Homologous Series Members
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Transcript of Homologous Series Members
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Homologous Series Members
Alrick Moodie
February 2007
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Hydrocarbons
Hydrocarbons are compounds which
contain carbon and hydrogen only .
The two families of hydrocarbons which
we will explore are :
- the alkanes- the alkenes
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Alkanes
The alkanes have general formula
CnH2n+2
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Physical properties of alkanes
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Chemical reactions of alkanes
Alkanes are saturated hydrocarbons i.e.
they have the maximum number of
hydrogen atoms .
Alkanes undergo substitution reactions
where one of the hydrogen is replaced by
a next element .
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This process is called halogenation
CH4 + Cl2 CH3Cl + HCl
This reaction is carried out :
1. In the presence of ultraviolet light
or
2. At temperatures of 200oC
The reaction also occurs with brominealthough with less vigour
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Chemical reactions of alkanes
Alkanes also undergo combustion
reactions.
They burn in oxygen to form carbon
dioxide and water .
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Alkenes
Alkenes are hydrocarbons which contain
one carbon to carbon double bond
C=C
The double bonds means that alkenes are
unsaturated . They can hold more atomstherefore they undergo addition reactions .
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Reactions
Hydrogenation
Halogenation
Hydration Hydrohalogenation
Combustion
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Like alkanes , alkenes will burn in air to
form carbon dioxide and water .
C2H4 + 3O2 2CO2 + 2H2O
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Hydrogenation
Alkenes will add hydrogen gas to form the
alkane .
C2H4 (g) + H2 (g) C2H6 (g)
This reaction is carried out at 200oC over ametal catalyst , usually nickel .
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Margarine manufacture
Some margarine is made byhydrogenating carbon-carbon double
bonds in animal or vegetable fats and oils.
You can recognize the presence of this in
foods because the ingredients list will
include words showing that it contains
"hydrogenated vegetable oils" or
"hydrogenated fats".
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Halogenation
Alkenes will add the halogen to form the alkyl
halide .
C2H4 (g) + Br2 (l) C2H4Br2 (l)
This reaction is carried out at room temperature .
This is the qualitative test for alkenes , the easeat which bromine in a solvent is decolourized .
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The bromine is usually in a non-polar
solvent such as carbon tetrachloride CCl4
Aqueous bromine , bromine water can be
used .
The colour would go from red-brown to
colourless .
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Hydration
Alkenes will add water to form the alcohol .
C2H4 (g) + H2 O (l) C2H5OH (l)
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The reaction is carried out :
1. The alkene is passed with steam over a
catalyst .
or2. Concentrated sulphuric acid is added to
the alkene and the mixture boiled with
water to 120
o
C .
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Hydrohalogenation
Alkenes will add hydrogen halide to form
the alkyl halide .
C2H4 (g) + HBr (g) C2H5Br (g)
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Alkenes also undergo oxidation . The
reactions are not necessary to be taught at
this stage . The products could be :- the diol
- the carboxylic acid
- the aldehyde / ketone
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Comparison between
alkanes and alkenes
alkane Alkene
Main type of rxn Substitution Addition
Combustion Produces CO2
and H2O
Produces CO2
and H2O
With Bromine Slow rxn and at
high
temperature
Rapid
decolourization
With acidified
manganate
No rxn Decolourizes