The chemistry of
ALKENES - unsaturated hydrocarbons
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Index of GCSE level Oil - Useful Products
Chemistry Revision Notes
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The uses of alkenes (an important
chemical feedstock) and the occurrence of the alkene functional group in 'biological' molecules
and their uses
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
EXAMPLES of the industrial
importance of alkenes
industrial manufacturing processes, a direct use of
ethene gas 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, manufacture and uses
with using polymers, recycling, biodegradable plastics
polymerisation of alkenes to
form addition polymers - structure, properties, uses of
poly(alkene) polymers (extra notes for advanced level
Stereoregular polymers -
isotactic/atactic/syndiotactic poly(propene) (extra
Alkenes are used
as the monomer unit from which addition polymers are manufactured e.g.
Ethene makes poly(ethene).
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 +
H2C=CH2 + Cl2
===> ClH2CCH2Cl ===> H2C=CHCl
Stage (1) This addition
reaction catalysed by iron(III) chloride (FeCl3) and
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
anhydrous hydrogen chloride formed can be used to make other
chloroalkanes or chloroalkenes, or dissolved in water to make
Ethane-1,2-diol (see below - antifreeze
manufacture) is also used to manufacture Terylene, a
polyester and a condensation polymer.
For details see
The manufacture, molecular structure, properties and uses of
Manufacture of alcohols
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.
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
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
Ethane-1,2-diol is also used to manufacture
Terylene, a polyester and a condensation polymer.
For details see
The manufacture, molecular structure, properties and uses of
Many chloroalkanes are used as
e.g. 1,2-dichloroethane is used as a
degreaser and paint remover.
Other miscellaneous examples of the uses of
Ethene is the starting molecule for
the manufacture of ethanoic acid, used in vinegar and many
organic syntheses e.g. making ethanoate esters.
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!
EXAMPLES from thousands of organic molecules in living organisms
alkene hydrocarbons such as limonene smell good are used to give
pleasant aromas to such
things as candles and cleaning products.
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
R/S stereoisomerism - the bottom
carbon of the hexagon ring is chiral - four different groups attached to it.
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
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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and fats - hydrogenation and margarine
The term 'unsaturated' when applied to
oils and fats tells you the molecule contains at least one C=C double bond.
Unsaturated vegetable oils and more saturated fats (e.g.
butter or lard) are important sources of energy in our diet.
In the two diagrams above, you can see the basic difference between
an unsaturated vegetable oil and a saturated fat molecule, due to the presence or
absence of C=C double bonds.
They illustrate the general structure
for all naturally occurring triglyceride esters.
The manufacture of margarine
Margarine is manufactured as a butter
substitute and is made from animal or vegetable fats mixed with skimmed
milk and salt.
Animal fats and vegetable oils a
re primarily made up of triglyceride molecules illustrated above
with different degrees of unsaturation due to the presence one or
more C=C double bonds in the long chains of the glycerol ester
Margarine has become more popular
because it is supposed to be more healthy than the animal fats in butter
which is much higher in saturated fats.
Margarine has high content of
mono and polyunsaturated fats which are supposed to be more healthy
than saturated fats, and they are often obtained from olive,
rapeseed and sunflower oils
However, unsaturated vegetable oils
are liquid at room temperature and not very spreadable on foods such as
and a nickel catalyst, adding hydrogen across a C=C double bond,
unsaturated oils/fats can be converted to more saturated molecules that
are soft and spreadable solid at room temperature.
The hydrogenation produces more saturated
molecules between which the intermolecular forces are slightly increased
- sufficient to increase the softening point by a few degrees.
By varying the degree of hydrogenation, varying the
number of C=C bonds remaining, you can produce a range of margarines
with different softening points - the point at which the margarine
hardens and solidifies.
However, in the process of hydrogenation, some of the cis/Z linkages are
'isomerised' into trans/E linkages as a by-product and 'trans-fats' are
supposed to be less healthy in our diet (see the first triglyceride
For more on oils, fats and hydrogenation see
alkenes section 2.5
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)
which contains the
alcohol group –OH, is classed as a sterol, a sub–group of organic molecules called
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
and shows it to contain one C=C alkene double bond as well as the hydroxy group.
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
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