Addition of hydrogen, halogen, water, hydrogen halides(Markownikov’s addition and peroxide effect)
Addition of Hydrogen to Alkenes
The addition of hydrogen to the carbon-carbon double bonds of the alkenes is called hydrogenation. Overall, it is the reductive removal of the functional group which has double bonds. This addition of hydrogen to an alkene causes the production of alkane. The mechanism of hydrogenation is an example of the heterogeneous catalysis process and generally, this reaction takes place on the metal’s surface. The molecules of hydrogen react with the atoms of the metal at the surface of the catalyst. It causes the breaking of the relatively strong hydrogen-hydrogen sigma bond and it is replaced with the two weak metal-hydrogen bonds. Then there is an interaction of the pi bonds of the alkene with the metal catalyst and it weakens the bond. As a result, the second hydrogen atom is transferred from the surface of the catalyst and it makes the alkane. This alkane is released from the surface of the catalyst and it allows the catalyst to accept the additional hydrogen and the alkane molecules.
Addition of Halogen to Alkenes
Mostly bromine and chlorine are used for the halogenation of alkenes. As the halogen approaches to the double bond of the alkene, the electrons of the double bond repel the electrons of the halogen and causes the polarization of the halogen bond and creates a dipolar moment in halogen molecule bond. This addition reaction is stereoselective. For this reaction of halogenation, the inert solvents can be used. This halogenation will produce vicinal dihalides with the anti-addition.
Addition of Water to Alkenes
In the presence of a catalyst, the addition of water to the alkenes will cause the formation of the alcohols. Usually, this reaction is proceeding via the standard carbocation mechanism and follows the few steps. At first, there is an attraction between the hydrogen atom and the pi bond. The second carbon is converted to the carbocation. Then an acid-base reaction occurs between the carbocation and water molecule and oxonium ion is produced. Then this ion is stabilized by losing the hydrogen ion and resultantly alcohol is formed.
Addition of Hydrogen Halides to Alkenes
The alkenes react with the hydrogen halides, and these hydrogen halides include the hydrogen bromide and hydrogen chloride. Hydrogen bromide interacts with the alkenes in the cold. There is the breaking of double bond and bromine is attached to one of the carbons and hydrogen is attached to another one. When ethene reacts with the hydrogen bromide the bromoethane is formed as a result.
There is an empirical rule that is based on the Markovnikov’s experimental observations for the addition of hydrogen halides to the alkenes and according to this rule the reaction between the hydrogen halides and unsymmetrical alkene to yield an alkyl halide, causes the addition of hydrogen to the carbon of the alkene having the greater number of hydrogen substituents and the halogen is attached to the carbon of the alkene having the fewer number of the hydrogen substituents.
The free radical addition of the hydrogen halides such as hydrogen bromide to the alkenes is known as the peroxide effect. Usually, the hydrogen halides react with the alkenes by an electrophilic system of addition, but in the presence of the organic peroxides, hydrogen bromide is added by the different mechanisms.