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Organic chemistry is a subdiscipline of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.
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Organic Chemistry I – Alkenes
Summary: Organic chemistry is a subdiscipline of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.
Alkenes
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.
Naming Alkenes
These are hydrocarbons with a general formula CnH2n 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
Isomers of Alkenes
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:
Occurrence and Extraction of Alkenes
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 + HydrogenWhen irradiated with high energy radiation, Propane undergo cracking to form methane gas, ethene and hydrogen gas.
CH3CH2CH3 (g) → CH4(g) + CH2=CH2(g)
Physical Properties of Alkenes
Chemical Properties of Alkenes
Combustion/Burning
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 + WaterIn 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 + WaterBurning 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)
Addition Reactions of Alkenes
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 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.
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 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
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.
Uses of Alkenes