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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