Alkenes Chemical Properties

Although the double bond between two carbon atoms is stronger link than a single bond, it is not twice as strong, (i.e. the second bond formed between the carbon atoms is weaker than the first). Thus, the second bond is more vulnerable to attack by suitable reagents, even under fairly mild conditions. Thus, the reaction of this second bond tend to be addition reactions, where the unsaturated carbon atoms become saturated. The alkenes are much more reactive than alkanes.

Combustion of Alkenes

The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water,. For example, ethene burns as follows :

	C2H4   +   3 O2   ==>   2 CO2   +   2 H2O       

Addition Reactions across the Double Bond

Because the alkenes are unsaturated hydrocarbons, their most important reactions are addition reactions across the double bond.

The alkenes are readily oxidised by potassium permanganate to form glycols. For example, ethene is oxidised to ethylene glycol.

	3 H2C=CH2   +   2 KMnO4   +   4 H2O     

			==>     2MnO2  +  2KOH   +   CH2OHCH2OH

							Ethylene Glycol 

During the oxidation of alkenes, the purple colour of the permanganate solution disappears and the reaction constitutes a test, known as Baeyer's Test, to detect unsaturation in any compound.

Addition of Hydrogen

The alkenes are readily reduced by the addition of hydrogen across the double bond to form alkanes (i.e. reduction of alkenes). For example, when an alkenes is passed over a nickel catalyst at 150 degC, the alkene is reduced to an alkane.

	H2C=CH2   +   H2      ==> 	    	CH3CH3  

	Ethene                          	Ethane  

Addition of Halogen

Halogens readily add across the double bond of the alkenes to form dihalides

	H2C=CH2   +   Cl2       ==>     CH2Cl CH2Cl     

	Ethene                          DiChloroEthane  

H2C=CH2 + Br2 ==> H2Br CH2Br
Ethene DiBromoEthane

The decolourisation of bromine is a second test for an unsaturated organic compound.

Addition of Hydrogen Halide

Hydrogen halides readily add across the double bond of the alkenes to form alkyl halides The reactivity of ethene, with the halogen acids is in the order

	HI	>>	HBr	>	HCl     

Thus, ethene reacts readily with hydrogen iodide and with hydrogen bromide at room temperature to form ethyl iodide and ethyl bromide, respectively.

	H2C=CH2     +    HI      ==>    	CH3CH2I

						Ethyl Iodide    

With ethene, the hydrogen atom of the halogen acids can add to either carbon atom to yield bromoethane.

However, with higher members of the ethene series, the orientation of the addition of asymmetric molecules across the double bond is governed by the Markownikoff Rule.

Addition of Water

Water can add across the double bond of the alkenes to form aliphatic alcohols. This is hydration reaction is catalysed under a number of different conditions.

When ethylene and steam are heated (i.e. at 300o Centigrade) under high pressure (i.e. at 70 atm.) in the presence of a catalyst (i.e. phosphoric acid, , on a silica support), ethanol is formed.

	H2C=CH2   +   H2O       ==>     C2H5OH  

Reaction with Sulphuric Acid

Similarly, fuming sulphuric acid absorbs ethylene at room temperature to form ethyl hydrogen sulphate, with much evolution of heat.

	C2H4   +   H2SO4	==>     C2H5.HSO4       

If this is treated with water and warmed, ethanol is formed.

				heat    

	C2H5.HSO4   +   H2O     ==>     C2H5OH + H2SO4          

Polymerisation Reactions due to the Double Bond

When ethylene is heated under great pressure in the presence of a catalyst a large number of the molecules combine to form polythene, (C2H4)n, (i.e. Polyethylene). This particular kind of reaction is called an addition polymerisation and the mechanism by which it takes place is a reaction is a free radical chain reaction. The overall reaction is

		n(C2H4)         ==>             (C2H4)n 

		Ethene                          Polythene

Alkenes Physical Properties

Compound	Formula	MP degC	BP degC	Density (g/ml)
========	=======	=======	=======	========
Ethylene	C2H4	-170	-102	0.6128
Propene		C3H6	-185	-47	0.6142
1-Butene	C4H8	-130	-6.5	0.6356

Alkenes Preparation

There are two general methods for the preparation of alkenes. Both methods involve the dehydration of the appropriate aliphatic alcohol (i.e. the removal of a molecule of water from an alcohol).

• dehydration of the alcohol using concentrated sulphuric acid as the dehydrating agent, or

• by passing the vapour of the alcohol over hot alumina.

Alkenes Structure

The chemical bonding in alkenes can be illustrated by reference to the simplest alkene, ethene. This compound has the following structural

		H       H    

		  C = C        

		H       H    

The double bond represents a four electron bond (i.e. two shared pairs of electrons). However, the two bond between the carbon atoms have significantly different chemical properties, and are formed in different ways. The first bond between the carbon atoms in ethene is a s bonds (sigma bonds) and is similar to the carbon to carbon bond found in the alkane series. However, the second bond between the carbon atoms in ethene is a p-bond (pi-bond), which is much more reactive than the sigma bond and behaves differently in a variety of experimental conditions.

The ethene molecule is planar (i.e. all atoms lie in the same plane) and the bond angle between all the bonds (i.e. carbon to carbon and carbon to hydrogen) is 120 degrees. This observed structure for ethene can be explained in terms of sp2 hybridisation of the orbitals on the carbon atom.

Thus, ethene is a flat molecule, the distance between the carbon atoms being less than that in ethene.

Styrene, C6H5.CH=CH2, is the monomer used for the synthesis of the industrially important plastic, Polystyrene.