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CHEMISTRY
TOPIC: ALKENES-Ethene
By Kingsley Idiagbor, B.Sc.(Hons), PGDCs, NCE,
MNSM
Ethene
Ethene is the first and typical member of the alkene family . It
is produced in large amounts by the cracking and the fractional
distillation of petroleum and from natural gas. Ethene finds a lot
of applications in our everyday life and probably the most versatile
in its uses in the petrochemical industry.
Laboratory Preparation
Ethene is prepared in the laboratory by the careful heating of a
mixture of ethanol and tetraoxosulphate (VI) acid. The mechanism of
action is the dehydration of the ethanol molecule by the acid. Due
to the inflammable nature of ethanol, the flat-bottom flask
containing the mixture of the H2SO4 and
ethanol, has to be placed on top of a sand-bath. Furthermore, it is
a wise practice to also include a thermometer over the reactant
mixture to know whether the mixture is being overheated or not.
The reaction which is a two stage process is represented by the
following equations:
- C2H5OH + H2SO4
----------> C2H5HSO4 +
H2O
- C2H5HSO4----------------------->C2H4
+ H2SO4
Properties
- Ethene is a colourless and slightly sweet-smelling gas;
- It is also slightly soluble in water and burns with a bright
flame;
- Ethene melts at -169oC
(-273oF)
- It has a boiling point of -103.8oC
(-155oF)
Because of the presence of the double bond in ethene, it is
very reactive and readily forms many products such as
chloroethane, bromoethane, ethane-1,2, diol and polythene
- Addition Reaction: This is a reaction that involves the
addition of a molecule across a double bond or triple bond to form
a single covalent product.
Formation of
dichloroethane:
Ethene undergoes addition reaction with chlorine to form
1,1-dichloroethane or 1,2-dichloroethane as shown below:
- Polymerization reaction: This is the subjection of
small molecules (monomers) to high temperature and pressure to
form long-chain and heavy molecular weight compound (polymer).
- Ethene undergoes polymerization reaction in the presence of
suitable catalyst such as Ziegler catalyst (triethyl aluminum),
high temperature of 500oC and presence of 250atm to
form polythene, which is a simple plastic.
n(CH2=CH2)-------------------------->(-CH2-CH2-CH2-CH2-)n
Ethene 250atm,600oC
& Ziegler Polythene
Note that both addition and polymerization reaction are the
typical reactions of unsaturated hydrocarbons, i.e. those
hydrocarbons with double or triple bonds like alkenes and
alkynes.
Uses of Ethene
- In agriculture, it is used as a colouring and ripening agent
for many fruits;
- It readily forms many products like chloroethane and
bromoethane
- Polythene, which is a polymer of ethene, is a simple plastic
that is used for packaging;
- Ethene is a petroleum gas that is useful as industrial gas;
- It is used as starter for many industrial products like
detergents and textiles.
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