ALKENES

WHAT IS ALKENES?

An alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond.The simplest alkenes, with only one double bond.

1. General formula: C_nH_2n
Where n = 2, 3, 4 … (n = number of carbon)

  

2. Alkenes are unsaturated hydrocarbons which contain one or more carbon-carbon (C = C) double bonds in molecules.

3. The functional group in alkenes is carbon-carbon double (C = C) bond.

Ten First Alkene

Name of alkene	Molecular formula of alkene
Ethene	C2H4
Propene	C3H6
Butene	C4H8
Pentene	C5H10
Hexene	C6H12
Heptene	C7H14
Octene	C8H16
Nonene	C9H18
Decene	C10H20

Physical properties of the alkenes

1. Physical state

Ethene, Propene, and Butene exists as colorless gases. Members of the 5 or more carbons such as Pentene, Hexene, and Heptene are liquid, and members of the 15 carbons or more are solids.

2. Density

Alkenes are lighter than water, therefore, are insoluble in water. Alkenes are only soluble in nonpolar solvent.the density of water is higher than density of alkene.
When going down the series, relative molecular mass of alkenes is higher due to the higher force of attraction between molecules and alkene molecules are packed closer together.

3. Boiling Points

Boiling points of alkenes depends on  more molecular mass (chain length). The more intermolecular mass is added, the higher the boiling point. Intermolecular forces of alkenes gets stronger with increase in the size of the molecules.

Compound	Boiling points (oC)
Ethene	-104
Propene	-47
Trans-2-Butene	0.9
Cis-2-butene	3.7
Trans 1,2-dichlorobutene	155
Cis 1,2-dichlorobutene	152
1-Pentene	30
Trans-2-Pentene	36
Cis-2-Pentene	37
1-Heptene	115
3-Octene	122
3-Nonene	147
5-Decene	170

4. Melting Points

Melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, all alkenes in general have low boiling points and melting points. Alkenes are held together by weak attractive forces between molecules (intermolecular forces) van der Waals’ force. When going down the series, more energy is required to overcome the attraction. Hence, the boiling and melting points increases.

Compound	Melting Points (0C)
Ethene	-169
Propene	-185
Butene	-138
1-Pentene	-165
Trans-2-Pentene	-135
Cis-2-Pentene	-180
1-Heptene	-119
3-Octene	-101.9
3-Nonene	-81.4
5-Decene	-66.3

5.  Electrical conductivity 

All members in alkenes do not conduct electricity.
Alkenes are covalent compounds and do not contain freely moving ions.


6.  Reactivity of alkenes
 
Alkenes are more reactive (unsaturated hydrocarbon).
Alkenes have carbon-carbon (C = C) double bonds which is more reactive than carbon-carbon (C-C) single bonds. All the reaction occur at the double bonds.



Chemical properties of the alkenes

 

1.  Combustion of alkenes

• Complete combustion of hydrocarbons (alkenes) 

          C_xH_y + (x + y/4) O₂ –> xCO₂ + y/2 H₂O                                                                              

          C₂H₄ +  3O₂ –>  2CO₂ +    2H₂O

      (Alkenes burn with sootier flames than alkanes. It is because the percentage of carbon
       in  alkene molecules is higher than alkane molecules and alkenes burn plenty of oxygen  
       to produce carbon dioxide and water)
                                        

• Incomplete combustion occurs when insufficient supply of oxygen
  C₂H₄ + O₂ –> 2C + 2H₂O
  C₂H₄ + 2O₂ –> 2CO + 2H₂O
  (The flame in the incomplete combustion of alkenes is more smoky than alkanes)

2.     Addition of hydrogen (Hydrogenation)

       Addition reaction is atoms (or a group of atoms) are added to each carbon atom of a   
       carbon-carbon multiple bond to a single bond.

       C₂H₄ + H₂ –> C₂H₆ (catalyst: nickel and condition: 200°C)

      Example: margarine (produce from hydrogenation of vegetable oils)

 

   Addition of halogen (Halogenation)
 
   Halogenation is the addition of halogens to alkenes (no catalyst of ultraviolet light is     
    needed).

     Alkene + Halogen –> Dihaloalkane                   ( C₂H₄ + Br₂ –> C₂H₄Br_{2 )}
 
     In this reaction the brown colour of bromine decolourised (immediately) to produce a   
    colourless organic liquid.

    Bromination is also used to identify an unsaturated (presence of a carbon-carbon  
    double  bond) organic compound in a chemical test. 

   Additional of acidified potassium manganate(VII), KMnO₄


   C_nH_2n + [O] + H₂O –> C_nH_2n(OH)₂
   C₂H₄ + [O] + H₂O –> C₂H₅(OH)2


   The purple colour of KMnO₄ solution decolourised immediately to produce colourless organic 
   liquid. Also used to identify the presence of a carbon-carbon double bond in a chemical test.


3.   Polymerisation reaction of alkenes

     Polymers are substances that many monomers are bonded together in a repeating sequence.
     Polymerisation is small alkene molecules (monomers) are joined together to form a long chain 
    (polymer).

                  nCH₂ = CH₂ –> -(- CH₂ – CH₂ -)-_n


   ethene (monomer)(unsaturated compound) –> polyethene polymer (saturated compound)
   It must be carry out in high temperature and pressure.



  
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