Organic Chemistry - Part V

Alkenes
Alkenes are hydrocarbons that have one double bond. They have, as we have seen earlier, sigma and pi bonds. The pi bonds are unsaturated and hence alkenes are known as unsaturated hydrocarbons. Alkenes are more reactive than their alkane counterparts due to the presence of unsaturated pi bonds. Ethene is the first member of the homologous series of alkenes (CH₂ CH₂).  The others are propene, n-butene, n-pentene. We shall study the preparation, properties of this simplest of the alkenes. The properties of others would be similar as they belong to the homologous group. Ethene is also known commonly as ethylene. Its IUPAC name is ethene.  

Ethene  

Ethene is an odourless, colourless and tasteless gas at ordinary room temperatures. It is insoluble in water. The molecular, electronic and structural formula of ethene is shown below.  

Ethene is found in coal gas and in natural gas to some extent. In laboratory, ethene 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 ethene gas
In the lab ethene is prepared by cracking kerosene or candle wax.  

Kerosene is poured over sand and this is kept at the bottom of a hard glass test tube. A few pieces of pumice stone or porcelain is kept a little distance away. The sand is slowly heated. After a while the porcelain portion of the test tube is heated. This is done alternately. The heated kerosene first vapourizes and then cracks. When the vapours pass over the hot porcelain, they crack again into smaller and smaller molecules. The gases are then passed over water. Ethene is collected by downward displacement of water. It can be understood that this method for collecting ethene gas does not give pure ethene gas. This is because from cracking, we get many types of molecules. All those, which are lighter than water and insoluble in water, will be collected.

To obtain pure ethene gas, another method is followed. This is from a chemical reaction with ethanol and concentrated sulphuric acid.  

The temperature of the mixture of ethanol or ethyl alcohol and concentrated sulphuric acid is increased to 160°C. The acid acts as a dehydrating agent and picks up a water molecule from the ethanol molecule, leaving the reaction product as ethene gas.

The laboratory equipment to produce ethene gas is shown below. About 20 to 25 ml of ethanol is taken in a round bottomed flask.  

Concentrated sulphuric acid is added to it from a thistle funnel slowly. Heat is supplied from a Bunsen burner and the temperature of the flask is raised to 160°C. Ethene gas starts evolving and it can be collected over water by downward displacement of water.  

Physical properties of ethene
The low molecular weighted alkenes such as ethene, propene and butene are gases at room temperature. Alkenes with five to 15 carbon atoms are liquid at room temperature.  Higher molecular weighted alkenes are solids at room temperature. Alkenes are insoluble in water but are soluble in organic liquids.  

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

     H₂C CH₂     +  3O₂               2CO₂   +    2H₂O   +  heat

The sooty flame is due to higher amount of carbon in ethene than in methane.  All the carbon         atoms cannot get oxidized while burning, this makes the flame sooty.

2. Reactivity : Alkenes are more reactive than the alkanes 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 alkene will become an alkane. For example if ethene is reacted with chlorine, it becomes 1, 2 dichloro-ethane.

When hydrogen is added to ethene, and heated in the presence of nickel, it becomes ethane.  

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. Such processes are used for hydrogenation of oils.

When hydrochloric acid is added to ethene, it becomes chloro-ethane. The reaction is shown below.  

Similar test with bromine is also used extensively. Bromine water decolorizes on reaction with ethene. This is a prominent test for testing unsaturated nature of hydrocarbons.

3. Polymerization : Alkenes like ethene undergo polymerization, which is a process whereby long chain molecules are formed. Ethene is an unsaturated gas. For making long chain polymer, all its pi bonds can be broken and another ethene can be attached. The second ethene molecule’s all pi bond can be broken to add another ethene molecule. In this way a very long chain molecule or polymer can be produced.

When ethene gas is heated at a temperature of 200°C and at a pressure of 2000 atmospheres, it polymerizes to become a solid called ploythese. The reaction is shown below.  

A large number of ethene molecules join together to make polythene. “n” can be as large as 1000. The molecular weight of the polymer is very large. Monomer of polythene is ethene.  

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