Alkynes are the homologous series of unsaturated hydrocarbon with a general molecular formula CnH2n-2.
Alkynes show a high degree of unsaturation than alkenes, hence, they are chemically more reactive than the corresponding alkenes or alkanes.
Examples are :
Ethyne is the first member of the alkyne series. It has a molecular formula, C2H2, and a structural formula of HC = CH.
Ethyne is usually prepared in the laboratory by the action of cold water on calcium carbide. The reaction is carried out on a heap of sand to prevent the flask from cracking as a result of the large quantity of heat evolved.
- Write and name all possible structure of hexyne
- How can you prepare a few jars of ethyne in the laboratory?
NOMENCLATURE: The naming of alkyne are obtained by substituting “ane” in alkanes with ‘ene’.
- Ethyne is a colourless gas with a characteristic sweet smell when pure.
- It is only sparingly soluble in water
- It is slightly less dense than air.
- It is unstable and may explode on compression to liquid.
- Combustion:- it undergoes combustion reaction in air to form water and carbon(iv) oxide
2C2H2 + 5O2 2H2O + 4CO2.
NB: In limited air, it burns with very smoky and luminous flame because of its high carbon content. But in plenty of air and appropriate proportion, it burns with non-luminous very hot flame of about 3000oC.
- Additional Reaction:-Ethyne undergoes addition reaction to produce unsaturated product with double bonds and then a saturated compound with single bond.
a. With hydrogen in the presence of nickel as catalyst.
b. Halogenation: e.g Cl2, Br2, I2
c. Addition of Halides;
Hydrogen halide reacts with ethyne to produce halo-alkene and further halogenation produce halo-alkane.
- Addition reaction with water through dilute tetraoxosulphate (vi) acid in the presence of CuSO4 as catalyst to form ethanol.
- Addition reaction with alkaline KMnO4 added to ethyne, it first turns to green from purple and then to colourless.
In the presence of complex organic –nickel as catalyst to produce benzene.
3 C2H2 C6H6
3 (H – C = C – H ) C6H6
- Substitutional Reaction
- With ammomiacal solution of copper (1) chloride to form reddish brown copper (I) dicarbide
C2H2 + 2CuCl Cu2C2 + 2HCl
H – C = C – H + 2CuCl Cu – C = C- Cu + 2HCl
- With ammomiacal silver trioxonitrate (v) to form white silver dicarbide
C2H2 + 2AgNO3 Ag2C2 + 2HNO3.
H- C = C – H + 2AgNO3 Ag – C = C – Ag + 2HNO3.
N.B: Alkynes can be distinguished from alkene by reacting with ammonical metals of copper(I) chloride and silver trioxonitrate (vi).
- It is used to produce oxyacetylene flame for cutting and welding of metals
- Used in the manufacture of PVC plastics
- It is used in miner’s lamp
- Used in making synthetic fibre
- It is also used in making artificial rubber
Test for Unsaturation
Unsaturated compound decolorizes bromine water.
- Give a chemical test to distinguish between alkyne and alkene.
- Describe a test for unsaturated compounds
New School Chemistry by Y. O Osei yaw Ababio Page
- The concentration of hydrogen ion in a neutral solution is
(a) 10-6 moldm-3 (b) 10-7moldm-3 (c) 5 x 10-7 moldm-3 (d) 1 x 10-8mol dm-3
- Hydrogen can be prepared in a large scale using the
(a) Harber’s process (b) Down’s process (c ) Bosh Process (d) Contact Process.
- Which of the following hydrocarbons is alkyne?
(a ) C2H4 (b) C2H6 (c) C2H2 (d) C3H8
- The final product of complete reaction between ethyne and hydrogen gas is (a) ethane (b)methyl ethane ( c) ethane ( d) hydroethyne
- Ethyne polymerizes in the presence of organonickel complex as catalyst to form (a) polythene (b) benzene (c) polythene (d) methyl benzene.
- a. Calculate the H+ of a solution whose PH is 5.
- State three (3) uses of ethyne
- a. With the aid of labeled diagram, describe the laboratory preparation of ethyne. b. Give a chemical test to distinguish between ethane and ethyne.
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