ALKENES
Alkenes area class of HYDROCARBONS which contain only carbon
and hydrogen. Two other terms which describe alkenes are unsaturated
and olefins.
UNSATURATED hydrocarbons contain either double or triple bonds.
Since the compound is unsaturated with respect to hydrogen atoms, the
extra electrons are shared between 2 carbon atoms forming double bonds
in alkenes.
Alkenes are also called OLEFINS because they form oily liquids on
reaction with chlorine gas.
The example compounds of ethene or ethylene and pentene are shown on
the LEFT.
Ethylene is the number one organic chemical synthesized in the U. S. and
the world. The small quantities of ethane, propane, and butane found in
natural gas are converted into ethene. It can be produced by thermal
cracking of ethane to produce ethene and a hydrogen molecule.
Alkenes are the raw materials for a number of plastics such as
polyethylene, PVC, polypropylene, and polystyrene.
Alkene chemistry is found in unsaturated fats, beta-carotene, and seeing
light through vision.
Structure
CnH2n
Example: CH2CH2
Properties
Physical Properties
Boiling points depend
on chain length,
slightly less than
alkanes.
Non polar
Insoluble in water
Less dense than water
Chemical Reactivity
Alkenes are quite
reactive because of the
presence of the double
bond. Many small
compounds react by
addition i.e. molecules
add to the alkene to
form one product.
All compounds:
Combustion Reaction
ALKENE NAMES
Root names give the number of carbons in the longest continuous
chain. Alkene names are formed by dropping the "ane" and
replacing it with "ene"
The following list gives samples:
Example: root = propane - drop "ane" = "prop"
alkene = "prop" + alkene ending = "ene" = propene
No. of
Root Name
Carbons
2
ethene
3
propene
4
1-butene
5
1-pentene
Formula
CnH2n
C2H4
C3H6
C4H8
C5H10
Structure
CH2=CH2
CH2=CHCH3
CH2=CHCH2CH3
CH2=CHCH2CH2CH3
The dehydration of ethanol to give ethene
This is a simple way of making gaseous alkenes like
ethene. If ethanol vapour is passed over heated
aluminium oxide powder, the ethanol is essentially
cracked to give ethene and water vapour.
To make a few test tubes of ethene, you can use this
apparatus:
It wouldn't be too difficult to imagine scaling this up by
boiling some ethanol in a flask and passing the vapour
over aluminium oxide heated in a long tube.
Dehydration of alcohols using an acid catalyst
The acid catalysts normally used are either
concentrated sulphuric acid or concentrated
phosphoric(V) acid, H3PO4.
Concentrated sulphuric acid produces messy results.
Not only is it an acid, but it is also a strong oxidising
agent. It oxidises some of the alcohol to carbon dioxide
and at the same time is reduced itself to sulphur
dioxide. Both of these gases have to be removed from
the alkene.
It also reacts with the alcohol to produce a mass of
carbon. There are other side reactions as well, but
these aren't required by any current UK A level (or
equivalent) syllabus.
The dehydration of ethanol to give ethene
Ethanol is heated with an excess of concentrated
sulphuric acid at a temperature of 170°C. The gases
produced are passed through sodium hydroxide
solution to remove the carbon dioxide and sulphur
dioxide produced from side reactions.
The ethene is collected over water.
WARNING! This is potentially an extremely
dangerous preparation because of the
close proximity of the very hot
concentrated sulphuric acid and the
sodium hydroxide solution. I knew of one
chemistry teacher who put several
students into hospital by getting it wrong!
That was many years ago before safety
was taken quite so seriously as it is now.
The concentrated sulphuric acid is a catalyst. Write it
over the arrow rather than in the equation.
The dehydration of cyclohexanol to give
cyclohexene
This is a preparation commonly used at this level to
illustrate the formation and purification of a liquid
product. The fact that the carbon atoms happen to be
joined in a ring makes no difference whatever to the
chemistry of the reaction.
Cyclohexanol is heated with concentrated
phosphoric(V) acid and the liquid cyclohexene distils
off and can be collected and purified.
Phosphoric(V) acid tends to be used in place of
sulphuric acid because it is safer and produces a less
messy reaction