Unsaturated Hydrocarbons: Alkynes

Alkynes are the  homologous series of unsaturated hydrocarbon  with a general molecular formua CnH_2n-2.

Alkynes show a high degree of unsaturation than alkenes, hence,t hey are chemically more reactive than the corresponding alkenes or alkanes.

Examples are :

ETHYNE

Ethyne is the first member of the alkyne series.  It has a molecular formula, C₂H₂, and a structural formular of HC = CH.

Laboratory Preparation

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.

Evaluation

1.         Write and name all possible structure of hexyne

2.         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’.

Physical Properties

1.         Ethyne is a colourless gas with a characteristic sweet smell when pure.

2.         It is only sparingly soluble in water

3.         It is slightly less dense than air.

4.         It is unstable and may explode on compression to liquid.

N.B:  Alkynes can be  distinguished from alkene by reacting with ammomiacal metals of           copper(I) chloride and silver trioxonitrate (vi).

USES

1. It is used to produce oxyacetylene flame for cutting and welding of metals
2. Used in the manufacture of PVC plastics
3. It is used in miner’s lamp
4. Used in making synthetic fibre
5. It is also used in making artificial rubber

Test for Unsaturation

Unsaturated compound decolorizes bromine water.

Evaluation

1. Give a chemical test to distinguish between alkyne and alkene.
2. Describe a test for unsaturated compounds

READING ASSIGNMENT

New School Chemistry by Y. O Osei  yawAbabio Page

WEEKEND ASSIGNMENT

1.         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

2.         Hydrogen can be prepared in a large scale using the

            (a) Harber’s process     (b) Down’s process   (c ) Bosh Process   (d) Contact Process.

3.         Which of the following hydrocarbons is alkyne?

            (a ) C₂H₄           (b) C₂H₆            (c) C₂H₂            (d) C₃H₈

4.         The final product of complete reaction between ethyne and hydrogen gas is

            (a) ethane                    (b)methyl ethane         ( c) ethane       ( d) hydroethyne

5.         Ethynepolymerises in the presence of organomickelcomplext as catalyst to form

            (a) polythene               (b) benzene                  ( c) polythene    (d) methyl benzene.

Theory

1.         a. Calculate the H⁺ of a solution whose PH is 5.

            b. State three (3) uses of ethyne

2.         a. With the aid of labeled diagram, describe the laboratory preparation of ethyne.

            b.  Give a chemical test to distinguish between ethane and ethyne.