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 :

H

H – C = C – H                           HC  – C =  C- H

[

Ethyne.                                               H                 prop-1-yne

 

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 alkaens 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.

 

Chemical Properties

1. Combustion :- it undergoes combustion reaction in air to form water and carbon(iv) oxide

2C₂H₂ + 5O₂2H₂O  +      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 3000^oC.

2.  Additional Reaction :-Ethyne undergoes addition reaction to produce unsaturated      product with double bonds and then a saturated compound with single bond.

 

1. With hydrogen in the presence of nickel as catalyst.

H   HHH

H- C =  C – H  + H₂          H – C = C – H   + H₂                  H – C    – C  – H

ethene                                    H      H    Ethane

1. Halogenation: e.g Cl₂, Br₂,  I₂

ClClClCl

H- C = C – H + Cl₂              H – C = C – H  +  Cl₂         H – C –   C – H

1,2- dichloroetheneClCl1,1,2,2tetrachloroethane

 

Br    BrBrBr

C = C – H  +  Br₂        H –   C  =  C – H   +   Br₂           H–C  –  C – H

1,2 – dibromoethane                       Br    Br                                                                                                                                                 1,1,2,2 -tetrabromoethane.

1. Addition of Halides;

Hydrogen halide reacts with ethyne to produce halo-alkene and further halogenation     produce halo-alkane.

E.g:                                                                          ClCl

H – C = C – H  +HCl       H – C = C – H   +   HCl        H – C –   C – H

 

H   Cl                                 H    H.

Chloroethene             1,2- dichloro ethane.

 

d.Addition reaction with water through dilute tetraoxosulphate (vi) acid in the presence of     CuSO₄  as catalyst to form ethanol.

H      OH                                      H      H

 

H-C=C-H   + H₂O                H-  C   =   C – H                  H- C –     C – H

 

H     OH      Ethanol

Ethenol

1. Addition reaction with alkaline KMnO₄ added to ethyne, it first turns to green from      purple and then to colourless.

O       O

H – C = C – H  +  4[O]                  HO – C   =  C- OH

KMnO₄                Ethane dioc acid.

 

3. Polymerization

In the presence of complex organic –nickel as catalyst to produce benzene.

3 C₂H₂        C₆H₆

3 (H – C = C – H )         C₆H₆

 

4. Substitutional Reaction
5. Withammomiacal 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

1. With ammomiacal silver trioxonitrate (v) to form white silver dicarbide

C₂H₂ + 2AgNO₃Ag₂C₂  + 2HNO₃.

 

H- C = C – H + 2AgNO₃          Ag – C = C – Ag + 2HNO₃.

 

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

 

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.
2. State three (3) uses of ethyne
3. a. With the aid of labeled diagram, describe the laboratory preparation of ethyne.
4. Give a chemical test to distinguish between ethane and ethyne.

 

See also

UNSATURATED HYDROCARBONS

Saturated Hydrocarbons

SIMPLE MACHINE

AIR AND COMBUSTION

AN ALKALI