The functional group in alkenes is the carbon-carbon double bond. A carbon-carbon double bond consists of a strong sigma bond and a relatively weaker π bond. The typical reactions of alkenes are the reactions in which the π bond breaks and two strong sigma bonds are formed in its place.
A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction. Most of the addition reactions in alkenes are initiated by electrophilic species because the loosely held 1t electrons in alkenes serve as source of electrons for the attacking species. Thus, the characteristic reactions of alkenes are electrophilic addition reactions.
1. Addition of Hydrogen
Alkenes react readily with hydrogen in the presence of finely-divided nickel, platinum or palladium as catalysts. The process is known as catalytic hydrogenation. The ultimate products of this reaction are alkanes
Hydrogenation is used in the manufacture of margarine from vegetable oils.
2. Addition of Halogens
Halogens, especially chlorine and bromine add on alkenes readily producing a dihalogen derivatives. The reaction is carried out in inert solvent like CC14. For example, ethene reacts with bromine dissolved in carbon tetrachloride to give 1, 2-dibromoethane.
During this reaction reddish brown colour of bromine is decolorized
ADDITION OF UNSYMMETRICAL REAGENTS
Most of the addition reactions of alkenes are those with unsymmetrical reagents like HCl, HBr and H2O, etc. When such reagents react with alkenes, one part of the molecules attaches itself to one carbon atom whereas the other part to the second carbon atom of the multiple bond.
Some examples are as follows:
1. Addition of Halogen Acids
Alkenes react with halogen acids to form alkyl halides.
In case the alkene and the reagent are both unsymmetrical then two products are theoretically possible. For example, for the reaction between propene and HBr, in the dark and in the absence of peroxides there can be two addition products, but the experiments reveal that 2-bromopropane is the major product while 1-bromopropane is minor product.
Such addition reactions are governed by the empirical rule which was given by the Russian Chemist Markownikov (1869) after studying a series of similar reactions. This rule is known after his name as Markownikov’s rule. The rule states that:
During the addition across unsymmetrical double bond, the negative part of the adding molecule attaches itself to the carbon atom carrying less number of hydrogen atoms.
2. Reaction with Bromine Water (Addition of HOBr)
Alkenes react with bromine water to form bromoalkanols. The reaction involves overall addition of HOBr.
During this reaction reddish brown colour of bromine water is decolorized.
3. Addition of Tetraoxosulphate(VI) Acid (H2SO4) Alkenes react with cold, cone. tetraoxosulphate(VI) _acid to form alkyl hydrogen sulphates. The addition of tetraoxosulphate(VI) acid takes place in accordance with Markownikov’s rule.
This reaction may be used for removal of alkenes from gaseous mixtures.
Alkyl hydrogensulphates on hydrolysis with water or dilute H2SO 4 produce alcohols.
4. Addition of Water (H2O)
Water adds to alkenes in the presence of mineral acids. In this case also the addition is governed by Markownikov’s rule. For example, propene, on reaction with water, gives propan-2-ol as the major product.
The reaction of alkenes with water to give an alcohol is known as hydration and is an industrially important method for the manufacture of alcohols.
SOME OTHER REACTIONS OF ALKENES
Alkenes react with cold dilute neutral or alkaline KMnO4 solution to give vicinal dials (or glycols).
During the reaction the purple colour of potassium tetraoxomanganate(VII) solution disappears and brownish precipitate of manganese(IV) oxide are formed.
Alkenes, like other hydrocarbons, burn in the presence of oxygen or air to give carbon(IV) oxide and water. A large amount of heat is produced during this reaction.
2CnH2n + 3nO2 à7 2nCO2 + 2NH2O + Heat
C2H4 + 3O2 à7 2CO2 + 2H2O + Heat
In limited supply of oxygen, alkenes on combustion give carbon(II) oxide and carbon in addition to carbon(IV) oxide and water vapour.
The process of combination of a large number of molecules of same or different substances under suitable conditions to form a giant molecule with very high molecular mass is called polymerization. The product obtained by virtue of polymerization reaction is called polymer. When ethylene is heated with traces of oxygen under pressure, it polymerizes to polyethylene or polythene
Similarly, propylene on polymerization gives polypropylene