Alkanes and alkenes

1. Homologous Series
A homologous series is a group of compounds with the same homologous series, and successive members differ by a  –CH2 group.

Members in the same homologous series have the same general formula, similar chemical properties, and gradual change in physical property as a result of an increase in the mass of the molecules. The gradual increase in the mass of the molecules result in the increase in melting and boiling points, and the decrease in flammability and volatility.

2. Alkanes

Alkanes are saturated hydrocarbon with the general formula of CnH2n+2.

Saturated means that the compound consist of only single bonds. 
A hydrocarbon is a compound that contains hydrogen and carbon only.

Molecular Formula
Full Structural Formula

structure of methane, alkane structure
structural formula of methane


alkane structure, structure of ethane
structural formula of ethane


alkane structure, structure of propane
structural formula of propane

alkane structure, structure of butane
structural formula of butane

Alkanes are generally unreactive. The reactions they undergo include substitution reaction and combustion.

Alkanes undergo complete combustion (in the presence of excess air) to form carbon dioxide and water only.
alkane + oxygen ---> carbon dioxide + water

E.g. methane + oxygen ---> carbon dioxide + water
CH4 + 2O2 ---> CO2 + 2H2O

Alkanes undergo substitution reaction with halogens in the presence of heat or ultra violet light (uv light) to form halogenoalkanes.
E.g. methane + chlorine -------> chloromethane + hydrogen chloride
3. Alkenes
Alkenes are unsaturated hydrocarbon, that contain the C=C double bond. The general formula of alkene is CnH2n.

Unsaturated compounds contains multiple bonds (e.g. double or triple bonds).

Molecular Formula
Full Structural Formula
alkene structure, structure of ethene
structural formula of ethene

alkene structure, structure of propene
structural formula of propene


alkene structure, structure of 1- butene, butene
structural formula of 1- butene

alkene structure, structure of 2- butene, butene
structural formula of 2- butene

Alkenes are more reactive than alkanes, due to the presence of the electron rich C=C.

Alkenes undergo combustion and addition reaction.
alkene + oxygen ---> carbon dioxide + water

E.g. ethene + oxygen ---> carbon dioxide + water
C2H4 + 3O2 ---> 2CO2 + 2H2O

Alkenes undergo addition reaction. During the reaction, a compound is added across the C=C double bond, and a saturated product is formed. The following are examples of some addition reactions that alkenes undergo:

  • Addition of hydrogen to alkene, with nickel catalyst, at 200 oC will produce alkane.
           alkene + hydrogen ----> alkane 

  • Addition of halogen to alkene, at room temperature, will produce halogenoalkane.

          alkene + halogen  ---->  halogenoalkane

  • Addition of steam to alkene, at 300oC, 60 atmosphere, with phosphoric acid as catalyst, will produce alcohol.
           alkene + steam  ---->   alcohol

  • Addition polymerization is a process where the monomers are added together to form the polymer, without a loss in any part of the monomer.
            alkene ----> poly(alkene)

4. Distinguishing alkenes (unsaturated compounds) from alkanes (saturated compounds) 

Add bromine to each sample at room temperature. The sample containing alkene (or unsaturated compound) will turn reddish brown bromine colourless. The sample containing alkane (or saturated compound) will remain reddish brown.

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