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Part 2.
The chemistry of
ALKENES - unsaturated hydrocarbons
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
2.9
The uses of alkenes (an important
chemical feedstock) and the occurrence of the alkene functional group in 'biological' molecules
INDEX of ALKENE revision notes
All Advanced A Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Notes
Alkenes from the petrochemical oil industry
are rarely used directly, but they are important starting materials
to make lots of useful chemicals in the chemical and pharmaceutical
industries.
EXAMPLES
Direct use of
ethene itself
The only one I can think of is the
use of ethene gas to control the ripening of fruit.
It gets a mention in
Hormone control of plant growth and uses of plant hormones
(school biology notes)
Manufacture of addition polymers - poly(alkenes)
Already dealt with on three other
of my web pages.
Addition polymers - plastics e.g.
poly(ethene) and PVC, uses, problems, recycling (basic notes)
The
polymerisation of alkenes to
form addition polymers - structure, properties, uses of
poly(alkene) polymers (extra advanced notes)
Stereoregular polymers -
isotactic/atactic/syndiotactic poly(propene) (extra
advanced notes) Ethene is used
to make the monomer unit from which the addition polymer is made e.g.
Ethene is converted to phenylethene (styrene) to
make poly(phenylethene), polystyrene. Ethene is converted to
chloroethene (vinyl chloride) to make poly(chloroethene) PVC.
Chloroethene, the monomer for producing
poly(chloroethene), PVC, is made in two stages from ethene, which
originates from cracking oil fractions. In this case the haloalkane is
an intermediate compound.
ethene + chlorine == (1) =>
1,2-dichloroethane == (2) ==> chloroethene +
hydrogen chloride
H2C=CH2 + Cl2
===> ClH2CCH2Cl ===> H2C=CHCl
+ HCl
Stage (1) This addition
reaction catalysed by iron(III) chloride (FeCl3) and
is exothermic. Stage (2) Is
a thermal decomposition and elimination reaction, 500oC
and at high pressure (1.5 to 3.0 MPa (15 - 30 atm) and is a very
endothermic reaction. The
anhydrous hydrogen chloride formed can be used to make other
chloroalkanes or chloroalkenes, or dissolved in water to make
hydrochloric acid.
Manufacture of alcohols
See Part
2.6
The reaction of alkenes with steam synthesis of alcohols
Alcohols: including ethanol production and uses of alcohols
Ethanol as a fuels
Alcohols are used solvents and
manufacturing esters - the latter uses range from flavourings, odours
and drugs in the pharmaceutical industry.
Antifreeze
Anti-freeze liquid for motor vehicles contains
ethane-1,2-diol (ethylene glycol) which is made via a two
stage synthesis from ethene.
===>
===>
Stage 1: Ethene is mixed with air or oxygen and
passed over a catalyst (finely divided silver on an inert support such
as alumina) at 520-550 K and under 15-20 atmospheres pressure to make
epoxyethane
Note another example of a
transition metal catalyst, silver is in the 4d block below copper.
Stage 2: Epoxyethane is catalytically hydrolysed with
water to give ethane-1,2-diol
Alkenes or the alkene functional group in biological molecules.
The C=C double bond occurs in many naturally occurring molecules - many of
great use to us!
A few
EXAMPLES from thousands of organic molecules in living organisms
 Terpenes
Some
alkene hydrocarbons such as limonene smell good are used to give
pleasant aromas to such
things as candles and cleaning products.
Limonene
belongs to a group of unsaturated hydrocarbons called terpenes (terpenoids).
They are found
in plants producing natural oils with a variety of odours, some pleasant,
others not so pleasant.
One form of limonene can
be extracted from citrus fruits by steam distillation.
It has two
C=C double bonds and will add two bromine molecules per limonene
molecule.
It exhibits
R/S stereoisomerism - the bottom
carbon of the hexagon ring is chiral - four different groups attached to it.
Carotenes
Now this is what you call a 'real'
unsaturated hydrocarbon molecule with 11 C=C double bonds!
The middle 9 C=C bonds are
connected in the E E/Z stereoisomerism conformation (trans positions).
Beta-carotene is a
red-orange pigment found in plants and fruits, especially carrots and
colourful vegetables.
The name beta-carotene comes from the Greek “beta”
and Latin “carota” (carrot).
Beta-carotene is the yellow/orange pigment that gives
vegetables and fruits their rich colours.
The human body
converts beta-carotene into vitamin A (retinol), so beta carotene is a
precursor of vitamin A.
We all need vitamin A for healthy skin and mucus
membranes, our immune system, and good eye health and vision.
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Unsaturated oils
and fats
The term 'unsaturated' when applied to
oils and fats tells you the molecule contains at least one C=C double bond.
You can see the basic difference between
an unsaturated vegetable oil and a fully saturated fat, is the presence or
absence of C=C double bonds. Otherwise the general structure is the same, as
it is for all triglyceride esters.
Test for unsaturation in fats and oils
If you shake a vegetable oil or saturated animal fat
with bromine water or bromine in a solvent like hexane, the unsaturated vegetable oil will decolourise the
bromine water and a saturated fat will not.
For more details of this
reaction see section 2.4)
 Cholesterol
Chemically, cholesterol,
which contains the
alcohol group –OH, is classed as a sterol, a sub–group of organic molecules called
steroids.
Cholesterol is an essential steroid–sterol to humans but if
too much is produced or ingested in food, it can cause heart disease e.g.
hardening of arteries.
The image on the left gives the skeletal formula structure of cholesterol
Apart from the hydroxy group, you can also see the
alkene functional group too.
[Cholesterol
image from NIST]
The opiate drugs morphine, codeine and heroin
contain an alkene group to the bottom of the molecule.
Don't confuse with the presence of the benzene ring
(written in Kekule style) at the top of the molecule with the alkene
functional group.
What
next?
Index of all the ALKENE revision notes
All Advanced Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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BOX]
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