We are familiar that hydrocarbons can be categorized into saturated and unsaturated compounds. Alkenes and alkynes are categorized as unsaturated hydrocarbons having general formula CnH2n and CnH2n-2 correspondingly. They are characterized by the C = C and C° C bonds correspondingly. Remember the systems of IUPAC nomenclature for such classes of homologous ending with the suffix - ene for alkene and - yne for alkynes.
Fig: ethene and ethyne
Ethene and ethyne are the first members of the alkenes and alkynes series correspondingly. The concept of isomerism and cis-trans isomerism exists in the alkenes whereas Isomerism doesn't exist in alkynes as the molecules are collinear. The main feature of the chemistry of alkenes and alkynes is their addition reactions in which the C = C and CoC are transformed into C-C bond. For the alkenes, the products made up of such addition reactions by chlorine and bromine are oily liquids; therefore the older name of 'olefines' for the series.
Sources of Alkenes and Alkynes:
Natural sources of alkenes:
a) Cracking of alkanes:
Natural gas includes large amounts of ethane, propane and butane; and such alkanes can be cracked either thermally or catalytically. The alkenes made are then separated from the resultant gas mixture.
C2H6 → C2H4 + H2
C3H6 + H2 → C3H8 → C2H4 + CH4
b) Cracking of naphtha:
The mixture of naphtha and steam are heated to around 800°C. The cracked mixture obtained is then separated to a liquid fraction (gives fuel oil and petrol) and a gas fraction that includes hydrogen, C1-C4 alkanes and C1-C4 alkanes. Such gases can be separated through distillation under pressure.
Manufacture of ethyne:
Ethyne is the most common member of the alkyne series and will thus be employed as the representative member for alkynes.
Modem methods produce ethyne, on a large scale, from methane or naphtha by applying a temperature of around 1500°C for a fraction of a second. Hydrogen is obtained as a useful by-product.
2CH4 → (1500oC) → C2H2 + 3H2
This method has substituted the older method of obtaining ethyne by reaction between calcium dicarbide and water. The reaction still serves as a satisfactory source of ethyne on a small scale specifically for welding purposes.
CaC2 + H2O → Ca(OH)2 + C2H2
Laboratory Preparation of Ethene and Ethyne:
(a) Ethene by dehydration of Ethanol:
Ethene is generated by dehydration (elimination of water) of ethanol. This can be accomplished in two ways, either by passing the vapor of ethanol over finely divided aluminium oxide heated to 300oC; or heating the ethanol by excess concentrated tetraoxosulphate (vi) acid at 180oC. Both the reagents act as dehydrating agents.
C2H4 + H2O → (Al2O3 at 300oc) → C2H5OH → (excess conc. H2SO4 at 180oc) → C2H4 + H2O
(b) Ethyne from calcium dicarbide:
The reaction of calcium dicarbide by water, as illustrated above, is the method employed to get ethyne in laboratory.
Isomerism in Alkenes:
The theory of isomerism was illustrated and alkenes are state to represent cis-trans isomerism. All the alkenes above C3H6 exhibit isomerism. For C4H8, remember that the structure of cis-but-2-ene and trans-but-2-ene.
a) Addition reactions:
The Alkenes are unsaturated because of the presence of C - C and thus atoms can add across the double bond to give addition products and in the process transform C = C to C - C bond. Four kinds of molecules, H2, halogens (Cl2, Br2, I2), hydrogen halides (HCl, HBr2, HI) and H2O; can add on to the C = C.
- H2C = CH2 + H2 → (Ni or Pt at 200oc) → H3C - CH3 hydrogenation
(Helpful for the hardening of oil to margarine)
- H2C = CH2 + Cl2 → (room temp) → ClCH2-CH2Cl halogenation (Example: chlorination)
- H2C = CH2 + HI → (room temp) → H3C-CH2I Hydrohalogenation (example: hydroiodination)
- H2C = CH2 + H2O → (H2SO4 at 80oC) → H3C - CH2OH Hydration
b) Oxidation reaction:
The aqueous solution of potassium tetraoxomanganate (VII) acting as the oxidizing agent can add two -OH groups onto C = C of alkenes.
Fig: oxidation reaction
c) Polymerisation Reaction:
This is an industrially significant reaction of alkenes. Polymerisation is the method by which most of the simple molecules (termed as monomers) join altogether to form very giant molecules (termed as polymers), Alkenes, specifically ethene, is a significant monomer and undergo addition polymerisation. For ethene:
2nCH2 = CH2 → ... (-CH2-CH2-CH2-CH2-)-n
ethene Poly (ethene)
Polythene is broadly employed for making buckets, toys, bottles, pipes, cups, spoons, packing materials and cable insulating materials.
a) Addition Reactions:
Alkynes, similar to alkenes, are unsaturated and so undergo addition reaction. For alkynes the reaction takes place in two phases, the first addition transforms the C° C to C = C and the second addition transforms C = C to C - C. For illustration:
Fig: Addition Reactions of Alkynes
The chloroethene product is significant in making polyvinyl chloride (PVC); an inert polymer employed as insulating materials for the electrical cables and boots.
Ethyne doesn't polymerize so really as ethene. On heating, though, ethyne is polymerized to benzene, and significant hydrocarbon.
Test for Unsaturation:
The presence of multiple bonds (that is, Unsaturation) as found in alkenes and alkynes can be detected in the laboratory by employing two common reaction.
a) Bromine in carbon tetrachloride (Br2/ CCl4):
Whenever an alkene or alkyne is passed via Br2/CCl4 or bromine water (brown) there is decolourisation as the bromine which is responsible for the brown color adds across the multiple bonds.
b) Aqueous potassium permanganate [KMnO4(aq)]:
The purple color of KMnO4(aq) disappears (decolorized) whenever any unsaturated hydrocarbon (alkenes and alkynes) is passed through. This is an oxidation method in which the purple Mn(VII) is transformed to colourless Mn(V).
The two tests are utilized to differentiate saturated alkanes from unsaturated alkenes and alkynes.
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