Homologous Series Members

Alrick Moodie

February 2007

Hydrocarbons

Hydrocarbons are compounds which contain carbon and hydrogen only .

The two families of hydrocarbons which we will explore are :

- the alkanes

- the alkenes

Alkanes

The alkanes have general formula

CnH2n+2

Physical properties of alkanes

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 .

This process is called halogenation

CH4 + Cl2 CH3Cl + HCl

This reaction is carried out :

* In the presence of ultraviolet light

or

* At temperatures of 200oC

The reaction also occurs with bromine although with less vigour

Chemical reactions of alkanes

Alkanes also undergo combustion reactions.

They burn in oxygen to form carbon dioxide and water .

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 atoms therefore they undergo addition reactions .

Reactions

* Hydrogenation

* Halogenation

* Hydration

* Hydrohalogenation

* Combustion

Like alkanes , alkenes will burn in air to form carbon dioxide and water .

C2H4 + 3O2 2CO2 + 2H2O

Hydrogenation

Alkenes will add hydrogen gas to form the alkane .

C2H4 (g) + H2 (g) C2H6 (g)

This reaction is carried out at 200oC over a metal catalyst , usually nickel .

Margarine manufacture

* Some margarine is made by hydrogenating 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".

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 ease at which bromine in a solvent is decolourized .

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 .

Hydration

Alkenes will add water to form the alcohol .

C2H4 (g) + H2O (l) C2H5OH (l)

The reaction is carried out :

* The alkene is passed with steam over a catalyst .

or

* Concentrated sulphuric acid is added to the alkene and the mixture boiled with water to 120oC .

Hydrohalogenation

Alkenes will add hydrogen halide to form the alkyl halide .

C2H4 (g) + HBr (g) C2H5Br (g)

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

Comparison between alkanes and alkenes