °C.
These conditions cause a high level of branching,
meaning that the polymer chains cannot pack
tightly together.

High-density polythene (HDPE) is formed with a catalyst,
a pressure of 2 atm and a temperature of about 70 °C.
Little branching occurs under these conditions,
resulting in chains that can pack tightly together to
create a denser material.
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Other polyalkenes
Propene undergoes addition polymerization to form
polyproprene:
Chloroethene undergoes addition polymerization to form
polychloroethene:
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Which alkene?
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More about LDPE
LDPE is a soft, flexible and stretchy plastic, with a melting
point of about 120 °C. It is used to make:

plastic bags

squeezable bottles, and
general purpose
containers and trays

other items that need to
be soft and flexible, such
as tubing.
LDPE has the recycling symbol ‘4’.
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More about HDPE
HDPE is a tough and flexible plastic, with a melting point of
about 130 °C. It is used to make:

containers such as milk and
detergent bottles

rigid items such as folding
tables, chairs and pipes.
HDPE has the recycling symbol ‘2’.
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More about polypropene
Polypropene is a tough and flexible plastic, with a melting
point of about 160 °C. It is used to make:

ropes, carpets, rugs and other
textiles

medical, laboratory and
kitchen items that need to
withstand temperatures in
autoclaves and dishwashers

certain bottles, buckets, containers
and other items such as bottle
tops and moulded fittings.
Polypropene has the recycling symbol ‘5’.
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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