Chapter 30
Alkenes
30.1 Introduction
30.2 Nomenclature of Alkenes
30.3 Physical Properties of Alkenes
30.4 Preparation of Alkenes
30.5 Reactions of Alkenes
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New Way Chemistry for Hong Kong A-Level Book 3A
30.1 Introduction (SB p.122)
Functional group of alkenes:
The C = C double bond in ethene
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New Way Chemistry for Hong Kong A-Level Book 3A
30.1 Introduction (SB p.122)
Alkenes show geometrical isomerism
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New Way Chemistry for Hong Kong A-Level Book 3A
30.2 Nomenclature of Alkenes (SB p.123)
Nomenclature of Alkenes
1. Determine the stem name by selecting the longest
possible straight chain containing the C = C double bond
and use the ending ‘-ene’
2. Number the parent chain so as to include both carbon
atoms of the double bond, and begin numbering with the
end of the chain nearer the C = C double bond
3. Designate the position of the C = C double bond by
using the number of the first atom of the double bond
4. Designate the positions of the substituents by using the
numbers obtained by application of rule 2
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New Way Chemistry for Hong Kong A-Level Book 3A
30.2 Nomenclature of Alkenes (SB p.123)
Examples:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.2 Nomenclature of Alkenes (SB p.123)
5. If two identical groups are present on the same side of
the C = C double bond, the compound is designated as
cis; if they are on opposite sides, the compound is
designated as trans.
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.2 Nomenclature of Alkenes (SB p.124)
Example 30-1
Give the IUPAC names for the following alkenes:
(a)
(b)
Answer
Solution:
(a) trans-3,4-dichlorohept-3-ene
(b) cis-3,4-dimethyloct-3-ene
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30.2 Nomenclature of Alkenes (SB p.124)
Check Point 30-1
Draw the structural formula for each of the following
alkenes:
(a)
(a) cis-hex-3-ene
(b) trans-2,3-dihydroxybut-2-ene
(b)
(c) cis-1,2-dichloroethene
(c)
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Answer
30.3 Physical Properties of Alkenes (SB p.124)
Name
9
Formula
Boiling
point (°C)
Melting Density at
20 °C
point
(°C)
(g cm-3)
Ethene
CH2 = CH2
-104
-169
—
Propene
CH3CH = CH2
-47.7
-185
0.514
But-1-ene
CH3CH2CH = CH2
-6.3
-185
0.595
Pent-1-ene
CH3(CH2)2CH = CH2
30
-165
0.641
Hex-1-ene
CH3(CH2)2CH = CH2
62.9
-140
0.673
cis-But-2-ene
CH3CH = CHCH3 (cis)
4
-139
0.621
trans-But-2-ene
CH3CH = CHCH3
(trans)
1
-106
0.604
2-Methylbut-1ene
CH3CH3C(CH3) = CH2
31
-138
0.650
New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.125)
Cracking
• alkenes can be prepared industrially by cracking of high
molecular mass alkanes
600C
2CH 3CH 3  CH 2  CH 2  2CH 4
600C
2CH 3CH 2CH 3  CH 3CH  CH 2  CH 2  CH 2  CH 4  H 2
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New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.125)
Elimination Reactions
Dehydrohalogenation
• Dehydrohalogenation is the elimination of a hydrogen
halide molecule from a haloalkane
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30.4 Preparation of Alkenes (SB p.125)
Examples:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.126)
The ease of dehydrohalogenation of haloalkanes decreases
in the order:
tertiary
haloalkane
13
>
secondary
haloalkane
>
primary
haloalkane
New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.126)
Dehydrohalogenation of 2° and 3° haloalkanes can take place in
more than one way and a mixture of alkenes is formed
alc. KOH
CH3CH2CHClCH3  CH3CH = CHCH3 + CH3CH2CH = CH2
heat
2-chlorobutane
But-2-ene
But-1-ene
(80%)
(20%)
Note: the more highly substituted alkene is formed as major
product
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30.4 Preparation of Alkenes (SB p.127)
The relative stabilities of alkenes decrease in the order:
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30.4 Preparation of Alkenes (SB p.125)
Dehydration of Alcohols
Dehydration is the removal of a water molecule from a
reactant molecule
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New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.125)
The experimental conditions of dehydration depend on the
structures of alcohols
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.125)
The relative ease of dehydration of alcohols generally
decreases in the order:
tertiary
alcohol
>
secondary
alcohol
>
primary
alcohol
Like dehydrohalogenation, the more highly substituted
alkene is formed as the major product
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New Way Chemistry for Hong Kong A-Level Book 3A
30.4 Preparation of Alkenes (SB p.128)
Example 30-2
Classify the following alcohols as primary, secondary or tertiary
alcohols.
(a) CH3CHOHCH2CH3
(b) CH3CH2CH2OH
(c) (CH3)2COHCH2CH2CH3
Solution:
(a) Secondary alcohol
(b) Primary alcohol
(c) Tertiary alcohol
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Answer
30.4 Preparation of Alkenes (SB p.128)
Check Point 30-2
Classify the following haloalkanes as primary, secondary or
tertiary haloalkanes.
(a)
(b)
(c)
(a) Secondary haloalkane
(b) Primary haloalkane
(c) Tertiary haloalkane
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Answer
30.4 Preparation of Alkenes (SB p.129)
Addition Reactions
Hydrogenation
• hydrogenation of alkynes using Lindlar’s catalyst
produces alkenes
• prevent further hydrogenation of the alkenes formed to
alkanes
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.129)
Alkenes are more reactive than alkanes
Reason: presence of the C = C double bond
Energetically
favourable!!
 alkenes undergo addition reactions and the reactions
are exothermic
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.129)
Electrons of  bond are more diffuse and less firmly held
 susceptible to attack by electrophiles
Electrophiles such as H+, neutral reagents such as bromine
(can be polarized) react with C = C double bond
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.130)
Electrophilic Addition Reactions
Addition of Hydrogen Bromide
Addition of hydrogen bromide to C = C double bond
yields a bromoalkane
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.130)
Examples:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.131)
Propene reacts with HBr to give 2-bromopropane
(major product) and 1-bromopropane (minor product)
The formation of two possible products can be explained
by the reaction mechanism.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.131)
Reaction Mechanism: Electrophilic Addition
Reaction of Hydrogen Bromide to Alkenes
The mechanism for the addition of HBr to an alkene involves
2 steps
Step 1:
Step 2:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.131)
Theoretical Explanation of Markownikoff’s Rule
If the alkene is unsymmetrical, two different carbocations
can be formed
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30.5 Reactions of Alkenes (SB p.132)
2-bromopropane is the major product because the more stable
secondary carbocation is formed in the first step
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.132)
Markownikoff’s rule states that in the addition of HX to an
unsymmetrical alkene, the hydrogen atom adds to the carbon
atom of the carbon-carbon double bond that already has the
greater number of hydrogen atoms.
Example:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.132)
Markownikoff’s rule is related to the stability of the carbocation
intermediate formed in the electrophilic addition reaction.
The relative stabilities of carbocations:
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.133)
Addition of Bromine
Alkenes react rapidly with Br2 in 1,1,1-trichloroethane
at room temperature and in the absence of light
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.133)
The behaviour of alkenes towards Br2 in CH3CCl3 is a useful
test for the presence of carbon-carbon multiple bonds
Add Br2 in CH3CCl3
to excess alkene
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New Way Chemistry for Hong Kong A-Level Book 3A
The reddish brown
colour of Br2 is
decolourized
30.5 Reactions of Alkenes (SB p.133)
Addition of Bromine Water
In an aqueous solution of Br2, the following equilibrium exists
Br2 + H2O
HBr +
HOBr
Bromic(I) acid
The bromine atom bears a partial positive charge while
the oxygen atom bears a partial negative charge
∵ oxygen is more electronegative than bromine
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.134)
When bromic(I) acid reacts with alkenes, bromohydrin
is formed
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.134)
Addition of Sulphuric(VI) Acid
Alkenes react with cold and concentrated H2SO4 to
form alkyl hydrogensulphates
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.134)
The large bulky –OSO3H group makes the alkyl hydrogensulphate
very unstable. Two possible further reactions take place:
1. Regeneration of alkenes
2. Production of alcohols
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.135)
Catalytic Hydrogenation
In the presence of metal catalysts (e.g. Pt, Pd or Ni), H2 is
added to each atom of C = C double bond to form an alkane
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.135)
• Hydrogenation is useful in analyzing unsaturated
hydrocarbons
• The number of double or triple bonds present in the
unsaturated hydrocarbon molecule can be deduced
by the number of moles of hydrogen reacted
• Catalytic hydrogenation is
used to convert liquid
vegetable oil to semi-solid fats
in making margarine and
solid cooking fats (known as
hardening of oils).
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3
(a) What chemical tests would you use to distinguish
between two unlabelled bottles containing hexane and
hex-1-ene respectively?
Answer
(a) Either one of the following tests:
Hex-1-ene can decolourize bromine water or chlorine
water in the dark while hexane cannot.
Hex-1-ene can decolourize acidified potassium
manganate(VII) solution while hexane cannot.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3
(b) What is the major product of each of the following
reactions?
(i)
(b) (i)
(ii)
(ii)
Answer
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3
(c) Give the reaction products for the following reactions:
(i) CH3CH = CH2 + H2 
Ni
conc. H2SO4
(c) =(i)
CH3CH
CH3
(ii) CH3CH
CHCH
3 2
(iii) CH3CH =(ii)
CHCH3 + Br2 
(iii)
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Answer
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3
(d) Arrange the following carbocations in increasing order
(d) The increasing order of stability of carbocations is :
of stability.
Explain your answer briefly.
Answer
Tertiary carbocations are the most stable because
the three alkyl groups release electrons to the
positive carbon atom and thereby disperse its
charge. Primary carbocations are the least stable as
there is only one alkyl group releasing electrons to
the positive carbon atom.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3
(e) Upon the reaction with hydrogen chloride, it involves the
(e) Basedformation
on yourofanswer
in (d),
arrange
the following
carbocations.
Therefore,
the order
of reaction rates
molecules
order
increasing
rate of
ofcarbocations,
reaction i.e.
followsin
thethe
order
of theof
ease
of the formation
with hydrogen
chloride.
the stability of
carbocations:
Answer
Therefore, the rates of reactions of the three compounds
with hydrogen chloride increase in the order:
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30.5 Reactions of Alkenes (SB p.136)
Ozonolysis
Ozonolysis is a widely used method for locating the double
bond of an alkene
(unstable)
The unstable ozonide is reduced directly by treatment with Zn and
H2O
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.137)
Overall process of ozonolysis:
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.137)
Example 30-3
Predict the structures of the following hydrocarbons A, B and C
using the information given below:
Hydrocarbon
Molecular
formula
A
C3H6
B
C6H10
C
C10H16
Products after ozonolysis
Answer
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.138)
Solution:
A: As C3H6 can be expressed as CnH2n, the hydrocarbon is
a molecule with one C = C double bond. When A
undergoes ozonolysis,
formed.
and
∴The possible structure of A is CH3CH = CH2.
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New Way Chemistry for Hong Kong A-Level Book 3A
are
30.5 Reaction of Alkenes (SB p.138)
Solution:
B: As C6H10 can be expressed as CnH2n-2 and only one
dicarbonyl compound is formed on ozonolysis, the
hydrocarbon is an alicyclic molecule with one C = C
double bond.
∴ The possible structure of B is
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New Way Chemistry for Hong Kong A-Level Book 3A
.
30.5 Reaction of Alkenes (SB p.138)
Solution:
C: As C10H16 can be expressed as CnH2n-4. Two products with
totally five carbon atoms are formed. So the original
compound is an acyclic molecule with three C = C
double bonds.
∴ The possible structure of C is
CH3CH = CHCH2CH = CHCH2CH = CHCH3.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reaction of Alkenes (SB p.139)
Check Point 30-4
Draw the structures of the alkene molecules that give the
following products on ozonolysis.
(a) CH3CH2CH2CHO and CH3CHO
(b) CH3CH2(a)
CHO and CH3COCH3
Answer
(b)
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30.5 Reactions of Alkenes (SB p.139)
Polymerization
Polymers: Compounds that consist of very large molecules
made up of many repeating units
Monomer: Each repeating unit
Polymerization:The reaction by which monomers are joined
together
Addition polymerization: alkene monomers are joined together
without the elimination of small
molecules
Addition polymer: The polymer produced by addition
polymerization
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.139)
Poly(ethene)
Monomer: ethene
Depending on the conditions, two kinds of poly(ethene) are
formed
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.139)
Low density poly(ethene) (LDPE):
• Molecular mass: 50 000 to 3 000 000
• Light, flexible and low melting
temperature
• Uses: make soft items like wash bottles,
plastic bags and food wraps
High density poly(ethene) (HDPE):
• Molecular mass: up to 3 000 000
• Tougher and higher melting temperature
• Uses: make more rigid items like milk bottles and water
buckets
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30.5 Reactions of Alkenes (SB p.139)
Some products made of poly(ethene)
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.140)
Reaction Mechanism: Free Radical Addition
Polymerization of Ethene
1. Chain initiation
The diacyl peroxide molecule undergoes homolytic
bond fission to generate free radicals
The radical reacts with an ethene molecule to form a new
radical
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.140)
2. Chain propagation
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.140)
3. Chain termination
The radicals react to give a stable molecule and the
reaction stops.
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.141)
Poly(propene)
Properties: more rigid than HDPE, high
mechnical strength, strong
resistance to abrasion
Uses: make moulded furniture; make crates,
kitchenware, food containers; make
ropes and hard-wearing carpets
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New Way Chemistry for Hong Kong A-Level Book 3A
The helmet is made
of poly(propene)
30.5 Reactions of Alkenes (SB p.141)
Poly(phenylethene) (or Polystyrene)
Preparation of monomer (phenylethene):
Formation of poly(phenylethene):
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.142)
Poly(phenylethene):
Properties:
• Transparent, brittle and chemically inert
Uses:
• Make toys, specimen containers and cassette cases
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.142)
Expanded poly(phenylethene):
Properties:
• Extremely light, while solid foam
Uses:
• Make light-weight ceiling tiles
in buildings, food boxes and
shock absorbers for packaging
Some products made of
expanded poly(phenylethene)
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New Way Chemistry for Hong Kong A-Level Book 3A
30.5 Reactions of Alkenes (SB p.142)
Name and
structural formula
of monomer
Name and structural
formula of polymer
Poly(ethene)
Ethene
CH2 = CH2
Uses
LDPE: plastic bags, wash bottles, food wraps,
pipes and tubing
HDPE: tough bottles and jugs, buckets,
washing-up trays, toys, pipes and tubing
Poly(propene)
Moulded furniture, crates, kitchenware, food
containers, fibres for making ropes and hardwearing carpets and athletic wear
Propene
CH2 = CHCH3
Poly(phenylethene)
Phenylethene
CH2 = CHC6H5
63
Poly(phenylethene): moulded objects (combs,
toys, cups, brush and pot handles), refrigerator
parts and insulating materials
Expanded poly(phenylethene): good shock
absorbents in packaging, light-weight ceiling
tiles in building, disposable foam cups and
food boxes
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The END
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New Way Chemistry for Hong Kong A-Level Book 3A