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.
C2H4 + 3 O2 ==> 2 CO2 + 2 H2O
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.
H2C=CH2 + H2 ==> CH3CH3 Ethene Ethane
H2C=CH2 + Cl2 ==> CH2Cl CH2Cl Ethene DiChloroEthane
H2C=CH2 + Br2 ==> H2Br CH2Br
The decolourisation of bromine is a second test for an unsaturated organic compound.
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.
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
C2H4 + H2SO4 ==> C2H5.HSO4
If this is treated with water and warmed, ethanol is formed.
heat C2H5.HSO4 + H2O ==> C2H5OH + H2SO4
n(C2H4) ==> (C2H4)n Ethene Polythene
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
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).
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.