Organic Chemistry - Part VI

Alkynes
Alkynes are hydrocarbons that have triple bonded carbon atoms. The carbon atoms are bonded together by one sigma bond and two pi bonds. The pi bonds are not localized and hence these hydrocarbons are unsaturated.  Alkynes are more reactive than their alkene and alkane counterparts.  Ethyne is (CH  CH) is the first member of the homologous series of alkynes.  Its molecular formula is C₂H₂.  The other members are propyne C₃H₄, butyne C₄H₆, pentyne C₅H₈, etc. We shall study the preparation, properties of this simplest of the alkynes. The properties of others would be similar as they belong to the homologous group. Ethyne is known commonly as acetylene. Its IUPAC name is ethyne.

Ethyne
Ethyne is a colourless and tasteless gas at ordinary room temperatures. It smells slightly like ether. It is insoluble in water. The molecular, electronic and structural formula of ethyne is shown below. Ethyne or acetylene gas is a linear molecule.  

Ethyne is found in coal gas to some extent. In the laboratory, ethyne can be prepared by cracking of large alkane molecules. (Cracking is a process whereby large organic molecules are heated and broken up into smaller organic molecules).

Laboratory preparation of ethyne gas
In the lab ethyne is prepared by the action of water on calcium carbide. The chemical equation is:

The apparatus used for producing the ethyne gas is shown below.  

Water is slowly dropped on small pieces of calcium carbide kept in a conical flask. Calcium carbide reacts with water to give off ethyne gas (or acetylene gas). The gas is collected by downward displacement of water as it is insoluble in water.

Physical properties of ethyne
The low molecular weighted alkynes such as ethyne, propyne and butyne are gases at room temperature. Alkynes with five to 13 carbon atoms are liquid at room temperature.  Higher molecular weighted alkynes are solids at room temperature. Alkynes are insoluble in water but are soluble in organic liquids. Gaseous alkynes can produce a feeling of anesthesia when inhaled.

Chemical properties of ethyne
1. Combustion : Ethyne burns in air with a sooty flame. It forms carbon dioxide and water and gives out heat.  

      HC CH     +  5O₂                       4CO₂   +    2H₂O      +  heat

The sooty flame is due to higher amount of carbon in ethyne than in methane.  All the carbon         atoms cannot get oxidized while burning this makes the flame sooty. But if ethyne is burnt with a proper control, for example, if the gas is made to pass through a small nozzle, then it gets ample air mixture to burn completely.  This type of complete combustion is used for acetylene lamps in industries. Acetylene lamps produce very luminous non-sooty flame.

Ethyne combined well with oxygen can burn to give a flame whose temperature is 3000°C. This oxy-acetylene flame is used for welding metals, where very high temperatures are required.

2. Reactivity : Alkynes are more reactive than the alkanes or alkenes due to the presence of unsaturated bonds. The pi bond is not localized and hence can be broken easily in a reaction.  Such a reaction is called addition reaction. In an addition reaction, the pi bond converts into a sigma bond and the alkyne will  become an alkane. For example if ethyne is reacted with chlorine, it becomes 1,1,2,2 tetra-chloro-ethane.  

Similarly, addition reaction with bromine will give rise to 1,1,2,2, tetra-bromo-ethane. Bromine water decolorizes on reaction with ethyne. This is a prominent test for testing unsaturated nature of hydrocarbons.

When hydrogen is added to ethyne, and heated in the presence of nickel, it becomes ethene and then proceeds to become ethane. The bonds become saturated.  

This is known as the process of hydrogenation. The addition of hydrogen to a double or triple bonded hydrocarbon leads to saturation of the bonds.

When hydrochloric acid is added to ethyne, it becomes first chloro-ethene and then 
1,1, dichloro-ethane. The reaction is shown below.  

3. Polymerization : Alkynes like ethyne undergo polymerization, which is a process whereby long chain molecules are formed. Ethyne is an unsaturated gas. It polymerizes in two fashion, one way forms cyclic or aromatic hydrocarbons and another way forms long chain polymers. For making aromatic or cyclic hydrocarbons, only a few of the pi bonds are broken. For making long chain polymer, all its pi bonds can be broken and another ethyne can be attached. The second ethyne molecule’s all pi bond can be broken to add another ethyne molecule. In this way a very long chain molecule or polymer can be produced.

When ethyne (gas) is passed over a solution of copper chloride and ammonium chloride in HCl, it undergoes polymerization reaction to form solid divinylacetylene. The reaction is shown below.  

When ethyne gas is passed through a red hot tube, it polymerizes to form an aromatic compound called benzene (C₆H₆). The reaction is shown below.  

Next        Main        Previous