Alkenes - Organic Chemistry 1: High School Chemistry Form 3

Hydrocarbons, which contain two hydrogen atoms less than the corresponding alkanes, are called alkenes.
They have one double bond and are unsaturated carbon compounds.
Alkenes cannot be obtained directly from crude oil.
They can only be obtained by cracking of alkanes.

These are hydrocarbons with a general formula C_nH_2n where n is the number of Carbon atoms in a molecule. The C=C double bond as the functional group.
A functional group is the reacting site of a molecule /compound.
The carbon atoms are linked by at least one double bond to each other and single bonds to hydrogen atoms.

NOTES ON ALKENES

• Since carbon is tetravalent, each atom of carbon in the alkene MUST always be bonded using four covalent bond /four shared pairs of electrons including at the double bond.
• Since Hydrogen is monovalent,each atom of hydrogen in the alkene MUST always be bonded using one covalent bond/one shared pair of electrons.
• One member of the alkene, like alkanes, differ from the next/previous by a CH₂ group.
• They also form a homologous series.
• The C=C double bond in alkene is the functional group.
• The C=C double bond in alkene can easily be broken to accommodate two more monovalent atoms. The double bond in alkenes make it thus unsaturated.
• Unsaturated hydrocarbon easily reacts to be saturated.

Isomers of alkenes like alkanes have the same molecular general formula but different molecular structural formula.
Isomers of alkenes are also named by using the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature/naming.
The IUPAC system of nomenclature of naming alkenes uses the following basic rules/guidelines:

• Identify the longest continuous/straight carbon chain which contains the C=C double bond to get/determine the parent alkene.
• Number the longest chain form the end of the chain which contains the C=C double bond so the C=C double bond lowest number possible.
• Indicate the positions by splitting “alk-positions-ene” e.g. but-2-ene, pent-1,3-diene.
• The position indicated must be for the carbon atom at the lower position in the C = C double bond.
• Determine the position, number and type of branches. Name them as methyl, ethyl, propyl e.t.c. according to the number of alkyl carbon chains attached to the alkene. Name them fluoro-,chloro-,bromo-,iodo- if they are halogens
• Use prefix di-,tri-,tetra-,penta-,hexa- to show the number of double C = C bonds and branches attached to the alkene.

At industrial level, alkenes are obtained from the cracking of alkanes.
Cracking is the process of breaking long chain alkanes to smaller/shorter alkanes, an alkene and hydrogen gas at high temperatures.
Cracking is a major source of useful hydrogen gas for manufacture of ammonia/nitric(V)acid/HCl.

Long Chain Alkane → Shorter Alkane + Alkene + Hydrogen

When irradiated with high energy radiation, Propane undergo cracking to form methane gas, ethene and hydrogen gas.

CH₃CH₂CH₃ (g) → CH₄(g) + CH₂=CH₂(g)

• Like alkanes, alkenes are colourless gases, solids and liquids that are not poisonous.
• They are slightly soluble in water. The solubility in water decrease as the carbon chain/ molar mass increase but very soluble in organic solvents like tetrachloromethane and methylbenzene.
• Melting/boiling point increase as carbon chain increase. This is because of the increase in van-der-waals /intermolecular forces as the carbon chain increase.
• The first four straight chain alkenes (ethene,propene,but-1-ene and pent-1-ene)are gases at room temperature/ pressure.

Alkenes burn with a yellow/ luminous sooty/ smoky flame in excess air to form carbon(IV) oxide and water.
In excess air/oxygen, the following reaction occurs

Alkene + Air → Carbon (IV) Oxide + Water

In limited air Alkenes burn with a yellow/ luminous sooty/ smoky flame in limited air to form carbon(II) oxide and water.

Alkene + Air → Carbon (II) Oxide + Water

Burning of alkenes with a yellow/ luminous sooty/ smoky flame is a confirmatory test for the presence of the C=C double bond because they have higher C:H ratio.

Ethene burning in excess air:

Ethene + Air → Carbon (IV) Oxide + Water
C₂H₄(g) + 3O₂(g)→ 2CO₂(g) + 2H₂O(l/g)

Ethene burning in limited air:

Ethene + Air → Carbon (II) Oxide + Water
C₂H₄(g) + 2O₂(g)→ 2CO(g) + 2H₂O(l/g)

• Bromination/Halogenation
  The double bond of an alkene will undergo an addition reaction with aqueous bromine to give a dibromo compound.
  The orange bromine water is decolourised in the process
  

  H₂C=CH₂ + Br₂ H₂BrC-CH₂Br

• Hydrogenation
  Hydrogenation is an addition reaction in which hydrogen in presence of Palladium/Nickel catalyst at high temperatures react with alkenes to form alkanes.
  
  During hydrogenation, one hydrogen atom in the hydrogen molecule attach itself to one carbon and the other hydrogen to the second carbon breaking the double bond to single bond.
  

  

  This reaction is also called saturation of the double bond.
• Oxidation
  Alkenes are oxidized to alkanols with duo/double functional groups by oxidizing agents.
  This is accomplished by using acidified potassium manganate (VII) or potassium/sodium dichromate(VI) solution at room temperature and pressure.
  The purple manganate (VII) solution is decolourised during the reaction.
  The orange acidified potassium/sodium dichromate(VI) solution turns to green.

  

• Hydrolysis
  Hydrolysis is the reaction of a compound with water/addition of H-OH to a compound.
  Alkenes undergo hydrolysis to form alkanols .This takes place in two steps.
  

  - Alkenes react with concentrated sulphuric(VI)acid at room temperature and pressure to form alkylhydrogen sulphate(VI).
  - On adding water to alkylhydrogen sulphate(VI) then warming, an alkanol is formed.

  Alkenes + concentrated sulphuric(VI)acid → Alkylhydrogen Sulphate(VI)
  Alkylhydrogen Sulphate(VI) + Water (then warm) → Alkanol

• Addition Polymerization
  Polymerization is a process where small molecules called monomers join together to form large molecules called polymers at high temperatures and pressures.
  Only alkenes undergo addition polymerization.
  Addition polymers are named from the alkene/monomer making the polymer and adding the prefix “poly” before the name of monomer to form a polyalkene.
  During addition polymerization; the double bond in alkenes break, free radicals are formed, the free radicals collide with each other and join to form a larger molecule.

• In the manufacture of plastics e.g Polythene, polychloroethene, P.T.F.E
• Hydrolysis of ethene is used in industrial manufacture of ethanol to produce beer.
• In ripening of fruits.
• In the manufacture of detergents.